Backfiring through S&S carb... : V-Twin Forum: Harley Davidson Forums

JoeShovel · 09-22-2009, 11:09 AM

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?

MegaGlide · 09-22-2009, 11:25 AM

Are the plug wires new?
If not, check to see if the one going to the non-firing cylinder is rubbed through somewhere and shorting intermittently as the engine vibrates.

Waffen · 09-22-2009, 11:28 AM

Manifold leak, carb jets plugged up with some debris.

W

JoeShovel · 09-22-2009, 11:35 AM

Quote:

Originally Posted by Waffen

Manifold leak, carb jets plugged up with some debris.

W

How would I go about cleaning this?

Hillside · 09-23-2009, 09:54 AM

NAPA sells a good carb cleaner kit. Very agressive cleaning solution.

Scott

Waffen · 09-23-2009, 01:04 PM

Quote:

Originally Posted by JoeShovel

How would I go about cleaning this?

First to the manifold. If you can get it running, spray some WD40 where the manifold is bolted to the jugs. If you notice a change in idle or how it runs you may have a leak which creates a lean environment which you experience as the backfiring through the carb.

To clean the carb, personally I think it's best to take it off the bike, pull it apart and clean it throughly and don't forget to check the float.

I don't know your carb specifically but you should be able to find a manual showing the schematics which will help. Also if you do take the carb apart it might be a good idea to change the o rings and gaskets.

W

JoeShovel · 09-24-2009, 08:48 AM

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?

Waffen · 09-24-2009, 12:12 PM

Quote:

Originally Posted by JoeShovel

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

JoeShovel · 09-24-2009, 02:12 PM

Quote:

Originally Posted by Waffen

Timing might also be off.

W

Ok, this is something I was also thinking but should I advance or retard...?

I would think by retarding it, it would still be too lean if that is the case.

wfolarry · 09-24-2009, 07:07 PM

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?

JoeShovel · 09-28-2009, 10:41 AM

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.