6000 7000 RPM trouble

[cb400joe90]

Hello all,

Im starting a new thread on this because I think its separate from my carb issues. I say this because I had this issue before I removed and cleaned the carbs.

Basically when Im in 6th gear and doing 120kph or around that range The bike can sometimes hit a flat spot, as in the rpm wont raise above 6500-or 7000 rpm. On some occasions the bike actually dies and I have to coast to the hard shoulder which is scary on the motorway. After a minute I can start the bike and off I go again. Here is what I have done so far.

I put fresh oil and filter in today.
I have checked the breather pipes from the tank which are all clear.
I removed the petcock and it looks clean and the rubber diaphragms look fine.
I fitted a MOSFET type reg/rec last year and the battery is charging fine.

I cannot be 100% on this but I think putting the petcock to reserve eliminates this problem. It might have been just luck that the issue didnt occur when it was on reserve.

What do you think lads??? Bike is a 1994 cb400 superfour NC31

[Superjack44]

How the petrol tap main feed and reserve work is, the main feed has a small pipe inserted into the top of the tap that screws into the tank this is covered by a very fine mesh filter the reserve works by fuel feeding directly into the tap below the main feed pipe.
If you are correct in your suggestion that the problem does not occur while running on reserve suggests that the main feed pipe has a blockage somewhere.
I suggest you remove the tap including the internal filter again and strip it completely to make sure it is clean.

[cb400joe90]

Thanks superjack, Ill have a look again. I didnt notice a mesh filter before?

[Superjack44]

Thanks superjack, Ill have a look again. I didnt notice a mesh filter before?

Its probably stuck inside the tank where the petrol tap screws into, it should be screwed into the top of the petrol tap itself, before you start trying to get it out check the pipe that is protruding from the top of the petrol tap for debris or even a kink in the pipe anything that interrupts the flow of fuel through the tap could cause you the problems you are experiencing.
If you strip the tap take note of what goes where especially the seals

[Gidanski]

I had that problem for a while, took the strainer out from above the fuel tap and it was filthy. Cleaned it up and helped alot with the problem. I also fitted new ignition coils today and that seemed to have helped a bit as well.

[cb400joe90]

Thanks Gidanski, Im flat out busy the next few days but ill check it out asap. One thing i did notice is the new breather pipe I fitted a while ago is slightly too but and air gets into it. Ill replace that asap also

[cb400joe90]

Ok im sure the petcock is fine. Fuel flows freely. I also checked all vac tubes which are clean and clear. I have removed the carbs again and will strip them tomorrow and recheck float heights and so on. I have looked through the manual and I think it says the float height should be 8mm but this doesnt sound correct. Can anyone confirm what the float height should be ????

Im also going to order new rubber boots for the carbs tomorrow as the one I have are not in great shape.

[cb400joe90]

Finally got new rubber boots fitted to the carbs and I installed a new battery. The problem persists though. I took the bike for a quick spin today and when I had it in 6th gear doing roughly 100 kph the bike would not accelerate any more. It stays around 6000 rpm. If I drop it to 4 th gear it will pull past 100 kph no problem.

I cleaned the carbs and everything is 100% clean. The jets that are fitted are size 98 & 95.
Petcock is working fine. Breather pipe coming from petcock is also fine.

Any more suggetsions

[kayzaki78]

I think Im having the same problem, top gear wont rev past 6k rpm I have to downshift to 4th to increase speed. My mechanic told me it was my rpm gauge that causing this problem so now Im waiting for my replacement parts will update you once I got it replaced cheers


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