C1 1954 vacuum fluctuation

[Joe R.]

My 54 Blue Flame has 7000 miles on a complete rebuild, and have had recurring stalling issues when the engine gets to normal operating temp. I have rebuilt all carbs and reset the float levels, adjusted the valves to spec, checked timing (at 3 BTDC), but the problem persists.

Last night I put a vacuum gauge on the manifold port that goes to the Powerglide and the vacuum fluctuates wildly between 15 and 22" HG. Question, can that indicate a leaky valve, and are there other possibilities?

Thanks in advance!

Joe

[Troy P.]

Re: C1 1954 vacuum fluctuation

Could it be a bad tranny modulator diaphram or when you put the gauge on did you disconnected the hard line to the tranny? Do you have the sealing rings (not sure what the technical term is for those 3 parts) installed between the intake manifold and the cylinder head? If not that could be the source of a vacuum leak.

[Joe R.]

Re: C1 1954 vacuum fluctuation

I "tee'd in" to the vac line to the modulator when measuring the vac. Did not disconnect the line to the trans. I do have the split intake rings installed in the head ports.

How can I check that mod. valve? The trans shifts well.

Thanks!

[Troy P.]

Re: C1 1954 vacuum fluctuation

Joe, you could disconnect the hard line to the tranny and and plug the intake where it was connected and see if that makes any difference in the vacuum readings.

Going back the basics of vacuum gauge 101 - excessive fluctuation at all speeds = faulty head gasket; steady at idle but fluctuates when speed is increased = valve springs are weak; fluctuation of up to about 6" = faulty carb adjustment...but you probably already knew that.

[Troy P.]

Re: C1 1954 vacuum fluctuation

Last night I put a vacuum gauge on the manifold port that goes to the Powerglide and the vacuum fluctuates wildly between 15 and 22" HG. Question, can that indicate a leaky valve, Joe[/QUOTE]

If it was a burned or leaky valve the vacuum would only drop each time that valve operated. It would not be constant.

[Joe R.]

Re: C1 1954 vacuum fluctuation

Very good point Troy. Didn't think of it that way.

I'm going to disconnect that mod valve vac line tonight and measure again. What carb adjustment could cause that fluctuation? Uneven butterfly openings? Ball check valve?

[Troy P.]

Re: C1 1954 vacuum fluctuation

I would guess that too lean on the idle mixture could cause stumbling but I am far from an expert on carbs so don't count on that. I would think a 54 running lean would be unusual.

[Joe R.]

Re: C1 1954 vacuum fluctuation

I agree. If anything, it runs on the rich side most of the time.

I'll let you know what happens after tonight. Thanks again Troy.