• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Unresponsive At Open Throttle

RidingSince1969

CB750 Member
Messages
6
Reaction score
0
Points
1
Location
St Joseph Missouri
I recently got my CB750K3 on the road -- the bike was given to me last summer, and I've spent the last year fixing it up.

I'm happy with the way it runs -- except for one thing. It's highly responsive at low throttle openings, but when above half throttle or so (main jet?), the engine bogs down. If you just leave the throttle open, the rpms will "catch up." I just had it out on the highway, and it will run 70+ mph and stay there -- but it takes a while to get there. It feels like it's not getting enough gas, but it eventually gets what it needs.

I eliminated the possibility of restricted air intake by removing the air filter and bottom of the air box. No change. The carbs originally were gummed shut from sitting for 15 years, so over the winter I cleaned them, including carefully reaming all the jets. I've also installed new plugs, changed the oil, and set the timing.

Any ideas. I'm under the impression that it's something simple, since the bike runs great otherwise . . .
 
It's probably what Tracy said but also consider that new jets are easy to come by and it's near impossible to "ream" them without damaging them. I'm assuming you either own a set of gauges to synchronize the carbs, or had it done.
 
Take it for a test ride and when it is bogging reach down and start to close the choke slowly. If it gets better then it is lean if it gets worse then it is already to rich. You can try this at different throttle openings once you get to tuning so you can see what adjustments need to be done to the main jets and the needle position. Never ream jets, you will never get them the same and they wont be accurate. you can buy them at JetsRus.com and they are cheap and they ship fast.
 
If OP set the timing CORRECTLY he should already know if the advance is working.

'Reaming the jets'....................danger lies there for sure.................

How about a compression check just to see if engine is even worth working on?

Late I know but may be useful to others, so many think you just 'fix the carbs' and merrily wheel away with 100% fully restored power. The great fallacy.
 
I had a similar problem for a long time... finally checked the fuel petcock, and the rubber inside had decayed, so that "ON" was really 'off', and vice-versa! Fuel flow was really erratic due to the messed-up innards. Replaced the petcock, no more problem!
 
Back
Top