Compression and Valves

[timpetou]

Hi, after taking advantage of this forum for the last couple of weeks this is my first post. So, I bought a 79 CB750 a month ago and after a 100 bucks worth of beer and another couple of hundred in parts in I am at wits end.

Here's what I'm dealing with. The number 4 cylinder seems to be hotter than the rest, there is considerably more and hotter exhaust coming out of that pipe too. When I crank the throttle real quick there is a it doesn't rev up right away. It loses power past 4000 RPM and that prompted me to check compression and valve clearances. The results are: 140(4), 140(3), 140(2) and 30(1). Number one didn't change when I did the wet test.

The valves checked out like this

1: Intake 0.04, 0.04               Exhaust 0.13, 0.13
2: Intake 0.06, less than 0.04  Exhaust 0.09, 0.09
3: Intake both less than 0.04   Exhaust 0.05, 0.05
4: Intake both less than 0.04   Exhaust 0.23, less than 0.04

What I did so far:

new battery
new spark plugs (have spark on all 4)
cleaned carb (twice, dammit, after using gasket sealer that turned into fun little black snowflakes )

Now I am wondering, what the fuck is going on? Are my valves responsible for all that trouble?

[Lucky 1]

Post was mistakenly identified as a SOHC.
My apologies.

[TOOLS1]

Gee Lucky, He said it was a 79 CB 750. Time, for new glasses.
Any way, it sounds like there is a major problem with the No#1 cylinder. You could do a leak down test to determine what is exactly wrong, but the head will need to come off, for repairs either way it goes. Also it sounds like your carbs are still plugged up. These CV carbs are a pain to clean. I have learned my lesson, and take them to a shop that has an ultrasonic cleaner.
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[timpetou]

Hello Lucky, thank you for the swift and kind reply. It is a 79 cb750, like I wrote in the post. All measurements pertaining to valve clearance are metric (as in 0.04 meaning four tenth of a millimeter), whereas the compression test was performed using a psi gauge.

[timpetou]

Hi, thanks for the tip on the carb-cleaning. The way I understand these compression test is if there is no pressure buildup past a fairly low value that changes with a spoon of oil in the cylinder there is a problem with the rings. If thats not the case I am looking at valve-crap.

What am I looking for once that head comes off? Good

[TOOLS1]

Bent/burnt valves, damage to piston/cylinder, broken rings....
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[Lucky 1]

My apologies.

[Lucky 1]

Yes....Mistake o'Grande.

[Rodgil]

The spoonfull of oil will help the piston rings to seal. If there is only 5-10 psi improvement, then the culprit is the valves.

[timpetou]

yes, that is exactly what happened. I am curious, could the valve clearances for the number one cylinder (both exhausts 0.13 mm) be responsible for the lack of pressure?

[LukeM]

Welcome to the forum.  On the home page of our little playground, you'll find a link to Manuals.  Find the Factory Service Manual for your motor.  Go to the Maintenance section.  You'll find all sorts of information about valve clearances and so forth.  This is a DOHC motor, so let's focus on the stuff you can do while it's still in the frame, and still in one piece.

I'd recommend getting the valves adjusted first.  You can do this while it's still in the frame.  You've already done the feeler gauge thing, so you know where you need adjustment.  Then hie yourself to the Honda dealership with your measurements, and ask them to set you up with the appropriate shims.  That will help you get some consistency in the valve train.

I agree with the TOOLS-meister: there may be some valve work needed in your future.  But go through the maintenance section and give it a good tune up.

The carbs are a bit finicky, but not impossible to clean up. Just need to be fastidious. They can't be too clean.  The upside is: after they are working, the more gas you run through them, the better they will get.

Good luck, and keep us in the loop.
Luke M

[TOOLS1]

If you have valve clearance, the valve should seal. If you did not have any clearance then I would suspect the adjustment. You are fortunate in the fact that, you will not need to remove your engine from the frame, for the repairs.
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[timpetou]

Guys, thanks for the great advice. Now I am wondering, will adjustment of the valve clearances take care of the compression issue in cylinder 1? Or will I have to pull the cylinder head after all, no hope fixing my issue with the valve clearance alone?

Cheers.

[TOOLS1]

I" In reply to this post by timpetou
I know you are wanting to hear something like "put some air in the front tire, and it will be just fine" And I hate repeating myself, but.
If you have valve clearance, the valve should seal. If you did not have any clearance then I would suspect the adjustment. You are fortunate in the fact that, you will not need to remove your engine from the frame, for the repairs.
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[robs750]

You have stated that some of the valves have less than .04MM clearance.That equals to less than .0015".If you cant get a feeler guage in,then they may be holding the valves open if your lucky.

I would suggest swapping out the shims with a couple of quarters in the low reading cylinder to open the clearance.No need to invest money on new shims until you know that the head is good.
With the valves fully closed,do a leakdown test.

[timpetou]

lol, that's exactly what I wanted to hear. Anyway, the engine is on the table now. I removed the CH and alas, one of the exhaust valves on the low-compression cylinder has a part missing. I can stick a screw driver in there. Seems some parts are missing, little ones and some of the bolts were sheared off in the past. Ordered a used cylinder head, exited to see where this is going.