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Debbie Harris's 1969 Grand Prix

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Intake vacuum normal?

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This is from a post I made on the Farmall site, I was wondering I anyone here could have any insight on my problem.

So, I started my Super A, everything seemed fine, it was idling smoothly and everything seemed to be in order. I left the garage for 15-20 minutes while it was running to do few things in the house, I wanted to warm it up so I could finally retorque the head bolts. I came back out and the tractor had stopped running. I assumed it was because it ran out of fuel, as I was having carb issues (could have still ran out of fuel). After I retorqued the headbolts I went to put the rocker assembly back on. I went to turn the engine using the hand crank so I could pop the pushrods back into place without fuss and here is where my heart stopped.

It was really hard to turn the engine. It wasn't a smooth turn, and I heard a strange grinding/scuffing noise followed by what sounded like the "hhhhuuuuu" (like exhaling, mouth open) noise of air being let out of the manifold. My first thought was the cylinders/pistons are messed up. I had straight 20w oil. This is the longest run of the engine since I put the head back on. Could the oil have thinned out too much or something?? I put the proper amount in and it was flowing out of the rocker assembly all the way up top like it should. Coolant level good.

Everything I have done to the engine is the following:

- New head with seated valves.

- Adjusted governor bumper screw to stop hunting

- adjusted governor rocker/clevis to carb according to I&T manual

- 20 weight non-detergent oil, filled until it came out of top petcock on pan. (I previously used 30 weight non-detergent)

- gapped/lashed rockers at .016 (cold)

- New spark plugs (Champion D21)

... 3 hours later

Okay, so it might not be as bad as I thought... I still don't understand what is going on. After the engine cooled again, I went to turn the handcrank just to see what it what was going on again. I noticed something that red flagged instantly in my mind: As I tried to turn the crank it got real easy to turn and then hard again the easy then hard and so on, It wanted to rotate back a couple degrees towards my hand, not just stop in place. I instantly knew there was a vacuum in the cylinders. I took every spark plug out and tried rotating it again by hand.

Guess what happened? It rotated freely without any resistance and was perfectly smooth. Like perfect. I really can't explain it.

So, is this vacuum effect normal? My old head had bad valves, is it possible that I'm not used to this kind of resistance from a good, solid seal on the valves/head when trying to turn the engine manually? Or maybe an intake valve isn't opening correctly?

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Sounds like a valve is stuck. Check pushrods, are they adjustable? Check your rockers. Are they correct?

Its a 4 stroke engine. (example) Piston top dead center in #1. 1)The intake valve opens to let the dropping piston suck in air. 2) Bottom of stroke, intake closes and the piston shoots back up to TDC compressing the air/fuel. BANG... 3) piston is forced back down and exhaust valve opens, 4) piston pushes out exhaust as it comes back to TDC.

If you have "pressure" (wouldn't be vacume) in the cyl that's not being released.....It's in the valve's. Pulling the plug will let air in/out weather or not valves are working. Could be a bent valve....worse case, or a bent pushrod or a broken spring. I'd be leaning towards something wrong with pushrods or rockers.

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I don't think it would be a bent valve at all. Pushrods are brand new, I triple checked them for bends and faults. If an intake valve isn't opening on the induction stroke then it would cause a vacuum, the piston is trying to move down while pulling nothing in, like putting your finger over a plastic syringe and pulling the plunger out. I may have an intake valve gapped too much and it isn't letting in air as fast as the cylinder is moving down. According to what everyone has told me, to lash the valves you put cylinder #1 at TDC on the compression stroke and lash valves 1, 2, 3 and 5. Then you bring cylinder #4 to TDC on the compression stroke and do valves 4, 6, 7, and 8. I did that exact procedure. Maybe I am just fighting compression. I mean remember, I'm doing this by hand with a crank on an engine with newly seated valves and a properly sealed and torqued head gasket. My old valves were pitted and junk so maybe I'm just used to cranking with poor compression.

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Interested to hear whatcha find.

The problem i'm still having is with this damned Zenith updraft carburetor. Tractor will run for a couple minutes then sputter and die. I got it out of the garage for a little test drive. I got down my driveway, but on the way back it would sputter lose power, and die. Id wait 10 seconds or so and it would fire back up and I'd get about 10-20 feet in before it died again. I had to repeat 3 times before I got it back into the garage haha

The good news is that I haven't seen any weeping of oil or coolant from the head gasket and no radiator leaks. Does anybody know why the tractor would spit a real quick puff of dark smoke when I start it up warm? Its never done that before, the exhaust is clean and smoke free, its just that one quick puff on first ignition.

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Valve seals might need a lil longer to break in. Leaks a lil oil in the cyl when not running. Or could be too rich on the carb. Glad to hear ya got er running.

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