Topic: CB750 K7 No compression mystery

[M.Henry]

So I just put my motor back in my bike after doing some cleaning up of the internals and trying to fix a few oils leaks. I only dissassembled it down to the removal of the jugs, but no case splitting junk. Well When I got everything plugged up and connected, I could barely get her running. I thought it was just a cold engine start-up being so challenging because of an untuned carb, and a cold cold garage. Well I finally got her running but it doesn't sound right, I notice that the exhaust puffs out of cyl 1 is cold, cyl 3 is spitting fire every now and then out of the exhaust, and there was air flowing out of my cyl 4 carb that appeared to be exhaust (including a stream of fuel from the carbeuration process in reverse flow. kinda cool)

Well anyways I checked compression, cyl 1 & $ never moved the needle, while cyl 2 & 3 barely moved the needle up to 60-90 psi, but always returned to zero after each pulse (not building up the pressure on the gauge). I figured I had timed my cam wrong when I assembled the top end. So I started the tedious process of pulling the motor out again. Once I got it out I pulled the valve cover off and the cam timing marks lined up correctly at the right setting of the crank position. Though my notch was on the bottom instead of the top. (I did my research and knew this doesn't really make any difference, it just switches the strokes between 1 & 4 and vice versa for 2 & 3. which still works cause the bike has a waste spark). I corrected flipped the cam to be right, flipped my cam chain sprocket cause I had assembled it with the wrong face in contact with the cam's cam chain sprocket support portion. I readjusted the valve lash while I was in there. (tight then undo a quarter rotation) I put it back together, threw it back in the bike, started connecting her back up.

Well when I was about to put the carb on my helper thought we should check compression. Well we got out the compression checker. I climbed on the bike and started kicking her. (still had some electrical to set up before I could use my starter again) I immediately noticed that there was very very little resistance, I could have spun her over by just standing on the kicker. He said no compression. We then checked all 4 cylinders and yep, none of them had compression. We thought about this for a while and couldn't think of what could be going on. We opened up one of the valve lash access ports and noted that valves were opening and closing when the kicker was spinning the engine over, so the valve action is happening. We also poured a touch of thick oil in the chamber and rechecked compression to see if it might be rings that were not sealing, and there still was no compression readings. I then got so confused by this failure after pulling the motor for the situation to be worse, that I called it a night and we drank some good rum. So my question is what do y'all think this issue could be caused by? I've been pulling hair out wracking my brain over this issue.

[M.Henry]

Note: I have a harbor freight compression checker, and my helper has a nicer one. We verified the lack of compression with both to assure that it was not a issue with a compression checker failure.

[M.Henry]

hmm. Good point. I don't think I've ever referenced the plate before, it could be out of phase.

[M.Henry]

Yeah, I have certainly been concerned with poking holes in pistons, but I would have felt that problem or heard failures happening if that happened. Right?

[754]

If your valves are too tight, no compression .
 Set on the wrong stoke this happens, easy to fix
 Start with 1, watch the intake valve, see if it closes  as piston is coming up, then it's probably right.   (Both should be loose , but not if set wrong.. look at the threads sticking out above the nut, should be close to same)Then you set both rockers, then turn over till 1 comes up again,  at least one rocker be closed and you go and set 4.

[M.Henry]

Hmmm, I can check that. My helper set the valve lash. I do remember him going through and making sure there was just a smidge of wiggle on the unengaged rockers before we put the valve cover on. Which leads me to think that the rockers may not be too tight.

He just mentioned the idea of collapsed (worn down?) springs that aren't pulling all the way tight. Is that a possibility? (His background is V8 motors, so I'm not sure if tahts really even a thing with the cb engine or more of just some big v8 engine thing.

[seanbarney41]

wait wait hold up...how did you set valve clearance again?  Tight then back off a quarter turn?  That ain't right.

[M.Henry]

by tighten then back off a quarter turn, I mean tighten until it just barely makes contact with the valve, not bottoming out the screw or anything. By that method it gets it in the ball park of the shimmed gap without tediously playing with shims to set it.

