Compression

[PaulP]

I have a GSX1100esd and have had it since 1984, originally it was my everyday bike but when kids came along it didnt get much use, then none at all for many years, so ihave given it an oil and filter change, cleaned the carbs and fitted a new battery, it runs, but only on 3 cylinders, i cannot get any heat on the exhaust on no 2, i have swapped the plug and lead over with no3 but no change, i bought a cheap but supposedly working set of carbs as i could not find any fault with the originals, but again it will not run on no2, it would be an unlucky coincidence for both sets of carbs to have a problem on no2.

I bought a compression tester and find that i have very low compression on all cyclinders, 60 to 90 psi, with no2 being at 70  psi, this seems way to low, or maybe my tester is not so good, any thoughts on how to progress ??

[richyrich]

I had an oil cooled engine where the valves sealing face had oxidised and  'furred up' , this gave low compression .   After a few spirited rides , if I could get it started ,  the compression came back up .  Presumably the fur had been burnt and knocked off.   It still wasn't great tho, I took the head off and lapped in the valves in the end . Was proper job after that.

[Gixer1460]

Possibly due to the lack of use the piston rings have 'gummed up' / stuck in their grooves? If you want to sacrifice a sump full of oil, you could try degumming them by taking all plugs out, set crank so all pistons are at same ht. ie 90 deg between 1-4 and 2-3 TDC and fill cylinders full of petrol / gas and leave for 24hrs. The fluid will leak through and around the rings and into the sump which MUST be drained and refilled inc. filter with fresh oil. If any petrol remains in the cylinders, disconnect the coils so no sparks and spin engine over on starter to eject through plug holes - a rag over will cut down on fuel everywhere. Then new plugs back in and start to see if any different. And yes the compressions are low - should be over 120psi even with a cold engine and they shouldn't vary by no more than ideally 10% - yours are 33% ! ! !

[Swiss Toni]

+1 on the valves/rings being the culprits! And welcome to OSS. 

[Wolf]

Have you done a wet test ? 

[PaulP]

Do you mean just pour a little oil in and test compression?, if so then no I haven’t, how much should I use per cylinder?

[Swiss Toni]

Just stick an oilcan nozzle in the plughole, and give it 3 or 4 squirts. Just enough to seal the rings. Don't forget to vent it out when you're done. 

[Kasuberoa]

Hi there,

Was interested in subject, have to point out something, test have to be done at normal engine running température, all spark plugs removed to help rotation, this give right values, if low compression is detected pourring oil help determine rather it's segments (increased compression) or valve issue (no change). at this point compression values comparison is hazardous

[Gixer1460]

Whilst a comp test is a good 1st stage diagnostic tool, it's good to try a leak down test which will give a definitive point toward either rings or valves without engine disassembly. Regarding the post above, a wet test is still informative. A single lower increase could point toward defective rings in that cylinder, a similar pick up across all cylinders could point towards the valves, no / low pick up across all would likely prove poor rings but i'd still prefer to use the leak down for confirmation.

[Kasuberoa]

6 minutes ago, Gixer1460 said:

Whilst a comp test is a good 1st stage diagnostic tool, it's good to try a leak down test which will give a definitive point toward either rings or valves without engine disassembly. Regarding the post above, a wet test is still informative. A single lower increase could point toward defective rings in that cylinder, a similar pick up across all cylinders could point towards the valves, no / low pick up across all would likely prove poor rings but i'd still prefer to use the leak down for confirmation.