Modern Vespa

Ok, so thanks to you guys regarding my first start advice my PX125 started up fine and seems to run ok.

I took it out for a ride at the weekend and I'm getting stuttering/bogging at higher RPM in 4th.

The revs level out at about 45mph , then after 2-3 seconds it starts stuttering until I roll off the throttle.

Any ideas? Too much fuel? Not enough?

It's got a Polini original box exhaust, drilled air filter and I think a 102 main jet.

Everything else is standard.

Thanks

Pop the carb box off , air filter and unscrew the main jet stack, it's the bigger of the two. Read off the value of the jet , sits at bottom of stack . Then come back on here to confirm it's a 102. 101/102 is normally bang on for standard px125 which is 96 main, with that setup, plus 4/5 for exhaust and 1 for the air filter.

Tell us all the numbers. There will be an Air Corrector at the top, which will be a 120, 140, or 160. The middle part is the Atomizer. It'll be something like a BE3, and then the main jet, which FerrisWolf already described.

It's more for tuned engines, but I wrote up a wiki article about SI Carb Tuning a while back that may be helpful.

Hi guys. Thanks for the replies.

The numbers from top to bottom are 140, BE5, 102

Hmm...stock is 160/BE3/100. Someone richened up jetting across the board at some point.

What's the provenance of the bike? How long have you had it? Has it always been like this?

Given that it's fairly stock, I'm wondering if it either had a bigger top end on it that was put back to stock for sale without changing the jetting, or it has an air leak that someone tried to jet over.

I took ownership of it in the summer, and it did run fine albeit being a bit tricky to start sometimes. Could get 50-55mph out of it and didn't stutter.

Exhaust was a rusty old thing so I put the new Polini box on and put 102 jet in as recommended. Was 100.

Other things I have done is remove carb/airbox and given everything a good clean.
Used new gaskets etc. and confident all is as it should be.
New fuel and oil lines too.

For a UK PX125 the AC140/BE5 is normal. Might be too rich with the 102. Could be a seal. Might just need a new spark plug.
If you just hold it full throttle in 2nd gear, what happens?

Yes I have my standard jet stack still in 20mm SI to hand and its ac140/be5/96 ; non cat model , the UK MY model 2003. I upgraded to a 24 mm Si and Dr177. Previously with a road 2.0 on 125 the 101 was bang on but went to 102 for safety.

Thanks again guys.
I won't be able to see what happens at full throttle in 2nd until the weekend but I'll feedback.

I'll pick up a new spark plug and see if that makes any difference.
I've got a 99 and 100 jet, so I can try those to see what happens too.

Anything else to try or check?

Ok, so after 3 weeks of it refusing to start, it spluttered into life at the weekend... but only after I had changed the mainjet from the 102 to 100.

Took it out for a little run and it ran sweet, with no spluttering in 4th at full throttle, and was showing 50mph+ on the speedo.
Much more where I wanted it to be.

Now, it'd be nice to know what the potential issue was with the spluttering and top speed of around 45mph ish... and then the not starting.

It can't surely be the main jet?

The main will certainly affect top revs but should have no effect on starting.

I think you need to spend some time checking through the motor. *Not* tearing it down (that's more like to make things worse by causing an air leak), but testing for signs of (impending) failure.

Here's a list of things that I'd be checking given your symptoms before I started worrying about jetting. You can't jet around basic mechanical failures. You can try, but you'll eventually wind up stranded. Trust me on this.

1) New spark plug
As Jack suggested, this may be all it is. If it's fouling, that'll impact performance overall. Easiest thing to test, too--buy a new one, put it in.

2) Good spark -- using the new plug, install it in the plug boot, then ground it to the case and kick the bike over. Make sure it's sparking by visual inspection. This is best done with the old plug out so the motor will spin longer and give you more sparks to observe. If spark is good, install and keep going.

3) Clean the carb jets.
You may have something clogging things up, e.g. a clogged idle jet making it hard to start. Remove the jets and blow them out good with carb cleaner. Don't use anything harder than brass (i.e. steel) to clean them if you want to scrape them out.

4) Check the carb float and needle -- Does it leak/flood if you stop it for more than a couple minutes with the fuel tap on?
Easy test is to stick a paper towel under the fuel bar (the little brass piece that sticks into the mouth of the carb). If it's leaking, the towel will be wet with fuel after a while.

Bonus Round:
(Things that require special tools)
- Compression test -- make sure that the cylinder has at least 90 PSI of compression. It'll be hard to start and will never run well if compression isn't adequate. Needs a...wait for it...compression test gauge.

Not to be confused with the
- Pressure test -- Make sure it doesn't have an air leak. I won't be surprised if it does, but we can discuss that later. This topic has entire threads of its own (search for "getting serious about air leaks") on here.

Hi gents,

Been a bit busy but finally got round to having time to spend on this.

The usual thing of it absolutely refusing to start is going on at the moment!!

Tried all choke and throttle positions, bumping etc. and nothing.
Tried a dash of petrol in the head.
Even tried the old lawnmower trick by heating the spark plug up before installing and kicking it.
Nothing... no sign of it even wanting to start.

In reply to your suggestions chandlerman:

1) New spark plug installed.

2) Good spark (in my newb opinion).

3) Jets all cleaned.

4) Not sure what part you mean when you say fuel bar, but I can't see any leaks anywhere.
Note: I have had to leave the fuel tap on when not in use as it's much harder to start if I turn it off between runs.

5) Compression seems good, around 115-120 PSI

6) I don't have any means of testing this


Is it worth me tearing down the carb again and checking/replacing bits?

Again, thanks for all your help.

Since youve been kicking frantically, pull the spark plug, turn off the fuel and let the cylinder air out. Give it a few kicks and then leave it alone.

While thats airing out, inspect the carb and 1. ensure the starting jet is clean and seated, 2. the float is working as designed (test it with fuel on and then lift the float up- fuel stops dribbling, release it should dribble), 3. Ensure idle and main jets are seated. 4. Air/ fuel screw on the back of the carb is 2.5 turns out from seated 5. Both red O washers on sandwiching the fuel banjo.

When all thats done, put a spark plug back in and try your normal restarting procedure.

Thanks MJRally.

Looks like I've got a little job list for this evening.

Stand by for either a) jumping for joy, or b) launching the bloody thing!

Well jumping for joy was the order of the evening!!

Removed the carb (again) and gave it all a damn good soaking with carb cleaner.
I think there may have been a blockage between the fuel bowl and the sprayer somewhere as it took a little while for the cleaner to start dripping out.

Put it all back together and started 3rd kick!

Any ideas what would cause the blockage? Old fuel? Evaporated fuel?

It had a full strip down and clean over the winter, and I've only used it a couple of times before it decided not to start.

It's fairly easy to get random crud in the carburetor. When in doubt, clean it out.