Back fires under a load on the engine

AA Ford Discussion Group relating to the repair and restoration of your AA Ford.
Post Reply
Jim505
Posts: 14
Joined: April 5th, 2008, 8:32 am
Model Year: 1930

Back fires under a load on the engine

Post by Jim505 »

Today must be carb day!!

When I drive my AA and go up a hill I get a very sharp backfire just as I down shift. Timing is on, float seems to be ok. When driving on level ground it runs fine no backfires. Any body got any hints.
User avatar
spectria
Posts: 1874
Joined: May 15th, 2008, 9:53 pm
Body Type: Mail Truck, Stakebed
Model Year: 1931
Location: Quincy, Ca.

Re: Back fires under a load on the engine

Post by spectria »

Jim505 wrote:Today must be carb day!!

When I drive my AA and go up a hill I get a very sharp backfire just as I down shift. Timing is on, float seems to be ok. When driving on level ground it runs fine no backfires. Anybody got any hints.
Hello Jim, I am very new to this forum, and although I have 2 AA's I have never worked on one that actually ran.

Having said that, I was an Auto repair shop owner for 20 years, and also ASE certified Master tech with the L1 performance cert, and diagnoses was my specialty.

As a way to educate myself, and move this discussion, I need to know a couple things I don't about your Truck.

Does this carb have an accelerator pump?

Do you have to double clutch to down shift?

When you say the timing is on, does this mean the base timing is right or

Does it mean you have mechanically added timing "on".

Usually, when you get a backfire (out the tailpipe, I know this might sound a little funny, but after years of asking these questions, I have found that many people think a hiccup is a hiccup, and a stack fire is a backfire) it usually means that excess fuel has entered the exhaust and when it gets the air it needs from a leak or gets all the way to the tailpipe and gets that air it fires off.

Excess fuel happens when you have an internal carb leak, too high a float level, or very high revs when you suddenly close the venturi, and the vacuum runs up real high under the venturi sucking fuel out of the primary and transition ports of the carb, goes through the engine too rich to burn, then into a very hot exhaust with enough oxygen to fire.

Also pumping the accelerator with an accelerator pump carb will add fuel that may not fully burn during the sift process.

Double clutching will increase this likelihood, as you get 2 opportunities to flood the exhaust with unburned fuel.

Allot of advance would tend to fire off excess fuel in the intake under these same conditions, causing a stackfire.

That's my take on it, I would love feedback on my theories.

Dave
Dave in Quincy, Ca. I love Pics!!!! Post them All!!! :)
Join the Ford Model AA Truck Club - membership form at http://www.fmaatc.org
User avatar
rsierk
Posts: 95
Joined: February 23rd, 2005, 7:27 pm
Body Type: 185-B
Model Year: 1931
Location: Christiana PA

Re: Back fires under a load on the engine

Post by rsierk »

I think Dave is probably pretty close to it on this. I find from driving my A's that this back firing is more likely caused by having the low idle stop screw adjusted to allow too high of an idle speed. Sometimes this is done when the idle circuit is not working right and the screw is turned so that the power circuit is never shut off, as that is what it needs to idle, or to keep from stalling at a stop light, etc. The other thing to try is to set the float a little lower than you think is the nominal setting. Mine seem to work better that way. Randy
Post Reply