The engine runs badly and there's an unusual situation with vacuum under the intake manifold, in the lifter valley. When I remove the oil fill breather and put my hand over it, it nearly sucks my hand in. When I spray carb cleaner down the oil fill, the RPM increases. Spraying carb cleaner around the manifold makes no difference. The compression is 110-120 in all cylinders except #5 which is 80-lbs. It doesn't idle or accelerate smoothly, it runs like there's a bad vacuum leak. It idles (roughly) with about 5-hg of vacuum, and the needle bounces a bit. The engine is a 59AB, I believe it has low hours, but hasn't been driven in many years. I swapped the intake manifold and installed a new gasket after cleaning the mating surfaces. I tried different carbs, adjusted the timing, but no difference. Where do I begin to diagnosis why there's vacuum under the intake manifold?
I'm no expert on flatheads, but a cracked intake manifold would explain the condition you describe. The crack could be on the bottom of the manifold. Or a failed intake gasket, again near the bottom of the intake port. Has the engine ever run properly while in your possession or has the intake been recently changed? Maybe a porous casting somewhere or improper gaskets?
ClayMart, Thanks for responding, I replaced the intake manifold and carb with known good units and new gaskets. I looked for cracks or splits, and found nothing. The car is new to me and I never heard it run any better then it does now.
So what do you have for crankcase ventilation? Vented fill cap and a road draft tube, or has someone added a PCV system? If there's a PCV valve installed somewhere make sure it hasn't gotten fouled and is always stuck in the open position. Some pictures of the engine from a couple different angles might be helpful as well. Now I'm really grabbing at straws here, but... intake valve guides...??
I would think a valve guide would have to be worn to a half inch wide hole before it would make such a sucking vacuum that you could feel it with your hand on the fill stand. Is the proper original breather still in the front corner of the pan rail? Are you using a pre-48 intake? Were there any modifications to the venting tube inside the valley? you said you replace the intake and gasket again? Did you note anything at all on the ones you replaced? Were does your vacuum brake for the distributor hook to the intake?
Yeah, I know. That's why I said I was grasping with my initial reply. It looks like at least some flatheads used a split, two-piece valve guide. Don't know if they were just a press fit or if there was some other way they were retained that might be a common weakness on those engines.
Crap, I had a senior moment. Yes his 59A might have split guides. But even if they are worn more than any others in history, it still shouldn't be causing enough vacuum in the valley to hold his hand to the pipe.
With my limited knowledge of flat heads ,all of the up thru 48, 59 A had split guides.8ba,8cm, started in 49 with the single piece guide and regular stem valves. 48 and earlier had the tulip valves and split guides. There are keepers that hold both types in the block.
Watching this close as I'm having the exact same problem! I too have checked all the usual places. I'm pulling the intake and seeing what's what inside next Sent from my Pixel 2 using The H.A.M.B. mobile app
To pull the vacuum described by the owner, there has to be a direct leak into the manifold from the valley SOMEWHERE! Now you just need to find it! Possibly shoot a short squirt of white paint into the oil filler that's pulling all that vacuum, shut off the engine, Pull off the manifold, & see where the white paint went ?? The suction will put the paint where the hole is better than just looking & guessing, & I seriously doubt it's the valve guides : if it was, the motor, if it ran at all, would make enough metal rattles to scare a 1st sergeant.
You said you believe it has low hours so it must have been rebuilt. Maybe someone forgot to put the seals on the valve guides. Sent from my SM-G950U using The H.A.M.B. mobile app
Just as “landseaandair” mentioned after removing the intake manifold, and looking more closely at the valves, we found an intake guide that was damaged, the bottom was broken and it was floating in the guide bore, hence vacuum in the lifter chamber. The engine seems fairly fresh, and the lifter bore doesn’t seem damaged, we’re changing all the valves and guides. Hopefully, have her back together soon and running well.
When you put it back together, take a little more care than the last guy on that gasket. Globbing form-a-gasket all over the place is dangerous for engine internals. Those thick paper intake gaskets might need a very thin swipe of sealant, but not really much at all. You aren't sealing out any water or compression, and oil won't leak out of there.
Well I'm taking at least partial credit, since I don't usually get to take any at all. You said the engine hadn't been run for maybe a couple of years. I wonder if it had a valve get varnished up and stick in the guide. Next time it gets spun over it raises the valve and moves the guide with it.
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