Vaccum Readings

[Bernica]

I connected a good new vaccum guage to the 3/8" port in the carb baseplate. At idle, it is pulling around 10 lbs. As I accelerate, the vaccum climbs to 20+. If I jump on the throttle, it briefly drops, then starts climbing up again. I thought I should be losing vacuum as I accelerate.
Brand new Pro Systems 850 dp; cam is 236 dur .050 with .572 lift.
Am I missing something here? Or did I just sleep through that class??...

[patrickt]

FWIW, I get a little over 13 at idle with my 245/245 at .050, solid flat tappet, and Holley 750. A typical, normal, vacuum gauge looks like this:

[Bernica]

Thanks for the video(s) Patrick. I feel better. I'll run mine on the gauge again tomorrow and record it too. Getting a little stumble / miss in the 2000 rpm range that I'm trying to track down.

[undy]

As the engine RPMs increase at part-throttle you're engine will be pumping much more air but your throttle plates will only be slightly more open, making the ratio (pumped air to throttle plate opening) slightly disproportionate. This will have a tendency to create more engine vacuum on an engine that doesn't produce much vacuum at idle.

Dave

[RICK LAKE]

Bernica If you are having a little stumble in this rpm range, it's in the area of where the primary jets are maxed out and the mains are coming on line. Too be sure about this, an A/F meter is the only way to know for sure. Depending on carb you are running you may need to change the acceration pump cam to start a little earlier to stop this. Going to a Dyno shop is the best bet at fixing the car correctly and getting the right 12.8-13.2 a/f ratios with the car running. Your floats in the float bowls could be low too. There is too many could be's for this issue. If you are running an MSD system, they are know for having problems too. Rick L.

[Bernica]

Thanks Rick,

I do intend on taking it to a chassis dyno in a week or so. I finally found one near where I live. Just been trying to get it as ready as I can. the carb is a brand new Pro Systems. This is the setup it came with:

858 CFM
Mech Secondaries
Main Jets: Primary = 80 / Sec = 88
Acc Pumps: Pri = 50cc / Sec = 30cc
Squirters = 42 (both)
PV: Pri = 6.5 / Sec = Plug
Air Bleeds: Idle Air = 76 / Hi Air = 36
Float Level is < 50% in sight plug
The cam looks like it's black or dark burgandy color
Holes drilled in throttle plates
Secondary throttle plate is only very slightly open, almost closed at idle

MSD Digital 6 Plus ignition and MSD billet distributor
15 deg initial timing / 34 deg all in at 3,250 rpm

New Autolite 3924 plugs, gap = .045
New Ford Racing wires
100 octane premium gas

[Archrms]

If he is pulling 10 at idle, shouldn't his PV be closer to a 5.0 instead of a 6.5?

[Gaz64]

Originally Posted by Bernica

I connected a good new vaccum guage to the 3/8" port in the carb baseplate. At idle, it is pulling around 10 lbs. As I accelerate, the vaccum climbs to 20+. If I jump on the throttle, it briefly drops, then starts climbing up again. I thought I should be losing vacuum as I accelerate.
Brand new Pro Systems 850 dp; cam is 236 dur .050 with .572 lift.
Am I missing something here? Or did I just sleep through that class??...

If the carb starts pulling vacuum again at wide open throttle, the carb is too small for the engine.

4 barrels are rated at 1.5 inches vacuum.

Or your air filter is too restrictive.

[Bernica]

Thanks all for the input. I finally figured out how to put a video on here. I tried my best, never done this before. I tried to show vaccum readings at different rpm's and you can see (hear) the missing I am chasing (see thread above).
I hope this works! Hey...I can build you a 70-story building anywhere, no problem, turn-key.... This is kindof new stuff to me.


http://www.youtube.com/watch?v=7sMKIQyCLSs

Thanks.

Michael

[Barnsnake]

Unfortunately, the testing depicted in your video is not very useful. The carburetor and the timing advance curve are designed to make the engine function properly under load. When revving an un-laden high performance carbureted engine you will almost always hear some blubbering or other odd behavior at some constant RPM. In some cases there may be an rpm range that the engine will not maintain without load, due to carburetor metering transitions and/or timing advances.

A detailed description of what the engine does "wrong" in on-road operation would be helpful here.

[Bernica]

Very good points Barnsnake. Thanks. I plan on putting it on load tomorrow and we'll see where I'm at.

[RICK LAKE]

Bernica Mike That motor has alot of camshaft in it. 900 rpm and vacuum of about 7-8. Throttle responce is quick and nice. 2,000-2,200 rpms the motor is just starting to come up on camshaft profile. Power range I am guessing is about 2,800 rpms to 6,500. Idling the motor will only cause loading up of the spark plugs, even with MSD. Where your spark plugs indexed to the motor before installing? Do you know the spec on the camshaft? I would tell you that the motor is running as well as you can get it with basic tools. Next is to the Dyno with A/F meters in both side pipes. This will clean it up a little. The problem with this is every day is different, Temp, humitity, Baro, moisture in the air. Altitude. All these make the car and motor run a little different. It's like if you live in a low area and get heavy air and the car feels like it just got a 20HP kick in the butt. We get this by the shore in the evening. Be careful not to lean this out, or too much timing. Rick L. Ps nice video. Sound good.

[patrickt]

Mike, the needle acts pretty normal to me, but I have to say I think your vacuum is maybe just a bit low. Maybe your LSA on your cam is tighter than mine, it's hard to say. But for comparison, here's a shot of my engine with my vacuum gauge. You can see I idle at about 850 and I pull a steady 13". My cam is a solid flat 245/245, .525 at .050 with a 114 LSA and I have it installed advanced about 5 degrees. My carb is the old style Holly 750 (4160). If your engine runs nice, pulls hard, and generally does well then I wouldn't worry about it -- although most people would raise an eye at only 7" at idle. I've used a vacuum gauge for decades, and (obviously) I tune my Cobra with one. It's not as good as an A/F meter, but watching the numbers, and how the needle dances, will help tell you if you have a problem.

[Bernica]

Thanks Rick and Patrick. FYI, the cam is 236/236 @ .050 and 296/296 @ .006. Lift is .572 and lobe separation is 108 degrees.