1930 model 40 backfiring out carb

I was having trouble with the Marvel (I thought) so I flipped the intake and installed a newly rebuilt and bench tested 1 barrel Rochester for a '1960-'62 261 chevy in-line six.

I closed the choke, gave it one pump and stepped on the starter. It started immediately, not even a full rotation! I slowly closed the choke.

There is however a problem. It is backfiring (flames) out the carb.

I am 99.999% sure I have the plug wires and timing correct.

Could the valves need to be adjusted?

Thanks in advance for the help.

Dwight

Or it could be a sticky or stuck valve. Has the car been sitting unused for a long time?

It sounds like your carb is too lean.

If it has the dual point distributor make certain the points are sync'd. One set operates half of the cyls the other operates the other half. I fought that on a 30 once.

Thanks for the suggestions.

Pete, the engine has been run off and on for about five years. The valves have never been adjusted, just set when the engine was rebuilt.

It has single points.

I will back out the mixture screw.

Check the spark plugs with a "megger 1000 volt" to see if they are 100% okey,an easy job without removing the plugs.If you don`t have a "megger" ask an electrikan worker.

Leif in Sweden

Dwight,

Fuel delivery is most likely the problem. The Rochester carb may require more pressure to operate on top of the engine. Are you using an electric fuel pump?

Actually, I am not using any pump. I have a nurse tank hanging above the engine compartment. So it is just gravity feed. If it requires pressure to operate, it is not getting any.

The carburetor would either:

(1) require pressure on the fuel

OR

(2) require a MUCH larger fuel inlet to be installed in the carburetor.

If the carb you have is for a 216, it is too small.

If the carb you have is for a 235, it is marginal.

A two barrel carb from a 265/283 would be better.

If you wish to stay with a 1-barrel, a Zenith from a GMC 270 (same bolt pattern as the 235 Chevrolet, but larger internally) would work better.

Jon.

Jon,

Thanks for the info.

I mistyped. It is for a 1960-62 235 or 261. I guess that is a little closer to what it should be. The engine is a 257.5 cu.

I will hook the nurse tank up to the fuel pump.

Thanks again.

Dwight

Dwight - most of the commercial rebuilders grouped the 261 carb with the 235 carb (because the 261 carb is quite rare!). 999 times out of a 1000 you would receive a 235 carb. I don't know of a single instance where someone bought one of these that was actually a 261 carb. The 235 carb should work other than for extreme high speed work.

Not grinding my own ax (sold completely out of 270 GMC carbs); but would suggest using what you have but watch the various sources for a 270 GMC carb.

Jon.

I will try to see if it is for a 261. I have one for a 292. I thought that would be too big. I will watch out for a 270.

Thanks Jon.

Good news. Bad news.

Good news: I checked the number on the base of the carb, and it is for a 261!

Bad news: I split the inlet of the diecast fuel pump hooking up the nurse tank fitting!

It's a good thing it will not run. I would drive it off a cliff!

Fuel pumps should be available.

Try Then & Now Automotive in the Boston area. I think they have more fuel pumps than the rest of the antique world combined. Nice people too.

Be careful about that cliff. There is bound to be some government regulation against it!

And be thankful the inlet split now. If it was that fragile, it could easily have split during rush hour traffic, creating a disaster rather than a mere inconvenience.

Jon.

Thanks Jon.

I guess there is always a silver lining if you look hard enough!

Update: Well, remember I wrote I was 99.999% sure the timing was right on? Wrong! After "playing with" an electric fuel pump and two regulators (1 totally stuck open from the factory, and one factory adjusted to much higher that the 1-4 psi. they promised) I got it running. Again it would idle fine but shoot flames out the carb on acceleration.

The timing was off. I loosened the bolt on the distributer and rotated it clockwise almost as far as it would go. Now there is no backfiring, no stumbling.

I am afraid to even think of it but maybe I now have a running engine?

You still may be off one tooth, if you had to move as far as it would go, MAYBE.

DALE IN INDY

Dale,

To check, I would lift the distributor and rotate it counterclockwise one tooth?

Thanks,

Dwight

YES.

As a kid we would then set it at the spot when going up a slight incline the engine would PING a tad. That would be in high gear at approx. 40 mph, and mashing the throttle somewhat. NOT saying that is best way in today's world.

I wish you well.

Dale. In Indy

Thanks Dale. I will give the one tooth a try.