Endless problems

[boostaholic]

Bike: 1980 Honda CB750k.
Mods: Carpy 4-1 Exhaust
         Stock Jets, stock air box New carb boots, plugs, wires, caps.


A few weeks back my bike was running flawlessly. Rebuilt the carbs, had them ultrasonic washed blah blah blah. Ran a bit on the lean side but made really good power and physically ran well.

I ended up riding it on a hot day and sat in traffic for 45 minutes. Bike got hot, obviously, but ran fine afterwards... UNTIL I let it cool. After it cooled, the bike wouldn't rev above 7k. Idled fine, revved everywhere else except couldn't get above 7k. So my initial thoughts were that I cooked the engine. Checked compression, everything looked fine. ~130 per cylinder, all across the board.

I'm at a loss now. My initial reaction is the carbs are screwed up, but how? I have an inline fuel filter so they shouldn't be dirty. Maybe the timing chain skipped a tooth? I have adjusted the chain tension so I'm not sure how that could happen. I have no idea at this point, just throwing ideas out there. Anything I should be checking?

I appreciate the help.

[TOOLS1]

Throttle not opening the carbs all the way?  Electronic ignition going bad? Charging system going bad?
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[shinyribs]

How does the bike act when it wont rev past 7k? Does it break up, like misfires? Or does it just act like it cant physically rev any further?

[boostaholic]

Its a hard sensation to describe. It sounds like a revv limiter.. It sort of just breaks up. And I should also note that it seems to be temperature dependent. When ambient temperature is around 70 degrees, it seems to run a lot better. The other day it was around 85 out and after about 20 minutes of riding, the issue began to get a lot worse.. To the point where it would barely rev at idle.

[TOOLS1]

That could be a vacuum leak, or the ignition going bad. Those two things can both be affected by temperature.
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[boostaholic]

After looking through some of the wiring and ignition parts, I noticed one of the coils has a small white line running down it. I've seen this on one of my cars coils. And come to think of it, if a coil is bad, they tend to get worse as temperature increases; as temperature increases, so does resistance.

I guess it's worth a shot? Anyone?

[Re-run]

Well it is hard to say. First thing would be to check for vacuum leaks around the manifold boots. Get it idling and then spray short bursts of carb cleaner around both ends of the boots. See if rpm changes. If it does, you have likely found your issue. From there you just have to see why they are leaking. Clamps too loose, maybe shot, ect.

You should also try pulling the choke while riding and see if things get better. If so, then you are likely too lean and need to check your a/f mix.

Check your plug wires. A car I had once would run fine cold, but once warmed up some, would run like shit. Came to find out that ONE plug wire was grounding on a something metal, can't remember exactly what it was anymore. As soon as I moved it away, no problems, other than the hole in the insulation. So, check your plug wires and may as well check the wires for your whole ignition system.

I can't say anything about a white line, my coils don't have that but they are different coils.

I would also check your valve clearance. As the engine warms up, your clearance shrinks. Double check to make sure they are not getting too tight.

[boostaholic]

I have solved my issues! In case someone ever encounters something like this, I will detail what I tested and how I solved said issues.

The issue was, the engine would not rev out completely. It ran fine, no smoke or anything but just wouldn't revv. All of this started happening after a very hot day of sitting in traffic.

Things I checked, compression, valve adjustments and timing. All of these things checked out and it seemed the only remaining items would be either carbs or ignition. Having just rebuilt the carbs and having it running supremely well, carbs seemed unlikely. I checked for any vacuum leaks, and aside from a bit of air leaking at the throttle blades, all was well.

I then tested the voltage while idling. Bike idled at like 11 volts and when revved would not pick up voltage. ALRIGHT, now were getting somewhere. I took off the stator cover and tested the stator. I un-clipped the stator plug from the rectifier which revealed some melted casing and really corroded connections. Although the stator passed all the tests outlined in the link provided below(*), I replaced it anyways to get rid of that nasty plug. I found a spare engine on craigslist with the stator and rotor for 75 bucks. Next I tested the rotor, between the two copper ? rings, only around 0.1-0.3 ohms. Its required to have around 5 ohms for proper operation. SOO, replaced the rotor. After having to beat the S%^t out of the bolt holding the rotor on, I got it replaced and all reassembled.

I did notice while I was tinkering that one of the coils had some cracking on it. I decided to pick up two new coils for 40 bucks on ebay (probably cheap off brands but whatever), and replaced the old cracked ones.

After buttoning it all up, I started her up and checked the idling voltage. It was ~12.5 volts idling, and 14-15 while revving!

Took it for a test ride and it pulls nicely through the gears again!

Here is the link which contains much useful information on proper charging system functionality and testing.

(*) http://www.cb750c.com/publicdocs/charging_system/genesound_charging_system.html