wont start

[Piatti]

You need to use a gauge. You can't feel the difference between 120 PSI (healthy and happy) and 90 PSI (sputters and won't start).

I only say this because I've been there. When you check, open the throttle and kick away till it stops rising. Borrow one if you don't own one, it's worth it just to rule it in or out. If you're less than 100PSI you will be chasing your tail until you address it.

Hope that's helpful. I hope to see you saying you've found it soon. These types of problem are maddening.

Am lending a compression tester, so will do a test next weekend. Kick start shaft broke at the weekend so need to replace that first

Some good reading on compression testing on this site:

http://www.dansmc.com/compression_test.htm

Will let you know how I get on...

Cheers

Piatti

[celt]

[
You need to use a gauge. You can't feel the difference between 120 PSI (healthy and happy) and 90 PSI (sputters and won't start).

I only say this because I've been there. When you check, open the throttle and kick away till it stops rising. Borrow one if you don't own one, it's worth it just to rule it in or out. If you're less than 100PSI you will be chasing your tail until you address it.

Hope that's helpful. I hope to see you saying you've found it soon. These types of problem are maddening.

Piatti borrowed a guage and true enough it was only putting out 90psi, 95psi if you were lucky.

Checked Squish slightly large at 1.59mm

Told him to check the rings, had a text/photo from Piatti to say ring gap was 4mm

Told him to check piston/ barrell clearance if he needs a re-bore if not new rings.

Thanks for the reply sdjohn

[coaster]

Piatti borrowed a guage and true enough it was only putting out 90psi, 95psi if you were lucky.
Checked Squish slightly large at 1.59mm

Told him to check the rings, had a text/photo from Piatti to say ring gap was 4mm

Told him to check piston/ barrell clearance if he needs a re-bore if not new rings.

Thanks for the reply sdjohn

I guess the initial strip down would have involved de-coking which would result in a loss in compression which would account for why it was running (erratically) before the re-build but not after....glad he's getting somwhere

[Bilko]

As Coaster says, looks like you're getting somewhere, which is great to see. I can feel your frustration. I'd say there's a lot of people watching this thread with hope!

I would never have got this sorted without actually knowing what the problem was, but with the low compression and Coasters explanation for lose due to de-coking it definately makes sense and is one to watch.

The compression tool looks a worthy buy and cheap too.

Good show Celt for the help there.

[Piatti]

Just ordered a new set a rings from scooter restorations, I can highly recommend the use of a compression tester, as stated in the link below 'You can have perfect ignition timing, clean carburetor, and fresh gas, but without good compression... the engine is not going to run.'

http://www.dansmc.com/compression_test.htm

Rings should be here by the weekend, i'll give an update after I have fitted...
Thanks for all the advice so far

Piatti

[coaster]

but with the low compression and Coasters explanation for lose due to de-coking it definately makes sense and is one to watch.

Back in the day the advice was to leave a ring of untouched carbon around the edge of the piston crown when de-coking. That was when we were using old school mineral oil rather than the low smoke formulations in use today. De-coking used to be a regular maintenance activity, never seem to hear of it now

[sdjohn]

Glad to see you are making headway!

This is exactly what happened to me when I got my Lambretta. I was new to 2 strokes and it was suggested to swap out the seals because the bike had been sitting and it smoked like a banshee when it ran. I had a friend help me through all that, great guy. Decoked it during the process and it wouldn't run after. I beat my head against the wall for a while thinking "it ran before it couldn't be compression". But it was.

A compression gauge is an essential diagnostic tool, as you've now discovered. Well worth the money!