1939 Buick Series 40 pistons and carburetion

When I rebuilt the engine for one of my '39 Buicks I could only find 1941 Fireball type pistons. They fit and everything is fine just as the machinist said they would. But, I have had problems with three rebuilt 1939 carburetors. I'm wondering, does the jet size in the carburetor have to be modified to somehow equal 1941 compression? Would the '39 carburetor with '39 jets be too lean for 1941 pistons? Have tried everything, and nothing solves the problem....rebuilt the distributor, etc. Of the three carbs only one will work and it runs lean when the car is cold. The person I got it from told me he always puts larger jets into his carburetors.

Check a sparkplug for color, White is lean, tan is correct, dark brown or black is rich.

Bob Engle

The car runs rich, but coughs back through the carb until it warms up

Having difficulty with three different rebuilt carbs may indicate there is an issue external to the carb.

Your initial post suggests that you have more than one 1939 Buick. Assuming you have one that is running correctly, I would swap the known good carb onto the engine with the issues. If it runs well, then take one of the carbs and go completely through it. If the known good carb does not solve the issue, then leave the known good carb on temporarily for testing, and find the real issue.

Jon.

Thanks. Yes, I've wondered the same, even though one of three rebuilt carbs runs okay when the car is warm. I've had the car in a restoration shop where they tried a new coil, rebuilt the distributor, changed the vacuum advance, played with the timing and nothing made much difference. The first carb was a complete rebuild by Daytona Carburetor and it wouldn't work at all. It came off of the blue car that I had put on NOS thirty-five years ago. I've owned the blue car 51 years. But, it had the old Bill Hirsch stuff in the gas tank and that came loose with Ethanol and ruined the whole gas line. I've wondered if the old carb still has some of that gum in it. They rebuilt the original carb that came off 35 years ago for the blue car and it runs wonderful. I bought a second rebuilt carb and it did only slightly better. Then I got one from Siebold and it does okay when the car is warmed up. He said to me once that he always uses larger jets. That is what has me thinking that possibly a 1941 Carter carb has different jets because the pistons are different....they dished with a shoulder instead of half flat with a built up dome on one side. The 1941 engine did have higher compression and more horsepower than the 1939, though the horsepower was minimal. It went from 107 hp to 110 hp with a single carb in '41. This engine has been totally rebuilt. I used 1941 pistons because I couldn't fine 1938-39-40 pistons. I've know others who did that. Thanks for your input. I just don't know where to look otherwise. Your switch idea is a good one.

If you are running rich, larger jets will make it worse. Put a vacuum gauge on the intake and see if you have good consistent vacuum. There are some excellent web sites explaining how to interpret vacuum gauge readings.

Bob Engle

Okay thanks. There aren't a lot of places to lose vacuum on a '39 Buick. I'm thinking maybe the carb base. Otherwise there is the windshield wipers and the vacuum advance. I wonder........maybe a tiny hole in the tiny metal vacuum line to the vacuum advance. That is a thought.

when you put a vac gauge on it tells you much more than how much/little you have, and if you have an old Chiltons manual, they have about 20 pics of the gauge telling what troubles are indicated based on the gauge needle, flipping back and forth, low reading ETC It can also be used to set timing for dist. Good Luck

Okay thanks. There aren't a lot of places to lose vacuum on a '39 Buick. I'm thinking maybe the carb base. Otherwise there is the windshield wipers and the vacuum advance. I wonder........maybe a tiny hole in the tiny metal vacuum line to the vacuum advance. That is a thought.

I must say that before my daily driver cars that now have totally computer controlled everything, if things weren't going quite right, the first thing I would always grab was a vacuum gauge.

Amazing how much you can tell about the engine from such a simple gauge.

Danny

Danny: So true! I got a vintage 50s vac gauge off Ebay and have mounted it under the dash in my 1940 so I can monitor it under way as well. When I was a callow youth I had a classic bullet-shaped one from J.C.Whitney mounted on the steering column of my 66 Fairlane. My friends thought I was nuts but it taught me a lot about auto engines.

Cheers, Dave

Earl, I assume you have Carter carbs.

Any chance you could put a Stromberg on just for test purposes ??

Just my opinion but I think it's a better carb.

Danny

Just noticed I've cracked the 10 year mark !!! Yea me :)

No Danny, no Strombergs on the place. In fact, I never liked the Stromberg's on my '42 and '48 Buicks I used to have. Hard to start hot. By the way, we did a vacuum test and got 18 but I don't know if there was an wavering. I'll get my buddy to do it again. Earl

I am new to this forum and work mostly on motorcycles. Popping through the carburetor is an indication of a lean condition on motorcycles an is usually caused by intake leaks. Very common on bikes. We trace it by spraying almost any liquid around anywhere a leak could occur and listening for changes in idle speed. Carb bases, intake manifold mating surfaces, vacuum lines...

I use carb cleaner mostly because it is usually handy and with a red hose on it you can get pretty directional. Don't use the carb cleaner around painted surfaces. An unlit propane torch will work also and so will starting fluid. Use caution of course with the flammable stuff.

Guy W.

Silver Spring, MD

The car runs rich, but coughs back through the carb until it warms up

This statement indicates either:

(A) multiple problems

OR

( problem(s) other than carburetion

If the carburetor is at fault with the above symptoms:

(A) the automatic choke may not be correctly set, or may not be functional causing the coughing until warm

( running rich could indicate:

(1) low vacuum causing the power circuit to function all of the time

(2) excessive fuel pump pressure

(3) defective float/fuel valve or mis-adjustment

(4) incorrect metering components

As to the comments concerning Carter/Stromberg:

If the engine is perfectly stock and you had one of each carb that was correctly rebuilt you could have a mechanic install one when your back was turned, seal the hood, and drive the car a 1000 miles. Then go back to the mechanic and have he/she install the other when your back was turned, seal the hood, and drive another 1000 miles. You would not know which was which, or even if the mechanic actually changed carbs after the first 1000 miles. Both are excellent carbs.

The Carter is slightly more difficult for an amateur to adjust.

If the engine has been modified, the Carter is easier to retune to a modified engine.

Jon.

Thanks guys