Dies at 5k RPM in gear but revs fine in neutral.

[mkaser81cb750c]

1981 CB750C DOHC

The bike revs fine in neutral but on the road it runs fine until you get to 5k RPM and it dies. I have to get off the throttle and then it is okay. Does this in every gear except neutral. Also once it is warm the bike overall runs terrible at all rpm (maybe because the power is even lower at that point?). Also battery never struggles or dies always starts up first try. Headlight gets brighter and dimmer depending on rpm... I assume this is normal? When warm it only goes about 25mph power constantly kicking in and out but mostly out. When warm barely makes it up a hill but picks back up going down a hill.

I believe it must be electrical since the bike acts fine in neutral (i'm assuming it draws much more power in gear and can't keep up?).

Carbs have been ultrasonically cleaned and synced, timing set, fuel flows freely from tank to carbs, SOS taken off, advance timing checked on 2 and 3 (figured 1 and 4 would be okay if 2 and 3 were, couldn't check 1 and 4 because their plugs were already fowled and I couldn't get a reading on the light), spark advancer is in great condition and works, 4 to 1 exhaust, stock air box, no vacuum leaks, new regulator/rectifier, air/fuel screw 3 turns out, tried dr8, dr7 and dr8 iridium ngk's, stock jetting (108, 68).

Whats confusing is trying to determine electrical or fuel issue here.

I'm thinking new point plate? Also when I open the cover for the point plate everything is covered in like a frothy oil... I guess this is normal and will not affect the spark? Help!

 

[tommycarl]

Well it's been awhile and you have no replies, so although I've never actually worked on a DOHC 750, I did have a CB900 that I assume would be close to the same. Take my comments for what they're worth, which may be nothing.

On my 900, the issue I had was carbs. I checked everything I could think of, but always came back to carbs. I ended up cleaning them I think 6 times before I finally got the bike to run right. Of course every time I cleaned them, I'd think it must not be carbs, but there was a noticeable difference after #6.

Your comment about 1 and 4 plugs being fouled would make me want to check the coil and wires for sure since 1 and 4 use the same coil. I'm surprised that you mention a point plate since I didn't think this bike would have points. Could it be that you're looking at pulsars? If so. I certainly don't remember any frothy oil on my 900. I did set the pulsars gap at .016" I think (.012" comes to mind too)... I just set them as low and the spec allows.

Intake insulators (carb boots) could be hard and not sealing, and any sucking air could cause major problems. You might check closely to be sure that's not a problem with yours.

I'd lean toward cleaning the carbs again, but the fouled plugs on 1 and 4 could point toward electrical... again, check to be sure that coil is getting spark to the plugs.

 

[mkaser81cb750c]

Thank you for the reply! I actually figured it out after thinking it had to be electrical the entire time. I also didn't think it could be carbs because I had them ultrasonically cleaned and I synced them. I ended up changing the reg/rec and that wasnt it... so I tried used spark units.... that wasn't it. I was about ready to give up and take it to someone when I read a random post somewhere about trying to take the airbox cover off and seeing how it runs. So I took it off and took it down the street and it ran like a top. Power all the way through the tach like it is supposed to. I couldn't believe it. So I'm assuming the air filter is old and not letting enough air through so because of that way to much gas was being sucked through the carbs when I twisted the throttle to 4k rpm. It runs great now. I'll probably try a new airfilter but if it goes back to how it was I'll just keep the cover off... There is still the chrome cover that I can use to hide the air box the lets air through freely.

 

[tommycarl]

Sounds a bit weird, but I'm glad you found something that might be causing it. The weird part to me is that if it's the air filter, why would it affect only 1 and 4? That and the fact that I'd expect it to run like crap with the cover off.

However, stranger things have happened, and it sounds like you're on the road to recovery... that's what counts, and learning something along the way is always a plus.