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  #16 (permalink)  
Old 08-04-2010, 12:13 PM
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Quote:
Originally Posted by DoubleVision
One point I`d like to mention. On Holley, demon and edelbrock carbs, the point of fuel entry is higher than the float level so fuel can`t drain out via the fuel line unless I`m missing something here. A Quadrajets seat is above the fuel level unless it`s using a "windowed" seat assembly.
Fuel can`t travel up so there`s no way that I can see it would be draining out of the fuel line so the fuel in the bowl is either evaporating or leaking out of the bowl itself. If the heat riser passages I mentioned aren`t block off properly when the vehicle is shut down heat soaks under the carb. take the car down the road about 10 miles. Remove the air cleaner and touch the carb, if it`s so hot you can`t leave your finger on it you can bet the heat riser holes are open and if the carb is getting that hot there`s no doubt the fuel is evaporating.
I just noticed that new edelbrock gaskets (7220) come with small pieces which I think are for blocking exhaust from entering the intake manifold.
My gasket set for 351W contained nothing to block the exhaust holes.
Why did OEM manifolds allow exhaust to enter the OEM intake manifolds like 65 mustang?
To aid in cold weather operation?
To allow exhaust banks to balance?

I am considering blocking the intake manifold exhaust passages. Sounds like carb will run cooler. Not sure if will effect engine performance?
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  #17 (permalink)  
Old 08-04-2010, 12:20 PM
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Quote:
Originally Posted by 001mustang
I just noticed that new edelbrock gaskets (7220) come with small pieces which I think are for blocking exhaust from entering the intake manifold.
My gasket set for 351W contained nothing to block the exhaust holes.
Why did OEM manifolds allow exhaust to enter the OEM intake manifolds like 65 mustang?
To aid in cold weather operation?
To allow exhaust banks to balance?

I am considering blocking the intake manifold exhaust passages. Sounds like carb will run cooler. Not sure if will effect engine performance?

That's a Fel Pro gasket, regardless of the name on it and is the way they have supplied 351W gaskets at least since the mid '80s. There are several different head designs and those gaskets cover most all of them including the 302/5.0.
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Old 08-04-2010, 12:31 PM
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wild hair

Wondering if I can squirt a large blob of hi temp RTV into the heat riser chock port to reduce exhaust flow under the carb?

Not sure if the RTV will fail and enter unwelcome places?

I am trying to be lazy.
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Old 08-04-2010, 12:48 PM
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Quote:
Originally Posted by Irelands child
That's a Fel Pro gasket, regardless of the name on it and is the way they have supplied 351W gaskets at least since the mid '80s. There are several different head designs and those gaskets cover most all of them including the 302/5.0.
I am not sure what I installed 11 years ago. I'll try to find my receipt.
I will also monitor heat riser channel vs. other intake manifold temps like DoubleVision eluded to.

Looks like I can pull 2 bolt choke block off plate and visually determine if exhaust ports to intake manifold are open or blocked with gasket,
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Old 08-04-2010, 01:55 PM
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Why would you want to block exhaust heat? It serves a purpose - to help atomize the fuel charge. It's an fuel economy measure that the Performer intakes use.
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Old 08-04-2010, 02:09 PM
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I`m not sure what years caddy engines did so, but I do know when small block chevy`s did. When the 265 was introduced they plumbed the heat riser to the carb`s flange so right beside the venturi holes there were 2 small holes connected together with a trench and the gasket assembly rode over the ports and sealed it so exhaust gas couldn`t get in with the incoming air/fuel mixture. They did this so it would quickly heat the carb to atomize fuel and to eliminate throttle blade icing, heating the incoming fuel made the vehicle so it would start up and run good in cold weather. 1969 Chevy did away with this and just let the exhaust heat riser plumb under the plenum under the intake. This way a regular carb gasket could be used. Many don`t know about the pre 1969 intakes being like this so if they happen to use one they put a regular carb gasket on it and never can get it to run right yet they don`t know why. Why is because the exhaust channel isn`t sealed off so the exhaust gas contaiminates the incoming air/fuel mixture. as we know exhaust gasses won`t burn twice. On blocking off the heat riser passages at the intake to head port it depends. If you have harsh winters I would leave it open or use a restrictor, which closes most of the port but not all of it. If you live in a year around warm climate I`d block them off. When the engine is cold it`ll still run on the shoddy side until it warms up.
Years ago when I did my Vortec build I used a performer Vortec intake, Vortec heads have no heat riser passage, I also used a Quadrajet with the electric choke disconnected. It started good in cold weather and you could drive it but it would sputter some. Most of the time I let it warm up a good 5 minutes before I took off.
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Old 08-04-2010, 02:59 PM
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Quote:
Originally Posted by Irelands child
Why would you want to block exhaust heat? It serves a purpose - to help atomize the fuel charge. It's an fuel economy measure that the Performer intakes use.
I am just curious if blocking exhaust from intake manifold will reduce rate of evaporation of fuel from bowl on a heat soaked engine. My guess is the fuel will still evaporate from the edelbrock bowl due to the mass of hot engine and modern fuel mixes flash off sooner than previous mixes.

