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starider said
Hi, it's a while since I was on the forum, but today I decided to have a good go to get my 175B running properly. Since restoration it has been very difficult to start and would not reach any sort of speed even after a carb rebuild,new contacts and condenser,accurate setting of timing. Basically I was so fed up I have not touched it for some time.I saw the advert in the magazine for an electronic ignition conversion which I purchased. My son fitted it today,added fresh fuel. It fired up almost instantly and ran better than ever had, so reset tyre pressures checked everything and my son disappeared in a cloud of blue smoke, he came back at speed and as he passed our gate there was a phutt and the engine cut. It started instantly and we set about setting mixture,tick-over when suddenly the engine revs increased alarmingly, the cut out button wouldn't work,pulling off the plug lead wouldn't stop the engine revving but a hand over the carb. did. We started it again after several minutes,it ran for a while then the same thing happened. Any ideas please. We are not sure if the new piston had nipped but there were no signs of this.  HELP!!!  starider  

Interestingly I just had a similar problem today. Bike engine has just been rebuilt, so definitely not carbon build up. My revs shot up after finding a false neutral and would not shut off until dropping down or up a gear. Engine was warm.  I have also noticed when ticking over, if I blip the throttle, the revs stay up for a few second rather than immediately dropping, also when hot. Perhaps some can explain this? I guess it could be drawing air from somewhere but I can't see where. I have a good strong throttle spring and the slide snaps back. 

Number6 said

Interestingly I just had a similar problem today. Bike engine has just been rebuilt, so definitely not carbon build up. My revs shot up after finding a false neutral and would not shut off until dropping down or up a gear. Engine was warm. I have also noticed when ticking over, if I blip the throttle, the revs stay up for a few second rather than immediately dropping, also when hot. Perhaps some can explain this? I guess it could be drawing air from somewhere but I can't see where. I have a good strong throttle spring and the slide snaps back.

I had this years ago, on my 850 Moto Guzzi and also, coincidentally, on my BMW R80. What I was doing wrong was setting the slide stops too high to get a tickover, instead of getting the idle mixture screw right (which it wasn't). The result was reluctance for revs to return to idle speed whenever blipped or revved, especially when hot. Later, after reading the instructions properly (says, starting at 1.5 turns out [idle screws], which is not necessarily what they must be) and getting the idle screws right, as it's them that basically set the idle (air bleed), so then the slides could be set lower and it wouldn't conk out, all was hunky-dory after that.

May or may not be something like that – compounded by the fact those bikes won't idle anyway until hot, so easy to have the sldies too high, by trying to get it to keep idling when setting up in the back garden.

 

 

 

 

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Just took a look at my throttle slide and yes it's a lot higher than I'd expect. So you could well have the answer. Thinking about my setting up of the carb, I don't think I gave it time to fully warm up. The pilot screw didn't seem to be doing much. I should known better . . . But I wasn't thinking. Too keen to get it out on the road. Thanks for you reply. I think you are spot on.