TOPIC:

I have just finished totally rebuilding my bike engine fully reconditioned engine wiring has every connection renewed new dyna coils rebuilt carbs etc everything on the bike i.e. lights etc work it turns over we have a spark but all i have had is a few loud backfires when trying to start it any suggestions from you guys out there on where i start to diagnose what could be wrong thanks

With backfiring I would recheck ignition timing and that the plug leads are on the correct plugs. I can tell you from experience that getting the leads in the wrong place is easy to do and can be one of those things you keep looking right past when checking.

RickG wrote: With backfiring I would recheck ignition timing and that the plug leads are on the correct plugs. I can tell you from experience that getting the leads in the wrong place is easy to do and can be one of those things you keep looking right past when checking.

+1 on what RickG says here. Backfiring usually indicates firing when valves are on overlap.. check your primary wiring to the coils. You may have crossed up 2 of them.

A mistake I made on my last rebuild was I mistakenly started my valve clearance check from the wrong side of the engine so my clearances were all off bike ran like sh*t until I figured it out so double check your clearances don't have my service manual here however I think you start from the opposite side to the alt side of the engine.
(Left side of engine sitting on bike)


Maybe something simple like this

Tight clearances - tight valves bypass combustion gasses - backfiring can result.

think i have found it the guy who built the motor did not put the pin in the crank on the timing side so timing is miles out just sourcing a pin

You had me going for a bit. I pulled the drawings out to find the "pin in the crank on the timing side" I think you're talking about the spring pin under the timing advancer.

Good for you for finding it. I absolutely hate trying to find crap like this.

Reminds me of what eventually became known as a common problem with Volkswaagen engines. If they backfired, it would shear the woodruff key on the crankshaft which drove the timing belt gear and the gear would spin just enough so you thought the timing belt jumped a tooth and cause a valve to bend. Replace the head, put on a new belt, set the timing and give back to the customer and within a week or 2 the car would be back with a bent valve. First time it happened I went through 4 heads before I completely stripped the engine to find the real problem.

As the saying goes "If it doesn't kill you, it only makes you stronger"