Carburetors and Vacuum advance

[buick840]

I have a 1934 8/40 Buick.

I have been having problems with the Stromberg carby for some time. It has been worked on a lot over the years and has a new rebuild kit in it but it still leaksl.

I have an option of getting a Brand new Stromberg carby which will bolt straight on but it does not have the port for the vacuum advance.

If I use this carby any ideas if the vacuum advance is not functioning how that will affect the performance/running of the engine?

I've removed the base of another original carb and inspected the vacuum advance port. It has an opening of aprox 7/16 at the front and from the base there is a 1/8 hole into that chamber.

If air pressure is applied the air will flow freely through the front fitting and out that small hole but if you block the small hole air will not go through to any other area.

The thing that stumps me is that on both the old carby and the one on my car that small hole is covered by the gasket and there is no groove in the base for any air to be sucked through.

Am I missing something here.......how does vacuum/air flow occur if this hole is blocked.

Is there another inlet to that front opening that I've missed.

Is it necessary to drill and tap the base of the new carby for vacuum or is is it more involved than that?

I have seen aftermarket spacers with Vacuum ports...would that give the correct amount of vacuum rather than going direct from the manifold

Appreciate it if anyone can offer some advice.

Edited August 11, 2013 by buick840 (see edit history)

[JohnD1956]

What little I know is that there are two types of vaccum. Manifold is a steady vaccum that comes from above the throttle butterflys ( plates). Cant think of the term for the other off hand, but it's source is below the throttle butterflys. The second varies faster due to the location of the input under the throttle plates. I believe both get their power from the suction in the carb and manifold as pistons pull the fuel mixture into the cylinders, but that can be wrong.

If I understand right, your vaccumport goes straight thru the base from the outside to inside the hole for the throttle plate? But is it above or below the throttle? That will impact how the vacuum advance responds to throttle activation.

Also is it possible to just switch bases? If so you may want to test the flatness of the surface of the used base before putting it on the new carb. Also if this is a NOS carb, you may want to consider a new gasket set for use with ethanol.

Edited August 11, 2013 by JohnD1956
fat fingers and no paitience with the smart phone (see edit history)

[JohnD1956]

Corrections to my previous post. Manifold vacuum is drawn from below the throttle. The other vacuum is" ported vacuum" and this is drawn from above the throttle. Also I misspelled vacuum.

as to your original question,how lack of vacuum advance impacts performance, it appears one side effect would be a hotter engine at idle. Another may be lack of pep from a dead start.

According to my 69's manual, most Buicks before now had manifold vacuum to the vac. advance. Advance helps give the cylinder charge more time to burn. It does not say why this then results in better heat transference to the coolant at idle but in 1969 they changed to ported vacuum at idle for all Buicks, and compensated for hot engine at idle with a thermo switch that changed the vacuum source if a 69 Engine hit 220 degrees at idle.

Without vacuum advance you would still have advance because there is a mechanical advance unit in the distributor. The faster the engine runs the more that mechanical unit would advance the spark. So you may not even notice much in the lack of power on the take off. You may actually see it later in the higher rpms when you do not have the combined advances working in your favor.

Edited August 11, 2013 by JohnD1956 (see edit history)

[Pete Phillips]

This being a 40-series, is it a downdraft carburetor?

[buick840]

Pete, I've added some photos and you can see that the 34 8/40 did use the downdraft Stromberg. They have not altered much at all over the years and new ones are being produced again today.

Thanks for the reply John.

I had another look at the old carby to see if I could trace where the vacuum may come from.

It's a bit hard to tell from the photos as there is a lot of crud still on the base but it appears (looking from the front of the carb) that there is a chamber leading off to the right from above the vacuum port. I assume it leads to the right idle tube and that opens through a small hole above the throttle.

So I'm assuming that this is the point where vacuum is drawn for the Vacuum advance.

That being the case then there isn't an option to alter the new base as it doesn't have that casting.

I've included a photo of the new carb I have been offered compared to the an original.

It will bolt straight on and work with adapters for the throttle and choke levers but won't have vacuum advance.

I could look at changing bases but that's not a straight forward change and I'd prefer to leave the new carb as is if I can get away with it.

The spacers with a vacuum port built in would seem to draw it from below the throttle, wouldn't that be the same as tapping into the manifold from say where the original wiper vacuum was taken.

Back to my original question.......how will it effect performance if I go with the new carb with no vacuum advance?

Edited August 12, 2013 by buick840 (see edit history)

[old-tank]

"Back to my original question.......how will it effect performance if I go with the new carb with no vacuum advance?"

Poor fuel economy, engine will run hotter, less power at part throttle (WOT is unaffected). Since most vacuum advances will advance about 10* you can set the base timing an additional 10* BTDC, but that can make for hard starting and bigger load on the starter, in addition to having run-on after shutting off. Just hook the vacuum advance to any vacuum source (I never could tell any difference in performance using ported or non-ported vacuum).

As tempting as it is to use a pretty new carb, it was not made for your engine and may have other issues (to be discovered).

Willie

[buick840]

Thought I should update this thread with the final outcome.

We used the original base with the vacuum advance port, swapped the accelerator linkage so as to use the original linkage setup, added a bracket for the choke cable and had it tuned by a carb guy in Melbourne and the result is the brand new carby is on the 34 and working like a charm.

The original had got to the point where we could not stop it leaking no matter what. The new carb has been on for some time now and no leaks....starts and runs like it should .........very pleased.

[danhar1960]

Love a happy ending.

Danny