Hi All

I have a CBR motor in a CB-1 and all has been running very good until now, I wonder if you have any ideas?

The bike will run smoothly on all 4 cylinders but refuses to make it past 6,000 rpm in any gear, but it will hold this rpm indefinitely on the flat in top. I think it's fuel as the engine just bogs down on anything past quarter throttle, it will spin up to 8-9k when in neutral, but just sounds pants on large throttle openings.

I have drained the fuel bowls as I seem to remember something similar turned out to be some water in the carbs, but No change.

Also checked to make sure the choke lever had returned as I though maybe that was stuck, but seems ok.

I have just had to ride 30 odd miles with it like this, whatever the problem is does to keep going. However on a long uphill section it did seem to loose maybe 2 cylinders and I though it was going to die completely so I stopped for a few seconds and it came back on all 4. I opened the fuel cap in case it was blocked, but that made no difference.

Also worthy of note is that the bike was really difficult to coax into life today too, after sitting for a couple of weeks.

Any suggestions welcome.

Chris

Hows the fuel pump ? Have you cleaned the carbs ? Is there any crap in the fuel filter ?

Hi

I cleaned out the carbs 2-3 months back, and the fuel pump seemed to be working ok, I didn't do anything with the fuel filter though.

Even if I have a semi blocked filter, or a poorly pump I would expect a few seconds of full power till the float chambers run down?

Unless the float chambers have a very marginal amount of fuel and need the pump pressure to run anything onto the main jet?

I also read somewhere on here about a possible speed limiter problem that might give this kind of symptom?

Any suggestions?
Chris

What sort of derestriction are you using ? Also maybe the float valves are sticking ?

Are the throttle slides moving all the way up what you rev it in neutral?
Take the airbox off and look down the carb mouths then rev it, you should see the slides lift all the way up together.
Try lifting them with your finger ( engine off) they should slide up nicely and come back down with a bit of resistance and make a slurping noise. If the just snap shut as soon as you let go the the diaphragm's are split.
This will cause poor full throttle response.
Check you are getting fuel to the carbs by sucking on the small vacuum hose that goes from the carb intake rubber to the fuel tap, you should get fuel flow only when you suck causing a vacuum.

Worth a check but are all 4 plugs getting a good spark?

Hi All

Many thanks for your suggestions and advice, turns out the carbs had re-filled with the dreaded gunge muck!!

I had stripped and cleaned them in June, so though all would be well?

The force of the gunge muck is strong, and we must continue to fight it in our tanks, in our filters, and in our carbs!!

C

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Years ago I a RD125LC, the knob on the fuel tap broke so I popped down to my local bike shop to get another. Had to get the full tap assembly.

Fitted no problem, would work a treat until I got on the local A road in top gear and then it would not go beyond 55mph, before that it would top out around 75mph. It would rev out easily in all the lower gears but would run out of revs at 55mph in 2nd to last gear, but into top gear it just wouldn't go any quicker.

Turns out the numpty had sold me a fuel tap for a RD50, the only way I found out was to refit the old one and compare fuel flow rate, it was a dribble from the 50 and a flow from the 125. They looked identical.

So the lesson is check don't assume a new part is a good part, or even the correct part.

Funny that as a mate had an RD 125lc years ago that was bought in bits. He rebuilt it and run it in properly but it seized, so he rebuilt it again and it seized again! So I had a look at it and found that the oil pump had a different number on it to my bike and it turned out to be from an RXS100, it was identical to look at but flowed less oil.