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G'day Chuck78,
Thanks for asking! It's been a long story... but after trawling through the electrics, replacing points, condensers, getting the timing spot-on, changing the plugs (again), swapping out the coils (nup, they weren't the problem), dismantling and cleaning the carburetors (again), AND SO ON...
... it has turned out to be getting the fuel mixtures correct in the carbies. From 3500rpm onwards the engine pulls like a train, but light throttle with the motor at 3000rpm will see it spit back through one of the carbies once in a while.
So after lots of fiddling, I have realised that I can EITHER (i) tune the pilot mixture so that the spitting-back is eliminated but the idling mixture is too rich and fouls the plugs, OR (ii) I can get the idling mixture spot-on but then I get the spitting-back at 3000rpm on a light throttle.
The nub of the problem is, I think, that I have pod filters + high-lift cams + free-breathing 4-into-1 exhaust system, and it's simply asking too much of the original spec. carbies to be able to deliver the correct air-fuel mixture right across all revs/ throttle openings.
I am going to try a little more to coax the carbies into some sort of acceptable compromise, but if I just can't get them to come to the party, then I'll pull them out and put in a set of CVs instead.
That's a bit of a pity, as I really do like the looks and mechanical simplicity of the original VM28 carbies, but at the end of the day I'm not going to ditch my prized Yoshimura camshafts. For the most part, this machine absolutely flies and is a blast to ride.
So I'll be looking for a set of CV's off a GS1100 or some such machine, to get things running correctly in all situations.
In short, I began by thinking that this was an electrical problem, but with a lot of elimination and many hours (more than I care to admit) of working at things, I came to realise that it's a fueling problem that I may not be able to solve with the existing hardware.
Of course, if anyone has any further tips or ideas for me in solving these fuel-mixture problems, I'd love to hear them. Otherwise it's going to be a set of GS1100 CV's that are going to be installed!
Thanks for asking! It's been a long story... but after trawling through the electrics, replacing points, condensers, getting the timing spot-on, changing the plugs (again), swapping out the coils (nup, they weren't the problem), dismantling and cleaning the carburetors (again), AND SO ON...
... it has turned out to be getting the fuel mixtures correct in the carbies. From 3500rpm onwards the engine pulls like a train, but light throttle with the motor at 3000rpm will see it spit back through one of the carbies once in a while.
So after lots of fiddling, I have realised that I can EITHER (i) tune the pilot mixture so that the spitting-back is eliminated but the idling mixture is too rich and fouls the plugs, OR (ii) I can get the idling mixture spot-on but then I get the spitting-back at 3000rpm on a light throttle.
The nub of the problem is, I think, that I have pod filters + high-lift cams + free-breathing 4-into-1 exhaust system, and it's simply asking too much of the original spec. carbies to be able to deliver the correct air-fuel mixture right across all revs/ throttle openings.
I am going to try a little more to coax the carbies into some sort of acceptable compromise, but if I just can't get them to come to the party, then I'll pull them out and put in a set of CVs instead.
That's a bit of a pity, as I really do like the looks and mechanical simplicity of the original VM28 carbies, but at the end of the day I'm not going to ditch my prized Yoshimura camshafts. For the most part, this machine absolutely flies and is a blast to ride.
So I'll be looking for a set of CV's off a GS1100 or some such machine, to get things running correctly in all situations.In short, I began by thinking that this was an electrical problem, but with a lot of elimination and many hours (more than I care to admit) of working at things, I came to realise that it's a fueling problem that I may not be able to solve with the existing hardware.
Of course, if anyone has any further tips or ideas for me in solving these fuel-mixture problems, I'd love to hear them. Otherwise it's going to be a set of GS1100 CV's that are going to be installed!
