Little compression after stopping?
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Vespa px125
Joined: 03 Apr 2021
Posts: 1
Location: Nottingham England
Sat Apr 03, 2021 1:06 pm quote

Can anyone give me some advise or ideas please?

Had a new malossi kit fitted and I've started to run my scooter in. I've done at least 200 miles since the fit. The last two times I've been on a ride, when I've stopped midway through, roughly around the 20 mile mark, when I've come to kick start it there is very little compression. It does eventually start after 2 to 3 kicks but it feels very sloppy when its kicked through, very little resistance.

When left overnight the scooter is back to normal, great compression and sounds great once started.

I've been told to get it dyno'd and jetted correctly once I've hit the 300 miles mark

Does anybody have any idea what could be causing this sporadic problem after stopping.

Joined: 12 Mar 2018
Posts: 94
Location: Texas
Sat Apr 03, 2021 4:01 pm quote
Sounds possibly heat related, maybe metal is expanding and you are losing the compression.

I would maybe throw a new spark plug in there and correctly snug it in (spark plugs don't take a lot of force to be tight).

I would also re-torque your cylinder head bolts to spec in a crisscross pattern.
GL, PK, PE200 with hack, Sears Rust Badge
Joined: 24 Apr 2012
Posts: 993
Location: Deep in the heart of Texas
Sat Apr 03, 2021 5:32 pm quote
Its hard to tell if you have low compression unless you do a compression test. It could be something simple like the kick start spring needs resetting or pulled out more for better resistance when compressed. Or it could be the clutch or a number of other things to cause a week kickstart. Prob time to start trouble shooting.

Good luck!
BV 350
Joined: 22 Nov 2016
Posts: 892
Location: Nebraska
Mon Apr 05, 2021 1:06 am quote
Maybe a sticking valve as well (I'm assuming 4-stroke) or perhaps inadequate valve clearance? Cam timing is right? No chance of a bent valve?

You checked the ring end gap before fitting the piston? And ring clearance in the piston groove? Add piston clearance in the cylinder?

Likely to be either a ring problem or a valve problem.

Easy to assume that new parts are all built to spec. But sometimes a good idea to check everything, just in case.

Definitely worth doing a compression test when hot. Might also build a compressed-air adaptor for the sparkplug hole, set the piston at TDC on the firing cycle (valves closed) and see where the compressed air is coming out, intake, exhaust, or crankcase.
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