350 Chev vac. leak?

[xanadu]

Hi all, i have a 350 chev (early 70's) in a pick up truck which is causing me some grief.
Pretty stock engine, small crower hyd FT cam, specs unknown, 600 holley VS on an
edelbrock speedmaster single plane, Accel HEI dist. running 12deg initial 34 total.
Have correct voltage to ignition system.

Initially put a vac. gauge on carb port, reading at idle / fluctuations between 12 and 7in.
Raising rpm to around 1500 produced massive fluctuations between 18 and 8in. Above 1500 to 3500 there is
about 1-2in fluctuation between 19-17in.
Popping from exhaust. Replaced fouled spark plugs and old ignition leads, PCV had minimal spring pressure so
replaced it. Clamped off brake booster hose and Rerun engine / popping gone but still erratic operation.
Carried out comp test suspecting blown head gasket between cylinders, all reading 150psi.
Carried out a leak down test to find all within 20% leakage. Removed inlet manifold thinking heat riser
passage or intake gasket faulty, minor leakage found, replace gasket. Checked camshaft for worn lobes / OK.
Restart engine to find the same operation. Remove rocker covers and check lifter preload / OK.
Did back each one off a half turn.
Remove carb to inspect / found to be clean and OK.
Have good reading on carb mixture screws and timing set at 12deg Initial.
Fuel pressure 4.5lb.
Visually checked for broken valve springs also.
Firing order is 18436572. Do all chevs run this firing order?
I am missing something...

[Schurkey]

In MY driveway, that'd get a cylinder-balance test.

Ground the spark to each cylinder in turn. Watch the vacuum gauge to see how much it drops when each cylinder is shorted. Cylinders that don't drop much are weak. Cylinders that don't drop at all are dead.

This at least narrows down which cylinder(s) are the problem children.

That firing order is used by all OEM small-blocks. Aftermarket cams can alter the firing order.

Grounding the spark to each cylinder is fairly easy--put a smear of silicone grease on the tips of eight nails. Slide each nail between the plug wire and plug boot (DON'T puncture the insulation.) Get the tip of the nail near the metal terminal of the plug wire. Connect a vacuum gauge to manifold vacuum, warm up the engine, With the engine at idle, touch a grounded test light or jumper wire to each nail to ground the spark to each cylinder in turn.

Cylinder_Balance_Testing_LG_01.jpg

[rebelrouser]

That is the classic signs of a problem in the valvetrain, most likely a flat camshaft or broken valve spring. Any time a vacuum gauge needle bounces on a running engine it means something is wrong with the way the air is entering the engine, and the cylinder causing the problem makes the needle fall, the good cylinders make it rise. Problems affecting all the cylinders cause low steady vacuum readings. As Schurkey mentioned a cylinder balance test should find the cylinder at fault. Also a dynamic or running compression test should show it up as well. Usually if you brake torque the engine a flat cam on the exhaust lobe will cause a popping noise out the intake, a flat lobe on the intake side just does not fill the cylinder so power is lower.

[rustbucket79]

Typed out something then read your post, my bad, you’re covered. Do you have a different carb to try?

[xanadu]

I have ground out each plug wire. Front 4 cylinders makes negligible difference / spark plugs wet with fuel. I have checked the cam lobes and are good. If it had a broken valve spring Would it not misfire above a certain rpm?
I checked the rise of each lifter with manifold off.
I don’t know how to check actual valve lift being a hydraulic cam.

[Schurkey]

If the plugs are fouled, the engine is not going to run right. Might want to clean or replace the plugs and re-test.

Big question then is "Why are the plugs fouled"?

[xanadu]

Thanks for the replies. Yes I did replace the plugs and leads as they were fouled and took a reading on them after about 3 minutes of run time. After checking lifter rise on each lobe I cranked engine over and noticed all pushrods turning also.

[Schurkey]

Did replacing the plugs and wires resolve the original issue?

[xanadu]

It resolved the backfiring out the exhaust, but still running rough.

[travis]

How does the inside of the distributor cap look? Might be worth replacing at this point.

It does sound like a valve train or valve sealing issue, but since your cranking and leak down numbers look pretty normal for a low compression engine with some miles, the next thing I would suspect would be valve springs. They may not be broken but could just be very weak from age/mileage.

Could be worn valve guides too...

[xanadu]

Distributor is fairly new as in approx. 3000miles, cap and rotor looks good. Yes I think that might be the next step / guides and springs. Ultimately it would be nice to strip the entire engine and assess. Will keep you posted when I find out more.