1980 cb750k chokes at 5000rpm

[surfish95747]

Hey everybody its been a while! I have a question for you. I had to rebuild my carbs again (full clean with pine sol first then a sonic cleaner, had to replace worn float needles as the rubber heads fell out and got stuck in the float hole, and kept the carbs completely stock. Pilot needles are turned out 3 turns, I have not done the drop down procedure with it yet and have not synced the carbs yet, and it has brand new air cut off valves, all holes are clear verified with air and carb cleaner. I just thought I would get some preliminary thoughts from the experts ;)

So, as I ride it, it runs so so smooth from 0 to 5000RPM in any gear, then it will not go past 5000 and just chokes. That's the best way to describe it. I will sync the carbs and THEN do the drop down procedure in that order, then if it is still choking, what things should I be looking at that would cause that to happen? Could it be a rejetting issue (oh I do have stock intake box with a K&N filter which I will be replacing with a paper filter today, and I have a MAC 4-2-1 exhaust), or timing, or electrical stator rotor issues? Thanks guys!

[TOOLS1]

It could be lean on the main jet, out of sync carbs pulling it down, or the timing advance would be my thoughts.
TOOLS

[Re-run]

If you pull the choke when it gets to 5000 rpm, does it run better?
How are the rubber cv tops? Sometimes it can be hard to see holes or tears in those diaphragms.
Have you checked to make sure the emulsion holes in the jets are actually clear?

TOOLS mentioned timing advance. Sometimes people only check timing at idle, they don't check advance. The manual should specify how to do this.

How old are the plugs and what do they look like?

I guess a good question is, does it only get to 5000 rpm in ALL gears or just a couple? If it is all gears, I would seriously check out your ignition.

[surfish95747]

So I tried pulling out the choke and it didnt do anything. It now starts choking at around 4000rpm. Seems to be getting worse. Re-run, what rubber cv tops? I am pretty sure they are. I may have to check again.

[Re-run]

Under the top caps are diaphragms that attach to the pistons, I don't know if your carbs have them, but the cv carbs on my v4 have them. If these get cracks or tears, vacuum doesn't work properly. But if choking it makes it worse, you may be running too rich already and may need to lean your idle circuit.

[TOOLS1]

The DOHC CV carbs use a metal diaphragm. Did you reverse the jets when you installed them (pilots where the mains go...) ? That is a common thing to do on the DOHC CV carbs.
TOOLS

[Re-run]

Metal huh, does it come out? If so, I would check the seal then. It still stands that is the diaphragm can't seal properly, the carbs won't work right.

[TOOLS1]

No, no seal. Just a really close tolerance fit, like the carb slides.
TOOLS

[shinyribs]

I think the lack of a solid seal like these carbs use vs the rubber diaphragms Rerun is talking about is a large part of the reason that the CB's respond so poorly to changes compared to bikes with the other style of slide.

In stock form the solid metal piston would be permanently reliable, though. Give and take.

[surfish95747]

Tools, I have the 68 jet in the jet closest to the pilot screw and the 102 jet in the jet furthest from the pilot screw. Is that right?

On Oct 24, 2016 7:47 AM, "TOOLS1 [via Honda CB750'S]" <[hidden email]> wrote:

The DOHC CV carbs use a metal diaphragm. Did you reverse the jets when you installed them (pilots where the mains go...) ? That is a common thing to do on the DOHC CV carbs.
TOOLS

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/1980-cb750k-chokes-at-5000rpm-tp4061452p4061468.html

To unsubscribe from 1980 cb750k chokes at 5000rpm, click here.
NAML