[george kujanski]

Trying to minimize heating of the carb, (don't drive it in cooler weather) will leaving the block-off portion of the gaskets work or is a proper thikness stainless shim better? Both of theses will lock the heat riser open.

George

[johnta1]

usually have to use a shim metal. The gasket will probably burn through.

[Kenth]

No offence, but why minimize heating of the carb?

The intake MUST become hot and heat the carburetor during driving to avoid ice formation in the carburetor throats, AND, heat is one, of three, factors that help vaporize and atomize the fuel into a combustible
mixture.
The other two factors are the gas velocity through the carburetor throats (which increases
with engine speed) and the amount of fuel (which must be increased correspondingly if
any of the two previous factors is reduced) which means increased
consumption.

Nota bene, this does NOT apply to racers, they starts at 4000 rpms, runs a 1/4 mile and dont care about fuel mileage, drivability or economy.

You might reconsider after reading this topic :-)

https://forums.maxperformanceinc.com...stible+mixture

FWIW

[steve25]

Well if the effect of today’s cut fuel boiling off hinders
You from restarting the motor once fully warmed up then I would gladly trade off some poor cold engine running for 10 to 12 minutes in cool high humidity conditions until the manifold gets some heat in it to not have a hot start issue.

The factory used the Hot air riser tube to the air cleaner snorkel far more so to keep the carb from icing up in cool humid conditions then relying on manifold heat from the crossover which takes longer.

If you’re not running a stock air cleaner and the related hot air tube then what can I say?

In the OPs case even though it’s more expensive then using a sheet of stainless steel to block off the crossover I would get some Ciramic high temp epoxy and use that to block off the crossover in the MANIFOLD.

If then want some heat to the choke pull off stove if you still have one then you can simply drill a 5/16” to 3/8s “ hole in the epoxy, and of course it can all be drilled out all the way should you want to remove it at some point.

Don’t try grinding it out with a Burr unless you don’t mind wasting that Burr since Ciramic is very abrasive.

[Half-Inch Stud]

I posted longk ago that the exh pulses will interfere in the "Y" in each head, and real bad if a plate is used to block. When EXH crosses-over they interfere with 1-one other CYL. Fill the "Y" to remove heat and reversion caused by the crossover. Of CYLS 4-3-6-5, fill the "Y" completely for best Header (and Manifold!!!) performance.

Yet a torque tube can also occur, if wanted by filling half the "Y". Reversion remains greatly suppressed too. Fill the branch of the "Y" for 6 & 3 to the EXH bowl window.

Reversion mitigation is important. If you run a RA-IV or bigger cam than factory, then i strongly recommend either crossover fill for best results.

[chiphead]

321 Stainless lasts longer than 304 in this application. The 321 is alloyed with titanium, which reduces carbide precipitation and burn through in continuous high temperature operation above 1400F. I did the block off shims, in addition to a phenolic carb spacer, to reduce E10 fuel boiling at operating temperatures. No negative effects once the engine warms up. I'm on the east coast where it gets hawt in the summer.

[PAUL K]

Watch out all the "carb experts" always cry about this modification because they can't figure out how to recalibrate the carburetor when this modification is performed.

Now, since you've been warned we prefer to fill the crossovers in the head with aluminum. It's a win win for performance and power but the heads need to be removed for this. The easiest method is to use the FelPro 1233 gasket set and it comes with block offs. Most gaskets will burn through unless a metal plate is used to block the exhaust before it hits the gasket. The block off plate that comes with the SBC intake gasket set works great on the 72 and newer heads.

[Kenth]

Hopefully some "good racer" can tell what George should do with his carburetor when he has ruined the conditions for his engine to function as intended, since it is not clear here.

FWIW

[PAUL K]

Hmmm..... Function as intended

Let's take the SD455 for example. One of the greatest engines to ever come out of Detroit. Guys at Pontiac started kicking around the idea in the late 60's and shortly after had the engineering department start developmental work. It started out with a cast crank, good rods and RAIV heads, it went through many changes including ported heads, BLOCKED EXHAUST CROSSOVERS, headers bigger camshaft, forged cranks and a Holley carburetor. Now it was turned over to the guys that had to figure out how to make the engine profitable. Head flow was reduced, compression lowered, camshaft tamed and a cast crank replaced the forged unit. Now the EPA guys had to make it pass emissions so the cam was tamed further, an EGR system was added and the hood scoop sealed up.

So fast forward to 2026 and you want to build your own Super Duty engine, which version will "function as intended"?

