Hard starting when hot

[SteveW]

I have a 51 Rapide that starts and runs well but when I shut it off, when it's hot, it will not start until it cools down. It is the not mag from what I can tell. It's getting a good spark, hot or cold. The mag and carbs were overhauled not that long ago (1 year). It has new gas in it.

Any advise would be much appreciated, Steve

 

[Bazlerker]

My guess is a bad condensor in the mag..

 

[davidd]

Steve,

It is the classic sign of a bad condensor. Do you know it is not the mag because you swapped it out?

David

 

[SteveW]

I've had bad mags in the past that have had a bad condensor and these are the same symptoms, but this time both cylinders are showing a good spark when it's hot. I've heard vapor lock will do the same but what's the remedy? Insulators on the carbs?
Steve

 

[Bazlerker]

A good spark with the plug laid on the cylinder is one thing....a good spark under compression is a different thing entirely. Is the condensor a new one-manufactured recently...or is it a NOS item, a paper condensor?

 

[SteveW]

I am using a timing light inductor on the spark plug's wires, so the plugs are under compression when I am checking them. They are working when the engine is hot. The Mag was rebuilt last year by Doug Wood who has an excellent reputation so I am sure the condensor is a good quality one. This stinks of a mag problem but I just don't see it, so far.
The bike is so unresponsive that I can not tell what combination of choke, tickling the fuel or starving it of fuel finally gets it going, except it cooling off will finally get it started.
The right way to check would have a spare mag and swap it out but I do not have a spare one. Thanks for the help.

 

[davidd]

I am still with Bob, although if you really believe the spark is good enough to work, then I would guess you have no or little compression when hot. There might be a Vincent that has suffered from vapor lock, but I have never had one do it, nor have I heard of one doing it. I would get the engine hot and do a leak down test. If it passes I would still believe it is ignition. You have not described the carbs as being unusual, but in general a carb that is good enough to start the bike and keep it running for hours until it is stopped is good enough to start it when it is hot.

David

 

[SteveW]

Thanks Dave, I'll check the compression hot. That could be the culprit.

 

[greg burt]

CONDENSER
75% of Fuel Problems are Electrical(Vapour Lock , Dont think so )

 

[Hugo Myatt]

I had this problem. It turned out to be a minute air leak between the rear carb inlet adapter and the cylinder head. This seemed to open up when hot causing weak mixture. When cold it would start first kick and did not spit back. It ran well but was a pig to start when hot. I should have realised earlier as the rear exhaust pipe was developing a blue tinge.