HOME and how to join Forum Open Area General Scott topics Starting when hot. Re: Poor hot starting

John Farrar

One of the more obscure reasons for poor starting when hot ,is the position of the carb and the length of the bellmouth. In my experience you must not use a bellmouth of any appreciable length. As an example a std Amal TT bellmouth is too long and gives rise to a rich mixture flatspot just off idle(it cannot be cured by changing the slide). I believe that the extra inlet length on the atmospheric side of the carb means that the inlet pulse wave is reflected across the primary “jet” (for want of a better word) several times , picking up extra fuel each time. This makes the mixture too rich for hot starting. If you have a std amal carb , at the std distance from the crankcase then this is not applicable.
When I corrected the bellmouth length, I was rewarded by much better hot starting.
The other point that has been mentioned is fuel height in the float chamber. This is very critical, and if the bike is leaning the wrong way when parked . fuel will bleed into the motor(even with the tap off) sufficient to give poor hot starting. As alredy ststed by others, in order to check this , it’s worth draining the carb, preferably by running it dry, before trying a hot start.
Stan’s point about the mag is well made, but I believe the comments about “modern” petrol are more to do with the fact that a poor ignition system will not ignite petrol that has lost some of its “volatiles”.
I have used modern petrol that is 18 months old in my old bikes (with good mags) that has been a nightmare with modern machines. Where I have been able to drain the float chambers on the trouble bikes, they start fine – I think that some of the volatiles “boil” off in the float chambers due to engine heat/natural evaporation, leaving a fuel which gives the ignition system a tough job.
Incidently I have a lawn mower which starts promptly with petrol that has been left in it rom the previos summer – it has a cheap and powerful CDI system which produces a mammoth spark!