Flat spot

[mark247]

This morning i found my bike quite hard to start so i opened the airbox up to put some engine start in ( which made it start. ) But when i opened the airbox i found that it had no foam or filter or anything in it. I have only just bought the bike so i guess the muppets who owned it last thought it wasnt needed. So anyway, i went to the bike shop where i cut into shape some foam and placed it in, which is all that was really needed. Now the bike runs rich ( i knew this would happen ) and it smokes a bit when i rev it up. Im going to need to tune it a bit more lean to compensate for the new filter obviously. But i found it now has a flat spot at about 13,000RPM as well. It red lines at 14,000rpm. I was running 91 octaine and i was told that i should run 98 in it, which i just filled it up with before. But i havent had enough time to see if the flat spot is gone with the new fuel.

Could the fact that the bike is running rich make it has a flat spot by causing it not to get enough air at high RPM? Or is it possibly my clutch is slipping under load or something? ( for some other reason )

Cheers guys.

[surfer]

a good method is to tape the accelerator back make it rev to redline and go to bed in the morning it will be as good as new

Never heard of this one before. If your bike is air cooled you will shag the motor for sure.

As to clutch slip you will know if your clutch is slipping if you open your throttle while riding up a hill. You will feel a noticeable difference between riding on the flat and opening it up, and the feel of it slip. It may be that you have the wrong oil in it (search threasd on this site for oil) as additives in the oil can cause this.

Sounds like the previous owner found the same problem and decided to open up the air box a bit by dumpin the foam.

Sorry don't know enough about your bike to know whether 98 or 91 will make a difference. There is a good thread on this site about different octane fuels though, check it out.

[surfer]

It may be that you have the wrong oil in it (search threasd on this site for oil) as additives in the oil can cause this.

Ooops meant to include here change oil and replace with a motorbike oil if you suspect clutch slip to see if that sorts it out before you do anything else like tear down the engine.

[F5 Dave]

Tom great advice as usual.

obviously a bodge to make the bike run better before selling it, thanks mate.

presume you used air filter oil & not engine oil? Engine oil would make it worse. Maybe no oil.

You could try raising the float heights & turning the idle screw in a 1/2 turn (clockwise), but this will only have limited effect at high revs. Check emulsion tubes (that the slide needles poke into) aren't worn (small ovalisation will cause issues, have to strip & look in bright light to see)). Check choke not stuck on.

[Sensei]

Pretty easy buy the correct Airfilter for the bike before you go playing with carb shit !! Clutch will slip well before redline as it is under far more load in the lower gears .

[mark247]

Tom great advice as usual.

obviously a bodge to make the bike run better before selling it, thanks mate.

presume you used air filter oil & not engine oil? Engine oil would make it worse. Maybe no oil.

You could try raising the float heights & turning the idle screw in a 1/2 turn (clockwise), but this will only have limited effect at high revs. Check emulsion tubes (that the slide needles poke into) aren't worn (small ovalisation will cause issues, have to strip & look in bright light to see)). Check choke not stuck on.

I used the correct oil. The flat spot seems to have gone since i have started running 98 octain fully ( for a while i had 50/50 kinda thing )

It still obviously needs a tune, but that can wait until i have money.

[mark247]

running 96 octaine fuel fixed the flat spot for a while but it soon returned, but i have just got a propper airfilter for it and replaced a fouled plug, it was only running on 3 cylinders when cold. Now it starts fine and it revs out right through a red line perfectly.

[F5 Dave]

Check that same plug in a week or so, if it has gotten black then carb float or emulsion tube. Don't always wear evenly for some weird reason.