Lean Condition back fire .

FlatheadTed

FlatheadTed

Well-known member
Any ideas on this 30 Model A New motor ,on a run accelerating at 40 MPH car looses power gives a small back fire ,then picks up a bit .We have found a lot of crud in the tank cleaned it out .Changed to known good Carb,running a filter just before the carb ,We have ruled out electrical .Thank Ted
 
Did you re-torque the manifold nuts? Usually this is for low RPMs. Check that the gas supply line is not bottomed in carb shutting off gas flow.
 
Hi Ted,

What kind of filter?

Many fuel filters were manufactured for fuel supplied with the pressure of fuel pumps as opposed to fuel provided by gravity flow which can cause fuel starvation -- the paper NAPA #3039 one in the glass sediment bowl works well.
 
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Hi,H,L , Thanks for the comments so far ,Yes we did re-torque the manifold.It has a temporary plastic filter timed on cc per minute checks outs OK .I am thinking a obstruction floating over the fuel out let .We water blasted inside the tank -over flowed .
 
Hello Ted ,
When you say you are running a new motor, does it now have a much larger cylinder bore than before?, the reason I ask is that a few years ago I did some flow testing on my carb jets as per instructions , and ended up with poor power and back firing at higher RPMs, I gradually increased the flow until it ran OK.
I later found that the engine was bored .090" over thus needing that extra fuel for the larger cylinder replacement.
The only fuel filters I use are the finger filter in the tank and the fine mesh in the sediment bowl, I removed the filter in the carb, as I had never seen it trap anything.
Regards Brian.
 
Hi Brian.You could be onto something there I know that some new jets are a little out in flow rate .I have done the mandatory checks with the drills .The carb we have on it now has been soldered and flowed .The motor has been sleeved, I think Std .
 
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Adjusting the GAV makes no difference ?? Once it backfires,, will it continue to backfire if throttle position doesn't change ?? Have you checked the fuel flow, gas cap vent ?? Try it with the gas cap off or loose..
 
FlatheadTed said:
Hi,H,L , Thanks for the comments so far ,Yes we did re-torque the manifold.It has a temporary plastic filter timed on cc per minute checks outs OK .I am thinking a obstruction floating over the fuel out let .We water blasted inside the tank -over flowed .
Click to expand...

Here's the complete set of filters I have tried and found to be problematic to some extent. There's a good chance yours could be causing trouble. My clear plastic one filled up with vapor and shut down the motor. They can behave differently on the road vs. on the test bench. Remove the filter and see what you've got.

post2007MARCrepairs036.jpg


Steve
 
I second the gas cap . Had the same problem you are having and it was the gas cap not venting as it should ,
 
Patrick& Duffy , I did try the cap but its worth looking at again .
Steve Thanks for allthe filter posts,it had the problem prior to attaching the filter but there was crud in tank then.
 
Ted,

Also, with the see through filters, (as Steve mentions in reply # 8 above), have experienced that everything can appear fine with the filter full of fuel for a spell; then all of a sudden, one sees an air pocket inside the see through filter & the engine begins missing.

How did the tank dry after filling it with water? Any possibility of droplets of water settling to the bottom of the tank?

Always takes a while to clean out a tank that set up for some time -- sometimes:

1. Fill tank with about 4 gallons of fuel & slightly shake car sideways; &,

2. Drain through a white cotton sock into 5 gallon container; &,

3. Then Repeat 1. & 2. several times until white sock stays clean.
 
You have probably heard; "90% of carburator problems are electrical". Something to think about! Ron W
 
Ted,

That's the main problem -- between drinks, you did not "drink" the quarts of alcohol -- it probably would have helped.

Flushing works.
 
I have heard that John ! I said the same thing to the owner, it certanly proves true with FH v8s ,
Yeah H,L . Maybe it was just the fumes .LOL
This is the offending vehicle ,Checking the Diz.
 

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Adjusting the GAV makes no difference .not really .Once it backfires,, will it continue to backfire if throttle position doesn't change.I think it does, and then it slows down .Its gets worse when you are accelerating


Patrick L. said:
Adjusting the GAV makes no difference ?? Once it backfires,, will it continue to backfire if throttle position doesn't change ?? Have you checked the fuel flow, gas cap vent ?? Try it with the gas cap off or loose..
Click to expand...
 
You probably checked this, but remember that the tank shut off valve has or should have a small screen in it to stop crud, and if its plugged it will slow down the flow. Iceman
 
FlatheadTed said:
Adjusting the GAV makes no difference .not really .Once it backfires,, will it continue to backfire if throttle position doesn't change.I think it does, and then it slows down .Its gets worse when you are accelerating
Click to expand...
check your points and codenser you never know does not sound like gas
 
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