Unusual Running

[scc 66]

Although this isn't an NC750 problem, hopefully the collective expertise on here may be able to help me.

Today I started up a bike that I have renovated. It is a Honda CB250 RS. 

It's red line is 10000rmp. It will only go as far as 7000 then it drops to 6000, and then goes back up to 7000 then back to 6000 and so on.

My suspicion is the accelerator pump in the carb. When the throttle is pulled back a diaphragm opens to allow more fuel. This is new, so shouldn't be the cause. Any thoughts about what else could be the cause of the problem?

[Andy m 23,543]

Coil breaking down, timing or running too rich on the main jet?

 

I foresee plug chops in your future to work out If it's carbs or sparks.

 

Andy

[scc 66]

5 minutes ago, Andy m said:

Coil breaking down, timing or running too rich on the main jet?

 

I foresee plug chops in your future to work out If it's carbs or sparks.

 

Andy

Thanks for your reply Andy.

The coil is new. The plug is a little black, but dry. The main jet is new ( it came with a gaskets kit and didn't have a number on it), but I could try putting the old one back. The plug is new, but I haven't checked the gap.

When you say plug chops, it sounds like you're calling me names, but I suspect you mean something more meaningful😁

[Andy m 23,543]

12 hours ago, scc said:

calling me names, but I suspect you mean something more meaningful

😁😁😁

 

Get the engine misbehaving then hit the kill switch. The colour of the plug will be frozen so tells you something of what's going on. If you go back to idle, let it get too hot, run without load etc. they can clear themselves and the picture you see is away from the problem zone.

 

I've never played with an accelerator pump, but if everything else is right it does sound like a possibility.

 

Andy

[Xactly 5,419]

You could try using a strobe light to see if the ignition timing is wandering around 6-7,000rpm. Are the fuel tap filter and tank vents totally clear? Not sue about accelerator pump. I had a Guzzi with these on del Orto carbs. My understanding is that they squirt additional fuel into the inlet tracts when the throttle is snapped open. If not working I’d have thought that the acceleration would continue but a lot more slowly. Sounds to

me like there’s a tiny hole in the diaphragm (assuming it’s a CV carb). Often when you think it’s the carb it’s ignition and vice versa..

[NC Mick 86]

Carb diaphragm?

[scc 66]

11 hours ago, Xactly said:

You could try using a strobe light to see if the ignition timing is wandering around 6-7,000rpm. Are the fuel tap filter and tank vents totally clear? Not sue about accelerator pump. I had a Guzzi with these on del Orto carbs. My understanding is that they squirt additional fuel into the inlet tracts when the throttle is snapped open. If not working I’d have thought that the acceleration would continue but a lot more slowly. Sounds to

me like there’s a tiny hole in the diaphragm (assuming it’s a CV carb). Often when you think it’s the carb it’s ignition and vice versa..

I don't rule anything out.

The timing should be fully advanced by 3500rpm. If there is a timing problem it means trying to find a replacement ATU and/or CDI.

Similarly if it is an air leak around the carb, it means new rubbers. Sadly, these replacement parts are getting scarce as they are no longer made.

Thanks for your tips, I'll see if I can have a play this weekend.

[MatBin 5,085]

Would anything further down the line cause an issue, exhaust sealing, blocked? Or further up, air filter?

Edited August 25, 2023 by MatBin

[Steve Case 955]

Be dead careful with plug chops on a 250rs the dct will not withstand too many if the plugs not earthed and will kill the coil.

I did this on mine and over time the top speed reduced until it would no longer top 70.

Replaced the coil and all was fine.

[Steve Case 955]

Ace bike, by the way.