backfire (2015)

If my Norton 750 is backfiring at between 3000 an 4000 rpm is my timing too far retarded or too far advanced. Help!!

kingcole said:

If my Norton 750 is backfiring at between 3000 an 4000 rpm is my timing too far retarded or too far advanced. Help!!

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Maybe a little lean in the mid range. How are the plugs?

Electronic spark or points? They can tolerate a little more advance with the electronic ignitions because the advance is in the black box and more precise than the dual points plate. Backfiring through the exhaust at what should be full advance sounds retarded (timing wise) like a late spark is making it fire past the exhaust valve. If it's popping back through the carbs, it may be too advanced.

kingcole said:

If my Norton 750 is backfiring at between 3000 an 4000 rpm is my timing too far retarded or too far advanced. Help!!

Click to expand...

Steady speed, as in a cruise? Engine hot or cold? Backfiring through the carbs, or out the exhaust (technically, an "afterfire"). Typically, if it's a backfire through the carbs, it's due to the mixture running so lean that it's still burning when the intake valve opens. That's why a cold engine will backfire.
If it's an afterfire, suspect an air leak in the exhaust.

Nathan

Danno said:

Electronic spark or points? They can tolerate a little more advance with the electronic ignitions because the advance is in the black box and more precise than the dual points plate. Backfiring through the exhaust at what should be full advance sounds retarded (timing wise) like a late spark is making it fire past the exhaust valve. If it's popping back through the carbs, it may be too advanced.

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Many thanks Danno, I have just changed from points to electronic and have just got the bike back to running after thirty years of being abandoned, I don't know why I changed to electronic as the bike started and ticked over easily on points, maybe I will go back to points if I have problems with electronic. I will try advancing little by little and see what happens. I set the timing up at 31 degrees running at 4000 rpm as per instructions but maybe my strobe light is a little bit suspect and the bike was jumping about a bit at these revs. Your advise has given me a direction to go in and if you have any more suggestions they will be more than welcome..

Electronic ignition relys on good electrical connections and solid grounding to throw the spark accurately. Having your timing light jumping around could be a "cheesy" electrical connection somewhere. An Isolastic engine is pretty well insulated from ground, so I added extra ground wires to my engine just to make sure a loose wire wouldn't leave me on the side of the road.

I think I timed my boyer at 31 degrees at 5000 rpm, so maybe you are a little advanced... but I doubt it would cause any popping or "backfiring".

If anything I would think you are getting some popping from an exhaust leak if it's not an ignition issue. With exhaust leaks, the popping tends to increase when you back off the throttle.... which is one of the telltale signs.

Normally when you lay hands on something and it goes differently afterwards, it was something you did that caused the difference.

Hi kingcote.
Check the rotor timing marks are accurate from top-dead-centre using a degree wheel and piston stop. If these are wrong then the timing you thought was correct may be the cause.
Ta.

Could be a coil going down?
Dave

I experienced a similar problem with an afterfire on deceleration. After throwing lots of money and time on it, pulled the head to decarbonize. While checking rockers, one seemed to bind a bit in the high position (closed). Noticed what appeared to be gasket maker chunk in oil tunnel Turns out the spindle and rocker were scored, presumably holding the exhaust open slightly which then allowed intake fuel to detonate in exhaust. I'm no expert but after replacing the spindle and rocker, afterfire stopped.