Zero compression on left hand cylinder

[billy sugger]

The 500 was due to get MOT'd so thought I would start it and ensure all was well, but lo and behold it wouldn't start. Took carbs off and cleaned out all jets and orifices, both mixture screws were fully wound in and head broke of one that was corroded but managed to remove it. 

New carb repair kit ordered, and replaced all seals, drain screws and mixture screws. Put it all back together, and she started, albeit reluctantly, and with a misfire on left cylinder After a lot of fettling I managed to get it to idle properly, so I kept her running to check for leaks, but she was still misfiring so took plugs out to check condition - both were fine so while engine was hot I did a compression test - right hand reads 160psi (should be 199), left hand doesn't register anything. 

Anyway, carbs, airbox and rocker cover off, all seems to be working fine, can see piston rise and fall, so, 

HELP! 

[Bianco2564]

Do a wet compression test, squirt a bit of oil in the cylinder and repeat the comp test, if the results rise it points to piston ring and bore wear.  Won't help with a hole in the piston or if there is a big chunk out the piston crown.

Check for top end problems; valve clearances, burnt valves, incorrect cam timing (cam chain jumped), bent valves (you haven't dropped something into the intake while carbs off?), head gasket.

Was it ok the last time you ran it?

 

[Mississippi Bullfrog]

If you can see the piston and there's no obvious damage to it then it sounds like a valve issue. Even a worn piston/ rings would give you some compression. 

[Stu]

It should be registering something if not you have a major issue like a holed piston of stuck/bent/broken valve 

 

In which case it would only really be running on one cylinder 

[billy sugger]

When the engine packed in it had bust the left conrod, and the loose end took a couple of lumps out of the area below the barrel, and there was nout wrong with the barrel, thankfully. Because the crankshaft was damaged I bought a used one complete with pistons and rings, and I fitted the complete replacement rather than strip the pistons/rings off my original one. The top end, (valves etc) remained untouched and I triple checked the timing when I refitted the head and the camshafts. 

I've just been and rechecked the timing and valve operation, and turning engine over by hand all seems to be in working order, from what I can see of the exhaust valves through the inlet manifolds they are opening/closing as they should so I'm stumped. I will drop the exhausts tomorrow and see if I can see the exhaust valves in action

[Old-codger]

As others have said do a wet compression test and also check the valve clearances in case one has closed up on that side or stuck open.

[Bianco2564]

When the rod broke, has the unconnected piston clipped the valves?

Have you access to a leakdown tester, you will be able to determine where the air is escaping from.

[billy sugger]

Hi @Bianco2564 I've not seen any damage to the valves, but that doesn't mean there isn't, and I don't have access to a leakdown tester

[billy sugger]

I'm going to remove the camshafts, support the camchain and do another compression test. If I have pressure then it has to be the valves 

[Bianco2564]

4 hours ago, billy sugger said:

I'm going to remove the camshafts, support the camchain and do another compression test. If I have pressure then it has to be the valves 

How do you plan to pressurise the cylinder with the cam chain disconnected?

Spinning it over on the starter with the chain lòose sounds a risky plan.

[Mississippi Bullfrog]

5 hours ago, billy sugger said:

I'm going to remove the camshafts, support the camchain and do another compression test. If I have pressure then it has to be the valves 

If you have a bent valve removing the camshaft won't close it.

[billy sugger]

I have tubing that sits in the camshaft recesses and the chain goes over them just as it does on the sprockets turning the engine over with the starter still moves the pistons up and down, so if there is any compression in the cylinders it should register movement on my tester dial.

Indeed, bent valves will likely stay open, so without the cams pressing down on them if not bent their own springs should keep the straight ones closed. 

 

As it happens doing it without the cams in place showed both exhaust valves were bent and not sealing completely. Removed valves, and with a bit of ingenuity I was able to straighten them so relapped them in then inverted the head and filled domed area around valves with WD40. First attempt was not good enough so one leaked very slightly, 2nd attempt no leaks

[Bender]

1 hour ago, billy sugger said:

I have tubing that sits in the camshaft recesses and the chain goes over them just as it does on the sprockets turning the engine over with the starter still moves the pistons up and down, so if there is any compression in the cylinders it should register movement on my tester dial.

Indeed, bent valves will likely stay open, so without the cams pressing down on them if not bent their own springs should keep the straight ones closed. 

 

As it happens doing it without the cams in place showed both exhaust valves were bent and not sealing completely. Removed valves, and with a bit of ingenuity I was able to straighten them so relapped them in then inverted the head and filled domed area around valves with WD40. First attempt was not good enough so one leaked very slightly, 2nd attempt no leaks

Old transit Di engine used to bend valves when belt went, soft as liquorice, straightend up no problem in vice  

 

Good luck on the re start 

[billy sugger]

The bent valves were probably my fault from putting rebuilt engine back together. I think I may have lined up the T mark on the alternator with the timing marks in the inspection cover instead of the O mark when I put the cams back on and lined their marks with the head, but if I had then they would be damaged more, and there are no marks on the replacement pistons so they must have been hit when the conrod broke

[Bianco2564]

Glad you have found the source of the problem.

Not sure I would straighten valves, the steel will be fatigued from the initial bend and further from being bent back.

Add in the heat when running and the valves will be weakened.

Edited September 26, 2022 by Bianco2564

[billy sugger]

To be fair they weren't bent all that much, just the tip, so they weren't sitting in their recess properly. I stuck the valves into a bit of tubing and gently tapped on the high points until they would spin in a drill without a wobble. 

 

Partially rebuilt engine, now have 120psi from a cold engine, so a result

[billy sugger]

Bikes now up and running. It's purring like a pampered Persian pussy in a feline equivalent of a health spa