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Intake manifold exhaust crossover

S

slow cruiser

Regular Contributor
Hi, I'm new here, first post. I've developed the heat issue on the intake manifold on my 1980 F25 with LM318. I've read about closing the exhaust passage, without welding or a gasket set with the metal plate, how can I seal the passage?

Also, where can I purchase the 1/4" carburetor gasket (Carter AFB)? No automotive stores here have one. Will the gasket for the Holley fit? The stud spacing looks different.
 
Fel-Pro high perf gasket set from Jeggs...Fel-Pro part # 1213, eliminates the cross over...about $20..Edelbrock carb gaskets will fit
 
That's foir serious performance use, not marine duty. Don't block that passage off or you'll be sorry!

Jeff
 
fastjeff said:
That's foir serious performance use, not marine duty. Don't block that passage off or you'll be sorry!

Jeff
Click to expand...

What is the bad effect, some here seem to block the passage as an ordinary thing to do not just for performance. After cruising (3000 rpm) the carb is so hot I can't keep my hand on it and obviously it will not run at low speed, below about 1500 rpm until the engine cools down.
 
Use a phenolic spacer to insulate the carb from the manifold. Something like this:

SUM-G1405-1_xl.jpg
 
Well, if thats mainly what you do.... Some people spend a lot of time at low speed, to save gas, and the motor tends to run cooler--lots of water out there to keep 'm cool!

Jeff
 
fastjeff said:
Well, if thats mainly what you do.... Some people spend a lot of time at low speed, to save gas, and the motor tends to run cooler--lots of water out there to keep 'm cool!

Jeff
Click to expand...

Not necessarily, but when I come off of cruise it's hard to dock at 1500 rpm. Indeed, when I last had the boat out last Fall it heated up after about 1/2 hour of trolling, again trying to get into my slip @ 1500 rpm is "uncomfortable". Shifting at this rpm is not good.

The engine does not run hot, 160-170. What is the negative result of blocking the exhaust passage?
 
Jeff..I installed those gaskets last year to eliminate the cross over because my manifold had rusted through at the old spring pocket for the choke,.So far after a full year of running with the gaskets in place I have had no issues at all...the engine runs just as well as the engine with the cross over intact, starts the same and gets just as good fuel mileage..as far as I can tell there is nothing to be sorry for. As for temperature both of mine run cooler when the boat is up on plane, more volume going through the engine.
Pete.
 
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Jeff: It sounds as if you've had experience with this. What about the Fel-Pro gasket without the cut out for the exhaust passage, seems it would burn right through
 
block the cross over passage. it was needed only to heat the divorced choke coil that was mounted on top of the passage and the exhaust help warm the carb to help cold running issues until the engine warmed up. also the cross over passage was also needed for the egr, (smog), system to operate. also the crossover passage has a tendency to burn the oil underneath the intake manifold, causing crystalization buildup and excessive vapors that cause poor idiling due to the pcv that has to evacuate them. that is why there is a insulator plate underneath the intake manifold in some applications. before you block the passage off with gaskets, you can pack the crossover with aluminum foil to help dissape heat. once the engine is warmed up, there is absolutely no need for the cross over passage, unless your engine has an egr valve.
 
Only true in warm weather and/or while the motor is working hard. At prolonged low speed running--and in cool water--heat is needed to properly vaporize the fuel.

Jeff
 
we have a mechanics disagreement. on carb engines, the crossover was needed for initial fuel vaporization. then then engine warmed up. that is where the engine thermostat comes in to play. that is what maintains the minimal engine temp. on computer controlled engines, the exhaust crossover has been eliminated entirely, because the computer/prom chip/oxy sensor, is able to deliver the correct fuel delivery to a cold start/run engine. the engine crossover actually causes additional smog vapors by the oil underneath the intake manifold becoming super-heated and then being evacuated via the pcv, into the carb, causing poor idling. over time, the oil does crystalize and bake to the splash plate underneath the intake manifold if it has one. in my opinion, if the engine does not have a egr, and your choke tube does not mount into the passage way of the crossover.......block it off. rule of thumb....ALWAYS......hot motor.....cool fuel !
 
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