Fill me in. I blocked mine in an attempt to keep heat away from my eddy carb. I've heard it can cure off idle stumble also.

This might sound silly but it seems to have changed the sound of my exhaust also. Is that right?

Just looking to learn all the reasons people block it out.

No more burned intake manifold paint!!!

always a plus!.....mine is bare aluminum.

I've always blocked them off.

Cooler intake charge is the main reason.

Never had a problem with running motors that way on the street?

i do it to keep the heat soak down on alum intakes with alum carbs

yes it will make the exhast note a tad different

aftyer a 40 mile drive I can not keep hands on the intake anywhere but the carb is like warm

cross over blocked,1" stacked alum plate/gasket heat disapater on mine

cured all the hot start problems,and the fuel perking stop

yes,it is for a much cooler intake charge,cooler denser air charge = more HP and is/was the main reason its done,now it mainly for heat soak on a daily driver

had a old timer tell me it was to replace the H pipe found on the mid 60s to late 70s exhast systems.

evens out the exhast pulse like the H pipe does.

now if that is true or not I have no idea,sound good though

I have had an icing incident or two with mine over the years. When the temp and humidity is just right, it will ice up and be a pain. If it does ice up, the best thing to do is to shut it off long enough for the heads to heat the intake and carb enough to defrost them.

That's the only downside I have ever seen.

I blocked mine and the base of my carb is around 68* when the engine is warmed up. I never checked it before but I can only imagine it was hotter than that.

A engines with 2 bbls are FAMOUS for blocking the crossover ON ITS OWN. Not a good deal at all.

I say on anything that is a street car .. you want to "heat" in the intake to eNhance A/F atomization.

What's the consequence of blocking if you are running a carter with the choke winding in the manifold? slower opening or no opening? how much air actually will flow back through the manifold once the heat riser valve is open.....or if the valve is not in place? assume the path of least resistance is down the passengers exhaust bank.

For a street car that gets driven all the time and it is a colder area then it is a good idea. It helps driveability alot for that. It heats the intake to heat the incoming fuel-air mixture which helps alot in cold weather and it heats the choke well in the intake to heat the choke coil in that style intake. But most of us gearheads (me included) that drive our hot rods in the warmer months we dont want it. We want the cool incoming air charge as it is more dense and a cool fuel-air make more hp as it is heavier and will fill the cylinders more. And as most high performance cars can have heat soak trouble in warm temps it will make that worse in warm temps. So most muscle cars dont really need it unless its a year round driver.

On my 69GTX I had to unblock mine to make the stock Carter AVS choke open correctly. It gets hotter but doesn't seem to make the fuel perk problem any worse though. As was mentioned exhaust would sometime block the crossover causing the choke to close, made for some bad starting on warm engines. Back in 1970 My cousins 1970 Swinger with a 340 had that problem.

When I went back to the stock cast iron manifold & correct Carter carb, I still bought the blocked off valley tray but on the choke well side, I drilled a few 1/4" holes to let a little heat in to aid the choke operation