Pontiac 1930 2door Big Six with Marvel Carburetor problems

[Guest jos.daalmans]

Hi there,

Here a message from the Netherlands.

A friend of mine owns a Pontiac 1930 2Door Big Six. He bougth it out of a crate en assembled it again. So far, so good.

The engine is equiped with a Marvel Carburetor. The fuel was running out of this carburetor, so I fixed the floater valve and floater valve seat. I cleaned the carburetors interior, made new gaskets etc. The fuel stays now where it has to bee.

Starting the engine is a big problem. Only playing with the throttle keeps the engine running (due the injected fuel) a bit higher in revs than idle. Idle without playing with the throttle lets the engine stall.

The Marvel carburetor is pre-adjusted according the maintanance manual with 1/8" clearance between the air valve and the carburetor wall/body.

The airvalve moves smoothly, and comes back slowly.

Here some pictures:

https://picasaweb.google.com/101253567782863215031/MarvelCarbureter?authkey=Gv1sRgCKavwJrx1sW93AE

The inlet and exhaust manifold where already mounted on the cilinderheads/engine and I'm afraid, there may be a vacuum leak.

After playing with the throttle, the carbureter is at the inner (bottem) flooded with fuel.

The two front cilinders hardly become warm, the last 4 cilinders get a bit warm.

The nut and special (unequal) clamp between the exhaust manifold "pipe" for cilinder "One" and the inlet manifold "pipe" for cilinder "One and Two" are missing, Same story at the end of the engine at cilinder "Six and Five" (assuming the front cilinder is "One" and the rear cilinder is "Six").

Compression on all cilinders seems to bee OK, according the "equal" heavy sound when trying to start the engine.

Does anyone know the Ignition firing order? I need to check this out to prevent this causing even more trouble. Normaly (most) a six cilinder firing order is 1-5-3-6-2-4.

I don't think it wil be that "rare" 1-2-3-4-5-6 sequence.

Any suggestions?

Thank you in advance and take care of your Pontiac,

Jos

[Guest]

153624 is correct. The fact that your manifold is not tight is a vacuum problem. If "Carbking" sees this and replys his technical information is certainly better than mine. Apparently the twin head Pontiacs had a reputation when new as been difficult to start in cold weather. I have driven mine daily for 53 years putting 499,000 miles on a 99,000 mile one owner car. For 20 years I drove it in Manitoba with winters as low as -54 degrees. It always starts and starts well. If everything is set right (correct timing, no vacuum leaks, float level correct and idle and main jets clean) On a cold start I pull the choke out full, step on the starter and the engine turns over about three or four times (unless you have the new wonderful ethanol added to your gasoline) as soon as the engine fires push the choke in until the engine runs smoothly. If the engine is warm it usually starts with no choke.

Seeing you pictures I would suggest that you should take the float bowl cover off, remove the three screws on the bottom and separate th two pieces of the carb. Unscrew both jets and make sure they are clean (don't push a wire through them). According to the manual, the top of the float (measured farthest from the float valve) should be 11/32" below the edge of the float bowl. As I have modified a later needle and a couple of years ago made a new float I set the float so that the level of the gas in the float chamber is 1/32" below the tip of the idle jet.

You problem at this time is likely a combination of vacuum leak, dirt in the carb and wrong float level.

[Guest jos.daalmans]

Hi Tinindian,

Thanks for the reply, we will remove the manifold and check if the manifold is flat and we make a new gasket. The carbureter was already cleaned and checked, al the channels transporting fuel.

The needle I made is originally the needle for an Triump sportscar. This needle has a rubber tip and closes perfectly the fuelstream when the floater is on level. The needle is connected to al little screw, rings and a bush, so the floater can lift up the needle if fuel is needed and pushes the needle down if the fuel level is OK.

Also I need to make two little "bridge-rings" to tighten the manifold to the cilinderheads at cilinder one and six.

