Vintage vespa with sidecar
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Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
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Location: London UK
Sun Jun 21, 2020 11:37 pm quote
Advance and retard are relative terms, depends what you're talking about. We might all be right. Two stroke variable does the opposite if four, now why is that?

If you had an air leak the jetting would be weak. Only continue to retard the timing if the plug chop at the target rpm looks good. Too weak it will be detonating and too rich it will seize. Only retard the timing under Goldilocks jetting conditions and nothing bad will happen.
Addicted
bare metal cafe racer
Joined: 01 Sep 2017
Posts: 809
Location: Aotearoa (New Zealand)
Mon Jun 22, 2020 12:04 am quote
charlieman22 wrote:
Not quite.
Haha... you mean - "exactly the opposite". Totally why I posted that. I felt lost.

This is what I was trying to figure out - because the chart doesn't actually refer to "retard" or "advance" - just + and -. Which is fine if you understand timing to begin with. But simply "-5" could be -21 - 5 = -26 or 21 - 5 = 16. I do understand that -26 = badness, so makes more sense now.

And as SoCal said - at high RPM you want the spark closer to TDC. The VAPE mostly retards (from the timing spot - guess it's all relative otherwise).

Found this which seems to explain the "why" pretty well.
http://www.dragonfly75.com/motorbike/timingcurves.html

Makes me think that I need a static CDI with my setup even more. Shelve the variable for if and when I actually tune the lump. Thoughts tho - does the VAPE Sport (variable) have any place on a stock setup? Does if offer any advantages or is it actually detrimental to a stock setup?

(I'm well aware my pipe is for looks and sound... that my setup is not using it for any meaningful power... but that I could...

I think I went variable because I was going to get a Kheper - sold on the "more power and fuel efficiency", and figured that the same would be true of the VAPE Sport. But then the Kheper's don't seem to last. And then the VAPE Sports were selling out all over the place - so they had to be good!
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 1426
Location: california
Mon Jun 22, 2020 12:11 am quote
Quote:
Haha... you mean - "exactly the opposite". Totally why I posted that. I felt lost.
A bit of California understatement.
What can I say.
It's trickier than it looks - but seems you've got it.

The difference between the static and variable systems is the CDI.
You could opt to buy a static and keep your variable for your back up.
Just a thought.

Your post unearthed another key point.
"goldilocks conditions".
So I will ensure all is right - before I go after my timing adjustments.

Takes a Village, People.
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
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Posts: 2134
Location: London UK
Mon Jun 22, 2020 3:44 am quote
Variable timing is always better when set up to the engine properly. Set up badly it's a disaster. On bone stock it's possible get away with some strong advance, which makes regular riding really comfortable.
bodgemaster
1963 GL, 1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
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Mon Jun 22, 2020 7:20 am quote
Quote:
Two stroke variable does the opposite of four, now why is that?
Boom. Question of the Day

Here’s Eric Gorr’s explanation (with my emphasis - notice he’s talking about an engine with an expansion pipe):

“Here is how changes in the static ignition timing affects the power band of a Japanese dirt bike. Advancing the timing will make the power band hit harder in the mid range but fall flat on top end. Advancing the timing gives the flame front in the combustion chamber, adequate time to travel across the chamber to form a great pressure rise. The rapid pressure rise contributes to a power band's "Hit". In some cases the pressure rise can be so great that it causes an audible pinging noise from the engine. As the engine rpm increases, the pressure in the cylinder becomes so great that pumping losses occur to the piston. That is why engines with too much spark advance or too high of a compression ratio, run flat at high rpm.”

“Retarding the timing will make the power band smoother in the mid-range and give more top end over rev. When the spark fires closer to TDC, the pressure rise in the cylinder isn't as great. The emphasis is on gaining more degrees of retard at high rpm. This causes a shift of the heat from the cylinder to the pipe. This can prevent the piston from melting at high rpm, but the biggest benefit is how the heat affects the tuning in the pipe. When the temperature rises, the velocity of the waves in the pipe increases. At high rpm this can cause a closer synchronization between the returning compression wave and the piston speed. This effectively extends the rpm peak of the pipe.

