Not really a backfire, but a puff out of the #3 carb. Is this a valve timing or spark timing problem? Some history...
I was putting the final touches on my restoration of 78' cb750. The last change I did was to replace the breaker plate with a new unit. The other points where sparking quite a bit and were original. I set the firing timing using the light bulb method, only I used a dvm with continuity beeper. I have both points set opening right on the F mark. The points are gapped to the closest listed setting as per one document I read. After doing all this, #3 was not running at all and #1 pipe was just warm. I pulled the carbs off to clean them for the fourth time and yes there was some dirt in all of them. While the carbs were off, I pulled both plugs and tested spark on both. looks fine. Tested compression with just the finger in the hole test. Tested OK. Valve chain tensioner has been adjusted. Valve clearance has not. Runs smooth, just that little inconsistent puff out the back of #3. Also #3 exhaust does not appear to be as hot using the wet finger test. Note that this puff is not fire, just air/fuel mixture. Also, this was doing this on more than one cylinder prior to the plate swap only the temp of all cylinders seemed more consistent. Head me down the right path.