Thanks again for the reply and keep you informed,

Jos

[nickelroadster]

Make sure that your spray nozzle is clear. You can also take the top of the float chamber off and check the level of the fuel and compare that to the level of the top of the nozzle. It could be helpful to mark the height of the nozzle on the side of the carburetor with a magic marker and compare it to the height of gasoline in the float chamber. This way you can check the 1/32" clearance quite closely. It sounds like you are having trouble getting gas. Certainly a vacuum leak could make you a bit lean. I don't think you have an alcohol problem over there but over here I sometimes have to drill out the nozzle about two number sizes to make the mixture rich enough.

[Guest jos.daalmans]

Hoi Nickelroadster,

Thanks for the reply. What I found out also was that de cilinder One and Two sparkplugs are completly dry. Seems to be that the cilinders dont get any mixture, but only "false" air. The other sparkplugs are drowned, so probably the wrong firing order. I'll have to check that tomorrow first of all.

Because the carbureter itselfs is also drowned, I assume that there is enough mixture available for the cilinders, but again, whats happening with it (spark on-time etc).

Take care of your Pontiac,

Jos

[nickelroadster]

The first thing I would do is check for proper valve timing even if you don't suspect a problem it will allow you to eliminate that. After that you should be able to figure out the firing order by removing the spark plugs and feeling for compression. Start at no. 1 and when you have it a top dead center feel at no. 6. If that is not right then that would not be the order and you could try your next choice until you have it. You should be able to sort it out this way.

[Guest jos.daalmans]

Hi,

This evening we tried to start the engine again after recleaning the carburetor en setting the right fuel level in the bowl.

The engine started, run a few seconds and it was over again.

The firing order was OK.

I made some pictures of the manifold en someone did some homework, but I'm affraid it was a bad job.

https://picasaweb.google.com/101253567782863215031/201210Pontiac?authkey=Gv1sRgCIiCsr3t-ZGqZA

The gasket needs to be replaced. Can I buy this gasket anywhere? or wil it be a self made gasket .

I take the manifold to a company specialized in rebuilding engines and have the manifold regrinded. The surface where the gasket was, is looking bad and this causes the leaking.

Keep you informed and take care of your Pontiac.

Jos

[nickelroadster]

Often times an intake leak can look like you have carburetor problems when you don't. looks like you got a lot of suggestions that were kind of off the mark. Hopefully you can use them in the future. Your gasket is probably available from someone like Olsens or Lamberts but in the Netherlands it might be easier to go ahead and cut your own. Good luck.

[Guest jos.daalmans]

Hi,

Past weeks, I fixed the carburetor by using two-component material. Its now leak-free. I had to make the gaskets for the carburetor myself.

https://picasaweb.google.com/101253567782863215031/Fotomap02?authkey=Gv1sRgCNigidbq-t2EQQ

The manifold is cleaned and was looking good.

A new intake gasket (almost 3mm thick) was ordered in the USA and we mounted everything again.

We tried to start the engine (with choke), a few times it wanted to start, but it stalled after a few seconds, no result.

Then we tried with starting and playing with the throttle and it it stalled again.

All the spark plugs out, 1,2, and 5 were wet, the others black. I've cleaned them, fitted them and no result.

Then we took an old spark plug to check if the ignition was working. The sparkplug was laying on the cilinder head. I turned the air valve adjusting screw one turn CCW (as a kind of last attempt). We started the engine en suddenly it was running (in this case on five cilinders:D). We stopped the engine, fitted the other spark plug lead and started again, gues what, running like a sewing machine:rolleyes:.

After a few minutes, the radiator started to leak, so I made this short movie.

When operating the throttle, the Engine kept in, but came to the higher revs and was running smooth.

- Question: Does this mean the mixture is to lean or to rich?

After filling up the coolant, we have tried to start the engine and no running at all. After a short time we tried to start again and it runs only whem playing with the trottle.

- Question: do you need to choke the engine also when its on normal running temperature?

Thanks in advance,

Jos Daalmans

The Netherlands

[D Yaros]

Application of the choke should only be required when the engine is cold.