Rochester 4GC question

[Mike]

I have this carb on my 56 - 365 c.i.d. and have been trying to trouble shoot an idle problem which has all the indications of a vaccuum leak, although it seems impossible to find  (have checked all the fundamentals on this topic).  Car is in winter storage so I cant check this right now, but is it possible or common for a vacuum leak to come from the small vacuum chamber that goes to the choke housing to draw that small choke  piston?  If so, how do you keep excess vacuum from drawing past the piston? There are no rings or anything.  Would appreciate any comments or thoughts. Thanks.

Mike

[Geoff Newcombe #4719]

Mike,
Since no one has answered your question yet I will put my 2 cents in.  That tube going to your auto-choke is to carry heat, it does NOT (should not) supply vacuum.  The heat it carries works on the coil spring in the choke housing and it then opens the choke as the engine warms.  If you have a vacuum leak it is elsewhere.  Best of luck.
Geoff N.  

[Porter 21919]

Geoff,

I have 66 Carter carb I rebuilt for my 429, it had an electric choke installed on it years ago, no tube for the heat anymore.

Lo and behold I had the engine running and doing some tuning and the hole where the heat tube would get installed was pulling vacuum so I had to plug it.

FWIW,

Porter

[Geoff Newcombe #4719]

Interesting.  Would that have been caused by a leak in the intake manifold where the tube used to be inserted?  My old 66 had an aftermarket choke heater where the heat tube was fastened (clamped), as I remember, to the exhaust manifold.  I didnt bother to return it to the factory setup since I live in Florida and my intention was to sell the car anyway after a few other things were fixed.
Geoff N.

[Porter 21919]

Geoff,

All that is left is the bolt on the manifold where some type of housing was located for the heat tube.

I simply removed the brass fitting on the carburetor, used a ten penny nail head cut 1/4 long, a little silicone and reinstalled the brass pipe fitting. End of vacuum leak, the engine did idle a little smoother.

Porter

[John Washburn]

Mike,

On the outside of the choke body, at the bottom of hole the piston fits into, is a plug. Sometimes this plug will leak, due to age or rust. Sometime the will come loose and leak or fall out. The vacuum leak is just enough to make the 56 run like crap. Take a close look and make sure this is not the problem. It 180 degrees from the top so is extrememly hard to check unless you take the carburetor off, or it has a big in it (or fallen out).

Good Luck

John Washburn
CLC #1067

[Mike]

John: That plug idea sounds worthy of investigating - I will try that in the spring.  What about the fit of that vacuum piston in its bore? Seems like a loose fit - could vacuum perhaps be pulling past that piston, even when its in a closed position?  I suppose I could try plugging that entirely just to see if the engine idles better.  What do you think?

[John Washburn]

Mike,

I dont think slop between the piston and the bore would cause the problem. I would check for a vacuum leak. It might be between the choke body and the carburetor, if not the plug as I suggested earlier.

Let us know once you find out.

John Washburn
CLC #1067

[Porter 21919]

I think John Washburn would be right, there is no vacuum issue within the choke piston. There is a nice gasket that seals the plastic choke housing on the Carter carb.

I was quite surprised myself to find that intricate brass jewelry piston within my 66 Carter carb choke housing.

One winter back in 1989 my 66 429 didnt start and I authorized my garage to fix the problem, as far as I know the heat tube was always leaking vacuum after they installed the electric choke.

I was quite surprised to learn after I rebuilt the carb last year that the choke mechanism pulled ported vacuum from the carb.

Porter ( close personal friend of John Washburn )