Modern Vespa

So this all started a few weeks ago when it was hot out and my 2005 et4 would stall at every red light amongst other issue (I’ll get to those). So I did the evap delete and that fixed the stalling issue (mostly).

However there is a few quirks it’s still does

- [ ] At a red light when I accelerate it hiccups or stutters for a bit. I can replicate this sometimes if I go at low speed and blip the throttle it’ll sputter and I can do this repeatedly. However it’s guaranteed every red light acceleration
- [ ] At WOT and at top speed it consistently stutters. Like it’s gasping for fuel or air. Only at top speed. Which I feel has been reduced since this happens. I recall easily getting to 100 km/hr on flat before. Now I top at about 85 km/hr
- [ ] When I slam on the brakes to a complete stop it occasionally stalls. Or usually comes close. As well when I pop it up on the kick stand it stalls, which honestly doesn’t bother me.

A few things on the maintenance:

About a year ago I did the following;
- [ ] New belt and rollers
- [ ] Cleaned the carb (didn’t need it)
- [ ] New spark plug
- [ ] New fuel line and added a fuel filter
- [ ] Changed oil

Recently I
- [ ] Cleaned air filter
- [ ] Evap delete

I am trying to determine what it is as I did the belt and the carb recently and those seem like the two possible culprits. However not out of the realm of possibility that something has happened in the past year. As well half the issues seem fuel and half seem mechanical so it could be a possibility of both.

Any help would be appreciated

Bump

Wiggle the carburetor around and see if you see any cracks in the rubber intake manifold that connects the carburetor to the engine. Common problem. This would cause problems like you're describing due to a vacuum leak.

Thanks for the reply! I went and checked and didn’t see anything. I tried spraying carb cleaner on it to see if the rpm would increase and nothing

When was the last valve adjustment?

To the best of my knowledge probably never. It’s about 11,000 km

It may be worth checking
that. It still sounds like a carb problem to me or possible a vacuum/air leak.

I’ll check the carb. It was recently cleaned. Is there anywhere an air clean could be. I swear I sprayed carb cleaner on every square inch under the pet carrier and saw nothing. The only hose hat seems relevant is from the intake to the fuel peacock and it’s crack free

The stuff you did and checked a year ago really doesn’t count for much any more. I’d reclean the carb and check that the electrical connections from the CDI to the plug are clean and tight, and that the plug is clean and well-fastened.

drain and flush your tank. check your fuel and vacuum lines. go back thru that carb and make sure nothing is missing.

your fuel line or intake could have a leak somewheres.

double check all the electrical connections, check the valves.

but without riding it I'd guess bad fuel or something amiss in the carb/intake/fueling system.

hth.

This sounds like it might be an auto choke issue. When it starts happening, unplug the power to the choke and see if that changes how it's running.

I will check the Auto choke.

One thing that has been eating away at me is when I cleaned the carb I took off the auto choke and I slightly bent the c-shaped washer that helps it mount. I don’t think it makes a different but concerns me now.

So question. Does the auto choke just play a crucial role on startup and then no role whatsoever after? Cause I noticed today it ran especially shitty on startup.

I was stalling at nearly stop again. Whenever I slowed down

there is an o-ring on the choke body that solidly seals off the the choke well. if this is compromised or the choke isn't fully seated-- needle into the guide and plunger into the orifice then that could be a problem. slightly bent isn't the end of the world as long as it secures the pull off portion (that there big black part) decently and in the correct position.

mostly start-up only. once "disengaged" the choke has little to do with running or restarts unless it is malfunctioning.

cover half of your air intake with tape.
test drive it.
did it make a difference?

remove the tape and create a small air leak.
could be by opening your airbox a touch.
does that make a difference?

This test will help you isolate where your issue may be.

So I tried unplugging the auto choke. And at first it seemed a bit better. At least at top speed and then kind of went back to normal. I’ll chalk it up as inconclusive.

However another thing I’ve noticed;

It runs WAAAAAY worse when it’s low on gas. Like around the time the light turns on I have to feather the gas at every red light.

I’ll try blocking off the air intake. You just mean taping over the filter? Or taping over the snorkel in front of the seat?

Ok another Update.

I cleaned the carb. it didnt seem too bad but when I looked through the pilot jet the hole was small and when i sprayed and poked it, it seemed a bit bigger, good sign!

Once I rode it, it seems like a big improvement. I dont hesitate nearly as much on acceleration and the stalling went down when braking.

However, now that I am low on gas again, I am noticing that it is a bit worse, and the stalling when braking is horrible. I am subconsciously trying not to stop at lights... like running them

Anyways I could adjust the idle jet but honestly when it is idling its fine, it maybe fluctuates in rpm but definitely doesn't seem to struggle.

My question is, the slipping when accelerating could be the belt, even though its new, but does anyone forsee anything that could cause a stall when I slam on the brakes? it even did it while on centre stand (before carb clean) and anything that would make the problem worse when gas is low?

My kind of rider. If the choke is damaged, it usually doesn't work right, so the bike runs very rich. If it's the original Walbro carb, it might be time to think of a replacement. Also replace the intake manifold and intake manifold gasket.

Thought I would give an update;

I did the valves which I thought were a lot more complicated but upon finding these instructions;

https://modernvespa.com/forum/article26722

And it fixed all the problems! so at this point if you have the same problem I described, you can know you just have to do the valves.

however, I may make a new post for this, but It ran perfect for over a day (lots of riding) and then starting acting up again, so I readjusted the valves, the exhaust valve was again slightly off. and I rode about a km and it started stuttering and now wont start. so I think my issue is valve related but they wont stay adjusted that issue I have no idea to tack so I may adjust it one last time in the highly unlikely even its just the lock nut that needs tightening, but otherwise I will try to take it in or get a new head. depending on that cost I may have to consider a new bike or go all out and do a 190 kit