Stutter and loss of power at high speed and high rpm

[Re-run]

He can also put about, what,  teaspoon of oil into the cylinder and that will temporarily seal the rings if they are bad. That will make compression go up just for a bit. If nothing changes, then it is not the rings. Simple, fast way to check if it is a ring problem.

[shinyribs]

He just did rings, but you're right. Always do the simple checks first. Regardless if they seem redundant or not.

[Re-run]

I wonder how many miles he has done since the rings. Sometimes they take a bit to seat fully. Maybe, just a thought? Or maybe the gaps in the rings are too close together? I haven't had to do the rings on my bike yet and so I am just shooting here.

[nilsthenomad]

So, I did not have the throttle wide open when I first tested the compression. The choke was wide open but not the throttle. Pulled all the plugs, open up the choke and opened the throttle all the way for this test. Re-ran the first cylinder with the throttle wide open with no change. Poured a cap full of oil down each cylinder and got these readings:

1 - 100
2 - 240
3- 100
4- 240

Any thoughts? The oil certainly jumped all of the numbers, but the low ones by only maybe 10 psi or so.

[nilsthenomad]

I also only did the rings about 500-700 miles ago. Honestly haven't got much riding in as problems kept cropping up and didn't wanna stray too far from my free AAA tow.

[TOOLS1]

Did you hone the cylinders?
TOOLS

[nilsthenomad]

beyond replacing the rings, I did nothing to the cylinders.

Further notes: I didn't warm up the engine before I ran the compression tests, I just saw that it says to also do that in the manual. I'm not sure how much that matters, but am willing to try them all again with a hot engine.

[nilsthenomad]

Ok, just warmed up the engine, much to the pain of my neighbors as there was still some oil in the cylinders that needed to burn off. I pulled all the plugs again, opened the clutch and throttle and ran a compression test on #1, it was 90 psi, pulled off the gauge, poured a little oil in there and tested again, still 90. Didn't retest the other 3, but I'm hoping this means its not the rings. I've had some erratic readings for sure with the compression tests, but just wanted to add this note.

[TOOLS1]

Your compression went down on two cylinders when you put oil in them? Are you using a Harbor Freight compression tester?
I am wondering if since you did not hone the cylinders if you are passing oil past the rings at high RPM and fouling the plugs. Or you could have a fuel delivery problem.
TOOLS

[shinyribs]

If the compression readings are all wonky when using oil to help the ring seal then I'd be inclined to suspect valvelash even more. JMO


And my Harbor Freight compression tester produces the same readings as my Snap-on. Just saying. But, since it is HF, not all units may be of the same quality.

New rings on old, un-honed cylinders may take an extremely long time to seat. Or maybe never at all. Really don't wanna burst your bubble there, just being honest. But, since the oil trick didn't improve your compression numbers, it seems like your compression is going away somewhere else.

FWIW, 125 psi is generally considered low compression, but useable. 100 psi is generally considered to be on the verge of now wanting to fire. My '76F and my '74K both had 125 psi and gave no issues at all, so...like I said, FWIW.

[Re-run]

Yeah, oil didn't seem to change his readings, so I would be inclined to say it is valve related.

I would really give the valves a good check to make sure things are not too tight. I would double check timing too, just in case. I don't know if the pistons can smash the valves or not on the dohc, if not, then his valves may be slightly open at tdc and lowering compression.

[rich]

I've read (on the Internet, so it must be true) that automotive type gauges will give a bit lower reading on compression tests on small engines due to the tubing and adapters that may be necessary. Again, FWIW.

Please check your valves, I'm dying to know if that cures it. Mine is having the same high-rpm miss/stumble. If your valve lash turns out to fix the problem, I might even break down and check mine.

[nilsthenomad]

Awesome. Thanks for all the info. I definitely do not trust my compression tester. I got it from AutoZone, but I doubt I would have the performance I have if the compression test was correct. Plus, I'm sure I drained my battery a lot as I ran and re ran each test. I am checking the valve clearances tonight. Will report back.

On Mon, Jul 13, 2015 at 10:13 AM, rich [via Honda CB750'S] <[hidden email]> wrote:

I've read (on the Internet, so it must be true) that automotive type gauges will give a bit lower reading on compression tests on small engines due to the tubing and adapters that may be necessary. Again, FWIW.

Please check your valves, I'm dying to know if that cures it. Mine is having the same high-rpm miss/stumble. If your valve lash turns out to fix the problem, I might even break down and check mine.

89 VN 750A
79 CB 750K
78 CB 750K
77 CB 750K
77 GL 1000 x 2
77 CB 550F
Holton, KS, US

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[nilsthenomad]

Alright, so I pulled my valve cover and started measuring valve clearances and here's what I found:


As you can see, they are all over the place. They are all supposed to read .005 correct? The valves with an X on them are so tight I can't even get a .002 in there. Considering I've got the tightest valves on 1 & 3 and they were the ones with the worst compression, I think it's safe to say we've found the problem.

Now, I've ordered the special hook tool to depress the valve buckets and seen how to pull the shims out, but I'm not sure where to find new ones and how to measure what size to get. Can someone help me with that? Is there only 1 shim per valve? I seem to remember finding that these take 25mm shims, but if someone can walk me through fixing all of these that would be great. Feeling a little relieved, but more work to come...

[shinyribs]

Here's the tool you need if you haven't already found one. The MP on this says its a Motion Pro brand, so it's their part number. You don't use the tool to depress the bucket. ONLY to hold it down. Spin the cam to push down which valve you want to work on, insert the tool, spin the can again to gain clearance to extract the shim. You're gonna need a sharp pick to lift the shim. The oil there creates a heck of a suction. A magnet works, too. Spin the shim as you try to lift it. That helps break the suction.


One shim per valve is correct. If for some reason a shim pops out and things get crazy do not turn anything until you have a shim back in there. It can wreck your cam and/or bucket. You can use a quarter ($0.25 piece) if you just need something temporarily to turn the engine, but not for running of course.

I think the exhaust is supposed to be .003 at the absolute minimum and .005 seems correct for intake, but I really don't remember. It's best to check the FSM on this before you shell out for shims. Pretty sure 25mm is correct, but again, best to check the manual.

Shims are listed as total size and in mm's. So you'll have to measure the existing shim against you installed clearance and do math to see what thickness shook you'll need. Hope that all made sense. Typing on the phone and shooting for brevity. :D

[shinyribs]

Oh, and your cylinders with the x's correspond with the cylinders with low compression. I'm pretty sure you're on the right path here. :)

[nilsthenomad]

So, after waiting for some new shims and a ton of measuring and shuffling of shims, I think the problem is fixed. With most of my valves out of whack I had a lot of shims to pull and was basically able to find most of them new homes where they fit. Ordered 5 new shims to make up for the rest. Put her all back together, and once all the oil that I had poured in the cylinders to check compression burned off, the bike seems to be great. Engine starts up faster than ever before and I'm getting a lot more power. Whole bike seems a lot happier and roar of the engine is unlike I've heard it.

Thanks a bunch everyone. Really appreciate all the input and help.

[shinyribs]

Right on.  Awesome. Glad it worked out for ya,bud. Ride safe and many miles.