Imola : Loss of compression

[TheSeeker]

Hello Folks

I hope you can help with a long story.

Several years ago I rebuilt an Li 125 engine with a second hand Imola kit bought from a friend. The engine ran well and I did a couple of rallies with it (Cobblesoul in France) and many local ride outs.





Coming back from Cobblesoul I holed the piston on the motorway when the petrol tank dropped below reserve. I pushed it home for several miles.





After that I rebuilt the top end with new seals and bearings and replaced the piston. The barrel lining looked ok and it ran well on the first few running-in trips.



On the last running in trip it lost all power, no compression at all and had to push it home (luckily not so far this time).

Initially I thought the barrel lining was at fault so I swapped the piston, barrel and head for a known working setup. Same problem with lack of compression. Then I started swapping everything I could think of with known working parts just in case (plug, cap, stator, carb, CDI etc). The spark is fine, the horn sounds if I keep my finger on the horn button when kicking.

I took the mag side off and the seal there looks ok.

What do you reckon? Is a blown drive side seal enough to have so little compression?

Best regards

Ben

[mick1]

Air test. There's a couple of threads kicking about on how to do a cheap air test, worth a search and give it a go.

[johnd]

Surely when the piston is travelling up the cylinder to compress the mixture once the transfers and exhaust port are closed the condition of the seals would be irelevant? I'd be checking the head gasket, head, rings and piston clearance. Leak test can't do any harm though.


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[TheSeeker]

Indeed, on the up stroke everything else should be out of the picture. That is why I was so puzzled when I put the old (working) barrel, piston and head back on and it did the same thing (but maybe that was for other reasons and misleading). Will try with new head and base gaskets and investigate an air test.

Cheers

Ben

[mick1]

Overheating could have warped the cylinder head. Before you get too far involved try soapy water around suspect areas and give it a few good kicks and see if you're blowing bubbles anywhere.

[ove]

Is the clutch stuck on,engaged? Just a thought.

[peter357bikes]

I lost compression on my rt225 after riding. Scratched my head ...next day fine..rode it again.. no compression...checked everything...opened up the casing and a couple of my clutch steels had locked onto the basket...ie clutch basically engaged....lots of filing later...not a problem since

[TheSeeker]

That sounds very possible and would explain why both top ends showed the same problem. The scoot had been standing in a garage for 18 months before the last ride when it lost compression. I'll have a look at the clutch this evening and report back.

Cheers

Ben

[bristolmod]

before opening it up, just kick it over whilst looking at the flywheel? Does it turn or not?

[thackers]

Could there be a crack in the casing around the crank opening?