Low compression after rebuild, won't start

[simonh]

Dear all, I hope that someone will be able to offer advice about my Model 8. The bike has just been rebuilt with a re-bore, hone, new piston and rings. I can't get the bike to start. Good strong spark, new carb, set up as original. It seems too easy to kick over and only 4 bar or 50psi compression using a tester. To try to diagnose, I have removed the head, inverted it and filled the combustion chamber with oil. It appears to be sealed, no leaks in 48 hours. I put 10mm of oil in the barrel and left for 48 hours, none or negligible got past the rings, I have re-checked that the push rods rotate freely at TDC. I took a known good spark plug from my other bike and tried that but not even a cough from the engine. I have also checked the position of the decompression mechanism and disconnected the cable.
The cylinder head gasket was originally a composite one from a club supplied set but I also tried a new copper gasket lightly smeared with Heldtite engine sealant and it was the same result.

I am at a loss now and would appreciate any help.

Thanks, Simon

[dave16mct]

The valve test is carried out with parrafin or petrol, not oil. Are you sure you have timed it on the correct stroke? I would remove the side plate on the head, put it in top gear and turn it over watching the movement of the valves, pushrods etc. That might show a valve that's sticking etc. I always do this anyway when timing as it shows you when its on the correct stroke. Did you see what the piston ring gaps were?
Dave.

[simonh]

Thanks Dave, I lined up the dots on the gear wheels when timing, but I can check again. I don't have any paraffin, so used 20 grade oil. I had just filled the engine with new oil, so I didn't want to use petrol. I timed the bike before I put the barrel and head on. A new 72.6 piston and rings came with the bike so I took that to the company who bored and honed the barrel, he bored to suit using the clearance settings from the piston manufacturers website. Thanks again for your reply

[SPRIDDLER]

I don't have any paraffin, so used 20 grade oil. I had just filled the engine with new oil, so I didn't want to use petrol.

You check the valve sealing with petrol or paraffin with the inverted head on the bench so it won't get into the engine/oil.

I timed the bike before I put the barrel and head on.

I've deleted my earlier point as I'm confused at exactly what you timed before the barrel and head were fitted, but regardless of that it doesn't explain the 50 p.s.i. low compression. Maybe you need to recheck the valve timing.
You need around 100 p.s.i. when kicking over with the throttle held wide open.

[Andy G]

Push rods may be able to rotate but with less than required clearance at tdc? Maybe slightly over gap the rockers at tdc and check compression to eliminate this possibility?

[56G80S]

I know this may sound daft but did you set the gaps in the piston rings at 120 degrees apart from each other?

It's a long time since I stripped and rebuilt a lightweight but others will know if the dots are more reliable on these than they are on the heavyweights?

Johnny B

[simonh]

I set the piston ring gaps at 120 degrees. I will strip again at the weekend and check the valves and timing. I didn't build the head myself, it appears to have had new valves and guides, so I checked it didn't leak and put it on. The bike was a basket case, but the guy claimed that the engine was newly rebuilt. The piston and rings were new and the head was spotless, so appears to be true. I only stipped the engine because a fin was missing and it came with a spare barrel.

[REW]

I have to repeat the advice re push rod rotation/tappet clearance. Rotation should be freely possible at TDC and I have experience of difficult starting where rotation was possible at TDC but clearance was not quite enough! Leave 'em definitely a bit loose while you track the fault.

[simonh]

Thanks to all for replies and suggestions which are greatly appreciated. I stripped everything down again early this morning before work. The cam/crank dots are aligned and I will re-do the ignition and valve timing from scratch after work today. I removed the valve springs and checked that the valves were free to rotate in their guides and not sticking. Following this, I inverted the cylinder head and filled the combustion chamber with petrol rather than oil. This did eventually evaporate but some did leak through to the tray underneath, but it took a couple of hours and it was not much. I will re-seat the valves and I will also loosen the valve clearance a little. The copper head gasket was new yesterday, but I will anneal it before re-using it. The composite gaskets ( I have 2 that were installed but the bike was never run) I think they are not re-useable (unless someone tells me otherwise, these will be binned). I will post another update if I can get the damn thing to run over the weekend. Thanks again, Simon H

[simonh]

Ok, well, I reseated the valves, stipped and reassembled the rocker shafts, re timed everything from scratch , left the push rods a little slack and reassembled the engine. At least it is consistent, exactly 4 bar compression and zero improvement. I am now thinking of wheeling the thing into a canal.