Modern Vespa

Color me confused, but "I have zero squish" and "I need to raise my compression ratio" seem kind've contradictory to me?

well, to be honest, I'm not sure I need to raise my compression ratio. but if this is the huge bowl that came before the sprint (VBB/super 6.5 CR), then I'd like to boost it. it's not necessarily contradictory, but you've got a point in that I'm not sure where I'm at.

I'll pull that stella head off tonight and have a look.

Ok, p series head on the left, Bajaj on the right.

24cc vs. 28.5cc. The bowl shape is different also. I was surprised the p series 150 head needs a long reach plug, I had a shortie in there and there were 4+ threads showing. Swapped for long for this test.

So, doing some of the maths....

57.8 bore and 46mm stroke from transfer to tdc gives 126cc.

(126+28.5)/28.5 = 5.4 CR = looks like the lower CR

using the p series head gives

(126+24)/24 = 6.25 CR

I believe there is missing volume going on here, and it has got to come from the domed piston! I was assuming flat. Now it makes sense.

If I reverse engineer the volume of the piston to make the top equation work out to 6.5, doing some algebra, we get about 5.6cc of volume lost to the piston dome.

(126+22.9)/22.9 = 6.5 CR

Then the p series head would come out to

(126+19.4)/19.4 = 7.5 CR

Eureka! And now I'll need another p series head from Kajiit to replace the one I'm stealing today. Frickin' Bajaj parts. I'm stoked to get some free power!

That's huge news! 1.6 HP is massive!!!! Reading up on scooterlounge, some of that was a better exhaust, but I'm doing that also, with the SIP road 2, which is better again.

PX head looks much better for more mid range.

Most stated compression figures are from BDC like on 4 strokes.

More useful for 2 strokes is compression from the exhaust port up.

Try not to remove too much packer under the cylinder, the 118 degrees you have now is good.

My VBC 2 port case has the additional factory drilling on the rotary intake. VBB does not. VBB also has the deflector style piston. On my engine, the crank, p&c and con rod were all one solid rusted tiki head.

Gotcha, this would change my volume of the piston dome estimate but either way it's the right direction. I'm not planning on removing the base packer or changing its thickness.

Any idea what the intake timings are on those 2? Curious which one is more like this Bajaj.

I went digging into my last 2-port motor thread to see if I ever documented the intake timings for a stock VBB motor.

I found this bit: So I guess my stock intake timing was 163. The crank was not OG, but I assume was stock because it was in the motor, which was otherwise stock when I got it.

Thanks for digging! I think we are same-same, since my eyeball delivered a measurement of 165.

So that's something left on the table for now…

I don't want to dig inside the motor at this point - there's an all 8" ride coming up on October 19th, I need to be ready for that. We're doing some mountain twisties as a group of slow bikes .

That looks awesome. Twisties on 8"s is always fun.

It took me like 25 minutes to remember how to put the flexi shaft on my dremel. DOH! Now daylight is gone, making big exhaust port must wait for better lighting.

Just goes to show I haven't been doing any cylinder mods lately. I think the last one was opening the exhaust on my p200 stock cylinder.

Hole enlarged, now to clean off the piston leftovers and give it a quick run with the hone.

Some 220 wet/dry sandpaper and a quick run of the hone.

2 Warnings to self: head should not be used. Engine also needs a proper head before use. That's the spare Stella engine.

55/160 idle
160/be3/110mj

Shout out if you have a better start point.

Back together. Despite my best intentions, I forgot to drill the spring hole in the top cylinder fin.

Oh well, just like most I will make do with one spring.

Letting sealers cure. Waiting….

Almost smoke makin time!

One spring will be fine. I'm not sure I have two springs on any of my motors.

Weird things of the day:
-carb fastener wouldn't fit into the hole for it on the carb and I had to dremel it out a bit. This is my old carb from my p200, it shouldn't be weird!
-new exhaust fit without fighting much. Worth the wait for the proper version for these cases.
-for some reason I installed the studs long side into the case last time, that makes no sense. Short sides in today.

just leaving this here for myself for later....

FMP says raise the exhaust port on the 177 pinasco kit by at least 3.5mm.

2 port kit, has 3 ports. 2 port OEM cylinder has 2 ports.

Yes. But when i eventually get that kit, the timings are tame, and can use some spice.

The Pinasco may have tame timings to start, but it will also go pretty good for a two-port, especially on a long stroke crank.

I'd definitely run it and see what you think before you bust out the Dremel. Recall that I trashed mine chasing power I was never going to get from a two-port motor.

Wise words, chandlerman!

OMG it is sooooo much better, first ride just completed. The jetting is close but I need to do it proper. Honestly it may be enough to not do the 177, but I will need some hills before I decide. It pulls so much better and it's gonna have more RPM no doubt about it. It went from boring to fun with just those upgrades.

In looking to replace the cylinder head I stole, it's kind of fascinating to see that the center bowl design that I stole isn't so common in the replacement parts market. Most of the replacement heads look more like my dud, though it isn't at all clear whether they are smaller volume but similar shape.