[TXSTARFIRE]

I wasnt aware that anything had to be done to the carb with a blocked off heat riser. I dont think I did anything special to my car. Only difference was it took a few minutes to warm up and was very cold blooded for the first few minutes. I always just used the 9777526 intake gaskets. In the parts book they had a note to use that part number when carb heat elimination was desired. If Pontiac had a part number for them and they were in the parts book I figured they were OK for me to use.

[72projectbird]

I use intake gaskets that have the opening sealed off completely, not just a gasket with another gasket that has to be laid in to block the crossover.

Summit actually has their own brand of intake gaskets that are completely solid in the middle, and they're pretty thick too. They fit great, although the rubber ring they supply for the coolant crossover to timing cover is the wrong size and wont seal, so use your old one or source a thicker one.

[Dragncar]

I wont even run a head that does not either have the ex crossover filled or welded. So what if it takes longer to warm up. They are old cars anyway, part of the deal. I swear I can tell if a iron headed Pontiac has stock crossovers vs filled.
Stock the ex note is kind of muddy. Cleaner- crisper filled.
Tells you all you need to know about the modification.
Paul would know this, I am not positive. I always thought Pontiacs, Olds and Buicks have 2 holes behind the center ex valves for warming the carb. But Chevy,s have one hole, not 2 side by side. And that is why the reversion is so bad with BOP engines. True ?

[694.1]

Street engine, cruiser?
This thread contains some drama but for the average driver, blocking one side of the cross over will still allow nice warm-up, a cooler carb and longer lasting paint on the intake,
The Dyno will never know & the carb therapist can assuage all guilt.
But DON"T use a putty knife blade! (right Dick?)

[Hikin Mike]

Originally Posted by PAUL K

Now, since you've been warned we prefer to fill the crossovers in the head with aluminum. It's a win win for performance and power but the heads need to be removed for this. The easiest method is to use the FelPro 1233 gasket set and it comes with block offs. Most gaskets will burn through unless a metal plate is used to block the exhaust before it hits the gasket. The block off plate that comes with the SBC intake gasket set works great on the 72 and newer heads.

I actually did that 30+ years ago on my car. I think I used a Mr Gasket set that included two steel stamped shims to block-off the cross over. When I need to replace an intake gasket, I re-use the "shim". Been driving it year-round with no issues.

[Formulajones]

Just block it. If you know carbs it's easy enough to tweak to a point you really shouldn't need extensive warm up times or have any cold drivability problems. Most of you guys don't run enough idle timing anyway so that's part of your problem to start with.

My current daily is running an Eddy rpm air gap so the cross over would be blocked regardless, the trick flow heads don't have one anyway. I drove it through winter and didn't even have the choke functioning. Fires off with one pump and runs on its own within a couple seconds and I drive away no issues.

[scott70]

Way back I started using intake gaskets that were solid and they would burn through then tried the metal block off type, they would eventually burn through. Then when I had sdperformance do my heads I asked him to fill them with aluminum. If you look up into the center exhaust port you can see where he blended it all together. Never had any ill effects of blocking the crossover, I don’t even run the choke hooked up and it idles by itself pretty quickly and yes you can’t just start it and walk away.

[Kenth]

If an engine starts and runs fine when cold, without choke and/or crossover, you can be sure that the engine is burning more fuel than necessary when it warms up
This is not rocket science, it is the science of the principles of the internal combustion engine.
Something that wannabe racers using a worn out Holley carburetor with stiffened/blown power valves will never understand.

FWIW

[george kujanski]

Thanks for all of the replies. This is just a street car, and my goal is to reduce heat soaking and trying to reduce fuel evaporation after shutdown.

Would just locking the heat riser reduce carb heating towards this goal? I figure there would be some flow thru the crossover , but then again during normal driving, it's open anyway so the same heating conditions will exist later in the driving cycle. BTW, I have an electric choke on the Q jet, going with a Edelbrock Performer 2156 to replace the cast iron with EGR; I have the EGR capped anyway. Just trying to achieve my goal without experimenting too much, R&R of intake , etc. I suppose the carb will warm up enuf when plugged anyway, bolted to a hot engine, but will there be less fuel evap after shutdown?

I'll measure some temps before and after, regardless of what method I finally decide. It's summer, time to drive rather than experiment with different methods.

George

[b-man]

Put a 1/4” phenolic or equivalent insulating spacer under the carb, easy enough and effective.

I have .280” thick phenolic spacers under all 3 carbs on my 421 HO, exhaust crossover and hot air choke fully functioning. The thermostatic flap in the passenger side exhaust manifold is working as well.

It’s great, warms up like it should and no fuel boiling issues. Carter M6907 mechanical fuel pump with factory vapor return line, vapor lock hasn’t been a problem.