My 351W engine heats up much quicker than stock SBF engine. I am guessing my 351W engine produces sufficient heat to atomize the fuel w/o use of intake manifold exhaust passages.

Would be nice if I could rig a temporary plug to install below the two bolt choke block off plate for testing.
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Old 08-04-2010, 05:55 PM
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You need an insulator type spacer between the carb and intake, not blocking off the heat X-over.
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Old 08-13-2010, 10:49 PM
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Quote:
Originally Posted by DoubleVision
I just had did a post on this, then I thought back for a second and had another thought come to mind. Remove the carb and look at the intake`s carb pad. Are there 2 small holes in the intake that are connected to each other with a trench? If so, these are the heat riser passage holes that are plumbed directly to the exhaust heat riser passages. The set up these engines used was, gasket, steel plate, gasket. The steel plate and gaskets together would seal off the trench between the holes causing it to stay in the trench. It has 2 jobs, one is prevent carb icing in the winter time, the second is to help cold starts since the heat would heat the incoming fuel charge, helping it burn easier. My nephew`s 69 Caddy 472 had a Quadrajet on it, but the intake had the 2 holes connected to a trench, he used a junk adapter plate and a holley carb and never could get it to run correctly, I showed him the 2 holes and told them how it worked. He got a Quadrajet for it and the correct carb base gasket which solved the problem. I`m not sure if they did it this same way in 1957, but it`s worth a look. If these holes are present and open, the engine is pulling in exhaust gasses, which can`t be burned twice, this will cause it to run rough and be low on power. the second thing it causes is the carb will get very hot and boil the fuel out of the carb once the engine is shut down. What happens after shut down? heat rises and collects under the carb. As for fuel draining back after shut down, yank the oil dipstick out and smell it, if it smells like gas the fuel pump is no good.
If the fuel pump has a return line, crimp it closed with a pair of vice grips, let it idle a few minutes, shut it down and see if it drains back. If it does then I would replace it. I`m not sure if the fuel pump or the fuel tank has anti reversal check valves or not, but it draining back would suggest it`s stuck open.
I've got the fuel line losing fuel narrowed down, but as Im rebuilding the carb at the moment, I know the the intake manilfold does have the holes connected to the heat risers and connected by a trench. The base gasket is open directly above the two holes and the trench. The bottom of the primary throttle plates are black with soot indicating the engine was sucking exhaust fumes back in for a while before I recently got it. The gasket/plate/gasket combo you spoke of, did the plate cover only the trench leaving the holes open up to the carb? Or did the plate cover the trench and both holes?
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Old 08-14-2010, 04:30 PM
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THe holes aren`t supposed to be open to the carb. The idea was since the holes are connected by a trench the exhaust flow could go one side to the other by the exhaust pulses. The gasket is supposed to keep these holes isolated from the carb itself. So while they worked isolated, it would get the metal plate hot which in turn would get the carb hot. If these holes are open to the carb then it`ll be next to impossible to get it in tune correctly. I don`t know if the exact gasket you need is avalible, if it`s not then you`ll have to find a way to block the holes off.
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