1975 no compression on right side cylinder

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acecafe1000

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Well, finaly broke down and bought a compression tester...found out why my right cylinder wasn't firing - zero, absolutely zero, compression. 120 on the left.

Done the basics, cam chain, valve clearance...with the valve access covers off, spark plug out, cycling the engine...from the minimum amount of surface area i can see on the pistion, not seeing any nasty holes. Valves are working as far as i can tell...and can see them peek thru the sparky hole. Tells me they are at least not hung up, not grossly bent. My qeustion is...shouldn't i be seeing at least some compression if my valves aren't fully seating? Shouldn't i see at least some compression if the rings are shot?

Before i checked the compression, when i discovered the one cylinder wasn't fireing, this is what i witnessed with the bike idleing off the left side only:
- put hand on dead cylinder pipe, sucks your hand to the pipe,
- rap the throttle, popping, air blow back thru the carbs...in sync with the piston
- pulled the plug and it shot out of the engine once i got it off the last thread - serious air push thru the spark plug
- absolutly now rattling, clanking, etc. while running.

Going to play with it some more tonight to see if i can track down where all my compression is sneaking out...before i learn how to tear into her.

Have i done absolutely everything i can do before opening her up?
 
acecafe1000 said:
Well, finaly broke down and bought a compression tester...found out why my right cylinder wasn't firing - zero, absolutely zero, compression. 120 on the left.

Done the basics, cam chain, valve clearance...with the valve access covers off, spark plug out, cycling the engine...from the minimum amount of surface area i can see on the pistion, not seeing any nasty holes. Valves are working as far as i can tell...and can see them peek thru the sparky hole. Tells me they are at least not hung up, not grossly bent. My qeustion is...shouldn't i be seeing at least some compression if my valves aren't fully seating? Shouldn't i see at least some compression if the rings are shot?

Before i checked the compression, when i discovered the one cylinder wasn't fireing, this is what i witnessed with the bike idleing off the left side only:
- put hand on dead cylinder pipe, sucks your hand to the pipe,
- rap the throttle, popping, air blow back thru the carbs...in sync with the piston
- pulled the plug and it shot out of the engine once i got it off the last thread - serious air push thru the spark plug
- absolutly now rattling, clanking, etc. while running.

Going to play with it some more tonight to see if i can track down where all my compression is sneaking out...before i learn how to tear into her.

Have i done absolutely everything i can do before opening her up?
Click to expand...

No compression is a sure sign of a valve problem.
 
I think you've done about all you can without tearing into the engine.

The sucking/blowback thing makes me wonder about valve timing. When they re-phase an engine the cam is rephased as well, which makes me think the left and right cams are able to be separated in some way (don't know, never had one apart) and that leads me to think it might be possible for the relationship between left and right sides to change if something let go.

Then again, I could be completely full of crap...
 
i'm waiting on this response:

"take big hammer, wack it on the right side of the engine...jump on, ride into the sunset"

If my valves were hung open, enough to get zero compression, would i not see some carnage at the valve springs? offset stems? clank'n like crazy? She's pretty in there, like she just rolled off the show room floor.
 
Yup read that thread, there are several more things you can do before you start taking the engine out.
 
acecafe,

Perhaps you adjusted your valves incorrectly and one of them is partially open. You would see evidence of compression when the engine is idling because the air cannot escape fast enough through the partially open valve, but when you do the compression test with the starter motor, the engine is turning over relatively slowly so there is enough time for the air in the cylinder to escape resulting in a zero reading on the compression gage.

So, check you valve clearance again. It is very common to incorrectly adjust the valves with the cam not in the correct closed position 180 degrees from the peak of the lobe. If you look in the valve cover hole, you can see the cam lobes.

Also, be sure and hold the throttle wide open or you will get low readings, like the 120 on your good cylinder.
 
I'll take a look at the valve clerances again tonight. Last night, pulled the carb and valve cover plates off both cylinders to compare where i'm feeling air during the test. The left side, no air release present at any locationa and getting 145 solid on the test. The right side, i get alot of air out of the carb intake, decent amount of air out of intake valve cover...little air from the exhaust valve cover.

guess i will tinker for a few more days before i get my hands dirty....thanks for all the responses.
 
