Loss of power above 6k rpm

[Kuvnoz]

I have a new to me 1980 CB750C that starts right up with choke and idles fine off choke almost immediately. At low RPM it runs fine but the power REALLY falls off when accelerating and RPMs get to 6k. The plugs are new, the engine was just re-sealed. The spark plug wires l seem to be fine and not dried out. I can't think of why a 750cc inline 4 is running out of steam so early?

Ive run a couple tanks of gas through it will fuel cleaner because why not. I also put fuel stabilizer in it when i cant find ethanol free gas Does anyone have any advice?

 

[brettp]

Valves adjustment needed. Exact same thing happened to me. Ran fine all the way to 6k but then stumbled and wouldn't climb. To make it more frustrating, the bike would rev fine all the way to 9.5k in neutral. But under load, it wouldn't.
How many miles are on it?

 

[Kuvnoz]

35k ish. It was just pulled apart down to the pistons to get re-sealed due to an oil leak, It (supposedly) didn't have that problem before.

 

[brettp]

Yeah, supposedly. Valves are my suggestion.
There might be others with ideas, too.

 

[Loose Chain]

Valve adjustment was my first thought also. Did you buy it this way or did you have the work done? Were the valves adjusted? Do you know when they were last done? If you can't confirm when it was done I would say get it done sooner rather than later. Even if it turns out not to be the problem at least you know it was done. It is amazing how many guys ignore this type of maintenance.

 

[Kuvnoz]

Bought it knowing it had the leak and had the engine re-sealed immediately because it wasn't a small leak.

I don't believe the valves were adjusted when out back together. The bike is good shape, but I dont have any maintenance history on it.

 

[Loose Chain]

Looks like a valve adjustment is in order.

 

[LordNocholas]

Hey man I struggled with this for about a year. Tried adding a 4 into 1 exhaust to fix it. Still had the rpm issue would completely lose power after 5k. Tried rejetting the carbs. Same thing.

Ordered a set of Murrays carbs same thing losing power still. So figured out it wasnt the carbs or exhaust.

Checked the timing and clearance on the pulse generator. The timing was off and i set the generator clearance to be closer than what it was.

This improved the bikes idle but i was still losing power after 5k.

Was starting to get super frustrated.

Ordered new ignition coils. I started to see improvements. It idle with much more power and got past 7k once.

Still didnt fix it completely.

Finally i bought new CDI boxes from ricks electrics.

As i was taking off the old cdi boxes i looked at them they were completely melted and screwed up.


Replaced them with the brand new ones.

Bike immediately starts with way more power and just sounds more mean.

I crack the throttle it pulled hard throughout the entire rev range for the first time.

Was so awesome.

I recommend you do all the stuff everyone suggested but replace your ignition system its probably that honestly if you notice no real improvements with anything else

 

[brettp]

Glad to see you're up and running and didn't give up! Congrats!
That will give Kuvnoz something to look at, too.

 

[LordNocholas]

To give you an idea like these both were melted. It 100% was the issue i had.

but now since i added all the after market stuff and ditch the crap carbs it comes with. Its pretty awesome now. I still have a few bugs to work out but i can at least use the full power of the bike

 

[Loose Chain]

but now since i added all the after market stuff and ditch the crap carbs it comes with.

The carbs are complicated for a novice for sure.

 

[kirky]

i think it shows there are so many variables with these sorts of problems, mine had a cracked coil melted cdi's and all original leads etc, it wouldn't rev above 5000 so i bought new parts, no difference. the problem on mine was me putting 102 jets in instead of the 75's that were in the carbs, my books said 102's and i thought someone had messed. once i put the 75's back in it was cured. these 75's i found out were the original jets for my Japanese market only bike for there emission controls. i tried the cracked coil and melted cdi's again and it was perfect, as i say lots of variables , glad you sorted yours.