Poor starting when hot

[ade]

I just got my XL600R on the road,after a top end rebuild.
After a run,it will cut out when at a junction etc and is a sod to start until its cooled down.
Valve clearances are spot on,valve timing no worries there,fuel present etc.
The only mod ive done is ive made a new plate in between the head and carb rubbers for the o rings(all properly machined,resessed ect) out of aluminium as the original plastic part is unservicable,all bolts tight etc.
I doubt this is the problem
Any ideas?

[mid life crisis]

No expert but the first thing springs to mind is the fuel evaporating, i would run it hot, let it cut out, take the plug cap of, give it a couple of turns and see if the plug is wet (could be the bit you made as the heat transfer will differ alot). If so test compression whilest its still hot. Hope it helps.

[ade]

i will try that,compression shouldnt be a problem as its had a rebore and piston.
another thought is the carbs might be at fault as when i got the bike it had not run for an undisclosed period.
ive only done about 12 miles on it to take it for mot(i carnt use it atm as its not taxed,waiting for the v5).
the part i made is called an insulator,so im on the look-out for a replacment.
thanks

[StumpyFingers]

Have you cleaned the carbs, it could be residue in them which is having an effect.

[ade]

yes i cleaned the carbs,they were surprisingly clean,plenty of carb cleaner and compressed air,although there was a pipe(overflow?) on the rhs of carbs with a bolt in it,have since removed this and it seems to start better when cold?(or my technique is improving,no leccy foot).
my guess it just needs using more to get it fettled.

[416cc]

It may well be that the bore is still a little tight and needs a few more miles to loosen up a bit.

Are you still using the auto valve lifter from the kickstart mechanism..? I removed the one on my XL250SA as it was more hassle that it was worth.If there is insufficient free play in the cable it can hold the valves open slightly, and when hot will make the problem worse. If the cable is too tight back it off a bit and see what happens.

The other fault that springs to mind is that the resistor in the spark plug cap can go open circiut or to infinite resistance when it gets hot. I have had this problem on my big road bikes as well as on my XR400R. You can disassemble the plug cap using a blade screwdriver to get the bit that goes over the spark plug end, and out comes a spring and a small bit that is the resistor. They normally measure between 5000-10,000 ohms using a multimeter, but when hot can go to infinity (& beyond!...sorry, Toy Story quote there...). I have got around this problem by cutting a small length of brazing wire which is the same dimension to fit inside of the plug cap, problem solved.

[ade]

compression is maniac,stand on kickstart lever time.
auto de compressor cable is tight,have tryed to get it loose,but theres no leeway.
plug cap is a brand new,motox special thing which cost £10,will try a normal one or the mod you suggested.
cheers

[Krtek]

I'm sure you're tried this... but mine needs a tiny,tiny bit of throttle to start when hot.

[416cc]

compression is maniac,stand on kickstart lever time.
auto de compressor cable is tight,have tryed to get it loose,but theres no leeway.
plug cap is a brand new,motox special thing which cost £10,will try a normal one or the mod you suggested.
cheers

If the decomp cable is tight, it may well be holding open the exhaust valve slightly when hot...& get worse if something not done about it. It will burn out the valve seats & can damage the valves.

[ade]

plenty of slack in both cables.will look into it more when the weathers a bit better
cheers