Vintage vespa with sidecar
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Molto Verboso
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Tue Sep 15, 2020 9:01 pm quote
Quote:
This kind of cylinder with the maxed out bridge port is a good point of reference. Won't all work for yours but some of the ideas are transferable. When the time comes we can do the detail.
Well - there is a worldwide pandemic occurring, the west coast of the United States is on fire, we have a contested election - that hasn't even occurred yet - and none of us can agree on any facts.

And yet - within our group of misfits - we know that 1mm of exhaust port lift = about 1HP of output and raises the power band. How I love this forum.

In that spirit - and because I like causing Jack's angina to flare - I give you this:
piston ramping.
It is popular in piston ported motor bicycling.
If you don't know what that is - its that really loud bicycle with a two stroke strapped to it - pouring soot at the tail pipe - that passed you at lunch the other day.
Think less refined vespa meets steam punk.

Rather than try and grind the roof of the transfers to lift them - these guys grind the top of the piston to lower it.
The net effect is to lift port timing- tho I imagine it doesn't do wonders for your squish band.
But then - they are not all about refinement.
They do it on the exhaust ports and transfers.
The interesting part to me was - this could allow some adjustment of transfer height to lower Blow Down timing - without having to try and grind transfer port roofs - something none of us really do.
It is also lower risk - a new piston is $80 - not $450 like a cylinder.

I post it in the spirit of the MHR cylinder project - and the previously noted Jack's angina.
My challenge on the MHR: If we were to create two paths - one to optimize grunt, and one to optimize peak HP - what would be the different exhaust port shapes we would go after - and what other tuning would we combine?

ramps.jpg
This is a fairly common practice for motor bicycles - they adjust both transfer and exhaust port timing by altering the piston top

imageproxy.php.jpeg
In situ - example of exhaust port "lifting" by lowering of the piston.

Ossessionato
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Tue Sep 15, 2020 9:15 pm quote
Well isn’t that interesting, did it come that way? Are you planning to alter it more?

By the for the next couple days I’ll be hanging out, upto page 15
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
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Tue Sep 15, 2020 9:42 pm quote
Hopefully a long way from angina just woke up all steady at 50bpm.

Nice idea, great for top bodging work but not so suitable for yours. The angle kicks up the transfer flow and all the torque disappears. The exhaust angle is ok but does promote detonation. Only suitable for raising peak power and rpm at the cost of torque. Or testing something with an old piston.

Who would raise rpm on a piston with rings that thick? I think we all know.

Exhaust port shape could go a lot of ways. If you changed that box for a quiet expansion, then even more options. With all the politics, fires and pandemics, who would notice.
Molto Verboso
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Tue Sep 15, 2020 10:16 pm quote
Quote:
Exhaust port shape could go a lot of ways. If you changed that box for a quiet expansion, then even more options. With all the politics, fires and pandemics, who would notice.
Could run motor oil in the fuel tank and no one would blink.
Mind you - they are already blinking and eyes are watering from the air.

Whats with the bottom edge angle of the MHR ex. port?
Why not have that more perpendicular?
What are they going for with that?

img_6767_17191.jpg
Red line shows floor angle - fairly steep rather than perpendicular.

img_7114_19855.jpg
BGM port (before and after porting). Radiused rectangle with flat top and perpendicular bottom in comparison. (I squared up the top a bit further after this trace).

Jet Eye Master
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Wed Sep 16, 2020 2:02 pm quote
This is to try and salvage some torque from the large exhaust port. We won't be changing that.
How straight is the top edge? Can you draw a line on it with some measurements?
Molto Verboso
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Wed Sep 16, 2020 5:34 pm quote
Quote:
How straight is the top edge? Can you draw a line on it with some measurements?
Did a second trace to see how that top edge looked.
The weird dip you see in the middle is directly related to the chamfering job they When they did the corners - there was more chamfer - giving a high point.
Managed to capture it in photos below.

It raises the age old question on measuring.
Do you measure the chamfers or not?
The port maps are actually maps of the chamfers - not the ports - so they can be misleading - unless of course that counts - in which case they are accurate.

Jack - do you have an opinion on that? Are we measuring the port of the chamfers when dialing in tight specs?

IMG_7665.jpg
Here is a trace of the ports. The bow at the top line is really just the chamfer size growing at the corners

IMG_7663.jpg
A careful eye along the top edge of the port - you can see the chamfer is larger at the corners.

