1962 300H.P. intake manifold gaskets

[Kenneth F.]

I am re-installing the intake manifold on my 1962 300 H.P., trying to smooth the engine when under load. My old gasket had the exhaust port totally open. The archives information mentions eliminating exhaust that runs through the manifold to heat the carburetor. Should I restrict the exhaust, or totally block it?Thanks,Ken

[Jim T.]

Re: 1962 300H.P. intake manifold gaskets

Kenneth the only reason the exhaust moves across the intake is because of the heat riser valve being cold at start-up. A 100% operating heat riser valve will open allowing the exhaust on the passenger side to flow into the right side exhaust system.

If you are going to block off the intake manifold exhaust cross over, recommend you tie open your heat riser valve. The exhaust heat riser valve on my 68 and 70 have been wired open for many years. A few years ago I blocked off the passage way below the 68's carb to completely restrict any exhaust flow. Cold start-ups on the 68 have not been affected with this change. MY 70 has a ZZ1 GM crate engine and the intake does not have an exhaust crossover in the intake manifold.

[Joe L.]

Re: 1962 300H.P. intake manifold gaskets

Kenneth the only reason the exhaust moves across the intake is because of the heat riser valve being cold at start-up. A 100% operating heat riser valve will open allowing the exhaust on the passenger side to flow into the right side exhaust system.

If you are going to block off the intake manifold exhaust cross over, recommend you tie open your heat riser valve. The exhaust heat riser valve on my 68 and 70 have been wired open for many years. A few years ago I blocked off the passage way below the 68's carb to completely restrict any exhaust flow. Cold start-ups on the 68 have not been affected with this change. MY 70 has a ZZ1 GM crate engine and the intake does not have an exhaust crossover in the intake manifold.

Jim and Ken------


If the exhaust manifold heat cross-over passage is blocked, the heat riser valve MUST be securely wired open or removed and replaced by a spacer.

For a 1962, the only function of the heat cross-over is carburetor heating. So, by blocking the cross-over, only that function is affected. For 1966 and later, the cross-over also is necessary for operation of the divorced choke. So, if the cross-over is eliminated, the choke will no longer function.

[Timothy B.]

Re: 1962 300H.P. intake manifold gaskets

Kenneth,

If it were mine I would tap and install small plugs in the passage directly under the carburetor on each end of the hot slot, there should be some good pictures in the archives showing a nice neat installation.

I would not block the exhaust crossover, you will get many opinions about this but I believe it's very important for proper fuel vaporization using a cast iron intake manifold.

In a worst case you can wire the heat riser open in the hot summer temps only if it's necessary. On a hot day with a fully warmed engine that valve will still tend to close when the engine is at idle speed but the spring will allow much easier opening as soon as the rpm's increase. Keep the carburetor set lean and full manifold vacuum advance and that heat will help vaporize the fuel load and allow better fuel distribution to the cylinders.

[Kenneth F.]

Re: 1962 300H.P. intake manifold gaskets

Jim, Joe, and Tim, I measured 235 degs on the manifold crossover with a wired open heat riser at idle, 177 degs on the AFB flange just above the mounting gasket, and 133 degs at a float bowl. The upper radiator hose is 185 degs, 6" from the thermostat housing. I should have put this information in my initial post, but did not realize there are different approaches to solve this. Temporarily replacing the top 10" of the steel line from the fuel pump made a big improvement. I will probably try the suggestion of plugging the hot slot. If you have any other thoughts, I am interested. Thank you, Ken

[Timothy B.]

Re: 1962 300H.P. intake manifold gaskets

Kenneth,

Describe the problem you are trying to solve, in post #1 you state there is some problem when the engine goes under load. I am not sure I understand what you are saying.

[Kenneth F.]

Re: 1962 300H.P. intake manifold gaskets

Timothy, I have a road test area near where we live. I go down a hill to a straight that is about a mile long. I can accelerate fairly hard to the point of breaking traction, and it runs fine, until coming up the hill near our house, in 2nd gear at about 1/4 throttle, and it seems like it is going to run out of fuel, but after cresting the hill, it smooths out in about 5 seconds. Sometimes it will run like this on a level road. The fuel pump will pump a pint in 15 secs. The fuel pressure is 6-8# with a cold engine, but the pressure drops when the engine reaches operating temperature.

[Timothy B.]

Re: 1962 300H.P. intake manifold gaskets

Kenneth,

I don't want to lead you on a wild goose chase as these types of problems are usually a process of elimination. From your description of the problem it sure sounds like a fuel issue.

The fuel pressure should be approx 5-6lbs at curb idle speed and stay constant. If the original AFB is used 8lbs may be high enough to overcome the needle and seat inside the carburetor, could your issue be a flooding engine when going up a steep grade? Have you had the carburetor apart to check the floats and float level?

Also keep in mind there is a fuel sender sock/filter inside the gas tank and that could clog and cause fuel delivery issues causing low pressure after the engine warms by not providing the needed fuel to fill the pump. Can you report how much the pressure drops when you have observed this and what type of fuel pump is on the engine.

[Kenneth F.]

