Husky 72 CR450 that revs by itself

[Gippslander]

Problem is Husky 72 CR450 with new Mikuni jetted by Hatrick holds revs by itself.

Have removed airfilter and can look in at the carby slide, when the idle speed screw is out so that there is no self idle (as I have it when racing) and crack the throttle the engine revs up cleanly and just as quickly drop revs, all as it should be.

But, with the idle screw in a bit so it self maintains idle when you crack the throttle the engine revs up cleanly and drops back but not quite to the set idle speed and then it slowly builds up speed and you can see the carby slide "floating" about 2-3 mm and the revs build to say 2000 rpm.

Engine was fresh including new seals about 15 hours ago, has always and still makes really good power.

Although it's probably not possible it seems that with the idle srew set for self idle and a little bit of air getting past the carby slide it is as though the engine can pull enough air to overcome the carby return spring.

Thinking of stretching the carby  slide return spring a bit but thought I would ask here first, any ideas?

Thanks in advance.

[09.0]

I think that's your problem, not enough tension on the spring.

[evo550]

I would have thought that increasing the idle screw would have put more tension on the return spring, therefore putting more downward pressure on the slide.

[PEZBerq]

Why have no self idle for racing?

Does sound like return spring is not operating properly. Does the slide close with a solid clunk when you open and close the throttle with the engine off? You should hear it thumping against the idle screw. Sometimes the throttle can stick on the twist grip and the spring can't close the slide properly. Check it twists freely and is not preventing the slide closing fully and having good spring tension holding it down.

[smed]

It could be running lean as the slide is slightly cracked open,Have you tried adjusting the mixture screw,what's it set at now?,or the pilot jet could be too small

[djr]

hello, not sure what your problem is but I would have thought it would be best to have the carb set to self idle on a 2 stroke running on premix.
otherwise when you shut the throttle for a corner or downhill the engine is turning over but getting no fuel & more importantly no Oil.
Obviously this wouldnt matter on a 4 stroke or 2 stroke with autolube.
anyway good luck its sure to be something simple but hard to find !

[Gippslander]

OK, 090's suggestion was all I needed to try and stretch the spring, was about 4 1/5 inches and stretched it to 6 (no jokes please  ) and it does come back down to idle and does not speed up so that was probably the problem, strange that it should be so with a new carby and everything else is in good working order, new home made free and smooth cable and cleaned up trapdoor twist grip, all ok.

So, that leaves the Q as to why I set up for no idle....

I just thought that was the correct way, the fellow who helped me set up my Bully in about 1971 told me that and I've never thought otherwise, when he was trying to teach me how to ride he said you never go downhill or into a corner with no throttle, you should be in the correct gear and at the right revs for the speed you want to go so you have more control and lessen the chance of the rear wheel sliding and engine stalling, not sure if that's right because I never could really ride, am sure they would have relagated me out of C grade if there was a lower place 

And... page 8 of my Penton/KTM 1974 owners manual says "We recomend little or no idleing if the cycle is used in competition", but I had not read that until I looked tonight.

[Montynut]

Back in the day ('70s) it was common to set a 'dead' throttle. When the big bores are set to idle they tend to 'push' or understeer in very slow corners as the engine is still driving. That was the theory anyway. Would set the idle so that the engine would idle for a few moments then stop. It also seemed to make the bike very sharp right off the bottom.

Was talking to Peter Paice and he always set the throttle on his trials bikes to a dead throttle. In very slow stuff the engine on his TY250, TY360 and 349 Montesa bikes would push the front wheel wide in full lock turns if the bike idled. That was in the 'no stop' period of trials when the rules changed in about 1979 to allow bikes to remain balanced and stationary bikes then needed to idle

250s and 125s would not have the same issues I would guess. Don't think it applies in more modern periods.