If Gorr’s correct, the slightly higher exhaust temperature that results from retarded timing is actually a benefit when running a tuned pipe.

With a box pipe? Maybe not so much ...

Last edited by SoCalGuy on Mon Jun 22, 2020 7:53 am; edited 1 time in total
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Mon Jun 22, 2020 7:53 am quote
SoCal - Great find.
When I read it - it confirmed something I experience related to heat - even with a box exhaust as well.

The scoot is crisper at revs and idles better when the pipe gets to temperature.
I don't mean 200° - I mean closer to 250°. That temp is often only reached when its put hard through its paces - or run for some time on open road holding a single gear.

There are diminishing returns I'm sure - but the exhaust operates better - at least on mine - between 250 and 275 degrees - making the bike noticeably smoother.
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
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Posts: 2134
Location: London UK
Mon Jun 22, 2020 11:47 am quote
Heat is definitely a help, as long as it's not too much. This is why my last exhaust was lagged with insulation. PX221 - Reed build with 38mm Dellorto
Heavy retard at max rpm and lagging make an expansion work harder. The trick is how much retard and what temperature.
CM will have no trouble getting over 300F with the BGM when it's tuned.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Mon Jun 22, 2020 11:59 am quote
Just stepped in from a Goldilocks checkout.
Found two suspicious items that could be effecting.
1. Cylinder studs strip at head bolt. Not visible - but would not torque down fully. Pic of head gasket suggests possible leak.
2. Piston top - no additional ping marks - I benchmarked with pic previously - though I have gone down two metering rods. Also - Piston remains pretty wet and looks like it is getting significant wash to me. Will continue to monitor.
3. Plastic cowl. BGM has a head design with fins going in different direction than normal. My plastic cylinder cowl may be sitting too tightly on the head - this is due to longer bigger stud on which the cowl mounts. Used spacer and offset it about 5-10mm from where it was.

About to go for a check ride to see if any of this makes a difference.
Will then do a plug chop in addition if I see temps still high.

Question - Do aluminum cylinders run hotter than iron by nature? For other BGM users - what temps do they see under various conditions? Chime in!!

IMG_6320.JPG
Plug - fairly new so not a lot of color. few tiny specs from some pinging perhaps - at some point - but not focused there as I have had timing all over the place so a bit was to be expected along the way.

Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Mon Jun 22, 2020 1:13 pm quote
and here is a WOT plug chop.
Color is a bit hard to see...
Looks like if I had held that for a mile - I would have had a gray ring base on the flat - and a 2mm brown ring at the bottom of the plug?
Or is this what lean looks like?
How long do you have to hold WOT to get a good read?
Scaring the neighbors.
-CM

IMG_6341.JPG
EQ31

IMG_6315.JPG
*edit - pic of the top of the piston at this point for reference. Looks wet to me - and wash seems large still - but early - less than 2 hours probably - 10 heat cycles.



Last edited by charlieman22 on Mon Jun 22, 2020 3:55 pm; edited 1 time in total
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
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Posts: 2134
Location: London UK
Mon Jun 22, 2020 1:44 pm quote
As long as possible, at least 30 seconds held at WOT is usually enough. Are you still running that 100 octane? That's going to confuse the set up. Better off on regular gas to avoid damage.

What depth head nuts are you using? Seems like you strip a lot.

Plug looks too weak. Crown looks like it's not been run enough to comment.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Mon Jun 22, 2020 1:50 pm quote
Plan to swap back to 1 richer and see how temps react.
Will do second plug chop on that plug - and speed run.
Will attempt to get a full 30 seconds - should have just left it in second gear and waited it out longer.
Stud nuts - using nice shoulder nuts - number of strips is ridiculous at 13ft lbs.
Have one 8mm stud - that was time certed at the cases.
That one has zero issues.
The 7mm studs are useless in my view - too easy to strip - single use.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Mon Jun 22, 2020 3:58 pm quote
Moved down one metering rod to EQ30.
Heat as bad if not worse - no improvement.
Still hunting for the root of my issue.

