Help with square four

[john.vessey]

My 1939 6oocc square four is not running well and I cannot find the solution. Any help/comments or advice most welcome.
The problem is that after about ten miles of perfect running it starts to loose power when the throttle is more than one third open. This gets progressively worse until it is only running one one or two cylinders. When cold it then runs perfectly again.
I fitted Newtronic ignition after two magnetos failed about ten years ago. I have changed the coil, battery, distributor head and plugs (to Champion LB 6c) all to no avail.

Any ideas?
Regards,
John Vessey

[brenton.roy]

Hi John, that would be annoying!

Assuming then that it isn't an ignition problem, have you checked fuel flow? Either gunge around the tap /filter or a blocked air vent in the cap.
After 1/3 throttle, you could be getting supply problems to the midrange / main jet. Does it run better if you slow down once you get symptoms?
It might be worth measuring the amount of fuel the tap delivers in say one minute, then blow back through the line and try again.

Where are you? - in summer, fuel vaporising can occur, particularly (but not only) if the line is too close to the motor.
Is the motor still new (tight)? I assume 4F's run just as hot as 4G's

You could try a plug chop when you get symptoms to see where the motor is at. Stop the motor at the throttle setting that is causing the problem - don't let it idle down.
http://www.barkshire.co.uk/AOMCC/Knowle ... spx?id=735
Brenton

[adrie.degraaff]

A friend had the same problem, after a endles surge it was the new electronic ignition, it's now on magneto again.

[john.vessey]

Roy,
Thanks for your comments.
I checked the carburettor and it was clean as a whistle. Fuel appears to be reaching the float chamber without any apparent hindrance. A spell of low throttle has no effect on the problem. I did wonder if the higher volatility of ethanol containing fuels could be the cause but the fuel pipe remains cool and the bike has a heat insulating gasket between the engine and carburettor which I checked was working.

The motor is not new having had a rebore about 15 years ago.

I am not familiar with a plug chop. What do I look at when I have stopped the motor?

One thing I did really puzzles me. Lester Grant suggested that I chenge the HT leads. I cannot see how all four plug leads would fail together but the HT lead to the distributor failed a few years ago and I replaced it with a carbon cored lead. I bought some copper-cored HT lead from Dragonfly and changed that lead. The bike refused to start although all plugs were sparking. I checked the resistance with a meter. The copper cored lead had zero resistance but the carbon cored had a significant internal resistance. I replaced the original carbon cored lead and the bike then started first kick!

Many thanks for your comments.
Regards,
John

[john.vessey]

Adrie,
I have not yet replaced the ignition. I asked Dragonfly if they had any reported problems but they have had no reports of it failing. I guess if there is no better suggestion I will have to install a new unit. The reason I have not done so is that I would not like to spend the time and money only to find that this was not the cause.

Many thanks for your suggestion.
Regards
John