Modern Vespa

Hello.
I trying to understand how to tune my SI 20/20 D carb.

what do the numbers on the throttle slide mean?
is there a progression of rich to lean or were they numbered by the same woman who did the Atomisers?

the fun I've had so far with my 2012 PX125E.
SIP road 2 exhaust and drilled air filter.

50/160 idle
140/BE5/99 main stack.
slide 11.
weak pull away after riding hard.

changed to 45/140 idle. (originally wrote 48/140 my bad)
slide 1.
runs much better.

now fitted a DR177 top end and running in including 1% oil in petrol.
48/160 idle ( going to try a 45/160).
160/BE3/112 main stack.
slide 1.

Cheers Del.

I suggest you put your jetting back to AC140 BE5 112MJ 48/140 before something unpleasant happens.

Hello.
Thanks for the advice. When I first fitted the DR177 kit, I used those jets first.
It ran like it was coal fired. I bought the new jets with the slide as that was the spec for an early P125X in the UK.

The engine is different every time I ride it. I'm running in the top end and am very gentle with it. I shall try the old jets again. I've done about 80 miles so it should have loosened up.

Update: had a play today.
It's now set up

45/140
160/BE5/115
slide 1.

It runs rich but only gone to 1/2 throttle but at least it pulls now. when it's run in I'm taking it to the Dyno shop in Essex.

Did you put in the AC140 or 160?

deleted as repeated

[quote="worrywort"] Hello My set up is now.

48/140 Idle jet

160/BE3/115 main stack

slide No:1

It's quite rich and I run it barely more that 1/4 throttle.

Hello jack221. I tried the AC 140 but it runs so rich I have to go way over 1/2 throttle just to do 30 MPH.

update: I went for a ride tonight and I'm going to put the 45/140 idle jet back tomorrow as starting the bastard is giving me grief.

I originally started this thread to find out what the throttle slide numbers mean. I can't find the answer anywhere but somebody with the Same question will hopefully benefit from this. (Even if their Engine blows up like mine will).

The throttle slide numbers mean nothing other than just being a label to tell them apart, similar to the mixer tube labels. Rumor has it they were numbered in historical order, ie, as each was created.


Each one has a some slight variation in how far back the beveled notch on the end is, whether or no there's any trimming of when the transition port comes online, or how much the underside is vented in various places. It's all about fine tuning vacuum and mixture. The only way to understand them all would be to study each one up close and even then, the numbering would merely represent the order they were designed.

Assuming your set up is all fairly regular DR177 and the ignition timing is static at 18 degrees, then you are closer to done than you think.

This will get it set up far quicker and save time out in the cold.

Firstly, if your autolube is running, stop putting oil in the tank.

If having trouble starting hot, then go with the 45/140 for now but if it leans out at 1/8 later on, change it before overheating.

Slide 1 is ok.

For the best results, you need to adjust the mixture screw every time you have changed any jet in the carb. They all affect this.

AC140 and BE5 will end up running cleaner and more reliable. Don't worry about how bad it runs now, it will get better quickly.

Your only variable is the main jet. 115MJ is clearly way too big. Not sure what jets you have to hand but all the jets from 100 to 115 are in single sizes, with good reason. I would first carefully try something near 105 to see what ball park you are in. If its too lean you know your jet should be between this and 115.
Only need to check the main jet at 3/8 throttle position for now.

Thank you pdxjim. I was suspicious that was the case.
it would be so useful if there was a table which gave which jets etc went with which slide. I notice this forum is full of jetting questions.

Thank you Jack221. I shall take your advice and change them now.
It's a bloody pain trying to rejet a carb when you're running in the engine.

Kind Regards Del

Thanks for some explanation. I've been trying to figure this out for a while and there appears to be no info anywhere online. Are there any compatibility issue or significant changes per slide that would warrant needing to switch to a different one? Or can the differences be adjusted out with the idle screw?

Hello Octobogs.

I have a 2012 PX125E which is euro 3 compliant. whenever I rode it hard for a few miles it would be running weak and I would stall the engine when pulling away.

I want to learn how to tune the SI carburettor myself. I just looked in the Haynes manual at what the early jets and slide were used. I fitted the No: 1 slide in place of the No:11. I found the jetting that worked well was the original set which was 45/140 Idle. 140/BE5/99 (99 jet fitted with a sip road 2 exhaust and drilled air filter heart).

then I found the head was warped so I fitted a DR177 kit which I was saving for next spring. I then changed the jets as was used in the original P150X.

I started this thread to try and make sense of the slide numbers. In the meantime Jack221 read it and advised me on what I needed, which is what the majority of this thread is about.

Elsewhere Phatanglo had the same problem which he fixed using a No:8 slide

I hope this helps
Cheers Del.