IMG_7667.jpg
chamfer to chamfer

Jet Eye Master
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Wed Sep 16, 2020 11:29 pm quote
I have an opinion on everything ☺️ always measure ports from inside. Port maps are just for reference, nothing accurate.
Lot can be done with that port. Still reckon we'll get 30 bhp out if it.

Can you put a straight line across the top of the port so I can see the general curve or angle?
Molto Verboso
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Thu Sep 17, 2020 6:32 am quote
Quote:
always measure ports from inside
Kinda guessed this might be the answer...

Top of port looks flat to me - once you take the crazy chamfer job out of the equation.

What's the trick for not sanding in to the stud holes?
How does one know the thickness of the wall between ex. port and stud hole?

IMG_7665.jpg

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Thu Sep 17, 2020 7:07 am quote
you guys make some weird art
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
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Thu Sep 17, 2020 1:25 pm quote
Going to have to raise that port a bit. You remember what we said before?

Stud holes is a real challenge. Experience is how you know. They can get thin without being an issue but no going through. Easy to say be careful.

Like they kept it straight across. Makes it easier to change. Will need a new packet of sanding drums for this job. That port will look a whole lot bigger.

First things first, need to run it in stock on the SC. Any news on rods?
Molto Verboso
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Thu Sep 17, 2020 1:39 pm quote
Quote:
you guys make some weird art
It's a butterfly - not a vjj - but hey - everyone sees something different.
(Vjj standing of course for Verband Junger Journalisten - just to be clear).
Quote:
Going to have to raise that port a bit. You remember what we said before?
Not whatsoever. Wider/taller/? (wild guess).
Quote:
First things first, need to run it in stock on the SC. Any news on rods?
Actually - yes.
Got a note today.
I have been granted one of the first lot of 28MM carbs.
It will come with the test rods.
Considering my plans - but can't see keeping this on the bench.

Have plenty of data on my SI best as is.
Will likely upgrade to the 28 and tune in.
See what that does for us before any change of jug.
Could be here as soon as the weekend.

Still interested to bench race the exhaust port plan for the MHR.
Motor will need to come out for that upgrade.
MHR transfers at jug skirt will require some JB weld on cases I suspect - to ensure sealing.
Will also allow some careful measures of port timing at stock - then as we lift and port.
I'm curious about port shaping as well - as is evident I'm sure.
Ossessionato
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Thu Sep 17, 2020 3:14 pm quote
Birdsnest wrote:
you guys make some weird art
Looks even better if you squint real hard, brings back memory's of my younger days.
Veni, Vidi, Posti
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Thu Sep 17, 2020 6:42 pm quote
Those SC guys don't do stuff in a hurry, do they? What size is the current one again?

Glad you are going to re-test the BGM with the 28 before swapping to Malossi. Pity you can't put the BGM back to stock to test!
Molto Verboso
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Thu Sep 17, 2020 7:54 pm quote
SC guys - like many in our world - are a small outfit.
Once Ginch turned me on to the GSF forum discussion on smart carb (about 100 pages - makes my thread seam quaint) and I realized that everyone was clamoring for the 28's - I figured it would be a while.
But the SC guys kept their word and allocated one of the first batch to replace my 25mm.
It'l be fun to see how a larger carb affects HP using Dyno.

Now that I have some data with the AFR meter with the SI - Im kinda interested to see how the SC goes when matched.
When I had the 25SC on previously - I suspect I was rich down low and lean up top.
Right rods will be interesting to experiment with.

If the 28 doesn't arrive by the weekend - I will likely go ahead and put the 25 SC back on - and meter it with the AFR - and see what I see.
I don't really have good numbers for the GSF on the tuned BGM with the SC due to lack of rods.

Is it possible that my new velocity stack (seen below) will have an effect on richness like a bell mouth does?

IMG_7444.JPG

Jet Eye Master
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Thu Sep 17, 2020 10:05 pm quote
Hopefully the MHR exhaust port doesn't turn into a vjj.

It's possible the SC 25 never worked right at least now there is a chance. The 28 will feel a lot more than 3mm bigger but still small. 35 is big enough.

Looking forward to seeing the MHR bolt on in comparison with the tuned BGM, should be a good fight.
Veni, Vidi, Posti
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Fri Sep 18, 2020 1:47 am quote
charlieman22 wrote:
Is it possible that my new velocity stack (seen below) will have an effect on richness like a bell mouth does?
Since nobody wants to dip into the bellmouth/velocity stack question - and yes I know you've asked at least once and maybe twice...