IMG_6355.jpg
EQ30 - small ring suggests I might be lean? Will keep richening and see if there is a turning point on temp - or if it is unrelated.

Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
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Posts: 7658
Location: Victoria, Australia
Mon Jun 22, 2020 6:43 pm quote
Typically 290 - 330 on mine.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Mon Jun 22, 2020 7:15 pm quote
Quote:
Typically 290 - 330 on mine.
Wow. Am I chasing a non issue?
What carb/timing/ tuning?

*Edit - and what RPM do you set your idle to? (aprox)
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Mon Jun 22, 2020 9:30 pm quote
Here is a shot of the three plugs - three different metering rods.
The lower the number - the richer the mix.
With each test - I managed to run the scoot further at WOT.
What can I say - improved process with time.
But may have an effect on how it is marking plugs as well...
Using second gear so I can just blast it for a good 30 seconds without driving a half mile.

Interestingly
- bike does not get hot doing these runs - WOT - 30 seconds - no problem.
- no apparent effect on temps with change of metering rods
- altering timing (so far) has also produced no alteration to temps

Temps:
6000 RPM, 50MPH, half throttle = 300 degrees and climbing.
Do I just need to grow a set of balls?
Does this cylinder just run hotter than the polini?
Or is there a hidden issue.
TBD.
🙂
-CM

(Jack - running mix of 100 octane and pumped fuel 50/50. don't want to change as this is what I had been using and prefer to keep it consistent).

IMG_6361.jpg

Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2134
Location: London UK
Mon Jun 22, 2020 10:28 pm quote
Looks like 29 is the winner. What temperature does it get to? If it gets to 330 and stops it's ok. If it keeps going, then we all know where that will end.

Backing off the timing will cool the CHT, just need to move it more.

I don't get it, what is actually stripping? The nut, stud or casing?
Molto Verboso
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Mon Jun 22, 2020 11:23 pm quote
Quote:
Looks like 29 is the winner. What temperature does it get to?
Cooled it off at 300° each time - not allowing it to continue to climb as of now.
Quote:
Backing off the timing will cool the CHT, just need to move it more.
Have that in the tool bag - was assuring 'goldilocks' conditions first.
Quote:
I don't get it, what is actually stripping? The nut, stud or casing?
Yes

One stud was stripped at the case - Time Cert recovered.
The other studs have been like potato chips.
Normally it is the studs - not the nut - who's threads strip while being torqued.
Something about the way they turn them to manufacture - IDK.
Have tried two brands - with similar results.
Meanwhile - the time cert'ed 8mm (think its a p200 stud) is super robust.

Anyone else out there with a BGM cylinder who can report their avg. temps?
I'll likely push this one further tomorrow and see it caps out or keeps rising.
I'll also re-look at my seal fix to see if there is any evidence of weeping - tho frankly - I don't know how it could.
Pressure tests in the garage could support a week long birthday celebration - no leaks.
How many pounds of pressure does the case experience in operation?
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2134
Location: London UK
Mon Jun 22, 2020 11:34 pm quote
I would hold at the 29 rod. Move the timing down until the CHT temperature drops. With ok jetting, if the CHT is too high the EGT is too low. And we discussed cold exhaust already. Is just a bolt on kit at this stage, so not at the cooling limit yet.

If you use deeper 7mm nuts, the studs won't strip. They make them in many depths but the more threads making contact the better. I even use 1.5 depth 7mm on my case bolts.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Mon Jun 22, 2020 11:38 pm quote
Quote:
If you use deeper 7mm nuts, the studs won't strip. They make them in many depths but the more threads making contact the better. I even use 1.5 depth 7mm on my case bolts.
These cases are seeing a lot of on and off of heads and cylinders.
Fire a link - above sounds interesting - but unfamiliar with it.
Quote:
I would hold at the 29 rod. Move the timing down until the CHT temperature drops. With ok jetting, if the CHT is too high the EGT is too low.
Check in end of your day. Should have further insights.
Molto Verboso
One or two fun scoots....nothing too precious
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Location: UK (South East)
Mon Jun 22, 2020 11:52 pm quote
CM, I've used these before and they're pretty good.

