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Dommi won't tick over when hot !

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Bike is 1959 Dommi 99, completely rebuilt except for cams and followers.

As part of the rebuild I purchased a new 376 carburettor from Burlen. All jet sizes and sttings are as stated in the manual. The bike starts and runs well. In the garage, from cold it will tick over beautifully for as long as 10 minutes (don't do this normally but just wanted to see if it would). However, going on a 20 mile run, getting the engine up to full working temperature, it will no longer tick over. Running at a very low speed coming to a halt at traffic lights for example the engine 'spits' and stops. It restarts OK but requires constant blips of the throttle to keep it running. Once moving again performance is as expected UNTIL the next stop. 

To give the full picture, in case it's a contributory factor, the timing (electronic ignition) is yet to be finely adjusted using a strobe. I've delayed doing this for a few hundred miles to make sure the engine is 'run in' enough to handle the high revolutions required when using the stobe. My feeling is, given the way it starts and performs, that the timing cannot be far from optimum, but that's just my opinion. 

There is a phenolic block between the engine and the carburettor, so I don;t think the problem is due to heat transfer between the two.

So, any suggestions on what to look for/what to change ?

Mike

 

 

 

 

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.. at the pilot jet. Is it tight in the carb body, and is it clear?

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I don't think the revs needed for a strobe check will do any harm as it is done in a no load condition. Your engine seems to behave opposite to what I'm used to. If it was mine, I should have warmed up the engine, then raised the throttle stop on the carburettor to get a decent idle speed. After that adjust the air screw.to maximize idle speed. The direction of moving the air screw gives an indication if it was too rich or lean. Then (hopefully) lower the throttle stop to a decent idle.

Checking fuel level in the carburettor is never wrong. The normal way of adjusting carburettors starting with the main jet on full throttle will have to wait until your bike is a bit is a bit more run in.

Good luck, Mike

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Have you tuned the idle on a warm engine, until you do it can't idle properly when warm. Once tuned for best warm running then when cold it won't idle as well which is what tickling and choke are for. 1.5 turns on the idle mixture screw is the starting point and always needs adjusting.

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I think you ignition timing is over-advanced. Retard it back at least 2* and then retune the carburettor.

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For what it is worth, I had the same issues and found two faults. Inlet flanges air leaks (identified using oil on flanges to eliminate air ingress) and exhaust pipes becoming loose in the head after a run.

Rectified both and now ticks over hot our cold.

 

I have addressed all the points mentioned with the exception of the timing. Will test again before adjusting the timing. I think, now that I have 400 miles on the clock since the rebuild, it should be safe to rev the engine to the required r.p.m, in order to use the strobe.

Mike 

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A fault when the engine and ancillaries are hot can also be a breakdown, somewhere in the electrical region,  if the fault persists once all the carburettor settings and timing are set.

 

John

 

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