Y Block Normal Vacuum w/mild Perf. Street Cam?

[dw-satx]

The freshly rebuilt engine (292 Y block) in my '61 F100 pickup stumbles on acceleration. The carb (Autolite 2100) fuel mixture seems to be very rich at idle and I cannot lean it out with the idle mixture screws. If I turn them out more than 1/4 to 1/8 turn, the exhaust is noticeably richer from the tailpipe. This is the same carb that was running good before the rebuild. The rebuild was all stock components except for the cam. I put a Howard cam (P/N 292002-10) back into it. Intake lift is 0.463 and exhaust lift is 0.463 with a duration of 266 degrees. I understand that the vacuum decreases as the valves stay open longer, but what should I expect with this cam? I am seeing about 5 inches of vacuum at idle. Is that normal for this engine with the new cam? I am measuring the vacuum on the carb by disconnecting the vacuum line to the distributor as I don't see any other place to tap into to measure vacuum. Is there a better place to measure the vacuum? Because of the low vacuum, I think I need to change my carb power valve to a 25 from the current 75. To my understanding the current 75 would be staying open at idle due to the low vacuum creating the rich mixture and consequently the stumble on acceleration. Vacuum does increase when I increase RPMs and stays steady until I decrease RPMs.

[39deluxecp]

i don't think you are getting an accurate vacuum reading. 5" is really low for a mild cam like that. did you check for a vacuum leak by spraying starting fluid at all vacuum points?

[KULTULZ]

I am measuring the vacuum on the carb by disconnecting the vacuum line to the distributor as I don't see any other place to tap into to measure vacuum. Is there a better place to measure the vacuum?

That is ported vacuum. You need to read directly off the intake manifold. Is there a pipe plug style fitting at the RF of the carb mounting?

[dmsfrr]

Originally Posted by KULTULZ

That is ported vacuum. You need to read directly off the intake manifold. Is there a pipe plug style fitting at the RF of the carb mounting?

dw-satx,
There's also a full manifold vacuum connection at the rear but the one in front of the carburetor should be much easier to get to.
.

[Sid]

There should be a port on the carb. approx 3/8 or a removable plug in the intake for cars that are equipped with power brakes. At a minimum I would expect vacuum to be over 12-14 inches at a full vacuum port. As far as power valve as a rule of thumb Divide your vacuum reading by 2 for a power valve. 12 inches of vacuum probably 5.5 p.v. If your vacuum comes up as an even number like 14 go with 6.5. Always take into account your elevation for jetting especially if you are over about 2500 ft.

[darrell]

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just because your carb was working good before doesnt mean it will after.when was it last rebuilt.how long was it sitting

[dw-satx]

Tried the starting fluid trick after the suggestion and no vacuum leak detectable with that method.

[dw-satx]

Removed a plug from the front of intake and am now seeing 15” of vacuum. That means the 7.5 power valve should be correct and my problem is elsewhere. I do have a rebuild kit ordered for the carb and will post an update after that has been completed. Thanks for the suggestions!

[wellcraft17]

My yblock runs a steady 23inches of vacuum with a bone stock engine and a 2bbl carb. It seems to me to be on the high side but don’t know why, engine runs great.

[KULTULZ]

Removed a plug from the front of intake and am now seeing 15” of vacuum

Make sure the valve lash and IGN Timing are correct.Then take reading.

[Daves55Sedan]

If I remember correctly, normal intake manifold vacuum at idle for a Y-block should be somewhere between 19 - 21".
First thing that comes to my mind is valve lash adjustment. You want to hear just a tiny bit of tappet noise. If you can't hear any tappet noise with the valve covers off, your valve lash adjustment may be too tight and your valves are staying open too long. The original specifications call for a gap of .019"

[Sid]

As far as your rich condition I wouldn't rule out a power valve problem. One test you can try for a power valve is turn your mixture screws all the way in. If the motor does not stall you may have a bad p.v. Having a backfire can damage a power valve. You may also want to check fuel pressure and float level. As far as the vacuum I think you are in the somewhat ball park with a performance cam. Not knowing what the lobe center line is. A cam with a lobe center line around 108-110 will have a lower vacuum then a cam with 112-116 lobe center line. In a perfect world at sea level and a bone stock engine it wouldn't be unusual to see 19+ in. of vacuum. One thing to watch for is a steady needle. Also elevation plays a small role in engine vacuum. It may drop about 1 lbs. per 1,000 ft. in elevation If it is a fresh engine it may even increase a little after breaking in. Also when using a performance cam you generally run a little more initial timing. I think I would probably go to about 8-10 deg. advance as a starting point with hose off the dist. and tune from that point. Also as mentioned earlier watch valve adjustment.

[Ole Don]

My 312 had some compression, and an Isky cam, and delivered about 11 inches of vacuum. It was enough to make the wipers run well, unless it was in fifth gear on a hill.

[KULTULZ]

It sounds like a fuel problem but the engine has to be in a good state of tune before going there.

If you take off an old carb that hasn't been serviced somewhat recently, you may disturb the settlement of debris in the carb bowl and that gets in all the circuits at fire-up. Most likely, it needs an overhaul, a quality filter and determining proper fuel pressure/volume.

You cannot determine PV selection until you know optimum manifold vacuum.

[Sid]

Originally Posted by Ole Don

My 312 had some compression, and an Isky cam, and delivered about 11 inches of vacuum. It was enough to make the wipers run well, unless it was in fifth gear on a hill.

That's about where the 406 was in my Starliner. It also makes a difference when the intake valve closes ABDC. Running 2x4 I think I was running 4.5 p.v.'s with progressive linkage and it ran like a raped ape.

[Daves55Sedan]

Not 100% sure about this, but I always thought that the power valve size had to do with the amount of venturi vacuum is expected to open the valve.
The standard size used in the Holley 2100's and Ford EBU's of 1954/55 on the 239/272 Y-blocks was 6 - 6.5
Not sure if the '57 and later carburetor configuration caused a difference in the expected venturi vacuum at the PV chamber.
Noting this because you mentioned that you are using 7.5 in your Autolite 2100. That is a '57 and later style carb, right? They are completely different from the pre 57 bug-sprayer type carbs.

[Sid]

The power valve is controlled by manifold vacuum. When manifold vacuum drops. When you mash on the gas pedal manifold vacuum drops and the power valve enriches with more fuel. If you have a high numbered p.v. enriching can happen too soon and even a rich idle on a modified engine and too low number can cause a hesitation. ( vacuum secondaries on a 4 brl are controlled by venturi vacuum.

[54vicky]

use the list on the page tool supplied before chasing your tail replacing parts.some of the suggestions are good others will empty your wallet and add to your frustration.remember the saying keep it simple that chart answers pretty well any questions and has since the advent of it.

[55blacktie]

dw-satx, I realize this is an old post, but where did you come up with 266 duration for the Howard's 292002-10 cam? I have the same cam, and the advertised duration is 255; the duration at .050 is 220.

I do realize that "Advertised Duration" and "RPM Range" for camshafts can be misleading. "Duration @ .050" seems to be the more accepted standard.

Is there anyone still onboard that has used/is using this cam? I have yet to install mine and would like to know what to expect performance wise. Please don't speculate, if you don't know.

[scicala]

If you can turn the idle mixture screws all the way in without the engine getting rough and dropping RPM (at a reasonable idle speed), then remove the carb to access the power valve cover. Square cover with 4 screws holding it on. If there is fuel under the cover, it means the power valve rubber diaphragm is ruptured and needs to be replaced.

Sal