1981 CB750 wont rev out.

[brettp]

I suspect the cables to the spark plugs. If you kept the spark plug cables on the plugs, and exchange them on the coils, your wires could be corroded. Take off this spark plug wires from the spark plugs and the coils and see if you can get a resistance measurement from the plug cap to the end of the wire.

 

[Guitarguy1996]

It was only running on 1 and 4 regardless of which coil, wires, and plugs I used in each cylinder.

 

[brettp]

That is odd. Valves, even being way off, should allow the bike to run at least a bit. I can't believe this is a valve issue.

 

[Guitarguy1996]

I adjusted them once already, that's why it has compression at all to begin with, but maybe it's off just enough? That's the only thing I can think of. When it runs great on 2 of 4 and there's no other differences I can find.

 

[brettp]

I'm honestly stumped. I hope somebody else has a good idea.

 

[Guitarguy1996]

Yeah aside from valves, I'm running out of ideas too.

 

[brettp]

Do the valves. I ordered a shim kit from 4-into-1 (and still needed to order some specific ones) and my bike just now went through all the RPMs to 10k. I've been fighting it for a few weeks. In addition to the valve job I adjusted the timing. Mine was off a bit. 1 or both of those just fixed my rev limiting problem.

 

[SenseAmidMadness]

So went through the 2 middle carbs again and found a clogged tube under the pilot jet! I was thinking it wasn't removable like my wife's cl350 but wrong I was! It's still not great though. Sounds like it's running a whole lot better but it's just got no go. Topped it out at 71. Best yet but even that was a fight

Knew it.

Check the valves too, but you're wrestling a hydra here.

One of the many problems with pods and the stock carbs is unequal air supply across all four. The outer two carbs consistently get much more airflow than the inner two, and this problem only gets worse when you're riding. So your mixture, and the vacuum supply to raise the slides, will never actually be even no matter what you do. I have heard of people doing band-aid fixes like taping the pods unevenly and intentionally setting the carb synchronization uneven at idle so the outer two carbs have less open butterflies.

Tell your buddy whose bike this is to buy different carbs and save the hours of frustration. I have heard that certain years of GSXR carbs will fit with minor modification and tolerate pods much better.

And if you need in-person help, I'm like a half-hour down I-24 and have direct hands-on experience with a DOHC CB750. Happy to hang out and drink beer and work on stuff.

 

[brettp]

+1 to GXSR carbs. Heard that, too.

 

[Guitarguy1996]

So I adjusted the valves on the middle 2 cylinders to be as close as I could get them to the outer 2 and a few did need adjustment and I gained a bit of compression but still no luck. The guy that owns it brought spare coils he had to work so ill try those tomorrow.

 

[brettp]

Did you check clearance on ALL valves? Or just 2 and 3? What did you end up with for clearance numbers?
I don't know you, so I'll ask: do you know how to check the coils?

 

[Guitarguy1996]

I know they can be tested, I've done it before with my xs750, but I'm not sure what exactly to test between, what the resistances should be ect. I've got a manual for the cb but it doesn't have anything about testing coils in it. It just says, if no spark, replace coils....thanks Clymer...super helpful.

 

[brettp]

I know they can be tested, I've done it before with my xs750, but I'm not sure what exactly to test between, what the resistances should be ect. I've got a manual for the cb but it doesn't have anything about testing coils in it. It just says, if no spark, replace coils....thanks Clymer...super helpful.

Ok, I just did this a couple days ago. I'll try to explain.
Primary coil: this is all about the inputs to the coil with the spade connectors. put one lead on each of those connectors and your primary coil should read about 5ohms.
Secondary coil (put multimeter on 200k ohms setting!): for each coil, take the caps off the plugs and test across those caps. For instance, take the plug caps of cylinders 1 and 4 off the spark plugs, then put a multimeter lead in each plug cap. This will test the resistance between plug 1 and plug 4. Do the same for 2 and 3. Your resistance ratings should be around 22-25k ohms.

If you can't get a reading, take the wires off the coils and the caps off each wire. Look for corrosion. Then test the resistance of the caps and the resistance of the wires.

I hope that helps

 

[Guitarguy1996]

Thanks! I'll give it a try tomorrow and see what happens!