I ordered a jet set for 115-138. Almost every vendor is out of the BGM set at the moment, so I had to do a brand I hadn't heard of (EVO?) through scooterpartsco.

https://scooterpartsco.com/main-category-c-801_4181_4187/dellorto-vespa-si-carburetor-main-jet-kit-115138-p-37597.html

so I had to swallow my pride and use the magnifying glass function on the iPhone today to look at the jets I do have. I swear I was going to throw them all out, I couldn't see any numbers, WTF? but with it I managed to find a 113 and a 115 to go a bit bigger for my jetting exercise. my others will come, and they will be organized and legible, but I did actually have something to work with.

problems of middle age, I suppose...

No shame in that. I've had to keep a big ol' Sherlock Holmes magnifying glass in the workshop for reading jets for several years now.

Best advice I've ever got here was from C'man22:
Don't throw away those jets.

Get a micro-drill bit set, and if you want to go further, a $$ reamer set from Germany.

Do NOT go by printed #'s on jets.

Thx Ray. Explains why on my last jetting odyssey, a BGM 120 main felt close but a 122 was sputtery and out of the ballpark (it was actually a 127).

If you want exact numbers, KMT jets are the ones you want…

But… Pricey…

well i did a bit of jetting today

up from 110 to 115, it's rich up there
112 - also rich
back to 110 - may still be rich

next up 108

the scooter gets about 42 mph in 3rd, about 6500 RPM.

it's not a much higher rev range so far, but it's more fun getting there and when you shift from 3rd to 4th, it picks up well right away. it's just a lot more capable, even if the speeds haven't moved a lot.

a bit funky at low throttles, I'm thinking the atomizer be3 might not be the best choice.

so, thinking about atomizers, I go back to my favorite threads:

Lets talk atomizers

Which atomisers for the SI carb are leaner/richer?

and one of my favorite charts:



I have BE5, BE4, BE3, BE2.

I think richening up the 1/4 throttle kind of stuff might call for BE4 or BE5.

The 55/160 idle jet seems reasonably happy and reasonably big on the idle scale. It's falling down to idle pretty well, it doesn't seem like I need to increase the idle jet.

Other idle jets in the stash: 48/160, 45/140, 45/120, 50/120, 38/120.

Sounding like a definite improvement in performance. With the 2 port the revs will be slightly lower than expected.
If I remember you now have a 20/20. This carb likes AC140/ BE5 with a 07 slide.
I'd stick with that until the main jet and pilot jet are both done, then evaluate the atomiser.
Set the main jet in the old fashioned way. Wide open, 2nd gear, begin obviously way rich, reduce one size at a time, until it revs out clean.

OK so here's the jetting update.

Started with 140/BE5/108. Definite improvement on the low throttle weirdness, so the BE5 is a keeper. Tried moving down to 105, it didn't feel so good. Then I ran back into one of the things I'd forgotten about this particular carb - the front air filter screw has been stripped at some point, so you need a fastener that's a bit longer than normal to grab the remaining threads. During my 105 run, the air filter got all loosey goosey and I had to pause the work to go to Home Despot and get some longer screws.

Returned to it and tried 105, just didn't feel right. Back to 108 and around my bigger test loop of about 11 miles. I actually had lower top speed (50MPH), and it still just didn't seem like all was correct. There is a kind of cyclic power surge at full RPM, it's like power is sort of on for 2 seconds, off for 2 seconds, sometimes longer, sometimes shorter, but something isn't right.

A quick look at the plug said I wasn't crazy rich, if anything maybe slightly lean.

I tried going lower to 102, that was nothing different. Back up to 110, nada. This engine seems to run the same with jets between 102 and 112.

So screw it. In goes the 120. Now there's what rich splutter on this engine actually feels like! I hadn't actually experienced it yet on this one, though I thought I had. 118, still rich. 115, just a tad. So back to 112 and it's relatively clean. This means my 160AC running was all on the lean side. I also did drilled filter for this last iteration (stolen from my Stella motor, will have to replace it along with the cylinder head).

The kind of poor combustion / cycling power thing at max RPM is still a thing. Honestly, I'm guessing the mixing of the charge isn't that great on this 2 port top end. Also, probably would like that rotary valve opened up more.

At any rate, the max speed on my 2nd go around of the testing loop was 53MPH. I'm getting about 6500 RPM in 3rd, about 6000 in 4th. It's jetted safely, I even did a bit of acceleration down hill (holding my breath).

Current jetting 140/BE5/112 with 55/160 idle. Timing is at 22*.

This cylinder is not gonna make plaid, but I didn't really expect it to. The bike is cleared for local rides and shenanigans.

Next up - I think I'm gonna repair this bench seat that I got during all my parts trades. It's not a bad tear, and getting the cover off is easy. The repair should be simple. The saddle seat has me a bit forward, and I have tiny legs, I can't imagine how tall people do it.

Just to describe further, that max rpm thing is kind of like you are running good, then subtly less so, a bit like the tiniest bit of brakes coming on, but super super light. It's just a shift in the power, but you can feel it. It doesn't feel dangerous, just like a headwind, then a tailwind, then repeat…

If the main jet is more or less correct, it's likely to be something to do with the 2 port poor scavenging issue.

55/160 pilot although will run acceptable, is not going to be best suited jet. Try 52/140, I think this will be better.