[seanbarney41]

maybe that works? but I have never heard of anyone doing that successfully, so just use the feeler gauges...otherwise how do you know you have enough clearance?  This is not a small block chevy with hydraulic lifters.   Valve clearance is very important.

[M.Henry]

yeah. I guess that makes sense, the only thing we really changed was the valve lash.

Now I get to play the fun game of setting valve lash through those access ports. any tricks to make the process any easier? I always have trouble getting everything in thier, especially on the cylinders 2 & 3

[HondaMan]

"...tight and then back off 1/4 turn..." - wasn't that how the old Ford 289 engine's hydraulic lifters were set? They had 0.200" travel before the oil pumped them up(!).

[HondaMan]

I always have trouble getting everything in thier, especially on the cylinders 2 & 3

Yeah, me, too!
There is a nice valve-adjuster tool made by Motion-Pro that has the screwdriver and 10mm wrench built together in one tool, so you only need the feeler gage beside it. I have two of these because they are so handy, and I always lose one somewhere.

[M.Henry]

hmmm thats a good looking tool, wonder if I could rig one up with some JB weld, a 10mm socket, and a screwdriver.

Also my helper says he feels caught red-handed with your 289 comment.   

[seanbarney41]

It's not that hard, just takes a little patience, practice, and double check your work...all things required for any successful engine rebuild.  I like to bend the tip of the feeler gauge about 45 degrees about a half inch from the end...

[M.Henry]

yeah, I've got the practice down, and patience is the key to success. I'll just have to take my time and do it tonight or tomorrow.

I'm rather glad my solution means I don't have to take this engine out again. I'm getting to practiced with that.

What gap am I setting it to anyways?

[754]

Takes a little getting used to I snug it a bit check, then give it a bit more.. it moves a bit
 I hold the feeler about 1/2 inch or so from the bend and slide my UNwelcome finger against to bend to help prevent buckling.
 T think you can buy holder that the feeler fits in at a 90 or lesser angle, look atvtge bike supply shops see what they offer on line. Fairly easy after a few times.

[DV Red Herring]

https://www.motionpro.com/product/08-0392

After buying this little guy, valve adjustments were a one and done ordeal for me. Best of luck!!!

[seanbarney41]

whoa there dude, totally no guarantee that valve clearancing is necessarily what your no compression problem is, just the the first thing we caught that might be the culprit and it's easy to check

[Ridem32]

Did you put in new rings while it was down?  I would have.  Maybe you broke some rings putting it back together. But I would say it’s out of time.


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[M.Henry]

No new rings. Probably should have, but I am building a performance motor that I'd rather spend the money on. This current motor is just supposed to work in the mean time.

Yeah. I'm about to get down in the shop and check the valve lash gaps.
We did try the compression check test of putting some oil in the cylinder to see if the leak was valves or rings, and it made no difference which kind of points the finger at valves more then rings.

Yeah. Sure sounds like timing. Maybe the reference plate actually is changed. I've had like 5 eyes on the bike to check timing was right.

[754]

 Are the pistons facing the right way. .
 IF YOU FEEL LIKE cutting the frame to avoid pulling motor to take top end out, I designed  a kit to put it back in and be removable later on , can save some time..

[M.Henry]

I've actually seen your thread about your frame kit. Could you pm me the details on it.

[pjlogue]

I think Hondaman is correct about your excess valve lash.  Get yourself a set of feeler gages and a shop manual, and take the time to adjust the valves properly.  If you run the engine for any length of time with a large excess of valve lash you will mushroom the ends of your valves. 

-P.

[M.Henry]

There’s a very faint registration mark on the plate just outside the points cam to align the notch. Please post a picture of yours

Where is this mark exactly? I can upload the reference pics.

[M.Henry]

Alright. I started in the garage by checking compression. It has the same characteristics as where we started, a touch of compression on 2 & 3 but no movement of the needle on 1 & 4. This is after adjusting the valves on cyl 1.

My helper for tonight (different helper, also American muscle based knowledge)
Had the good idea to just try backing out all the valve tappets to assure that my valves are closed. Then compression check it. I'll have to do a full valve adjustment but it will help me better understand if it is the valve train giving me the issues, or the rings.