Full revs without throttle!

[ZaphodBeeble]

Hey guys,

Finished putting my bike back together this evening (had to do all the wheel bearings, suspension bearing, chain and sprockets). I also adjusted the exhaust valve as it was closed as far as it would go.

So, I started it and warmed it up and it was revving nicely, finally had a nice ring to it. Before I adjusted the exhaust valve it wouldnt rev very high but now it has a powerband. Oh boy does it have a powerband.

I pulled off on it and opened the throttle. Firstly it was a bit boggy low end (as I expected) but then I hit full power ARGH the damn thing went nuts. I shot down the road and got up into 3rd gear and the end of my street was approaching. I closed the throttle but the damn thing didn't care. It revved it's bollox off and I had to jam the brakes on to slow down.

Now when I slowed down the revs dropped back down to normality again. I thought that the throttle might have stuck so I shut the bike off and checked it. Nope, no sticking, was operating smoothly, I could hear the slide tapping up and down.

So, I pulled off again and RAAWR it hit powerband again and wouldnt stop when I left go of the throttle, so now I'm screaming up a narrow alley (I was heading back to my bike shed) and had to jam on the brakes again. Each time I slowed the engine down the revs dropped back down and stayed there.

So, after all this, I need to know what the hell is going on. Is it the fact that I have opened the exhaust valve fully and it is not supposed to be that way? Or could it be that the mixture is too rich?

Help would be appreciated guys.
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Current bike: VTR1000F Firestorm. Previous: Firestorm (written off) - XJ600 Divvy - NSR125 - DT125 and a load of offroaders.

[baldy]

my old scoot used to do that and i find out it was a mixture of dodgy timing and the mixture
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One day i will rule the world, just not today.

[AcIdBuRnZ]

DIY cruise control...nice one

As always, ask Kickstart
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Past: Honda NSR125R >> Kawasaki ER-5a2 >> Kawasaki ZZR-600e1 >> Suzuki GSXR-750Y >> Honda VTR 1000 Firestorm
"Chickity China the Chinese Chicken..."

[Hex]

If the throttle cable etc ain't sticking its probably an air leak mate.
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The BCF's very own Party boy! Though he's getting old and feeling it!
Monkey hanger, Born and bred
My little photo portfolio

[Kickstart]

Hi

I would say an air leek, but normally that means that although it is getting loads of air it is not getting sufficient fuel so have no power despite the revs. Tends to only happen at very small throttle openings.

I think my next guess would be that the throttle slide is sticking, possibly from being sucked hard against the engine side of the carb (can happen, but not normally on small 2 strokes). For this check that the spring is in good condition.

Also check that the carb is not opening far enough to allow the needle jet to snag on the nozzle. It should not happen, but I suppose it is possible if someone has played with the clip to raise the needle.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[ZaphodBeeble]

OK guys cheers for the help. I'll be stripping the carb tomorrow to see if I can suss it out.
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Current bike: VTR1000F Firestorm. Previous: Firestorm (written off) - XJ600 Divvy - NSR125 - DT125 and a load of offroaders.

[Diehard]

My GSXR was doing that. But I found that one of the richening/leaning clips on a needle had come off from wear and thus the needle moved up and down at will, so this could be worth checking. I also changed the diaphrams at the same time so could be that (if your carbs have them).
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If I Die... I Die!

[Kickstart]

[Diehard]

I was lucky, I got a full Bandit 400 carb set for £60 . Still fiddleing to get it right though.
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If I Die... I Die!

[Kickstart]

So about the cost of one new diaphragm.

By the way, that is not that cheap for 2nd hand carbs. I paid £25 for a new set for the FZ750.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Diehard]

Go on then spoil me ignorance . I though it was a good deal because all of the other breakers were charging more and the one on E-Bay went for more also. A new one would cost £350, or so I was quoted. Hell it's paid for now. But while I'm on, any ideas of the best way to get out a stuck in mixture screw that has been de-headed? Have a spare for the hole so screw can be sacrificed.
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If I Die... I Die!

[Kickstart]

Hi

If you can the cut a slot in the top of the screw and try again. Only down side is that someone might have damaged the carb body by screwing the mixture screw in that hard.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[ZaphodBeeble]

Just to let you all know...

The bike is fixed!

I stripped the carb off completely and took out the reed valve and cleaned it all up. Seems that the carb hadn't been fitted properly and all the bits and pieces were a bit on the loose side. The inlet gasket was OK too so that was a relief (fed up of going to buy parts).

I got it all back together, after about 3 hours work (including changing the rear inner-tube because of 2 screws in my tyre ) and switched the fuel on. It started 3rd kick. Sounded really nice.

So I went for a decent ride on it yesterday with my brother on his NSR. It's running really nice now, too much top end power though, I have no bottom end at all hehe. I'm going to have to close the exhaust valve about 1/4 of the way maybe. All-in-all it was a good day.
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Current bike: VTR1000F Firestorm. Previous: Firestorm (written off) - XJ600 Divvy - NSR125 - DT125 and a load of offroaders.