High Revving - air leak - failed gasket

[loadedscooterist]

Hi All,

As some of you may know I have a li150 which was bought last year. I haven't been out on it much so far - mainly due to poor weather and sorting bits on it (maybe a blessing in disguise if you read on). Its latest issue is a high revving engine on start up. i.e start it and it sounds like its going to take off. So much so I can only run it for a few seconds before switch off.

Anyway, I have been running tests and my novice prognosis came down to an air leak. I have checked the carb with the WD40 test and cant find anything there (in fact I have stripped it twice). However, I moved onto the leaking exhaust (which is now off the scooter). I have included a couple of pics here from my Flickr album - there are a lot of pics so just thought I'd include a couple to begin with). It looking very likely to be the exhaust gasket thats done the damage. BTW, there was no exhaust clamp between U bend and main body of the exhaust and the joint between the two was leaking. All in all a bit of a mess!

https://www.flickr.com/photos/153041800 ... ed-public/
https://www.flickr.com/photos/153041800 ... ed-public/

As you can see I have now got a very scored piston - looks like carbon from the air leak??

Going forward I definitely need to replace the gasket as a starting point- but do I need to replace the piston as its too far gone? The exhaust has a fair bit of wear and tear as well.

As ever grateful for any advice on this issue.

[coaster]

Well that has certainly been leaking but I'd be surprised if it would cause it to rev as badly as to describe. That piston has seen some seizing so you need to get the barrel off and I'd suspect there might be some evidence of more serious air leaks. You probably don't need a new piston, that one will probably clean up ok

[HxPaul]

Daft question.Have you checked your throttle cable,it may be to tight or snagged on something making it rev.

[loadedscooterist]

Hi Paul, I checked that as well. It seems to be moving ok when I turn the throttle (it lifts the slide ok and the spring in the carb seems nice and strong too). Is there a way to lubricate it when its in situ anyway?

[HxPaul]

If its nylon lined you shouldn't have to.

[loadedscooterist]

My suspicion is that the leaking at the exhaust manifold as has been like that for a while and needs a new gasket and a good clean up as a starter for ten. The only other improvement that I have done over the Winter is to fit a new flywheel and 12v kit. Once this was fitted the scooter has been fine (some very small rides up and down the street where I live and it was ticking over nicely - all ready for a proper run). I left it for about a week and the first time I kicked it over it was really revvy - even without the choke on. My first thought was the carb - I have had this stripped down twice and it still makes no difference. I would like to swap out the carb and see if that makes a difference so I can totally discount that.

[loadedscooterist]

Ok thats good to know. One item off the list then.

[ForemanBob]

I would doubt whether a leaking exhaust gasket has caused that much damage.

I would suspect that you have bigger problems elsewhere.

You really need to strip to investigate. Are you in a local club? If not reach out to some of the local guys from the Falkirk Steeple etc to assist you

[loadedscooterist]

Unfortunately I suspect you are on the right lines. There is more to this than meets the eye. more updates as I find them.

[Martin20]

I fitted a 22mm Jetex on my Special last year and had to change the manifold to accept it. Somehow, when I put the gasket on between barrel and manifold, I managed to miss the kickstart side stud! Tightened everything up, started it, and it revved exactly as you describe. Soon found what I had done, corrected it, and all was/is fine!
You probably have not changed your manifold, but if you have, worth checking!