1926 Buick Master Six -Engine back firing

[Duchess 1926]

[nickelroadster]

Please post a little more detailed description of the backfiring.  Often times it means that you are running lean but there can be several other things that can cause backfiring.

[Mark Shaw]

Are you running the vacuum tank?  If so, you may have a vacuum leak causing intermittent fuel starvation.

[Pete O]

 

41 minutes ago, Mark Shaw said:

Are you running the vacuum tank?  If so, you may have a vacuum leak causing intermittent fuel starvation.

Good point.  That fuel filter appears empty to my eyes.  Looks like the picture was taken with the engine not running, which could explain the empty filter, which looks to be installed where the vacuum tank would be drawing the gas up through the filter.  It's not good practice to suck gas through a filter.  It's better to push it through.  The widening of the fuel line at the filter causes a low pressure area where vaporization can occur.  If you are sucking the gas up through the filter, you can get bubbles in the flow because of this, which as Mark said can cause temporary fuel starvation, lowering the level of gas in the carb bowl causing a lean mixture causing the backfiring.  I'd relocate that filter somewhere between the vacuum tank and the carb, where the gas would be pushed through the filter.

[Grimy]

If you use a filter with a vacuum tank, the filter should be placed horizontally in the line near the frame of the car, but away from-- or shielded from -- the heat of the exhaust.  I have 35,000 miles on such placements with no issues, even in the hottest driving conditions.  In line with what @Pete Osaid, I think this is the worst possible place and orientation for a filter.

Edited June 7, 2021 by Grimy
clarification (see edit history)

[Mark Gregush]

I have an original glass bowl fuel filter installed on my 1920 Dodge Stewart vacuum pump. It is installed on the in line with a banjo fitting to top of the tank, but the bowl does not drain out between runs. Makes sense that the filter in this case would be horizontal, between runs it could drain out installed as shown and would have to be refilled each time the pump does its thing. Putting the filter after the pump between it and carb would defeat its use, getting any crud from the tank out before it reaches the vacuum pump. There is no pushing of the gas out of the vacuum pump, it is gravity feed so there is not a lot of head pressure to force the gas thru the filter.

You could also have a valve(s) that are hanging up. The rocker arms may need to have the pads reground. What I found out doing a little checking because the valves on my 28 Chevrolet head were hanging up, the pads needed to be cleaned up, made smooth again. Also, when rocker arm pads are ground, there is a small angle across their face so as the pad brushes over the valve stem it rotates the valve. We have an old Black and Decker valve grinder at the shop, it has a special attachment to do this. So it's not just a one car brand thing.

Edited June 7, 2021 by Mark Gregush (see edit history)

[Mark Shaw]

A sediment bowl should be located under the vacuum tank because the vacuum tank only gravity feeds the carb.  A paper filter may work OK for awhile, but as it filters more dirt the differential pressure will increase and eventually starve the carb.

[Pete O]

On 6/7/2021 at 11:06 AM, Mark Gregush said:

There is no pushing of the gas out of the vacuum pump, it is gravity feed so there is not a lot of head pressure to force the gas thru the filter.

Thanks Mark, that's interesting information that I did not know.      Is there a sediment bowl built into the vacuum tank that will catch debris and condensed moisture that can be drained out?  If so, is an inline paper filter even necessary?   I know in the Model A community, where the fuel tank is in the cowl and it gravity feeds to the carb, guys tend to want to add a filter, and all that does is cause flow problems as the filter clogs up.  The factory sediment bowl that's in the fuel line is really all that's needed if you're not dealing with a fuel tank that's full of rust.  The add on filter is a band-aid.  The only real cure is to clean the tank.

[Mark Gregush]

30 minutes ago, Pete O said:

Thanks Mark, that's interesting information that I did not know.      Is there a sediment bowl built into the vacuum tank that will catch debris and condensed moisture that can be drained out?  If so, is an inline paper filter even necessary?   I know in the Model A community, where the fuel tank is in the cowl and it gravity feeds to the carb, guys tend to want to add a filter, and all that does is cause flow problems as the filter clogs up.  The factory sediment bowl that's in the fuel line is really all that's needed if you're not dealing with a fuel tank that's full of rust.  The add on filter is a band-aid.  The only real cure is to clean the tank.

Same with the Model T's re adding a filter between the tank sediment bowl and carburetor, esp with early cars and tank under the seat. Yes a clean tank is best. This is what I have on my 1920 Dodge vacuum tank, it is aftermarket era correct. In my case there really is not room under the tank for a sediment bowl/filter. The fuel tank on mine does need to be cleaned (the brown at the bottom is rust and scale). Most glass bowl types also have a screen inside so larger particles don't back flow into the pump or carburetor depending on where you mount the filter. 

[DavidMc]

There is a vertical  pipe from the carburetor inside the exhaust manifold, to heat the fuel air mixture.  It can rust through allowing the engine to draw in exhaust gas with the fuel..  The pipe can be pressed out and replaced.  That would be the first thing to check.