https://www.amazon.co.uk/Naraku-cylinder-head-nut-set/dp/B01K11489Q

You may need to use a shorter one on the top right stud, in order to get the extension on for mounting the cylinder cowl. Regarding the usual M7 nuts, you can get half or full. The full size is going to be better as they have more thread.
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2134
Location: London UK
Tue Jun 23, 2020 12:04 am quote
This is a 1.5 nut
https://www.sip-scootershop.com/en/products/nut+m7x1+mm+nickel+plated+h+70mm_87170000

But I would use 4 (3 in your case) of these
https://www.sip-scootershop.com/en/products/distance+nut+m7+mm+_14470000

Looking forward to seeing if the BGM has more power with less timing. It's going to be close.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7658
Location: Victoria, Australia
Tue Jun 23, 2020 1:03 am quote
Yep SWA's ones are good.


I have a 200 mazz bell crank in LML cases with about 6mm of packer, 1mm squish, just matched to the cases/packer and covers opened of course. Currently 24/24 and LML reed with sip road twin pipe (still can't find my LTH box!). Did have 190/BE4/180 with 50/120 and ran quite well but a bit warm, now I have 140/BE3/130 and 55/160, could stand to go slightly richer on both main and idle. But as I said before, 290's to 330 was typical, with higher temps to 350 was about the max I saw on some long steep hills... although the carb box was a little loose I discovered, so would expect the same roads, about 300km n the weekend.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Tue Jun 23, 2020 8:20 am quote
Quote:
CM, I've used these before and they're pretty good.
SWA - thanks. Those are my favorite of the lot on looks-appeal of these longer options. Much appreciated all options you guys posted.

Ginch - thanks for the details.

It's surprising to me that the cylinder runs so much hotter than the Polini.
I'm still a little skeptical I may have an un-detected issue- but its a tricky one if that is the case.
On the exact same run - driving in the exact same manner - I would have expected 50° less on the Polini.
Have some time today to run it through it's paces with less advance and see if the temps settle with a ride also.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Tue Jun 23, 2020 10:25 am quote
Follow up:
SWA - didn't you have both and early piston BGM and then the new one? Do you recall what kinda temperatures you saw and what your set up was?

Chandlerman - are you out there? Same question on your BGM temps and set up.

Others?

🙂

-CM
Molto Verboso
One or two fun scoots....nothing too precious
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Tue Jun 23, 2020 10:48 am quote
I'm guilty of never having used a CHT gauge on any of my Vespas, but yes, I had the original BGM 177 kit and then the v2 kit with the black graphite coated piston. All I can say is that the later kit performed better and didn't leave strange vertical lines on the bore wall. The lines were definitely not seize related (although I did eventually seize that one), but they indicated some design or manufacturing fault with the early BGM kits. The replacement piston for the old kit was slightly longer, so they have changed piston spec. at least twice.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Thu Jun 25, 2020 2:01 pm quote
Update:
I went back and re-worked my seal fix.
Suspect it was moving under heavier RPM/load.
Improved version seems terrific.
Temps have come under control - 250's on speed runs. 275-just under 300 max on long 4th gear pulls at 50MPH (6000 RPM).
This is where the issue was previously.
Much more confident now.

Reworked sound recording best I can.
Tried a handful and ended up with homemade dead cat solution on mic.
Sorry about the blanket honey.
Honest.

Have some data now.
EQ29 (richer) gives best WOT plug read.
EQ31 (leaner) gives highest absolute speed by 1-2 MPH.
Bike is running strong and lots of fun again.
We can move on to cylinder mods when we want.

BTW - thanks to Ginch and Chandlerman for providing some numbers based on their experience.
Sounds like these cylinders are going to run between 275 and 325 under load.
On my speed runs - I am seeing 250s.
On long haul in 4th I am 275 - 300.
Haven't gone for miles yet - but temp guage was not climbing much with 3/4 mile run at 6000RPM in 4th.