One thing it has to do is smooth out the gas flow, which I'm guessing has the potential to improve atomisation. And it looks like a good one works better than a bad one, duh. So due to less turbulence at the actual opening, it really does increase the amount of air entering the carb.
If I was to guess based on your picture, I'd say it'll make a real difference. It will increase air flow. More air needs more fuel. Where in the rev range that fuel is needed most... I don't know.

But it it works out, you'll need to put it inside a filter.

The other thing I keep seeing is talk about airboxes on more modern machines... apparently the volume has a big effect, and swapping from an airbox to a plain filter almost always reduces power noticeably. I wonder if this is the 'still air' effect, or optimised flow within the airbox itself?



bellmouth.jpg

Ossessionato
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Fri Sep 18, 2020 3:09 am quote
Ginch, I know that filters reduce flow, both indirectly, because whenever you look at HIGHLY tuned motors, e.g. the Germans dyno'ing their 75 HP smallies, they're not running filters, and also from direct observation of my Sprint, where as soon as I put a filter on the PWK, it doesn't want to rev out.

I hadn't considered trying a bell mouth to increase the venturi effect until just now. Maybe I'll see what I can find and give it a shot, just for comparison...eventually...
Molto Verboso
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Fri Sep 18, 2020 7:13 am quote
Ginch wrote:
charlieman22 wrote:
Is it possible that my new velocity stack (seen below) will have an effect on richness like a bell mouth does?
Since nobody wants to dip into the bellmouth/velocity stack question - and yes I know you've asked at least once and maybe twice...

One thing it has to do is smooth out the gas flow, which I'm guessing has the potential to improve atomisation. And it looks like a good one works better than a bad one, duh. So due to less turbulence at the actual opening, it really does increase the amount of air entering the carb.
If I was to guess based on your picture, I'd say it'll make a real difference. It will increase air flow. More air needs more fuel. Where in the rev range that fuel is needed most... I don't know.

But it it works out, you'll need to put it inside a filter.

The other thing I keep seeing is talk about airboxes on more modern machines... apparently the volume has a big effect, and swapping from an airbox to a plain filter almost always reduces power noticeably. I wonder if this is the 'still air' effect, or optimised flow within the airbox itself?

I'm curious if I can find one for my Mikuni TMX 30. Its a little rich with the foam filter installed. Would be interesting to see if a velocity stack leaned it out at all.
Molto Verboso
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Fri Sep 18, 2020 8:09 am quote
Jack -
When this is all said and done - I imagine I will locate a second set of cases - maybe even something fancy - and use the spare cylinder/parts to create a reed valved beast with welded in reinforcements/ full monty - but still full monty touring...
That one can have the 35mm.
The 28mm is built on the same billet as the 25mm - so it should fit my almost downdraft™ intake manifold.
Like that idea.

Suspect the gang would be interested in seeing some sketches on top of my "butterfly" of an exhaust port - showing trade's between peak and torque options if you feel like showing off a bit. I know I would!

Good stuff Ginch!
Thanks!

For anyone interested in experimenting - there is a company on Aliexpress that is stocking these in the US.
$6.00 apiece in a range of cool colors - makes it the price of an in and out burger.
Come in a range of lengths.
My opening is 45mm.
I used an O ring to make a nice seal.

https://www.aliexpress.com/item/4000381534562.html?spm=a2g0o.productlist.0.0.7a9d6331D6JtQ9&algo_pvid=6d22e787-0003-445d-912c-d65b9db2adfc&algo_expid=6d22e787-0003-445d-912c-d65b9db2adfc-0&btsid=0bb0624316004437718548624e5461&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_

(Use the search term "carburetor cup" on aliexpress if you want to search it.
Then pay attention to the USA vs China button you can click to choose shipment from US.
Amazon has some as well - bit smaller set of choices.)

Kinda surprised that just using the bell mouth without a stack can perform so well! Interesting chart.
Molto Verboso
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Wed Sep 23, 2020 4:29 pm quote
BGM Tuned SI24/24 Vs Smart Carb Results
Alright folks - so here is a look at how we did.
These were performed with the SI as well tuned as I could manage - and the SC with EQ rods.
Since I am doing these tests with the throttle pegged wide open (WOT) - The only factor on the Smart Carb was the tip of the rod (the rest of it is not in use for this test).

