Blocking off heat riser

Johnnydidd

Johnnydidd

Well-known member
I been wanting to block off the heat riser on my intake manifold, Obsolete auto parts has a intake gasket that will block off the heat riser has anyone used this gasket? Does it hold up?
 
Block it off completely?
Not a good idea if you'll be running the car in any cold & humid weather, it keeps the carb from icing up.
There are intake gaskets with a smaller hole for less paint burning heat in the crossover.
 
Dang, that was fast. Not even going to wait for another opinion? I shouldn't have said anything so soon.
 
Ok, I'll chip in dmsfrr. I always thought the heat riser did more on fuel atomization in cold weather, or a cold start up, and less for carb, as in Venturi icing. I know the hot air supply (carb heat control) on light aircraft is aimed at the Venturi ice problem. I don't have the temperature swings outside Seattle that you have in Abq, but I've probably got more humidity on average. I was also told the riser heat is now more important, as now they don't do as wide a Reid Vapor pressure change on winter/summer gas blends. Fuel injection doesn't need it as much.
 
The small hole gasket was a truck item, they were presumed to keep running with less cold start-up problems. And , perhaps not heat the intake charge as much.
So I have been told.
 
Sounds like you are cooking the fuel? Do you have a dual exhaust heat riser valve in place on the passenger side exhaust manifold? If so make sure it is open - maybe wire it open for a test. A sticky riser valve can cook a carb.

Use the truck intake gaskets - and insert a piece of stainless shim stock on one side to fully block the opening. Exhaust can enter the heating space - but not flow through it.

Got a heat isolator under the carb? A single gasket won't do the trick.

Do you have access to an infrared heat "gun" - what is the temp at the water exit from the block when this is happening?
 
GREENBIRD56, thanks for the reply, there not a heat riser on the exhaust manifold on either side, I will try to block off the heat riser as you suggested, under the carb is an aluminum spacer about 1/2 inch thick, I will order a insulator spacer, I will need to leave the aluminum spacer on it has the bolts holes to hold carb, would I use one like the aluminum spacer with 4 holes or a spacer with 1 large hole? I do have a infrared gun.
 
There is evidence that carb spacers can have considerable effect on performance - aside from the thermal break aspects. On a streeter - best keep the manifold carb bores separated side to side - its OK to slot them front to rear (that maintains the OEM "cross H" flow distribution between cylinders). That slot is a common and effective practice - but opening up the plenum to both sides can often eat up your low end torque (and make it hard to set the idle mixture).

The thermal break phenolic spacers can be found with four holes in thicknesses down to 1/2. Just work to make sure the carb bores smoothly match the spacers and manifold openings - no steps in the flow. Your infrared test gun will show a measureable difference between the carb and the aluminum adapter below the isolator while the engine is running. The carb will cool down closer to the intake air temp.
 
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