79 CB 750K tuning issue(Possible Accel pump issue?)

[lakeguy556]

Hey all,

Long story short, I'm working on rebuilding the above mentioned bike. 7K original miles on the odo.

Got my carbs rebuilt last night. Bench sync'ed them and threw them back on this am. A/F mix screw is set at 2 turns out.

Bike fired up and ran sewing machine smooth. Let it run for a while the whacked the WOT. Once it got to around 5-6kK the bike began to stutter and wouldnt climb any higher. Rolled slightly off the throttle and she began to climb to around 6K but wont go any higher.

After doing some research I fear I may have neglected to properly clean/check the accelerator pump while I had the carbs apart. I did check the diaphragm with a back light at the time and it looked ok. tube was straight and not bent. It did have a bit of corrosion/rust/varnish on it which I cleaned up with 000 steel wool. I put it all back together but was not aware of the ball/spring/plug issue at the time.

Could this be my problem, that I need to pull the carb bank back off and remove the brass/nylon plug in the accel pump and check for proper operation.

Im running the stock airbox with new AF and spark plugs. Valve are in spec.

Appreciate any input

[robs750]

Sounds like it is starving for fuel at that point.
I could be wrong but dont think it is the accelerator pump but more of a main/needle jet issue.

[TOOLS1]

I'm with Robs. The accelerator pump only works when you are accelerating. It has nothing to do with high RPM running.
TOOLS

[rrgunslinger]

Or and maybe a vacume leak?

[seestheday]

I agree with Rob/TOOLS.  Accelerator pump literally squirts in extra fuel when you quickly roll on the throttle.  It doesn't do anything when you're just holding the throttle in one position.

If it was not working properly it would bog when you hammered the throttle, but it would then slowly climb (assuming everything else is working well).

[lakeguy556]

Appreciate the advice fellas, however after doing some reading and thinking, I'm having trouble buying that the carbs are even the issue.

Short of giving the accel pump the attention it may have needed, I know that the rest of the carb rebuild was meticulous. Everything was replaced. Carbs were soaked, sprayed, rinsed and blown out. The other issue I have with this is that it just runs to perfectly down low to the 5-6K point. Its almost like it hits a hard line at 6 K and wont budge.

Just spit balling here and it could well be something in the carbs but I'm leaning more towards electrical now.

Ignition timing/Spark  advance maybe?.

When it gets to that 5-6K mark it does almost seem as if its cutting spark. I also seem to be getting some popping out of the exhaust at that point.

Battery shows 13.8 volts above 3K rpm, so the charging system seems to be working. I'm going to pull the pulser/generator cover and hit it with a timing light.

[robs750]

Well you could test the electrics per the shop manual.

[TOOLS1]

The only ignition defect that would affect all 4 cylinders at the same time would be the advance mechanism.
TOOLS