IMG_6421.JPG
wind noise reducer.
Or
Blanket - what ever.

Screen Shot 2020-06-25 at 2.37.55 PM.png
Not sure I captured my best EQ31 - but bottom line - its producing just over 15HP - which is the high end of what they quote.

Screen Shot 2020-06-25 at 2.42.51 PM.png
EQ29 - hitting about 55 sustained max in 3rd

Screen Shot 2020-06-25 at 2.56.39 PM.png
EQ31 - bit better top speed - but plug chop at this WOT barely registers a ring at all

Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2134
Location: London UK
Thu Jun 25, 2020 7:51 pm quote
Temperature behaving now. Maybe a little cold?

Where did the timing end up? 23 degrees? Port timing was 123/173/25, squish 0.8mm. The EQ29 is the correct rod, as the Dyno looks as it should. Another run with the timing advanced a little would be interesting.

How does this compare with the Polini? Can you do a status comparison? From memory, Polini was 122/177/27.5? And came out 2 bhp higher?

BGM most likely has a better feel to it at the moment but just slower. With some Dremel work it will be considerably quicker.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Location: california
Thu Jun 25, 2020 11:21 pm quote
I'll run final tests and post recap.

The leaner jetting (eq31) is faster.
The GSF dyno doesn't show it - but that may still revolve around quality of recordings.
I can feel it in my seat.
I can measure it on the GPS.
But the plug chop has almost no ring around the base.
Meaning - one assumes - that it may be the fastest but it won't be the most durable.

If I wanted to drag race - the leaner option would be appropriate jetting.
For robust running - the 29 is a wiser bet.
I'll try and improve my recording methods for the Dyno and run a final tomorrow.
I'll also see if I can sneak a hair more advance in without blowing up our experiment.
Data and numbers to follow.

BTW - the Polini was 123/177 with a 1mm squish.
It had a modified Malossi head on it with center plug.
BGM is torquey - sounds and feels great now - and I am a fan.
Im looking forward to caressing it a bit with a sanding wheel to get more power - and expect it to meat or exceed the Polini if I was a betting man.

Need to check on status of the MHR kit. Should be in the mail.
🙂
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
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Posts: 2134
Location: London UK
Fri Jun 26, 2020 8:39 am quote
charlieman22 wrote:
The leaner jetting (eq31) is faster.
The GSF dyno doesn't show it - but that may still revolve around quality of recordings.
I can feel it in my seat.
I can measure it on the GPS.
But the plug chop has almost no ring around the base.
Meaning - one assumes - that it may be the fastest but it won't be the most durable.
1st thing this made me think of is, that's why we do a plug chop. If it was just the case of weakening the main jet until it went is the fastest, then there would be no need to check. On a two stroke it will only not blow up if, it is near the fastest but still has colour on the plug. Usually a few sizes richer. 10 miles at WOT is the test.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
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Fri Jun 26, 2020 2:35 pm quote
BGM Stock vs Polini
BGM is humming - and its a lot of fun to ride.
The power is all in by 7K RPM.
Stats of comparison below.

Now the fun starts with some port enlargement/ timing.
Let's find out what kind of difference it makes.
Sime's build was:
Quote:
Finished Timings: 125/181/28
The game is a balance between outright max power - and usable power - or power band.
Blow down should determine the width of my power band, and exhaust timing should determine it's peak HP.
Lower blow down = > torque/ broader power band/lower max rpm.
Higher exhaust port timing = greater peak power.
Unfortunately - these two things are interdependent.

Let's say for argument sake I wanted to limit my blow down to 27 (like the Polini) but I wanted to max my exhaust port timing to 181(like Sime's).
That would define my timing as 127/181/27
But would that be any better than 123/177/27 (as Polini currently is)?

What would the trade offs be - and if there are none - why would I stop at 181 rather than take exhaust timing up as high as I could, apply a 27 blow down, and determine what ever that would give me for my transfer port timing?
Jack - you wanna 'splain this to us?!

