Heat Riser and cross over heat slots?

[Rob M.]

Question:

If I use freeze plugs to plug the heat slots in the intake manifold (under the carb.) of my 327/300 and the heat riser valve is working properly would there be any issue with back pressure with the riser closed? Or, do I worry too much?

[Joe L.]

Re: Heat Riser and cross over heat slots?

Question:

If I use freeze plugs to plug the heat slots in the intake manifold (under the carb.) of my 327/300 and the heat riser valve is working properly would there be any issue with back pressure with the riser closed? Or, do I worry too much?

Rob-----


No problem will be created. However, if one blocks the exhaust crossover passages in the intake manifold (e.g. using intake manifold gaskets with blocked cross-over passage), that's a different story. In that case, one has to block open the heat riser or eliminate it and replace with a spacer.

[Rob M.]

Re: Heat Riser and cross over heat slots?

Thanks Joe. No new gaskets, just freeze plugs.

[Timothy B.]

Re: Heat Riser and cross over heat slots?

Rob,

To add to Joe's post I would suggest using small tapped plugs if the intake is off the motor. If it's on the motor then the small freeze plugs is the way to go.

With the slot plugged on my 67 300hp engine, the only thing I noticed at slow idle 550-600 just after the choke fully opens the idle will search up/down about 50 rpm. This only lasts about two minutes and it's because the temperature in the intake manifold is not hot enough for good vaporization of the gasoline.

I believe with the hot slot active you will not see this but with today's ethonal gas we don't have much choice and the inactive slot really helps keep the heat away from the carburetor body. If you used some gasoline without ethonal I bet you would not have any problems with the hot slot but the described warm up condition is not hard to live with. If you increase the idle speed you can cover this up but after the car warms the idle will probably end up at 750.

Check out the pics in this post

https://www.forums.ncrs.org/showthread.php?t=71175

[Rob M.]

Re: Heat Riser and cross over heat slots?

Tim,

The manifold is still on the car, so I will use the freeze plugs. I checked the post out and like how you did yours - looks nice.

The closest non-ethanol gas to me is 100 miles away (not working). Thus, I want to plug the heat slots.

Thanks again

[Timothy B.]

Re: Heat Riser and cross over heat slots?

Rob,

If the non ethonal gas is a long distance then by all means plug the holes with the plugs. You will be amazed how much cooler the carburetor feels with the slot plugged.

[Joe L.]

Re: Heat Riser and cross over heat slots?

Tim,

The manifold is still on the car, so I will use the freeze plugs. I checked the post out and like how you did yours - looks nice.

The closest non-ethanol gas to me is 100 miles away (not working). Thus, I want to plug the heat slots.

Thanks again

Rob-----


I would plug the heat slot regardless of the use of ethanol-blended fuel, or not. I plugged mine LONG before ethanol was widely used in gasoline. In addition, GM ceased to use the heat slot design effective with the 1970 model year.

[Stuart F.]

Re: Heat Riser and cross over heat slots?

Many of the late 50's early 60's slots got all plugged up with carbon any way rendering them useless. It was a common maintenance item to scrape all the carbon out of the slot. I did it many times as part of a carb overhaul, or as they used to say; "boil out the carb".

Stu Fox

[Rob M.]

Re: Heat Riser and cross over heat slots?

Thanks all for the push!

I've been meaning to do this project for a long time and I have a couple road trips planned. I want to avoid any chance for issues.

Cheers

[Timothy B.]

Re: Heat Riser and cross over heat slots?

Rob,

Just out of curiosity, have you had any problems up to this point with the hot slot active.

Prior to plugging the intake on my car I can't recall any problems with the heat from the slot. The carburetor temperature is much cooler though..

[Rob M.]

Re: Heat Riser and cross over heat slots?

Tim,

The only issue I had was coming home from Lake Tahoe (Regional) crossing the high desert near Bishop. The temp was 105-107 and if I stopped for gas or lunch, it was close to vapor lock.

But I'm hoping that this will help to prevent that from happening again.

[Joe L.]

Re: Heat Riser and cross over heat slots?

All-----


Carburetors live a much longer life on engines without the heat slot. The heat slot is probably the #1 destroyer of carburetors. In addition, it often renders their major components unsuitable for rebuild due to warpage or other damage.

[Timothy B.]

Re: Heat Riser and cross over heat slots?

Joe,

I agree that having the slot plugged is best, that change makes a big difference in the carburetor temperature. On my 300hp engines, I have tried to set idle at 500-550 rpm and the heat under the throttle blade helps with vaporization especially during warm up.

Rob,

I am not sure if your heat soak condition after a long drive will be any better with the slot blocked. On my 67 the carburetor still gets very hot after a drive, it's today's gasoline that seems to be the bigger problem IMO.

[Stuart F.]

Re: Heat Riser and cross over heat slots?

Tim;

I assume you are still running a phenolic spacer under your L-75 AFB, Right?

On my L-76 which normally does not have one, it made a difference of some 35 to 40* which, along with all the other heat isolation steps I've done (and described before), practically eliminated any heat soak/percolation problem. It still smells up the garage some, but no where near what it used to.

It will be interesting to see how the 3461S does in comparison. I'm still debating on whether to transfer the Tomco inlet Valves to it from my 3721SB now on the car. They flow and seal better than the needle/seats, and allow me to lower the floats about 1/16" - all of which may have helped some. I'm a big fan of my electric choke as well, as you know, for driving. That clean air tube is a big source of heat. Having to do away with the steel tube from the fuel filter to the carb (replaced with hose) probably has little effect, but it covered all bets.

Stu Fox

[Timothy B.]

Re: Heat Riser and cross over heat slots?

Stu,

On my 63 the gasket, spacer and shield is under the carburetor and the slot is plugged. I used a traditional needle and seat without any problems and set the float with a 5/16 drill bit just like the 63 shop manual. I have not had any problems except heat soak after shut down which we can't avoid. IMO, the smell in the garage will be the same with the 3461 as it's vented the same as 3721.

Correct me if I'm wrong but the electric choke is still connected to the vacuum port on the carburetor base and pulls air to cool the electric choke coil so hot air still goes to the engine. On my 63, I have reduced the choke hot air tube to a diameter of .070 with some J&B weld, this change slowed the idle and still allows the choke to full open in approx 5 minutes. I may reduce to .063 again to see if that will work. PCV is fixed at .076 diameter and I have not decided if that small PCV restriction is going to be OK. Car will idle at 500 with idle speed screw in approx. 1/2 turn.

I am very happy with the results, I think the 300hp engine only needs about 10 CFM to idle but I have no data to back this up. I think you should think about using the intregal hot air choke, just make sure the tube in the exhaust is sealing and not rusted open and the tubes seal top and bottom. Set the throttle speed screw to 1/2 turn and check throttle blades to just show the transfer slot .010+-. Try to adjust idle air/idle speed with the choke hot air tube, pcv and inital timing. This way you can get a nice flow through the curb idle emulsion screws and avoid any hesitation because the throttle blade position is correct.

Sorry for my long post..