engine stops over 5000 rpm

Hi Gents,

When I rev my Commando over 5000 rpm, the engine stops running suddenly after a few seconds. A soon as I close a bit the throttle (to go down to 4500 rpm), it starts again . It doesn't look at all as if pistons started to seize. It happens systematically at a bit over 5000, and looks more like a sudden ignition switch-off than a gas supply or carburator issue (spark plugs look OK).

Ignition system is a Pazon Sure-Fire 12 volts.

Carb is a ingle Amal Premier 32 mm / 240 main jet / needle at lowest notch

I can't compare it to the previous situation as I am at the end of my rebuilt engine break-in and it is the first time I'm pushing her to 5000 rpm.

Apart from that, she runs very well (starts first kick, stable idle, no problem under 5000 rpm etc...).

Have you got an idea of where I should start my investigations?

Many thanks,

Laurent

To me it sounds like an electrical issue that might be set up thru vibration. I'd start by checking all connections in the ignition circuit. I don't think it's in the carb because the 240 main jet should be rich enough and your needle is in the richest position. Best of luck ,Skip

Good advice Skip! I'll start with double-checking all the ignition circuit connections.

Thanks a lot

phillyskip said:

To me it sounds like an electrical issue that might be set up thru vibration. I'd start by checking all connections in the ignition circuit. I don't think it's in the carb because the 240 main jet should be rich enough and your needle is in the richest position. Best of luck ,Skip

Click to expand...

+1

Slick

Move the needle clip up a notch and try a 230 main to see if you see any improvement.

I realize the plugs look proper but at this rpm the balance may change enough to cause the cut out.

All other ranges may be fine.

Could be electrical with some mechanical cause cutting spark about 5000 rpm which is the rpm electronic ignitions stop advancing. If just a voltage drop for some reason about 5000, sparks go nutzo in timing for mean mis-fires/back fires stall out not sharp rev limiter like power loss that instantly recovers below 5000. Carb gasket and other car air leaks tend to get bypassed on hi throttle and hi rpm but exhaust leaks can shut powe down above some rpm threashold on a steady throttle to just hold a speed. Its possible a worn down cam lobe puts extra slack in time chain to jerk wits out of electronic triggers near full advance. But for a new rebuild to do this would require lots of too timid operation below cam break in rpm, which could be aggravated by nil wet sump dry starts. Try key on [out of gear] tugging-flicking-jerking on trigger wires to power feed and Earth return with a plug on head to sense for a snap or spark w/o turning engine. After dark checking too. I have had water in fuel bowl do this, fine until enough flow of fuel needs lifted water into the mix.