Low compression on a total rebuild top end

[Jake]

I have a 1980 CB 750 I acquired a couple years ago. The engine only had about 120 pounds per cylinder so I rebuilt the engine new pistons 823 Wisconsin and had to cylinder head with all valves redone. All work was done by a professional machine shop. Did reassembly with All punch marks and locations exactly as specified now I only have 125 pounds for cylinder. I warmed the engine up approximately 1/2 hour still reading 125 pounds per cylinder. I am coming up with low compression and also on the vacuum side. I am reading late valve timing. Can the crankshaft chain stretch enough where it would give me these issues and if so can I slip the crankshaft chain one tooth without having to split the lowercase.

 

[dirtdigger]

Did they set the valve clearances for you? Get adjustable cam sprockets if they are available then you can time it correct with a degree wheel. Are you sure your compression tester is accurate?

 

[Jake]

The valves were set at 5° for the intake and 7° for the exhaust. I don’t think they make an adjustable camp sprocket for the 1980 hopefully I’m wrong. I went and bought a new gauge to put on my compression tester just to be sure and they both come up the same number as they did before the rebuild I do believe my chain that goes from the crank up to the cam is stretched out because all marks exactly where they’re supposed to be. I also bought a new cam chain that goes from Cam to cam still the same issue low compression and lead valve timing.

 

[dirtdigger]

I dont understand you valve numbers, valve clearance needs to be .005" to .006"

 

[Jake]

.005 on the intake and.007 I need your trust what I am trying to simply explain even with the.003 to .005 that I had previously before the rebuilt I still had low compression and also had a late valve timing on my set of vacuum gauges and now after I rebuilt the top and completely with new patient every done valves, I still have the exact same issue

 

[Jake]

Is there anybody out there? I can give me some advice on this and possibly what to look for thank you.

 

[dirtdigger]

Well if its the same as before then it is something you didnt change. If you are sure the valve timing is off then you should probably put a new chain in it to get it back to where it needs to be. If you had access to a mill you could slot the cam sprockets to get the valve timing closer on the stretched cam chain I suppose. You have already come this far with rebuilding the top end why not take it down one more time and put new chain in.

 

[Jake]

As I did put a new Time in chain that goes from cam to cam which made no difference I do understand what you’re trying to say if the cam chain that goes down to the crank it stretched out enough where the slack adjuster doesn’t do anything technically speaking I believe from Cam two cam Should not change at all can I get away with adjusting the cam chain that goes down to the crank by one tooth while keeping the two cams True in place and trying that and re-marking it at the bottom. Or will I slap into a piston with my valves

 

[dirtdigger]

One tooth is a lot, If your chain is that stretched I would be afraid of it breaking at some point. You would have to set up a degree wheel and degree the cam and see where 1 tooth will get you. Thats the only way you will know for sure where you timing is. Thats kind of why they make adjustable sprockets so you can get the cam timing exact, chain stretch, milling the cylinders and or head surface all changes the cam timing. Your cam to cam chain wont affect your running as much as the crank to cam. The stretch in the long chain affects the timing more as the chain wear is amplified by having many more links to wear.

 

[MP-x]

My two cents: I'd run that engine at least couple of hours or 200-300 km's more (what's that on miles?) before doing any compression tests on it, if it's been rebuilt. And use break-in oil or some Zinc additive on engine oil.
Just to allow top-end and pistons & rings to wear and settle a bit. If still low on compression after that, change the cam chain. It don't cost that much, and can be installed without taking the engine out and splitting the cases. Instructions on how, can be found on this site.
Yes, I know, many are against swapping cam chain like that, and I'm NOT saying that you should, just that it can be done. I've done my chains like that, but that's just me.

 

[Jake]

The camp chain that goes from cam to cam has been replaced already my concern is the train that comes up from the crank to the exhaust cam whether that will stretch enough where the slack adjuster cannot make it up there for giving me a lower compression

 

[MP-x]

If crank to cam (primary cam chain) has streched so much, that it affects timing and gives you lower compression, you should hear cam chain rattling when engine idles. I presume cam chain tensioner is at max? Cam chain would also lift a bit from sprocket when you try to lift it by hand if chain is loose.
It should not move at all if chain is ok and tension is properly adjusted.

 

[Jake]

All the things you mentioned are good here’s my question I put Wiseco Pistons in there 30 over I’m reading 125 pounds after this engine warms up and all the ring seat what do you think I’m gonna be reading will I ever get up to 180 or so on the psi

 

[Jake]

Can the crank position timing mark change over time as to give a false reading where the crank it

 

[dirtdigger]

You wont gain that much compression. With modern honing techniques rings seat almost right away. If it was honed old school then yes it can take a while to seat but you definitely wont gain 55 psi. I would be doing leak down test to see if by chance valves seats not seal or rings not sealing. If it passes with acceptable leakage numbers then you can look elsewhere. Take a little oil in each cylinder see if it raises compression or not...no more then a teaspoon.

 

[Bo82CB]

Do you have throttle wide open, air filter removed when doing compression check?

 

[Jake]

I did not even in Staller carburetors yet

 

[Jake]

Can somebody tell me which is the Trumark for Mechanical timing.