Below - final stats on stock BGM:
Port timings:123/173/25
Squish: .8
Ignition timing: Max advance 23°, 19°@ 6K RPM. 20.25 @ 1000 RPM
Peak HP 15.88
EQ28 metering rod

Vs Polini:
Port timings: 123/177/27
Squish: 1.0
Ignition timing: Roughy same as BGM
Peak HP with SI carb ~18
Peak HP with Smart Carb~ 17[/b]

What it sounds like just a little lean: EQ31 metering rod.
https://youtu.be/I-ipyyd5cA0

What it sounds like at ideal jetting: EQ29 metering rod.
https://youtu.be/RrXbZJOdCB0

Screen Shot 2020-06-26 at 11.50.34 AM.png

Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7658
Location: Victoria, Australia
Fri Jun 26, 2020 6:57 pm quote
Pretty nice for a barrel that is just matched. Thank you Charlieman.

I put one on a friend's late PX150 3 years or so ago. Was a rush job so just banged it on his stock motor with upjet and SIP road 2, didn't remove the 'covers' or touch the case. It'll sit on close to 110 (high 60's mph) with no fuss. He absolutely loves it.
Molto Verboso
08 GTS 250, 79 P200E, 62 Allstate
Joined: 11 Jan 2006
Posts: 1597
Location: Florence, OR
Fri Jun 26, 2020 7:04 pm quote
Oh man, I go away for a couple of weeks and I'm three pages behind! Time to pull an all nighter and get caught up on what's been happening.

Thanks for doing this - It'll come in handy in the future....

20200616_112528_resized.jpg
Ping-Ponging all over Oregon State...

Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 1426
Location: california
Fri Jun 26, 2020 7:35 pm quote
Having fun!
The bike is very close to my tuned Polini.
There is a little less at the very top - but how often are you really there?

A P125 or 150 with a BGM cylinder/box exhaust/ and up-jetted would be a game changer in my view - with minimal invasiveness. It's torquey and throaty and seams to have power everywhere up to 7000 RPM. Nothing high strung - no drama. Very impressed.

qas - that looks the perfect combo (cause if you took B-62 - it'd have to come inside at night to sleep with you guys!).

Jack - Dremel is idling - im trying to hold it back. See question from prior post.
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2134
Location: London UK
Fri Jun 26, 2020 9:41 pm quote
BGM as a bolt 187 is not so bad. As a 177 it probably feels a bit sportier but max power slightly lower still. With the torque to pull an upgear makes this kit useful.

Sime's timing 125/181/28 was carefully considered. I think this is what to go for. It's not a simple as more blowdown is more peak power, as all factors need to be balanced. If peak power is raised it does drag up the bottom end too, so not all bad. A really big blowdown is 36 degrees, so 28 is still at the lower end. The exhaust port being already full width is a big help.

With the 2 degree higher transfers, there is some margin for adjustment later. Assuming you don't get too excited with the Dremel.

Going to be interesting how all this compares.
Molto Verboso
One or two fun scoots....nothing too precious
Joined: 17 Jul 2013
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Location: UK (South East)
Fri Jun 26, 2020 10:59 pm quote
charlieman22 wrote:
A P125 or 150 with a BGM cylinder/box exhaust/ and up-jetted would be a game changer in my view - with minimal invasiveness. It's torquey and throaty and seams to have power everywhere up to 7000 RPM. Nothing high strung - no drama. Very impressed.
CM, this was exactly my impression of the BGM on a PX150 with a 57mm cut crank, the cylinder transfers 'unhooded' as you put it, and the case transfers matched accordingly. You're absolutely right about all the power being before 7000 RPM. My 2012 PX has the 7K RPM restricted ignition system and I never felt it kick in. With an upgear to 23T clutch (standard 68T primary) and 100/90 tyres, it was managing 67/68 mph at just 6750 RPM. I can only imagine what it's like at 60mm stroke with all the right adjustments as Sime has done.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 1426
Location: california
Sat Jun 27, 2020 9:15 am quote
SWA - yours looks to me like the exact recipe for converting your PX150 in to a touring 200 (maybe even a little better) at a relatively small price (with vespas - its always relative...). Lotta bang for the buck with low complication factor. Could see this putting a broad smile on a lot of NSMV participants.
Quote:
Sime's timing 125/181/28 was carefully considered. I think this is what to go for. It's not a simple as more blowdown is more peak power, as all factors need to be balanced. If peak power is raised it does drag up the bottom end too, so not all bad. A really big blowdown is 36 degrees, so 28 is still at the lower end. The exhaust port being already full width is a big help.