Couple notes and an asterisk*.
Notes:
- Smart and SI were both run with a bell mouth and no air filter.
- I used means to average out the runs - shown in chart.
- This does not reflect how the throttle feels - only WOT.
- The Smart Carb feels more crisp through the first 1/2 throttle - but I managed to get the SI as a close second.
- The SI has one run that was off the charts - with a much leaner jet. I couldn't repeat it - but it was real. GPS showed 63MPH on my speed run - a record.

Here comes the *

* I suspect that that one hot run that I couldn't repeat - no matter how many times I tortured the guys that insist on golfing next to my speed run (see what I did there), was exhaust pipe related. Meaning: perhaps I had it at just the right temperature - where it resonates at just the right way - in just the right moment - with just the right air density - and Venus aligned with Mars - and I hit 18.1HP., and it did it at a higher RPM (giving higher speed). The SI and the SC regularly gave back 17.4 or 17.5 on other runs - and perhaps the means shown here are lower due to single lower run being included (at least for the SC).

Think my 28MM arrives on Friday - and that will be the next thing I upgrade.
From there - I am going to put on a polini box to see how it compares to the Road II on this set up) and then perhaps a touch more lift to the cylinder that Jack has proposed - before retiring this cylinder for a future project.

Screen Shot 2020-09-23 at 4.17.52 PM.png
That one magical run with a hand cut 127 and just the right temp exhaust. Blue EQ30/ Red EQ32/ Blk is SI carb with 130MJ

IMG_7193.JPG
The Smart back on - with a velocity stack/bell mouth.

Screen Shot 2020-09-23 at 5.18.36 PM.png
These are SI 24/24 only. About 17 different runs. Red is the 130MJ. Blk is 128 and Blue is 132. Green is the magic run on the 127. The 130 was the most pleasurable to ride.

Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
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Wed Sep 23, 2020 10:42 pm quote
After the multiple test runs and probably twice as many not recorded, this is now pretty conclusive, SI24 or SC25 are just about identical. So 40 years on in carb technology makes absolutely no difference to the power but looks cool and is slightly smoother running. Not a lot for the money. The 28mm version will be more interesting.

Do the 28mm, Polini box and BGM packer in three steps in this order. To see if any improvement at each stage. Which will be the greatest increase? I wonder.
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Thu Sep 24, 2020 5:22 am quote
Still the best tuning show on the interwebs.
Molto Verboso
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Thu Sep 24, 2020 6:30 am quote
Quote:
Still the best tuning show on the interwebs.
Chuckled. Well - one of us is learning something anyhow...
Quote:
After the multiple test runs and probably twice as many not recorded, this is now pretty conclusive, SI24 or SC25 are just about identical. So 40 years on in carb technology makes absolutely no difference to the power but looks cool and is slightly smoother running. Not a lot for the money. The 28mm version will be more interesting.
And that somebody would be me.
Mostly agree with this - but would note two things.
That smoothness also comes with more responsive feel in 1/4-1/2 throttle - an area it turns out I use quite a bit.
Did all I could with the SI to try and optimize - and was surprised by how much I could squeeze out of it - even when I thought I already had it "right" - but was never able to get as much power/responsiveness to minor throttle change as I was the SC.

In truth - I had never really intended to put a 25mm side draft on - they just didn't have the 28 available. The idea was initially to be able to move up in carb size but not struggler with jetting.

That said: all this jetting has made me more competent at it - so I no longer fear jetting in a big SI - which would give a lot of value for the money in comparison to the 28MM SC. Looks like they go for about $300 - which is more than $200 of "savings". Not sure what kind of science project it would be to get it running as smoothly and powerfully down low as the SC. Has anyone had any experience with a 28MM SI?
Quote:
Do the 28mm, Polini box and BGM packer in three steps in this order. To see if any improvement at each stage. Which will be the greatest increase? I wonder.
That's the plan.

