Why Is It Backfiring

[captain marty]

While driving my truck last week, it started backfiring when I changed gears. It only started after I put $20 worth of premium gas in, like I always use.

This truck has never backfired on my before. I plan on draining the fuel bowl and checking what ever I can find that might be creating the problem.

I also think it may be related to incorrect polarity of my generator, which I never figured out how to polarize last fall. I have the early 28 Powerhouse generator.

Anything help I can get will ease my tension while driving it.

Captain Marty

[Chris Haynes]

Backfiring is usually caused by an exhaust leak.

[SHELBY MESSER]

The premium gas will cause it to backfire,polarity ot gen. will not cause it to backfire! Shelby, Louisiana.

[captain marty]

I have always put premium in and never had this problem. I understand if I have air leaks in the intake or carb, backfiring may occur. It's going to be a nice week so I will make time to search for leaks.
Any other tips will be welcome.
Marty

[1crosscut]

Why is it that you use premium gas? Correct me if I am wrong but from what I understand the higher octain rating of premium gas is to prevent pre-ignition (pinging) in high performance / high compression engines. The lower octaine gasolines are much better suited for lower compession engines.

[rollingsculpture]

it has been my experience with my AA that if i have backfiring it is because of fuel starvation .

[modeleh]

Perhaps running the GAV too lean and/or improper timing.

[vtwinsideways]

My guess would be air leaks, but like the rest it is only a guess. Could be warping of the manifolds or the carb. I fought with a Tillotson carb for the last week before giving up on it because the top half is just too warped to seal. As for premium gas, it should have no effect, although as stated above, you are paying more for no added benifit. Increased octane does not equal increased bang in the cylinder. Octane ratings can be thought of simply as a "anti-knock" rating. Racing fuel (and/or Avgas) doesn't make an engine produce any more horses, it just lets you run higher compression (or more boost) without pre-ignition (or detonation) knock. It's added squish that allows you to produce more ponies. No matter how much powder you use, the tighter you wrap a firecracker the more "BANG" you get out of it. Tighter "high compression" engines need the higher knock rating. The average A/AA does not. Luke

[tiredtruckrestorer]

Captain, you didn't mention how much "old" gas was in your tank before adding your $20 worth. The new ethanol flavored gas only has a shelf life of about 30 days unless you put an additive in it. After that time it can start to separate and collect moisture especially in a gas tank that is vented to the outside atmosphere like our Model A's are. If your truck was standing all winter with some gas in the tank, it is possible that the moisture may be causing your backfiring. The ethanol won't remix after it separates when you add new gas either.

I use the preminum gas in hot weather because it helps prevent vapor lock. The carb gets pretty hot on these Model A's and sometimes even boils the gas in the float bowl. The preminum gas helps prevent this along sometimes with a little 2 cycle oil added to it or I even heard that diesel fuel works.

Keith

[captain marty]

I've always heard that premium burns better and is more efficient. I had about 1/4 tank of gas from last fall and it did not backfire when driving with the old gas, this only started after I put in the new. I will be checking things out today if time permits.
Another reason why I love these old things, they are not rocket science and I enjoy working on and keeping them running.
Marty

[Hardhatz]

High octane is to gasoline as wet is to firewood.

Premium is more resistant to ignition, and contains fewer BTUs of energy per gallon.

[vtwinsideways]

I know it's unusual in the days of ethanol, but maybe you got a batch of "bad" (water saturated) gas. Premium burns neither faster nor more efficient than 87 oct. It just has a higher detonation pressure. Most carfolk understand that a diesel engine fires off of compression, not spark (and not the glo-plug once the engine is warmed.) Many do not know that gasoline engines will fire off of compression as well. A lower octane fuel will pre-ignight (that is, it will ignight before the spark comes, on compression) at a lower pressure (earlier in the compression stroke) than higher octane fuels. In higher compression gasoline engines (9.5:1+) this becomes a serious issue as gasoline will fire off of compression too early (before the spark comes) in the compression stroke and create a knock (not to be confused with a spark knock caused by timing too advanced). Unless your engine is running 9.5:1 or greater compression, there is no need to worry about this and there will be no difference in your engines performance or efficiency between 87 octane and 93 octane. Most of us are running between 4:1 and 8:1 in A/AA engines, negating any effects between the grades of fuel. Luke

[spectria]

Why is it that you use premium gas? Correct me if I am wrong but from what I understand the higher octane rating of premium gas is to prevent pre-ignition (pinging) in high performance / high compression engines. The lower octane gasoline's are much better suited for lower compression engines.

Additives along with higher octane prevent pre-ignition , and lower octane is fine in a low compression engine but...

If you have carbon build up in the combustion chamber, the carbon will act like a glow pug and fire off the lower octane stuff before TDC, causing the piston rattle that comes from two separate fire fronts colliding, carbon and spark plug as they burn towards each other.

If you have two much advance you will get ping also. An air leak on the intake side will cause a LEAN misfire and unburned fuel passing through the engine intact, and igniting in the exhaust. If it is too lean and too much advance you can get stack-fire (sometimes called backfire) back through the carb. The lean intake and igniting in the exhaust often shows up on down hill no load closed throttle coasting, tons of unburnt fuel in the motor, and tons of fresh air in the exhaust, so BLAM!!!

[captain marty]

I found a little time to search for the backfire. I cleaned the fuel bowl, fuel line and pulled the carb for a good cleaning. After reading all the high and low octane reports, I think it may run just as well on regular. It would be a little cheaper. More later...
Captain Marty