With the 2 degree higher transfers, there is some margin for adjustment later. Assuming you don't get too excited with the Dremel.
Ok - so here is a snapshot of what the options would look like.
Timing: how much I have to grind the top of the exhaust port: base gasket:head gasket (assumes I keep a .8mm squish)
- 125/181/28: Grind 1.72mm: .58 Base: 1.12 Head
- 123/179/28: Grind 1.56mm: .2 Base: 1.5 Head
- 123/177/27: Grind 1.04mm .2Base: 1.5 Head

So I suppose I just answered my own question: "all things being equal, why choose 125/181/28 over 123/179/28?"
The answer is - I will get a larger exhaust port surface area with the higher ex. port timing, both from the additional material taken off the top of the ex. port, as well as the larger lift of the cylinder below - allowing more grinding of the bottom of the ex. port down to BDC.

Ok - so now I have two different questions...
1. If above is correct - why wouldn't we remove the head gasket all together. and put it all in the base gasket. This would look like this (roughly)
- 130/186/28: Grind 2mm: 1.7 Base: 0 Head
This would create the largest exhaust port.
Why isn't this "better"?
2. What levers can we affect that would help us bias toward more torque with any of the above timings to help reach best of both worlds solution?
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2134
Location: London UK
Sat Jun 27, 2020 11:13 am quote
Now this stage of any port job is just as you say. What you want against what can be achieved with the parts available.

130 transfers would lack torque but have the highest power, with an appropriate exhaust port. You'll get away with 126 transfers but I doubt any more with your hack. If you can adjust packers to get 125 this has the best chance of working but can be changed later if it doesn't.

The exhaust port can be adjusted to suit but 181 is a good duration for your needs. The shape, angle and pipe all have a big effect too but this duration is still low so no danger of going too far. I wouldn't go any less than this. I think Hibbert has similar (more) on the Bajaj.
Molto Verboso
1963 VBB2T
Joined: 07 Nov 2012
Posts: 1788

Sat Jun 27, 2020 11:31 am quote
Keep going CM , so much great information.
bodgemaster
1963 GL, 1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 5301
Location: So Cal
Sat Jun 27, 2020 12:38 pm quote
Quote:
1. If above is correct - why wouldn't we remove the head gasket all together. and put it all in the base gasket. 130/186/28: Grind 2mm: 1.7 Base: 0 Head
This would create the largest exhaust port. Why isn't this "better"?
Short answer: Power will be up in the higher revs. It would be a fun set up with an expansion pipe.
Molto Verboso
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 1426
Location: california
Sat Jun 27, 2020 1:29 pm quote
First - lemme acknowledge that my ability to both measure and actually grind to 1.72mm vs 1.56mm are both in question - so this discussion is somewhat semantics. My final porting job will probably have some of both!
🙂
Quote:
Now this stage of any port job is just as you say. What you want against what can be achieved with the parts available.
Quote:
Short answer: Power will be up in the higher revs. This would be a fun set up with an expansion pipe.
Alright you guys - but but what we are looking for here is some magic.
Quote:
The exhaust port can be adjusted to suit
Ok - so shape of exhaust port is one of those factors - that can help us get some "best of both worlds". See scaled drawing below of proposed shape.

What other levers can be used to extend our power band - improve our low end - while giving us more peak HP?

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collating some (2 stroke) wine facts

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