Revs sticking (not rising) when throttle shut.

[mike-s]

I went to the ctp protest rally and ever since then my 250 has been a bit problematic, once it has warmed up it seems to have adopted the tendency for the revs to stick and not drop down. Now i have replaced the clamps around the carb boots to rule them out, as despite the boots being replaced, i didn't have the foresight to replace the clamps too, and they are still the old ones which came with the bike.

I am going to have a look at it this afternoon and carefully spray wd40 or carby cleaner or something else flammible all around the intake boots to make 100% sure they aren't leaking. I'm almost certain that they aren't, but want to rule them out none the less.

Here's a bit of history to let you know what's been happening. Recently the head came off as the gasket was pretty naff and was letting a decent quantity of compression out the left hand side. Carbs were essentially untouched as they were spot on previously and the only mucking about i did was to put them back in and re-attach the throttle cable. The only major modification is the binning of the original 2:2 exhaust in favour of a 2:1 system, which i can't imagine would cause this kind of stuff up, could it?

If it turns out that the carb boots aren't the culprit, has anyone else got any ideas? i'm pretty sure that it's not going to be the throttle return spring as that thing is working fine and to be honest is bloody strong. I've resigned myself to the distinct possibility i might just have to take the carbs out and give them a once over to make sure, but any ideas to help point me in the right direction would be appreciated.

[MickLC]

Are they CV carbs?

[mike-s]

Yussum, you think potential diaphragm problem, or maybe the sliders aren't going down as readily as they ought to?
If you are, it's something i am considering as a possibility as well.

[robracer]

Carb overhaul should solve the problem, it could just be sliders sticking, you have checked the cable?

[MickLC]

Yeah I was thinking more the sliders sticking like Rob said....only one way to find out, pull those suckers off and give them a good going over.

[Strika]

Carb overhaul should solve the problem, it could just be sliders sticking, you have checked the cable?

This was my first thoughts. I have seen this a lot on older bikes with worn throttle slides. Another culprit, could be a sticking cable.

[Supafrog]

...and it has absolutely nothing to do with a choke cable?

[mike-s]

The choke is manually actuated on the left side of the carbies, no cable involved here.
Will take a look tomorrow morning, today shall remain a couch day. Cheers for the leads though.

however the choke o rings will get a look-in if the slides dont give any love.

[Gosling1]

sounds like an air-leak to me......

If the diaphragms have any pinholes, then the slides won't lift properly in the first place. You can check the slides easily enough for movement, just take the airbox off and lift them with your finger - they should return slowly and smoothly.....the old return springs might be a bit sacked out after all these years and need replacing ?

[mike-s]

Well I'll be doing some testing tomorrow and will be giving the old bird a good once over with every suggestion. I want to get this little bastard of a problem fixed as it's starting to shit me off to be honest.

[mike-s]

I sprayed an aerosol based degreaser all over the carb boots with no sign of the revs bumping up, so i'm pretty sure it's not that. I took the carb tops off and the diaphragms out and it seems that the left one DOES indeed stick ever so slightly when being lifted out a little bit and left to seat on its own. I've got some 1200 wet'n'dry and am going to give it a gentle go over on the bottom of the diaphragm and especially the carb body when i get a chance to see if i can improve the situation or if the whole situation is just going to elude the hell out of me.

[mike-s]

Sliders out, it looks like the left one was habitually sticking about 3-4mm open in about 50% of my many lift/drop tests. This combined with the barely visible burring on part of the slider surface may go a long way to explain the fun i've been having. The r/hand one was fine and never stuck once in the 20 odd times i lifted/dropped it. got the 1200 grit wet/dry out and gave it a wet rubdown on the entire slider as well on the lip on all of the edges of the slider/venturi recess.

A further 10-15 lift/drop tests shows i have most likely sorted the issue, it was probably just a ever so slight fragment of aluminium holding the slider up. The real test is when i turn her over tomorrow night after work as it was a shade late when i got to her tonight.

[robracer]

Sounds Good mike, hope she runs ok now

[mike-s]

that makes two of us

[Gosling1]

hope you got this one sorted Mike - sounds like you may have found the problem....