Sticking Throttle

[cyclobutch]

I seem to suffer this when the bike is cold - and possibly only after a dampening. The bike is fitted with a new concentric carb. I've had the carb apart several times as I also had to remove some casting flash and nibble the float bowl gasket to stop the float sticking down. I have a (nasty) after market twist grip fitted, same style as original. I've replaced parts of this from another (nasty) one I got on ebay, and tidies up the castings on that too. I've fooled with the cable routing several times. I'm suspicious the problem might be with the slide itself - I used to see this with the late model concentrics that were fitted to my '79 Bonnie and an Armstrong Rotax I had - really hateful carburettors those, I always thought.

Thoughts? I'm not enjoying the revving when I fire up my 1000 mile motor from cold.

Thanks.

[groily]

Just a thought, you probably had it already, but concentrics are notorious for distortion and slide seizure if the flange nuts are tightened up 'properly'. (As well as the other defects you mention!) They want to be not far past finger-tight, using a low-strength loctite or some fancy nuts. Loctite's what I use anyway and they seem to stay on while being lightly tightened. Also, decent correct spacers between flange and manifold, which must keep some of the heat off the delicate little swine I guess. They really are a pain ITA compared to Monoblocs I reckon. Plus the floats are going to be eaten by ethanol, some people say  . . .

[Goldy]

Have you got a tight bend in the cable were it exits the carb and across the frame top tube?

[muskrat]

Got my money on the overtight flange nuts.
Cheers
PS when running methanol at Eastern Creek the carbs would freeze full open by the end of the straight. Fun going into turn 1 at warp speed.
Cheers

[Brian]

I also think the flange nuts are most likely the problem.

I have found that throwing away the "O" ring helps. Just use a gasket. I think the "O" ring swells up and makes the problem worse. Have to admit to not being a fan of Concentrics.

[groily]

Interesting what you say about the O rings Brian. Seems all wrong, but I have ditched them on the pair of 626s on my hybrid G9 (for the bad reason that I didn't have any new ones to hand last time the carbs were off). Just used paper gasket, thicker spacer, paper gasket, loctite on the flange nuts done up lightly, and it's been fine.
On that bike it's the oversize non-factory manifolding that tends to slacken one side or the other when thrashed hard enough - but better to have the carbs still attached when they do!

[BSA_54A10]

The local Pommie Iron bike shop did some thick composite carb gaskets which he cajoaled me to try.
Now I use nothing else.
Replacing the O ring with a soft gasket allows the gasket to deform into the O ring grove and still make an air tight seal while the flange edges are actually supported.
If you always fit a nice new soft O ring then there is usually no problems but old hard ones become a fulcrum point to warp the flanges.