Topic: 1979 CB650 Head Removal?

[adr3naline]

Hey! So I've been trying to get my CB650 to run. I've worked out my electrical issues, and I can get the bike to crank, but I still haven't been able to get it to start. So, tonight I tested compression, and I've got 90psi at #1, 30psi at #2, 30psi at #3, and 90psi at #4. The service manual says 170 +- 28psi! Is that really where it needs to be? I adjusted the valves and the 90, 30, 30, 90 is as good as I could get. I also don't have any leaking gaskets at the head, from what I can tell.

So, I assume I need to get into my cylinders to inspect rings, pistons, etc.

The service manual says "All cylinder head maintenance and inspection can be accomplished without removing the engine." What is your experience? Is it just easier to remove the engine or can I remove the heads with the engine in the bike? What do you recommend with these old engines. Thanks!

[Fritz]

I have a 550 but the frame of the 650 looks similar. So yes, you ought to be able to get the head off without the need to remove the whole engine.

The results of your compression test don't need to match the ones stated in the manual. Honda most likely used a different compression tester to get their numbers. My brand new, just broken in, pistons/rings were doing 110psi on my particular tester...

The low numbers of cylinders 2 and 3 could have several reasons:
- Compression tester did not seal well in the spark plug holes in #2 and #3 (hard to get there)
- Carbs are very badly out of sync - all have to be WOT while doing the test
- Tappet clearance misadjusted
- Valves burnt in cyl #2 and #3
- Piston rings worn

First, I'd check and adjust the tappets, then I'd take off the carbs and repeat the compression test. If your numbers are still bad, pour some drops of oil into each spark plug hole (same amount in each cylinder) and repeat the test. If the numbers get better, your rings might be worn. If not, some valves might not be sealing.

[scunny]

+1 as above
to add more, after cranking are your plugs wet with gas, if not an introduction to your carbs is required.

[trueblue]

I assume this bike has been sitting for some time, if that is the case, as long as you have some compression ignore the readings until it has been run for a while, the rings can stick in the ring grooves and not seal against the cylinder, they free up after a good run, even with those readings it should at least try to fire if not run.  First check and make sure you are getting fuel to the cylinders (the plugs will be wet after cranking), then check spark while the plugs are out checking for fuel, then check the spark is happening at the correct time by checking/adjusting the static timing.  You already said you adjusted the valves, just a silly question, did you adjust the tappets with the cam on the back of the lobes? (the reason I ask is it wouldn't be the first time I have had seen this happen).  I have never seen a multicylinder engine that turns over freely that can't be made to run to some degree, even if it is only a cough and a fart, and I do work on a lot of engines , basically what I'm trying to say is don't be too quick in tearing the engine open, cover the basics first, AIR, FUEL, SPARK and TIMING.

[adr3naline]

The results of your compression test don't need to match the ones stated in the manual. Honda most likely used a different compression tester to get their numbers. My brand new, just broken in, pistons/rings were doing 110psi on my particular tester...

The low numbers of cylinders 2 and 3 could have several reasons:
- Compression tester did not seal well in the spark plug holes in #2 and #3 (hard to get there)
- Carbs are very badly out of sync - all have to be WOT while doing the test
- Tappet clearance misadjusted
- Valves burnt in cyl #2 and #3
- Piston rings worn

First, I'd check and adjust the tappets, then I'd take off the carbs and repeat the compression test. If your numbers are still bad, pour some drops of oil into each spark plug hole (same amount in each cylinder) and repeat the test. If the numbers get better, your rings might be worn. If not, some valves might not be sealing.

No I had didn't do anything to the throttle during the compression test. I've also been having some carb issues, like the leaky issues everyone seems to have with these carbs. I had already pulled it apart and cleaned it, but I guess it's going to have to get more involved than that.

[curemode2002]

Hey adr3naline just wanted to let you know I am here it utah as well nice to see someone close with the same bike.

I aggree when I first got my 650 it had been sitting outside for 3 years no cover nothing and my compression was #1 62psi #2 95psi #3 40psi #4 32psi. I was able to get good spark so I changed the oil rebuilt the carbs and fired her up. She coughed, sputtered and smoked really bad for like the first 5 minutes then she started running perfect. After the run I was getting #1 95psi #2 97psi #3 96psi #4 98psi. So I bet if you can rebuild the carbs she will run and clean up the rings. Check out www.siriusconinc.com they are a great source for the carbs on the 650 and the only reliable one for the 79-80 PD carbs.

And yes you can remove the head, cylinders and pistons without taking the motor out of the frame. But removing the carbs and the camshaft are a bit of work. If you have any questions or need a shoulder to cry on shoot me a message. (and the 650 will make you cry at some point  )

[adr3naline]

Hey adr3naline just wanted to let you know I am here it utah as well nice to see someone close with the same bike.

Nice! I'm just down the street in Orem! I'd love to see a nice running 650 sometime!

[curemode2002]

Just working on mine right now getting her ready for spring. Cleaning the carbs and such hopefully it stays a bit warm this weekend and I can get some stuff done.