Re: 1962 300H.P. intake manifold gaskets

Hi Timothy, I have a 3461 and a 3269 AFB, that I have swapped a few times after rebuilding. I tried different float settings, and metering rod springs. The fuel pressure at cold start is 6-8lbs. After idling about 10 mins, the fuel pressure starts to drop slowly, and by the time operating temperature is reached, the 0-15lb guage is at zero. The engine has never quit running, but has this miss that I can best describe as a slight shake, like the engine is about to drop one cylinder at light load on engine. It runs strong when accelerating. If it has a fuel delivery issue, I am surprised the pump would deliver a pint in 15 secs. but this test was done when the engine is cold. I have wondered if the fuel pump could leak internally at higher temps? The pump came from C/C, and is the type that the top and bottom is held together with screws. I suppose I should try another pump. I have moved this short hose on the suction side when the pressure is at 0 with the engine at idle, but the pressure doesn't change. Thank you. Ken

[Timothy B.]

Re: 1962 300H.P. intake manifold gaskets

Kenneth,

Verify the fuel line on the suction side of the pump is in tact first then change the pump. You have got to start there, the pump should maintain 5-6lbs. pressure at idle speed. Is this shake only at idle speed, how are you adjusting the A/F emulsion screws and are you certain there is no vacuum leak, what's the idle vacuum?

[Kenneth F.]

Re: 1962 300H.P. intake manifold gaskets

Timothy, It idles fine at 17-18" vacuum. I turn the A/F screws c/w until the idle starts to drop, and barely back the screws out until the idle smooths. I will be able to work on the car in about a week, and will do as you suggest with the fuel pump, and suction side of the pump. I will let you know how I do. I appreciate the help. Ken

[Duke W.]

Re: 1962 300H.P. intake manifold gaskets

Blocking off the heat slot with Welsh plugs or threaded pipe plugs is a good idea.

If you're having fuel percolation problems you can block one side of the heat riser. This creates a dead end passage that allows some exhaust gas heat to migrate under the manifold plenum, but much less that if the passage is full open. The conventional way to block the heat riser is to block both sides, which completely eliminates exhaust gas heat transfer.

Make a shim from thin stainless steel sheet stock - about .010". Place the shim between the manifold and gasket. Also bend a tab on the end in case you want to later remove the shim. You can remove the shim by loosening the manifold bolts. Then grab the tab with needle nose pliers and extract the shim. Re-torquing the manifold bolts should seal things up. (I would loosen all manifold bolts in the reverse of the torquing sequence. Then re-torque them using the shop manual recommended sequence.)

I blocked both sides of my 340 HP manifold long ago, but even in warm weather it had a severe stumble on start-up from a dead stop until the engine had been operating for about 20-30 minutes. No doubt the problem was exacerbated by the CR four-speed and 3.08 axle. Start-up without stumble or excessive clutch slipping was always an art.

I removed the shims, but later realized I should have left one in to determine how that setup worked.

For engines with divorced style choke coils that are usually on the right side, block the left side heat riser only. This should allow enough heat to reach the choke coil that it will function properly, but some adjustment of coil tension may be required. With the AFB hot air choke system it doesn't matter what side you block.

The biggest problem you have is fuel pressure. It should be in the range of about 3-5 psi at idle. (Verify in your ST-12.) If fuel pressure goes to near zero when the engine warms up you may have a defective fuel pump. Perhaps a check valve is leaking and not enough fuel is being delivered to the bowl to keep it full under load. You can try another pump or get a rebuild kit for yours.

Also remove and check the fuel filter for plugging.

Duke

[Kenneth F.]

Re: 1962 300H.P. intake manifold gaskets

Duke, Blocking one side of the heat passage sounds like a good approach. I will check into the fuel pressure first, as you have suggested. I appreciate all the great direction. Ken

[Kenneth F.]

Re: 1962 300H.P. intake manifold gaskets

I bypassed the mechanical fuel pump with an electric pump that would give 4 psi, using all of the original fuel line to the tank including the rubber hose that attached to the suction side of the mechanical pump. As the engine temperature came up to 185 degs, the pressure had slowly dropped to 0 psi. I removed the pressure line at the carburetor and dead headed the pressure guage to read about 3.5 psi. I reattached the fuel line to the carburetor. This probably took about 20 mins to re-start the engine. The pressure was at 3.5 psi, engine idling, but within 5 mins was at 0. I monitored the heat at the carburetor flange and read about 135 degs, but noticed that as the fuel pressure dropped from about 1.5 to 0, the carburetor flange at 3 corners read 185 degs. The manifold at the heat crossover read 235. I have the phenolic spacer and the stainless plate under the carb. Could heat due this to pressure? Ken

[Timothy B.]

Re: 1962 300H.P. intake manifold gaskets

Kenneth,

I don't think the heat has anything to do with fuel pressure but did you block the hot slot crossover under the carburetor.

From you description of the fuel pressure it sounds to me like the pump is sucking air somewhere. Could the fuel line be rusted, do you smell fuel in the garage. The pump should maintain full pressure when the engine is at idle, if you find that problem you may fix the problem you report when the engine is under load.

Could the pressure gauge be bad?