Choke tube

[rhschnauss]

How does the choke tube work?
does it pass into the exhaust manifold?
When I had my carburetor rebuilt recently, Daves carburetor told me the choke mechanism was corroded. I know that some choke tubes go into the exhaust manifold and if gases get into the choke tube will corrode the choke.
any ideas?

[Mark Norris]

Yes, the choke tube passes through the exhaust balance section of the induction manifold. At startup the right hand bank of cylinders exhausts mainly through this path and exits via the left hand cylinder bank and header due to the heat riser valve being largely closed on the right header. This forces exhaust gas to heat the induction manifold directly below the carb exit and heats the air passing through the choke tube. Once the heat riser valve opens (ie. as the under hood air temperature rises) then more/all the right hand bank exhausts through its own header and exhaust lines to the rear bumper.

Fitting a new choke tube is best done (perhaps only done) with the induction manifold removed. Its a fiddly job I last did using drifts and a gentle touch with a hammer. Note its removed in one direction only as one end is larger than the other (buy a new one first then you'll see what you are dealing with).

The choke tube is connected to the choke mechanism in the side of the Holley carb. If the choke tube is fractured then exhaust gas and carbon/soot will be drawn into the choke mechanism in the carb ...this is likely what your carb rebuilder had observed.

New choke tubes, and the air feed/delivery pipes, are available from Jack R. etc. If yours is a Carter carb then I have a set of pipes going for free since Jack accidently sent me them instead of the Holley versions that I needed.

[Barry Wolk]

Excellent synopsis, Mark. I always tell people to look for the soot.

I always understood that even after the diverter opens there's a back and forth pulsing that keeps the platter below the carb hot enough to aid in atomization.

My exhaust sounds funny to me since I blocked the heat that ruins the paint on the intake.

[Mark Norris]

Yes thats right, even after the heat riser valve opens there is going to be a continuous pulsation of exhaust gas back and forth through the passage in the induction manifold ..that keeps the ribbed vaporization plate below the carb hot ..especially useful then the accelerator pump dumps gas onto it.

[Roger Zimmermann]

However, this continuous pulsation of exhaust gas can also induce vapor lock...

[Barry Wolk]

Please explain?

[Mark Norris]

I'm going to guess and say it refers to the air above the induction manifold getting very hot due to the pulsating exhaust gases and so heating the non-insulated steel fuel supply line to the carb.

Now I'll have to admit 3186 does not like to move away at lights or a junction if she has been siting there idling for a few minutes ...there is serious fuel starvation for a 10 - 20 sec and she has on a few occasions stopped altogether. I give her a boost with the electric fuel pump sometimes to try and prevent it happening ...I was wonder if it was something to do with vaporlock and that maybe I should try insulating the horizontal section of fuel line with a silver cloth tube.

Interested to see what Roger meant.

[rhschnauss]

Mark,
Where did you mount the electric fuel pump? I would like to use one to fill the carb before initial startup.

[Roger Zimmermann]

Interested to see what Roger meant.
The exhaust gas entering the intake manifold is just heating it as well as the carb. Too much heat at the carb is no good especially with the fuel available now.
The Mark II engine has a fuel pump which is rather low installed but, on the '49-62 Cadillac engines, the fuel pump is located "nicely" almost in the middle of the V where a lot of hat is radiated to that poor fuel pump. By closing the exhaust passage under the carb, less heat is generated in that location, reducing the possibility to induce vapor lock.

[Mark Norris]

Mark,
Where did you mount the electric fuel pump? I would like to use one to fill the carb before initial startup.

The common location to fit an electric backup/priming fuel pump is on the inboard vertical surface of the driver's side chassis rail as it passes over the rear axle. You'll find the metal fuel pipe runs there. So this means cutting a section out of the fuel pipe and letting in the electric fuel pump with a rubber section before and after. Mine has an armored wire run from below the dash where a switch is mounted and fed from a wire run from the fuse box (fused power when the ignition is on, there are several spare fuse slots you can use, I think my car has a 15 amp fuse there and its blown once or twice). For extra safety this should be a momentary switch (ie. does not naturally stay in the "on" position because it has an internal spring to push it back to "off") so should there be a severe accident leading to an engine fire then the pump won't keep feeding it with gasoline.

The pumps themselves are easy to find (you don't need anything powerful, its just to push gas to the float bowl through the filter and mechanical pump). Something low pressure like this will do;

https://www.ebay.com/itm/192699559741?hash=item2cddc9ef3d:g:OLYAAOSwCd9dHyS G

FYI - 4 psi is equivalent to a 10ft tall tank of gasoline that is full to the brim.

[rhschnauss]

Mark we used to have a DB5 Vantage. Very powerful.

[Mark Norris]

I think the sound is the best though. I've had mine for 40 years ..they were just old cars then.