drlowz

1927 Chevrolet backfires through carb

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Hello all, new to the antique car world.  We purchased an all original 1927 Chevy with the 4 cylinder motor.  It idles ok, but upon giving gas backfires.  I pulled the plugs and they were all black.  I think they may be wrong plugs.??  Dont seem long enough.  Well any help will be appreciated.

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Backfire through the carb suggests either valves (or cam) or lean.

If it has even compression and idles OK it is probably running lean. So carburetor rebuild maybe.

Might be something as simple as a fuel filter.

Edited by JACK M (see edit history)

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Here are a few thoughts.  I don't disagree with the suggestion that your problem may be related to the fuel/air mix, but I would start on a backfiring problem with the basics.  First, ensure that spark timing is correct.   This car probably has a manual spark advance.  Set the timing right at top dead center with the manual advance in full retard position.  Start the car in full retard position, then advance the spark when the engine is under load.  If you have an owner's manual, it may explain this part.  You might also check the valve lash; I think it should be around .006 to .008 inches with the engine hot.  Clean plugs are always better than dirty plugs, but the cause of dirty plugs is rarely the plugs themselves.  Good luck!

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An all original 27 would have a Carter RX-0 or RAKX-0 carburetor and a vacuum tank for fuel delivery and not an electric pump. Assuming that is the case with your car, here are the things I would check in order of least difficulty. Does the carburetor still have the flex pipe from the exhaust pre-heater installed? The RX-0 carburetors like hot air. Without this pipe, the motor will be very cold-natured and tend to spit and pop from the carburetor when you touch the throttle. The only adjustment available to you would be the idle air screw. If it's idling properly, chances are it's adjusted ok. Also those carburetors have no accelerator pump so transition off the idle circuit takes some feathering of the throttle. Some hesitation is normal when you "give it the gas". Next thing I would do is disconnect the fuel line from the carburetor and make sure it has good flow. Then I'd drop the float bowl and check the float level as well as overall cleanliness. If the fuel system checks out, I would then look at the condition of the plugs, plug wires, check the ignition point gap, and timing.

 

Cheers!

 

Edited by rsb (see edit history)

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It is missing the hot air pipe?   Is there a way to aftermarket this?  Seems like might be missing part for exhaust.

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The hot air pipe is simply a flexible metal tube that draws air from the air filter you pictured, around the outside of the exhaust pipe, and into the carburetor. It is routed behind the motor, above the starter, and into the carburetor. It is available as a reproduction part. The air filter you pictured is basically an empty tin can with louvers around the bottom. The idea is the vortex of air will spin any heavy debris out. I think the vortex was only in the designer's head.

28Carb.jpg

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Update, I've found out it has a 28 head on the 27 motor if that makes any difference.  Still backfires when applying throttle.

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Let's think about this. If it backfires thru carb, it must mean an inlet valve is open when there is ignition in a cylinder or the inlet and exhaust manifolds are connected through the valves and the hot charge gets into the inlet manifold.

 

A stuck exhaust valve will let hot gas back into the inlet manifold when the inlet valve opens and a shot thru the carb. will follow. I know this from experience. Compression test will confirm. An indication might obtained be from the sound while cranking on the starter. An open valve will allow the engine to speed up when that cylinder is on the compression stroke, so the pitch of the cranking sound will change.

 

A bent valve stem might cause a stuck valve. Bent valves can follow a leaking head gasket: water in the cylinder doesn't compress much. You might find water in the oil in this case (or oil floating on the water in the sump).

 

Valves often stick in a well used engine sitting for a while.

 

Valve timing not right (early). Has it been set up correctly? Has it changed?

 

Inlet manifold leak leading to lean running. Vacuum wiper, its connections and tube OK? Connections and hose to vacuum tank and the tank itself OK? Inlet manifold gasket OK, nuts tight enough? Carb fitting to manifold OK? No huge leak due to wear around throttle shaft?

 

Overly lean idle. Later carbs had an accelerator pump to help with this condition.

 

It is my understanding that plugs are mostly black these days because of the lack of lead in the fuel. As long as the soot is not oily or wet it is OK (assuming the plug is in good condition and the gap is right).

 

It seems to me your engine has had a fair bit of attention in the past - e.g. the head has been changed. Did the repairers know what they were doing? Remember, these cars were worth nothing for a significant proportion of their life so things that we would frown on now were done (often by "bush mechanics") to keep them more-or-less running (thank goodness, else they would not have survived). So don't assume anything. Start checking the easy things: vacuum leaks and compression.

 

Keep us informed of your testing and results.

 

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Check out http://www.earlychevyparts.com for the flex pipe.

 

Cheers!

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Ok guys been a while since I posted.  We have the car running good now.  Believe it or not when purchased the spark plug wires were wrong and never thought to check..  

 

Now my question is for the preheater, do I need one for a 27 since i have the 27 block or do I need the one from a 28 since it has the 28 head on it....?

 

HELP

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I take it the other plug wires weren't wire at all but carbon?

Drive the car.......if it performs well forget the air heater.

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