Hot start problems 1974 350 scrambler

[Dave L]

Hello everyone, I'm having big problems starting when the bike is hot. It doesn't help that I'm in 35 degree heat all day at the moment but it won't catch even after tickling the float to try to release any vapour lock . I leave the throttle shut or at least a tiny bit open but this doesn't help much either. I have fitted a b6hs which does get a bit sooty. Would a B7hs or bp7hs help ? Bike cools down and starts first time. I reckon the carb is as old as the bike, although I've replaced the jets and seals etc. Any help greatly appreciated, cheers

[Duccout]

Hi Dave,

Three possibilities spring to mind: it could be the ignition coil breaking down, or the condenser. Or the mixture too rich, which would obviously help the cold starting, but would flood the engine when hot. You could try lowering the needle one groove and see if that helps. Is the float needle seating properly? If not it could cause some flooding. What about float height? A high fuel level would make things worse.

Colin

[ducwiz]

First of all, you must tell us which ignition system is in your bike - points/Battery, Ducati or Motoplat CDI? Devoid of this info, remote diagnosis is impossible

Hans

[Dave L]

I'll try the lowering the needle one groove first. I think I raised it earlier to help with a stutter when accelerating so it might make a difference. What about plug changes? Any thoughts? I'm thinking of making a heat shield, fitted between to rubber spacers (using longer studs) to try a keep the carb cooler. This should help with the vapour lock issue.The ignition is points/ battery.

[Dave L]

I'm also changing the coil

[Dave L]

My 74 year old friend, the retired Ducati mechanic , tells me that he rarely encountered vapour lock issues. It is more likely a dodgy coil

[Duccout]

I think that you have to look at parts that could be failing, rather than the carb insulation, as all the other Ducatis run with standard manifold spacers. I don't think that the plug is the problem, as you are running with the correct one anyway, unless it is faulty, probably better to lower the needle and change the coil and see what that brings. These sorts of problems seem to plague us, I suppose it is because the bikes are getting so old.

Colin

[blethermaskite]

Change your condenser first......thats the cheapest option then go for the coil.....make sure to only change one thing at a time otherwise you will never be sure what fixed it. ( I would leave the carb alone for the moment)
Cheers,
George

[Dave L]

Changed coil, dropped needle, checked condenser, changed plug, healthy spark, checked points and all associated wiring... guess what? I started it and went for a spin but the same thing happened again. It coughed and then faded and then stopped, and wouldn’t start again.Now it won’t start even when cold! I’ll put some petrol directly into the cylinder and see if it fires up that way, after a relaxing pint, that is.

[George]

As stated in previous post only Change one thing at a time, test, if no better put back first change move on to next change etc etc. This way you will find what is wrong and what isn't