The Stovebolt Site Forums The Shop Area Engine Shop Carburetor gasket

Looking to re-attach carb. My question is, is there supposed to be a gasket between insulator plate and intake? I have new gasket for between insulator plate and carb.
Motor is 59 235 with original rochester. It's been two years and I just can't remember. Should have taken more pictures!
Thanks
Brian M

Brian,

A gasket is not needed between the insulator plate and the intake manifold, but one will not cause any problems.

Be sure to align the slots on the inside of the carb-to-spacer gasket such that the slot is under a hole in the base of the carburetor (I hope that someone will post an answer with the correct terms for the slot in the gasket and the hole in the base of the carb).

If you use a gasket like this one, ...out alignment of the gasket to the carb.

Tim

Keith Hardy's wonderful site has a Rochester 'B' rebuild and troubleshooting guide. They merely call the holes 'vacuum ports'.

I have never understood why this seemingly important connection wasn't gasketed. The book shows no gaskets. I have used gaskets on both sides of the spacer before...maybe this wasn't necessary.

In the Rochester book I mentioned, if a leaking power piston is suspected which would result in a rough, rich idle, they instruct you to pull the carburetor and flip the phenolic spacer over, thus covering the vacuum ports to the power piston and taking it out of the equation.

They tell you that if the idle 'smoothens' out, you have found your problem! I didn't know 'smoothens' was a word. Google spell-check doesn't know about 'smoothens' but it does know about 'smoothies'.

Stuart

Maybe there is a mixup here. The power piston enriches when the vacuum is low. High vacuum conditions keep the power valve closed.

Fred,

You're right! Maybe I'd better read that paragraph again as what I posted makes little sense.

Stuart

Fred,

I think I understand the theory after reading several Rochester booklets.

Vacuum does hold the powervalve (piston) closed. Under a load with the throttle open, vacuum drops and the power valve opens and gives a little extra fuel to maintain speed...we all know this.

If the powervalve is suspected of leaking, Rochester suggests turning the phenolic insulator over, thus blocking the vacuum passages and rendering the valve 'dead'. Your question was that this also means the valve opens and dumps extra fuel into the system and might make the test NFG.

With the car idling, the throttle plate is closed and fuel is delivered through the idle circuit below the plate.

The fuel the powervalve adds is delivered above the throttle plate, in the venturi, and isn't a factor when the engine is idling with the throttle closed. That circuit is dead until the plate opens and air flows to syphon fuel through the tube.

Does that make sense...it does to me. The thrust of this test is to see if a leaking power piston is feeding fuel through the carburetor into the small vacuum port at the base of the carburetor. Flipping the phenolic spacer over blocks those ports mechanically and can answer that question.

Stuart

Well I’m not gonna take credit for the wording of this definition but; “A gasket is a mechanical seal that fills the space between two objects, generally to prevent leakage between the two objects while under compression or vacuum. It is usually desirable that the gasket be made from a material that is to some degree compressible such that it tightly fills the space it is designed for, including any slight irregularities.”

Now then, I will lay claim to making the same statement as above many times in my own words when the subject came up. I have always approached two machined cast iron surfaces seperated with a chunk of hard plastic (Bakelite) as a situation that needed “gasketing.”
There is absolutely no way that insulator is going to conform to the machined suface to form a perfect seal and since a vacuum tight system is the objective here I have always used gaskets with the insulator, top and bottom. Some are going to say that we can live with a slight vacuum leak between the insulator and the Manifold and/or Throttle body but I ask, why should we have to live with ANY vacuum leaks at all!
I’ve Never had a problem with installing a carb this way and don’t see why there would ever be a problem with it.
Also keeping a path to a vacuum port in the base of a carburetor open in any situation is only common sense if one fully understands the system.
Denny Graham
Sandwich, IL

Denny Graham,

Your last sentence...what does it mean?

Stuart

I'm just saying Stu, that the subject comes up all the time about some of the gaskets covering the vacuum port in the bottom of the carbs and it just takes a little common sense to see how to avoid that.
The original question was about using a gasket between the intake, insulator and carb and then the thread evolved into a discussion about trouble shooting the power piston function. I was simply addressing the gasket question, I'll leave the carburetor theory and trouble shooting up to yous guys.
Denny Graham
Sandwich, IL

Thanks for answering my question. The gasket that they gave me from the guy who rebuilt my carb does block the holes however I have the original gasket and it is usable and does not block them. I also assume the notches on the spacer block align with port or hole on carb bottom?
Once again thank you.
Brian M