Engine back fire

[Garry W.]

I have an ignition problem I think ??? When driving my M. N Special Everthing in is cool as long as I'm holding a constrant speed or eccelarating. However, periodically when I back off on the throttle slightly for traffic or something the engine seems to cut out momentarily then fires up again an produces one hell of a back fire, you;d think it would take the pipes right off the bike. The back fire seems to always be on the right side pipe, The problem seem worse after driving for a while . I'm thinking maybe a fault ignition coil ????? Any idea before I go to the bike shop ???

[MartyA]

Sound like it *might* be the pick-up coil wires. I had a 1950 Chevy panel truck once. An old trick in the mechanical points days and manual tranny was to cruise down the road, turn the ignition key off, pump the gas once, turn the key back on, and KA-BLAM, you got a back-fire.

I thought of this when you said the engine cuts out momentarily. If there was a fuel shortage I don't think it would back-fire, so I am thinking electrical. Then I think what wires change position when you are off throttle, and the pick-up wires come to mind.

Course, could be something else... Let us know when you find it.

[schpiff]

check the wires on the vaccum advance/pickup wires...sounds like one of em is broken.

[strom]

Im thinking more of a carb problem. It's the same with mine. Raise the float level or sth.

LP

[Jerry]

Check your exhaust manifold gakets, the ones where the exhaust headers fit onto the head.

My XJ was having similar problems when I first got it, loud backfire on letting up on the gas. I replaced the gaskets on the right side (#3 was at fault) and problem went away. Have since learned that others have had the same symptoms and did the same fix.

Set of 4 from Zanotti Motors, with shipping, came to $12.80 as of last month; I replaced all 4 when fitting a used Jardine system to the XJ. Local dealer had quoted $3.99 each plus tax. Don't be surprised if local dealer has to order them, Zanotti had them in stock. FWIW, Zanotti quoted the MSLP at $3.49 each, my local dealer isn't bike-friendly, sells mostly boats and a few ATVs.

Part number for the gasket is 341-14613-00-00

[MartyA]

Jerry might be correct if the crush rings are indeed sucking air off throttle.

But, if the engine is truly cutting out, it could be electrical. I had a friend with H-D Sportie. It had a loose battery with the side terminals, once in a while it would ground itself on the metal battery case and then suddenly unground itself. KA_BLAM! The last time it happened we trailered it to the dealer for new valves and springs.

If I remember the XS fires twice for each 4 cycles. The exhaust cycle might be carrying too much unburned gas out the exhaust valve. I still think somehow there must be a misfire in one of your cylinders on compression, then it fires and catches all that unburned gas going out on the exhaust cycle.

Then too if your spark is weak, and your exhaust gaskets are leaking, then you could be getting the same effect with the air leaking in. You might have incomplete combustion, which combines with the sudden inrush of air.

[Garry W.]

Originally posted by schpiff
check the wires on the vaccum advance/pickup wires...sounds like one of em is broken.

Hey! you're the man that was exactly the problem. Moved the wire and put on some liquid electrically tape and problem solved. Thanks a lot !!