Low compression both cylinders

[Triggerton III]

Hi all

I am looking fir a few pointers please. I have just rebuilt the top end of an 82ish engine. (First engine rebuild of any kind). I did have good compression. fired up with a little fettling. I checked the compression as it would no longer fire up

These are the results I am getting. left and right as though sat on the bike.

Left. Right
CT 65 65
OT 65 65
OIC 90 90

CT = Closed Throttle
OT = Open Throttle
OIC = Oil in Cylinder

The bike has done less than 4 miles and when it ran it was rough at low RPM.

Please sanity check my theory. The readings tell me that the valves are good. ( I did lap them)
The Rings wont be bedded in yet and I need to get that done to see if the compression comes up. I should be able to start it with the little oil in the cylinders.

I have checked the timing and it does look retarded, the marker on the rotor is well to the left of TDC marker when revving the engine.

Before I do this I will check the cam chain tension and adjust.

Any pointers or advice would be appreciated. I do get the feeling I will be taking the engine back out of the frame.

 

[GLJ]

I did have good compression. fired up with a little fettling. I checked the compression as it would no longer fire up

You said it did have good compression. Did you check compression before you started it? If so what was it.
Reason I ask is I just reassembled my engine. Yesterday I had a little time to kill and was curious what it had for compression. I used the starter motor to crank it over. 115 on right and 110 on left. This motor has not been started yet. It's still on engine stand. Your compression sounds pretty low.
I think you need to verify your cam timing and do a leak-down test.

I have checked the timing and it does look retarded, the marker on the rotor is well to the left of TDC marker when revving the engine.

It should be to the left. Where is it idling?

 

[Jim]

Check and see if your valve adjustment is too tight.

 

[gggGary]

Check the cam, crank marks? Cam off one tooth? Was your cam chain loose at some point?

 

[Triggerton III]

You said it did have good compression. Did you check compression before you started it? If so what was it.
Reason I ask is I just reassembled my engine. Yesterday I had a little time to kill and was curious what it had for compression. I used the starter motor to crank it over. 115 on right and 110 on left. This motor has not been started yet. It's still on engine stand. Your compression sounds pretty low.
I think you need to verify your cam timing and do a leak-down test.

It should be to the left. Where is it idling?

You said it did have good compression. Did you check compression before you started it? If so what was it.
Reason I ask is I just reassembled my engine. Yesterday I had a little time to kill and was curious what it had for compression. I used the starter motor to crank it over. 115 on right and 110 on left. This motor has not been started yet. It's still on engine stand. Your compression sounds pretty low.
I think you need to verify your cam timing and do a leak-down test.

It should be to the left. Where is it idling?

I dont have a compressor for leak down test. I can double check the timing. So far i havent been able to get the bike to idle. in fact when running it did pop and fart. now it wont start at all. engine out the frame again me thinks

 

[Triggerton III]

Check the cam, crank marks? Cam off one tooth? Was your cam chain loose at some point?

the bike was a none runner when i bought it. low compression right hand cylinder. yes the tensioner was lose at first dis assembly

 

[TwoManyXS1Bs]

A retarded cam would have late intake valve closure, lowering the compression readings...

 

[Triggerton III]

checked valve clearance, checked cam chain tension. Still low compression. engine back out to check cam and crank tdc marks. thanks guys.

 

[5twins]

That could have been done with the engine still in the frame and assembled, but a bit late for that now, lol. One of the things to check when you tear it down would be ring install. The top 2 rings usually have a designated top side and it must, of course, face up, or the ring won't seal. This top side is usually marked with some letters or numbers stamped into it. These types of rings have beveled edges or possibly steps machined into them, and these features must face a certain way to work and for the ring to seal.

 

[Triggerton III]

Oh right, i thought I would need to take the cover to check the marks on the cam sproket?? I havent taken the engine out yet. thank you 5Twins

 

[5twins]

The locating pin hole for the advance unit aligns with the TDC notch in the cam sprocket. You probably don't have an advance unit on your '82 model, but the hole in the cam should still be there. Down at the crank, align the rotor mark with the TDC mark, and this hole in the cam should point straight up or straight down if the cam is timed correctly .....

 

[Triggerton III]

Mate, you are a star. I didnt connect the dots!! thank you.

 

[xjwmx]

If compression really was good to start with, the problem would have to be burnt exhaust valves. I can't imagine rings going that bad in 4 mi., but newly (over) lapped valves maybe.

 

[Triggerton III]

I hope I haven't burnt the valves

 

[Triggerton III]

The locating pin hole for the advance unit aligns with the TDC notch in the cam sprocket. You probably don't have an advance unit on your '82 model, but the hole in the cam should still be there. Down at the crank, align the rotor mark with the TDC mark, and this hole in the cam should point straight up or straight down if the cam is timed correctly .....

No prize but you are the winner .looks like a retarded cam. Do you agree?

 

[5twins]

Yes, the cam looks to be off, probably one tooth. If you re-used the old cam chain it could also be possible it's very stretched out. But I think you should have been able to align things better than that, even with a stretched chain. Looks like she's coming apart again, lol.

What you may have done is align the sprocket notch but then not apply the tensioner to check it. The tensioner would roll the cam backwards like that when applied if the chain was used and had slack in it.

 

[GLJ]

I think you need to remove the nut and rest of advance unit so you can see the pin in the cam.

 

[Jim]

That's a 79 or earlier engine..... not 82ish.

 

[5twins]

Also, I think you have the notched ring that holds the advance unit on installed backwards. There is a raised ring or ridge around it's I.D. and that should face out .....

 

[Triggerton III]

Yes, the cam looks to be off, probably one tooth. If you re-used the old cam chain it could also be possible it's very stretched out. But I think you should have been able to align things better than that, even with a stretched chain. Looks like she's coming apart again, lol.

What you may have done is align the sprocket notch but then not apply the tensioner to check it. The tensioner would roll the cam backwards like that when applied if the chain was used and had slack in it.

think youre right. so i set it up and then adjust the tensioner and make sure the cam is still aligned? how do i ensure the cam doesnt role back?