Low Vacuum and RPM Drop

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Fuzzy Dice

Fuzzy Dice

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Cedar Falls IA
Hello all – Hoping the community can help me with a low vacuum situation on my 340. I only get 10" of vacuum at an idle of ~850 RPM. It holds steady there, it doesn’t bounce or drift. When I quickly open and close the throttle it dips to ~1” then rebounds to ~25”, then settles back in around 10”. I do get more vacuum if I bump the idle up to 1800 RPM, then I’m sitting at about 15”.

I’ve read up on some of the threads about this here and on moparforums so here are the symptoms I think are relevant
  • The engine starts easily with the choke when it’s cold and it fires right back up when it’s warm. It idles smoothly and runs down the road fine.

  • There is a big drop in RPM when I put it in gear, 400-500 RPMs, which I don’t think is normal. Due to this I have to bump my Park Idle up to ~1100RPM so it doesn't kill in gear.
  • The brakes are pretty weak (power brakes, front disc rear drum) but I’m assuming that’s just due to bad vacuum. They are bled and adjusted properly. I pulled the line off the booster, plugged it, and checked vacuum again thinking maybe a bad booster but there was no change in manifold vacuum.

  • I can’t find a vacuum leak using carb cleaner and listening for a drop in RPM. Everything seems fine there. I am also assuming it’s not a vacuum leak since I can get good vacuum if I increase the idle speed. Maybe this is flawed logic though…?
  • Changing the idle mixture screws on the carb doesn’t affect the vacuum or RPM, but again I think that could be several things. Screwing one of them all the way in does kill the engine though.

  • Fast idle cam is not kicking off. Initial choke sets at 2200 RPM, I can blip the gas pedal to bring it to 1500 RPM but never any lower than that until I put it in gear. If I leave it in gear for a minute and put it back into park then it goes back down to where it should be (currently ~1100)
I put on new plugs (NGK GR4 gapped at 0.040), Summit 8mm plug wires, vacuum advance distributor (holds vacuum fine), distributor cap, rotor, and PCV valve. I switched from a Holley 750 DP to a Holley 670 Street Avenger carb (was running very rich) but it was low vacuum before that swap too. Has an Edelbrock Performer intake and MP blaster ignition coil.

The cam specs are unknown but the idle isn't choppy and doesn’t have an audible lope to it so I’m assuming it’s either a stock or mild cam. Ignition timing is at 25 initial and 38* total with the vacuum advance unhooked and plugged. I didn't check it with the vacuum advance hooked up.

According to this article http://www.gregsengine.com/using-a-vacuum-gauge.html and this article http://www.secondchancegarage.com/public/186.cfm, it seems like the culprit is late cam/valve timing. I’m hoping it’s something more simple so I don’t have to tear this thing apart and degree the cam but you guys/gals probably know more about this than I do.

Any suggestions for someone fairly new to these old carbureted engines?
 
How many miles on the engine? Does there seem to be much slack in the timing chain? It sounds like maybe the timing has slipped a tooth on the chain..... RE: late valve timing..... 25 degrees initial sounds like a lot for a stock spec cam to not have starting issues.
 
mguner said:
How many miles on the engine? Does there seem to be much slack in the timing chain? It sounds like maybe the timing has slipped a tooth on the chain..... RE: late valve timing..... 25 degrees initial sounds like a lot for a stock spec cam to not have starting issues.
Click to expand...

Probably the reason for the high unstable idle, low vacuum and idle screws not changing anything all three.
 
mguner said:
How many miles on the engine? Does there seem to be much slack in the timing chain? It sounds like maybe the timing has slipped a tooth on the chain..... RE: late valve timing..... 25 degrees initial sounds like a lot for a stock spec cam to not have starting issues.
Click to expand...
Unknown mileage on the engine. Timing cover isn't off so I can't see to the chain. Wondering if there's external things (ignition timing, throttle blades, etc) I should be doing/checking before I start tearing into the engine.
 
It could be a mismatch between the heads and intake manifold if either has been machined without machining the corresponding surface.

If the block or heads were decked, the head will sit lower and that will change the angle between the intake ports on the manifold and heads... Or... if it''s a used manifold someone could have machined it to match their combination in the past.
 
Back in my day to check the timing chain play I would turn the engine over counterclockwise (by hand) until timing mark on 0* then remove Dist cap and turn engine clockwise (by hand) until there was movement in the rotor. Look at the timing mark, anything more than a few degrees.... Off comes the cover to inspect/replace chain. Just my 2 cents.
 
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