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Discussion Starter #1
A friend just got his K0 engine back from a complete overhaul. It was converted to conventional valve springs, but retains the same rocker arm setup. Not sure if valve clearances would be any different.
It's only run about an hour.. and makes about 130 cold on each side. It starts easily, idles beautifully and sounds very healthy.
BUT, it stumbles about 6500 rpm in neutral, and loses power at about 5500 rpm under load. The plugs get sooty fast.
It had points.. and I thought that this was a symptom of ether low voltage, or a failing condenser. But the voltage is a nice 13.6 at the (dyna) coils. So i had a charlie's place ignition waiting to get installed.. so this seemed like a good time. After installing, the idle was even more rock steady, and it starts so easily you can practically blow on it and it fires. Still, falling on its face at higher rpm and fouling plugs.
Before the refresh, the motor ran great, but it didn't always. Those early CV kehins liked to send the slides up too quick so it would stumble in the midrange and darken plugs, but was great at WOT. I added springs to the slides and that problem was solved. handily. for years.
But I'm running out of ideas here. I jetted the mains down from 130 to 125 with zero change. Tried no springs, tried stiffer springs.. same story.
I'm willing to entertain anything, but I do not believe it to be an internal issue.
Thanks!
 

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Are you getting full timing advance? 125 sized jets seem small to me. How are the airfilters? If they are original they may be blocking airflow. What did the engine refresh consist of?
 

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If your "stumble" or "power loss" is consistently at a given rpm rather than a specific throttle position I would suspect your new springs are not getting the job done and your valves are floating, especially since that seems to be the only significant change since it was running great.

BTW why are you checking the compression cold?
 

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Discussion Starter #5 (Edited)
JT, that''s where I'm leaning. Timing achieves full advance. No changes to carb alter the rpm that the power loss occurs. Float is the only thing that makes sense.

I took it cold because I had the ignition out, I had done a leakdown (it's tight), and then just wanted to insure it was making even compression.. regardless of temp. The rings are barely bed in, so I don't place too much value on the numbers just yet.
 

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I'd be looking at those carbs. I'm sure if Honda intended springs to be there they would have done so. Unless it's a common mod. A little bigger hole slows the slide down too. Maybe somethings blocked or leaking. I'd think if someone went to the trouble of installing springs they're get the right ones. My 2cents
 

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I'd be looking at those carbs. I'm sure if Honda intended springs to be there they would have done so. Unless it's a common mod. A little bigger hole slows the slide down too. Maybe somethings blocked or leaking. I'd think if someone went to the trouble of installing springs they're get the right ones. My 2cents

There was a Honda retrofit kit for the springs in those 4-speed carbs - a common mod back in The Day.
130 psi (if it's real) is probably the source of your misery. Even with new rings it should be better than that, especially after running for a few miles.
Again, that's if 130 is a real value......
This is what 450's do with low compression, loss of oomph at rpm.........

Now, most valve spring conversions I've seen switch to 5 mm valves - so maybe valves are as suspect (initially) as rings.
 

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Discussion Starter #8
Lefty, the k0 carbs were notorious for awful mid-rpm performance.. Honda did issue some retrofit parts and TSBs about it to try and solve it, but the damage was done from a PR standpoint.. and the word was out that they weren't the triumph killer they were built to be (even though it really WAS).. I found an article in an old motorcycle mag written by a dealer mechanic who said customers were bringing the bikes back constantly with driveablity complaints.
 
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