What would cause this?

[RECHRGD]

When trying to maintain a constant rpm in park (don't ask why) the rpm's drop a couple of hundred for about two seconds and then goes back to the original rpm.  It will do this over and over but not at lower rpm's.  This happens in the 2000-3000 range.  Thanks, Bob

[1BAD68]

maybe the timing is drifting around?

[RECHRGD]

maybe the timing is drifting around?

Hmmmm, I wouldn't think that should be a problem.  Both the timing chain (double roller Comp) and gear is only a couple of years old.  The distributor has about 35K on it though, (Mallory Unilite) but I replaced the module last year.  Carb is a Holley 750DP with a Proform main body.  Bob

[John_Kunkel]

I'd put a fuel pressure gauge at the carb and see if the fuel pressure is steady when the rpm's drop.

[Rolling_Thunder]

   i would guess something up with the carb...    I know jack about them but that would be my guess

[69charger440]

I would think it could be a vacuum leak!!! Check all your lines!! And check your manifold bolts, and carb bolts!! Make sure they haven't came loose!!

[firefighter3931]

Bob, most likely it's the mechanical advance springs in your distributor. Unless you have 2 light springs on the weights the advance will come in after 2k rpm. Try revving it up to 3k (Total timing "all in") and see if it does the same thing...if not, then look at the springs. 


Ron

[RECHRGD]

Thanks everyone!     I'll probably have a chance to check all that out this PM or tomorrow.  Bob

[RECHRGD]

OK, I got to play with it today.  I ran it up to 3500 rpm and it still did it.  The fuel pressure and flow is steady when this is happening.  With a vacuum gauge hooked into the intake I have 8" at idle and it maxes out to around 18" at 2400rpm.  When the rpm's drop the vacuum drops 1" and when it goes up it gains 1".  The 1" change is constant regardless of rpm.  I pulled the pcv valve and it seemed to change but it still did it, just not as quickly (could be just my imagination).  There is no baffle in the valve cover under the pcv valve.  Any ideas???  Thanks,  Bob

[firefighter3931]

Bob, are you running power brakes on this car ? If so....it's possible the check valve in the booster is malfunctioning. Try pulling the hose off the booster and plugging it to see if that's it. Sure sounds like a vacuum leak of some sort. Have a look at all the hoses & plugs on your carb. Make sure the carb is snug on the intake manifold.


Ron

[RECHRGD]

Bob, are you running power brakes on this car ? If so....it's possible the check valve in the booster is malfunctioning. Try pulling the hose off the booster and plugging it to see if that's it. Sure sounds like a vacuum leak of some sort. Have a look at all the hoses & plugs on your carb. Make sure the carb is snug on the intake manifold.


Ron

Thanks Ron!  No power brakes.  I'll check all hoses and plugs, some are pretty old.  I hope the recent milling of the heads and intake are dead on.  I had others put it back together and they said it was.  Bob

[John_Kunkel]

Since the vacuum and fuel seem OK I would put a timing light on it and watch what the advance does before the rpm's drop.

[RECHRGD]

I couldn't find any obvious vacuum leaks.  As John suggested, I put my timing light on it and had the wife rev it up to about 2300 and hold it there.  it appeared that the timing mark would move slightly in sequence with the rpm change, but I would think that would be normal.  So, I don't know if that's telling me anything.  Bob