Hi Everyone

Hoping someone can suggest ideas to help solve a problem that's driving me mad. 

THE PROBLEM:  The bike starts and idles (initially needs choke).   After a few minutes, she's warming to the touch and is happy to idle without choke but her revs drop away if throttle is applied, sometimes to the point where she actually dies. By feathering and twitching the throttle I can raise her revs past this baulking point but she repeats the  baulk on throttling until I've done this a few times, sometimes with a bit of a backfire. It can take a good 5-10 minutes of idling and fiddling before she's rideable and it's not clear whether it's simply the warming up that makes the difference.

WHAT I'VE TRIED: I've followed the Haynes manual to set the idle screw and air/fuel mixture screw then tweaked them (1mm at a time) to find an even idle, then tweaked some more to find a spot where she doesn't baulk (right term?) at the merest bit of throttle.  It seems to require hair-breadth adjustment which doesn't seem right. 

I'VE ALSO:  Cleaned the carb and replaced the needle valve with brand new Yamaha part, checked and adjusted the valve clearances, checked her fuel line, cleaned her air filter, checked and cleaned her spark plug, replaced her battery with a stronger model. 

The issue is that I adjust her idle and air/fuel screws, get everything feeling good, take her out and she rides fine, but she's right back to square one next time I start her.  It's like something has reset itself. I've been through this process over and over and over again. I clearly shouldn't have to touch the screws once they've been set. 

NB - This bike had been fiddled with in various unwise ways, so I;m not 100% sure if her carburettor is the original spec but it looks about right, and it doesn't seem to have visible model number so I can't check. 

If anyone can suggest anything at all that might help, I'd be immensely grateful.  If any clarification needed , please ask. 

Lolo

 

 

 

checked float level?

Thanks - very belatedly, my apologies! - for your reply drewpie. (Yes, the float level was good.)

I've been locked out of the site absolutely forever - login rejected and verification emails for a new account not arriving - but somehow I could get in today, so all this time later I'm back to say that I EVENTUALLY sorted the bike!! I thought I'd post the solution in case it's of help to anyone else.

I was at the stage of considering boiling the carburettor (suggestion from Reddit) or replacing it entirely. But I decided to soak it in carb cleaner one final time.. and as I picked up the jets section for reassembly, I noticed a couple of grains of "salt" on the dark cloth under it. No more appeared as I tipped the section to and fro. But there was nowhere for the grains to have come from so on a hunch I held the section open side down and tapped it with a screwdriver around the location of the jets. And another couple of grains appeared. I kept tapping and checking until no more came out. It took a while, but by the time I finished there was about 1/8th of a teaspoon of tiny bits of brown, black and white grit. I replaced the carb on the bike and she now starts, idles, and makes a perfect smooth progression from idling to revving (Let Joy be Unconfined!). It's like a different bike.

It's worth stressing that there was no loose debris in the jets or on visible surfaces of the carb . The grit came from somewhere else. Maybe some internal connecting channel between the jets which was clogged with this stuff (probably a million years worth of hardened petrol scum)? I don't know if the repeated soakings in carb cleaner eventually loosened it but it definitely wasn't going to come out without the tapping. (Perhaps boiling might have sorted it?).

So anyone with bizarre, resistant starting problems, it may be worth tapping your carb after cleaning it. I hope this helps someone - for me it's the end of several years of pain! LOL