hi chaps
what could be the cause of fast idle?
i have a amal 275 carb with a 160 jet fitted to my G3L, the needle is set on notch 3, the engine starts 1st kick, it runs very well and revs good. the throttle slide is shutting of as it should (not even touching the idle adjuster screw), choke is working ok, but i cant lower the idle speed, its just running a bit to fast
any one got any ideas? maybe it needs the 120 jet?
thanks barry

email (option): mrsbfuller@hotmail.co.uk

The main jet has nothing to do with the idle speed, you could remove the main jet and the bike would still idle. A fast uncontrolled idle is usually due to a worn throttle slide, air is being drawn in round the sides of the slide. In a worse case the carb body and or the jet block is worn.

Three things to look at the idle screw undo it right the way down . Has it got an adjustable throttle cable if it has undo it .And what about the mixture screw if it is to far in it will tick over faster .But only move one thing at once or you will not know where you are.

idle screw is right down and the is slack in the throttle cable, the throttle slide is sitting on top of the jet block, you got the arch on front of the throttle slide as you look in to the carb, the the rear of it theres about a 3mm gap, so could it be the wrong throttle slide? my slide is 5/4 with a 301 jet block, maybe a 5/5 slide as per WD might sort it?

email (option): mrsbfuller@hotmail.co.uk

Hi All,

Had same problem, and in the end was due to a too long nipple on the end of the inner cable, so slide would not come down all the way! There's not much clearance on the 275, between jetblock and inside of slide.

When someone had borowed the bike, they had replaced the inner cable, and hence the problem! Took me some time to find it!

Cheers,

Lex

email (option): welbike at welbike dot net

Before the engine is running if you open the throttle and snap it shut, you should hear a proper click as the slide bottoms on the jet block. But as already stated, I suspect a worn carb body/slide. Ron

email (option): ronpier@talk21.com

Should have mentioned, I build the carb from N.O.S. parts, so no worn stuff there!

Ron could well be right! a worn carb will not idle properly.

Cheers,

Lex

email (option): welbike at welbike.net

An air leak will also speed up the tickover..between the carb and head for example.
If the idle mixture screw isn't having any effect when you screw it in or out there is a major fault somewhere....
Incorrect slide cutaway (too large) or a sticking slide will do it as well...If you have over tightened the flange nuts it can distort the body and affect slide movement..A poorly routed throttle cable with tight bends can also result in an inconsistent closing action of the slide...Ian

when the slide is fully closed, apart fron the arch on the front, theres a gap at the rear where this part is flat. is there any chance it could be the wrong slide fitted, the carb is a 275s, also the engine is fitted with high lift cams so the throttle is very responsive. also would advance/retard settings cause this?

Well a 5/5 slide is correct for WD spec but sometimes these things were done for economy rather than performance. I don't think cams or ign timing will have any bearing on idle speed. You need to address all the other points mentioned in this thread. You could buy a new slide and try that in the hope that the body is not too badly worn. If the slide is a loose wobbly fit in the bore it's a good indication of wear. Ron

email (option): ronpier@talk21.com

there is slight movement with the slide, i spoes the slide does seem worn, it does look a bit scored, il get a new one ans se what happens
thanks barry

sorted the fast idle problem
the throttle valve was to short or the valve block was to long as there was to much of a gap causing the idle to be to fast. knowing that id have to get new parts or a new carb, i decided to have a play with the carb i got. i took out the jet block put it in a vice and got my trusty file out, i shaved down the top of the block until the gap of the throttle valve closed (opposit the arch), i left about a 1mm gap, smoothed everything of with a fine file, cleaned every thing, assembled the carb fitted to the bike. it runs a revs nicely and now i can lower the idle speed, so all worked out well.

the thing is you never know if some things gona work unless you try..

thanks barry

email (option): mrsbfuller@hotmail.co.uk