Contest: Anyone wanna guess what will produce the most gains?
Molto Verboso
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Thu Sep 24, 2020 10:29 am quote
charlieman22 wrote:
Quote:
Still the best tuning show on the interwebs.
Chuckled. Well - one of us is learning something anyhow...
Quote:
After the multiple test runs and probably twice as many not recorded, this is now pretty conclusive, SI24 or SC25 are just about identical. So 40 years on in carb technology makes absolutely no difference to the power but looks cool and is slightly smoother running. Not a lot for the money. The 28mm version will be more interesting.
And that somebody would be me.
Mostly agree with this - but would note two things.
That smoothness also comes with more responsive feel in 1/4-1/2 throttle - an area it turns out I use quite a bit.
Did all I could with the SI to try and optimize - and was surprised by how much I could squeeze out of it - even when I thought I already had it "right" - but was never able to get as much power/responsiveness to minor throttle change as I was the SC.

In truth - I had never really intended to put a 25mm side draft on - they just didn't have the 28 available. The idea was initially to be able to move up in carb size but not struggler with jetting.

That said: all this jetting has made me more competent at it - so I no longer fear jetting in a big SI - which would give a lot of value for the money in comparison to the 28MM SC. Looks like they go for about $300 - which is more than $200 of "savings". Not sure what kind of science project it would be to get it running as smoothly and powerfully down low as the SC. Has anyone had any experience with a 28MM SI?
Quote:
Do the 28mm, Polini box and BGM packer in three steps in this order. To see if any improvement at each stage. Which will be the greatest increase? I wonder.
That's the plan.

Contest: Anyone wanna guess what will produce the most gains?
Is the 25 mm Smart Carburetor actually 25mm? The reason I'm asking is, it has a 7.8% bigger area at WOT over the 24mm SI carburetor. I would have thought the graphs for the two would have shown an edge in top end power for the 25mm. Makes me wonder if 24 or 25mm is on the edge of usability for what you have. I'm also curious to know if the SC has a better or more stable AFR reading for you. Hoping to get my speedometer buttoned up tomorrow and get a few miles on the new motor. Want to get a few hundred more miles before leaning it out.

From seeing the numbers for the two carburetors already, I would have to think you would need a bigger change such as a packer under the cylinder to change timing or an exhaust pipe to take advantage of the larger carburetor.
Ossessionato
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Thu Sep 24, 2020 11:48 am quote
Jack221 wrote:
After the multiple test runs and probably twice as many not recorded, this is now pretty conclusive, SI24 or SC25 are just about identical. So 40 years on in carb technology makes absolutely no difference to the power but looks cool and is slightly smoother running. Not a lot for the money. The 28mm version will be more interesting.

Do the 28mm, Polini box and BGM packer in three steps in this order. To see if any improvement at each stage. Which will be the greatest increase? I wonder.
I bet the SC25 is like comparing apples to oranges for tuning in though?
Molto Verboso
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Thu Sep 24, 2020 12:40 pm quote
Birdsnest wrote:
Still the best tuning show on the interwebs.
So true. Thanks for doing this for the community Dr. CM
Veni, Vidi, Posti
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Thu Sep 24, 2020 6:55 pm quote
charlieman22 wrote:
Contest: Anyone wanna guess what will produce the most gains?
What's the packer idea? Went back a bit but didn't find it...

To a degree the carb and pipe rely on each other so you won't see the difference in isolation, if you take my meaning. But I think that the 28 will perhaps add a little more than the pipe.

Molto Verboso
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Thu Sep 24, 2020 9:21 pm quote
Quote:
Is the 25 mm Smart Carburetor actually 25mm?
I assume so - tho never measured.
Would probably be hard pressed to find other forum thread where someone put a sidedraft carb on at only 1mm larger than their SI?
Smart Carb gave me this one off the owners desk - when they were sold out everywhere - as a holding spot for the 28 - which was still in development.
Quote:
From seeing the numbers for the two carburetors already, I would have to think you would need a bigger change such as a packer under the cylinder to change timing or an exhaust pipe to take advantage of the larger carburetor.
One thing I'll be interested in is the resonance change with the exhaust.
I bought the Road II blindly. It was called the II so I thought it must be a recent generation. ¯\_(ツ)_/¯
I've had exhaust envy ever since.

Lynn - once you grind your way through some jetting on the SI - they are not so complicated. But... yes - with only one circuit in the SC carb - the maddening circling often required with jetting in the SI is eliminated.
Quote:
What's the packer idea? Went back a bit but didn't find it...
Jack had proposed that I lift the cylinder a bit more.
I've been reticent as I want to balloon the RPM curve out more around the 5000RPM range.
Power is just so useable there.

Ginch - looks from your chart like I might see my peak increase - but at lower RPM - with the Polini box. Will be interested to see how I compare to this chart.

