HELP:' 56 Vacuum problems

[Don Owen]

OK Fellas, I'm stumped: We have an early '56 fresh re-build, two fours, 097 cam, solid lifters, all built to specs. We have poor idling and performance as engine gets up to temperature, eventually sputtering out. Ignition is fine, fuel flow is fine, carbs are clean and functioning properly. Compression is strong and even in all cylinders, Avg. on first stroke of 120, and 160 on fourth strokes. No. 5 cylinder is the lone exception: 95 and 132. Nos.3 and 4 are 120,155-160 so I don't suspect a blown head gasket. Meanwhile No. 8 cylinder appears to not be firing at all and we have very poor vacuum: 9-10 inches at 900-1200 rpm. This should be at least 15 inches. The heat riser is fine and exhaust flow is fine. Visual inspection and pumping air into each combustion chamber have really not revealed a sticky valve in No. 8 or other cylinders, though that could be the case. Why else would No. 8 not be firing??

What do you think? Intake manifold crack on underside?( No leaks detected on top side and carbs ).Any ideas or other tests would be deeply appreciated as the car is "finished" and we hoped to put some miles on it this summer(it has about 800 miles on it since Summer of '98). Many thanks.

[Joe Fisher]

Re: HELP:' 56 Vacuum problems

Don, if I remember right, cylinders 5 & 8 are fed by the same runner on the intake. So if there is a vacuum leak on that runner, it normally affects both cylinders. Since cylinder 5 has the low compression, I have seen if there is a valve related problem, it will disrupt the fuel air flow to cylinder 8. Check for a sticking valve, a tight valve or a bent valve on 5 cylinder. If you suspect a vacuum leak under the manifold, or a leaky gasket underneath, cap off draft tube and the oil filler tube, remove the dip stick and put a vacuum gauge on the tube. Start the engine and watch the gauge. If it goes into a vacuum, then you must pull the intake and inspect it. I don't think this is your problem, normally you would see oil fumes out the exhaust. I would suspect something wrong with cylinder 5. I hope this helps. Joe

[G B.]

Please give some more information

Don -

What is the part number of the distributor you're using? I vaguely remember something about the '56 block requiring an oil passage notch in the distributor housing. I'll do some reading on this today to refresh my memory.

Also, please post the results of this test: After removing the valve covers, observe the amount of movement of each valve as the engine is turned over. You don't need to run the engine. Just spin it with the starter and watch each valve to see how much the rocker moves it. The lift of your cam causes more than 3/8" movement at the valve stem. What you're looking for is valves that barely move at all.

[Greg]

Re: Please give some more information

The serial no of the distributor should be either 1110872 or 1110879

[Wayne M.]

Re: '56 Engine Oiling

Jerry --- Let me point you in the right direction; RESTORER, Summer '87; discusses grooved rear cam journal, rear cam bearing, early vs. late distributor (why oil only to one one bank of cylinders), etc.

[motorman]

Re: HELP:' 56 Vacuum problems

if the ignition timing is set correct i would check the cam timing, it may be off a tooth.

[Don Owen]

Re: Please give some more information

Jerry: the distributor is an 872 and yes, it has a cutaway for upper engine oiling. Thanks for your idea on measuring valve lift. I'll give that a shot this weekend, although the valve stems appear to be rising about the same height.

[Bill Clupper]

Re: HELP:' 56 Vacuum problems

Don, based on the lack of compression in the hole, I'd use a leakdown test or just pressurize the cylinder and listen for where the air is leaking out, rings will leak into the pan and you'll hear it thru tho oil fill tube, intake valve will be heard in the carburetor and the exhaust out the tailpipe. this is a relatively easy way to get a handle on the source of your problem. Presurize to about 100 lbs and listen to what it tells you.

[Don Owen]

Re: Please give some more information

OK Guys, more tests today...we used a different compression gauge, screw in type. Results were quite different: all cylinders 55 -70 psi on first stroke and 120-125 on fourth. Two out of three compression gauges yielded these same results, so my prior posting is wrong as far as compression readings go. We measured valve lift with a dial indicator, ruler, and visually at cranking speed. All look fine,at about 3/8" or .290 to .400 thousandths...its hard to get good readings but I'm sure no valves are stuck open or closed. We did Jerry's test for internal intake manifold problems, closing off draft tube and oil filler tube then checking for vacuum at the dipstick tube: No vacuum there.

Meanwhile, we sprayed some carb cleaner and did get an increase of rpm,no increase of vacuum reading, at the gasket area right around runner for No. 8. We do have a vacuum leak there. Today's question: the increase in rpm was 25-50 rpm and we could certainly hear a difference for just a few seconds. Is this enough to cause the no firing in No.8 combustion chamber?? And, is it the sole cause for our vacuum reading of only 9" @ 900-1200 rpm???

As for our low but even compression I'll check with my re-builder. He may have used different pistons and/or rings. Many thanks foor all your help thus far. Donny