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Backfiring through S&S carb

34K views 15 replies 8 participants last post by  Billyroyjoebobj 
#1 ·
OK, 1966 74" shovelhead with Mallorery electronic ign, S&S super E shorty carb...

Was running fine with no problems at all and now it is backfiring through the carb very badly. Feels like one cylinder is not firing at all. This all started after I went to the gas station and put in lower octane due to them not having higher octane.

Here is what I have done so far to no avail:

I have drained out all of the bad gas from the tank and carb, put new plugs in, verified that I'm getting spark to the bad cylinder, adjusted timing...

I'm at a loss at this point. Every electronic component was replaced and rebuilt over the summer so I'm only left to think it is a mechanical issue which I hope it isn't.

Any thoughts?
 
#7 ·
Could anything else besides this create a backfire though.... I guess I'm just worried that I have a stuck valve or something...

I was informed that:

backfire though pipes = carb

backfire through carb = ignition.

Is this true?
 
#8 ·
Could anything else besides this create a backfire though.... I guess I'm just worried that I have a stuck valve or something...

I was informed that:

backfire though pipes = carb, Rich condition to much gas valves possible

backfire through carb = ignition. Lean condition, leak, plugged jet, jetted incorrectly and quite possibly ignition.

Is this true?
Timing might also be off.


W
 
#10 ·
Putting in 87 octane could make it backfire as it would appear to be lean. Putting premium in should have solved this. Did you do anything else [play with timing, jets etc.] when it was backfiring to try & correct it before you took it apart?
 
#11 ·
Well it was 89 octane that I put in it and drained it all out and put in fresh 93. Now, when I was stuck on the side of the road I did play with timing but have since put it back to where it was. It is very hard to start now. Have not had the time to play with it that much this weekend. Hopefully more this week since it is going to be nice.
 
#12 ·
Check to make sure your stator and rotor is charging the bike... With the bike running put a meter on your battery and rev it up to about 2k rpm watch the meter and make sure the voltage spikes up to around 12.9vdc if it doesn't move then your stator is bad or your regulator... No way to test regulator but you can test the stator by pulling the plug off the motor where the regulator plug into it.... You should have continuity between both posts of stator... Then each post to ground should have no continuity if it does anything different your state or is bad! Good luck
 
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