Tracking number in hand - 28MM and some EK rods arrive tomorrow.
🙂

screen_shot_2020_07_26_at_101535_pm_19642.png
This was from the stock BGM. Looks to me like the exhaust port work and slight lift gave me about 2HP increase - but it cost me a little in the 4000-5000 RPM range.

Molto Verboso
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Fri Sep 25, 2020 4:41 am quote
charlieman22 wrote:
Quote:
Is the 25 mm Smart Carburetor actually 25mm?
I assume so - tho never measured.
Would probably be hard pressed to find other forum thread where someone put a sidedraft carb on at only 1mm larger than their SI?
Smart Carb gave me this one off the owners desk - when they were sold out everywhere - as a holding spot for the 28 - which was still in development.
Quote:
From seeing the numbers for the two carburetors already, I would have to think you would need a bigger change such as a packer under the cylinder to change timing or an exhaust pipe to take advantage of the larger carburetor.
One thing I'll be interested in is the resonance change with the exhaust.
I bought the Road II blindly. It was called the II so I thought it must be a recent generation. ¯\_(ツ)_/¯
I've had exhaust envy ever since.

Lynn - once you grind your way through some jetting on the SI - they are not so complicated. But... yes - with only one circuit in the SC carb - the maddening circling often required with jetting in the SI is eliminated.
Quote:
What's the packer idea? Went back a bit but didn't find it...
Jack had proposed that I lift the cylinder a bit more.
I've been reticent as I want to balloon the RPM curve out more around the 5000RPM range.
Power is just so useable there.

Ginch - looks from your chart like I might see my peak increase - but at lower RPM - with the Polini box. Will be interested to see how I compare to this chart.

Tracking number in hand - 28MM and some EK rods arrive tomorrow.
🙂
It'll be interesting, to see your well tuned comparison of the two exhaust pipes. What you're seeing with torque and hp moving up the rpm range is why I don't want to lift my cylinder or increase timing. Unless your running 7000 rpm all the time your not going to take advantage of the ~ 12% top end power increase. But you will notice ~8% in the power band where you will use it more. To many people tune for WOT max HP and lose sight of the big picture daily driving aspect.
Molto Verboso
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Fri Sep 25, 2020 7:24 am quote
Christopher - here are the tuned vs not tuned curves.
It shows how the HP is now greater - starting at about 6400RPM.

However - this can be a bit misleading as it is at WOT only.
When I am at WOT - I'm frequently above 6000RPM - so the sensation is that it is more powerful.
I occasionally recognize a drop off in lower RPM torque - but rarely.
With 11HP at 5K RPM (WOT), I have close to a stock P200's peak HP - not that more wouldn't be welcome.

To put that in perspective, if I am in 4th gear at 6K RPM, I am doing about 53MPH.
So if I am on the hwy moving a bit slower and traffic speed pics up - I am right on the power with no downshift as I open up the throttle.
Likewise - when driving in traffic at 40MPH - in 4th - I still have 10HP at my disposal. That is fine for cruising and I don't find myself wanting to shift down to speed up - just a twist of throttle gets it done.

With all that said - my goal is to creep that HP up in the 5K range to hit a min. 12HP if I can. Power through the middle is never unpleasant.

Oh - and AFR. The SC seams to read in similar manner on AFR meter. My guess - AFR is jumping around because of resonance (burned and unburned fuel mixing) - but just a guess.

Screen Shot 2020-09-25 at 8.06.27 AM.png

Ossessionato
76 Sprint V, 63 GL, 62 VBB, 05 Stella
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Location: Chicago. Well, Evanston, but that's almost Chicago
Fri Sep 25, 2020 10:36 am quote
charlieman22 wrote:
However - this can be a bit misleading as it is at WOT only.
When I am at WOT - I'm frequently above 6000RPM - so the sensation is that it is more powerful.
Don't forget you can generate dyno runs at 1/2 throttle. That can be illuminating, too.
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
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Location: London UK
Fri Sep 25, 2020 2:29 pm quote
Nice comparison on the BGM. Didn't lose much at all. After the 28, box and packer lift, the mid loss should come back and the peak up a bit more. Too much blowdown at the moment

Tuning is not a digital thing. Carefully done you get the best of everything (apart from low rpm torque but we don't talk about that...... or fuel economy). There's a tuning job to suit everyone.