Update.
So last night, finally found time to attack, and i mean attack, my right cylinder valves. Re-tested the good cylinder...145, solid. Still zero on the right. Compared the two valves as they worked and noticed my intake on the bad cylinder had alot of crap on it and wasn't seating fully...so i shot that turd with BP Blaster, even attacked it with a long tooth brush, did other things that i'm not going to mention. Last thing i did was fully backed off the valve adjusters to almost .25 on both intake and exhaust on the bad cylinder...hooked up the tester, sure enough 130.

So i found TDC on the compression stroke, backed down the adjusters to 03/06 (or was it 02/04) anyway...tested again, zero compression. WTH. So backed the adjusters back to .25, test...130.

So tell me...can i leave the valves grossly out of specs, just to get the engine to fire, and run her for 15-20 minutes? I'm hoping this would blow out the offending build-up, soot, fuel varnish, (BP Blaster)....then let her cool, see if i can tighten up the specs and still get compression?

Runnig the valves at .25...will it hurt the engine? or just sound like a cylinder full of aluminum cans...

As you can tell, i'm doing everything i can before cracking that head.
 
acecafe1000,

Well, not that gross. Perhaps double what they are supposed to be. I still have this feeling that you may not be adjusting the valves properly (sorry). Did you peek in to see the actual position of the cam lobes when you did the adjustment? You also have to be sure that you hold the adjusting screw steady as you tighten the nut because the adjusting screw will move and close the gap down when you tighten the nut.

The fact that you are getting 130 would seem to indicate that you can fix the problem through valve adjusting or cleaning the valves as you have done. If you can get it running on that cylinder, the heat may help to dislodge the crap on the valves.

A 130 cold will probably result in a 145 to 150 once you get it going and warmed up.
 
i've had that in the back of my mind all along...am i not reading my instructions correctly? Here's my technique: thumb over the plug hole i'm setting, once i feel pressure, i watch my rotor and slowly bring her to the "T" mark...this resolutes in both intake/exhaust valves closed. Play in the adjusters - other cylinder, no play. Set the front, set the back. Then i rotate 360 degrees...land on the "t", do the same for the other side. I'v ASS-Umed i was doing it correctly as the cylinder that is happy fires right up.

I'm going to play with the adjusters and the tester tonight to see at what gap it starts to fall off of the 130 range...set them, try to fire her up. My ultimate worry is this little engine has been sitting for decade(s) with the right cylinder valves wide open, springs compressed...and the springs just simply don't have the juice to rebound anymore with a 02/04 or 03/06 setting.
 
"Last thing i did was fully backed off the valve adjusters to almost .25 on both intake and exhaust on the bad cylinder"

I would try to narrow it down to either the intake or exhaust valve. You might find that you still get 130 with one of the valves set at spec and the other at .25. Then I would try more cleaning of the bad valve.
 
acecafe1000 said:
My ultimate worry is this little engine has been sitting for decade(s) with the right cylinder valves wide open, springs compressed...and the springs just simply don't have the juice to rebound anymore with a 02/04 or 03/06 setting.
Click to expand...
I'm not an expert, but I doubt the springs are the problem. I don't see how a couple hundredths of an inch one way or the other will have any effect on spring tension.
 
excellent point DogBunny...never thought that possible the exhaust is okay and i'm only fighting the intake. I'll set one or the other to spec then go out from there...let you know what i find out.
 
You may have a sticking valve stem. From your narrative I am guessing intake here. With the valve covers off hit the stem with pb blaster let soak, repeat. you can also use a block of wood or soft drift and tap the valve open let it drop closed, do that a few times. How many miles on this? Really a top end job sounds like a good idea no matter what happens.
 
She's alive!!!!!
Saturday morning i did another tooth cleaning of the valves with BP Blaster. Set the exhaust gap to spec, ran test...110. Took the intake from .25 down to about .14, ran the test...120.

So i figured why not, button her up, kicked her a few times and she roared to life. After about 5 minutes of spent BP Blaster filling my garage...rolled her into the driveway and let idle for about 20 minutes. Jumped on and did a quick 2-3 miler around the neighborhood. Awesome...its amazing what two cylinders feels like compared to just one.

Sleet in Austin on Sunday so i haven't had a chance to see if i can tighten up the intake gap...

And DogBunny...broke down and added one guage, thanks for the tip.

IMG_0240.jpg
IMG_0242.jpg
 
Did you do the initial compression test with any oil in the cylinder? Or was it always a dry compression test?
 

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