Last edited by Jack221 on Fri Sep 25, 2020 10:52 pm; edited 1 time in total
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 1732
Location: california
Fri Sep 25, 2020 7:58 pm quote
Quote:
Too much blowdown at the moment
Your co- workers must love it when your right.
🙂

Chandlerman - good point on options for measure.
That ride I took up Ca1 into a coastal headwind eve me a good benchmark.
12+ HP at 5000 RPM can pull my rig pretty damn well.
Of course - can't have SoCal whipping my ass from 7K-9K when in a group ride - so you can see my dilemma.

28MM is here.
It's a modest increase in size - but interested to try.

Side note:
The PLX AFM has been a bit hopeless.
Think smart millennials with arrogance, poor quality, and no work experience.
Hint: you can only correspond by email with customer service - and if they respond - they will not speak on the phone even after they give you an RMA for a device that failed within weeks.
But I digress...
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
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Location: London UK
Fri Sep 25, 2020 10:57 pm quote
Guessing is great but not as accurate as a graph.

Does the 28 go on the same manifold? Results by tomorrow if it does.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Location: california
Fri Sep 25, 2020 11:25 pm quote
Quote:
Does the 28 go on the same manifold? Results by tomorrow if it does.
Fits same manifold.
I make some mods to the carb tho - including a brass elbow for fuel inlet - required as I have the carb tucked in tight between stock chassis and motor casing.
Ironic - I know - but I didnt want to dent or cut into my chassis.
Means I have to mod the idle screw, the choke pull as well.
Elbow is pressed in and then JB welded for security.
Curing now.

Would you expect the larger bore carb to require richer jetting or similar?
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
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Location: London UK
Sat Sep 26, 2020 2:14 am quote
So results Sunday.
Not denting the frame is funny. Have you had a look under that frame lately. It's not a concurs winner. Unless it ever becomes a race.

Now, that's a good question. If the 25 was a bit too small and the 28 allows it to be free flowing, the jetting may need to be richer. If the carb size was of no consequence it might be the same jetting. If the carb was way too small and jetted beyond its capacity the 28 may run leaner. So richer, same or leaner. Could be any. Means nothing without any Dyno data (real, gsf or butt).
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 1732
Location: california
Sat Sep 26, 2020 1:48 pm quote
Well - it's Sunday somewhere.
New carb - had to see how it went...
Didn't take much to tune in.
Certainly thirstier - moved down two rods.
Power band noticeably expanded downward - can jump ahead of traffic and zip with 1/3 throttle or less.
Small bump at top as well - suspect the pipe or lift are limiting factors there.

IMG_7232.jpg
It's really remarkable how vertical the carb is willing to sit/run.

IMG_7237.JPG
'Cause it's orange, it goes just a little faster.

Screen Shot 2020-09-26 at 2.24.47 PM.png
Blue is untuned BGM with 25mm carb.
Red is lifted and ported BGM with 25MM carb.
Green is same with 28MM carb.
Will new exhaust allow me to hit 12HP at 5KRPM? We're gonna find out.

Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2400
Location: London UK
Sat Sep 26, 2020 2:47 pm quote
You're keen. That is near 1 bhp across the board. Was thinking you might get something at the top but the mid is a bonus. So making more power with a smaller jet means the 25mm was maxed out or SC did some technical trickery with the atomiser. Either way going to be better to ride. Are they working on a 35mm yet?
Polini exhaust is sure to make more power somewhere.
And raising the packer some more will likely increase the mid even further. Will end up beating a P200 on half throttle, while pulling a sidecar and passenger.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 1732
Location: california
Sat Sep 26, 2020 4:53 pm quote
SC rods are backwards - so that 28 is actually richer than the 30 I was running on the smaller carb.
The move up from 25mm to 28mm required a richer rod (tho it has a smaller number...)

Yeah - I was super pleased with the feel.
Having to just about re-build the Polini box from scratch (only half kidding) to make it fit with my subframe.
Fab is done.
Let paint dry - so I could assemble (again).
Got out for quick trip in the hood.
Feels great at lower RPM.
Will be interested to see how it tests tomorrow.

Screen Shot 2020-09-26 at 5.48.01 PM.png
Paint booth

IMG_7245.JPG
just enough room not to hit subframe after I repositioned the mounting bracket on the box

Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
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Location: Victoria, Australia
Sat Sep 26, 2020 4:56 pm quote
I see you don't have spring holders on the Polini... that was something that I found annoying to have to do.
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