Idle Vaccuum

What camshaft is in the car and what is the initial advance at idle?

Do the power brakes work with assist or is the assist missing?

Comp Cams 292H

It appears the brakes work with assist.

Don't have the initial advance.

A 292 will probably give you about 12 at low idle. I wouldn't be shocked to see 10.

You need to run as much initial advance as the engine will tolerate to bring up the idle vacuum.

You also need to verify that the power booster isn't leaking.

You need to run the engine have a connected vacuum gauge handy and operate the brakes.

When you step on the brake pedal the vacuum should drop momentarily. Probably about 4 inches.

If it doesn't change the reading, the vacuum booster has a leak.

A decent Vacuum is about 15 -18

Adjusting the idle mixture screws by 1/2 turn will not have a noticeable effect on idle vacuum unless the adjustment was close too being to lean or massively too rich.

1 1/2 turns out sounds about right.

I agree with the suggestions of looking for vacuum leaks and changing initial timing.


A vacuum leak will increase idle speed if the idle mixture is made richer as it will act like opening the throttle blades with the idle speed screw on the linkage.

Many high performance cams have lowish idle vacuum eg: 10" but a bit of clever tuning can usually get them up to 15".



Good idle vacuum is a mix of:

compression,

ignition advance,

valve overlap,

idle speed,

throttle blade setting,

Fuel and

bypass air eg: air bleeding in from vacuum powered devices like brake boosters, vacuum operated heater-A/C controls, pollution controls and even leaky vac advance canisters on the distributor.

Or even deliberately bleeding air in below the throttle blades to get more idle air into the engine without opening the throttle blades further, as in some cases that can start mixture flowing through the carburettor's transfer slots making the idle mixture adjustment screws unable to control the idle mixture properly.



.

quote:

Originally posted by Hemihunter:
A decent Vacuum is about 15 -18

Yes but the largest factor is going to be in the timing of the camshaft, the overlap being the most effect on it.

A 292 cam probably has an overlap in the 70 to 75 degree range. That right there will drop you down to the 10-12 range.

A 60-65 overlap, 12-14.

You sometimes can gain about 2 inches by running around 15 to 16 degrees of initial advance.

Also the more static compression the more vacuum you will gain at idle. If you are running this cam with the iron open chamber heads that is around a 2 inch loss.

If you want 17 inches and a hotter cam, you need to get very tricky.

Simplest way to do that with a Cleveland now is to get away from the off the shelf cams from the major manufacturers and use George's special "Cobra Jet" cam.



Clevelands are not your average engines because of the canted valves and port configurations.

You need to trick the engine and the only way you can do that is with a very special cam grind.

Incidentally, I would expect anything with under 12 at idle, the brake booster needing an additional vacuum reserve reservoir. In fact with 10 inches, you may have problems with the brakes causing the engine to stall in bumper to bumper traffic?

What will happen is that the engine won't be able to fill the vacuum booster for more than one or two pumps of the brakes without it.

You probably wouldn't notice that unless you were in bumper to bumper traffic? Every time you apply the brakes, the engine vacuum will drop 3-4 inches and with anything under about 8 inches at idle, it may be stalling on you?



Having said that though, the Pantera is not a car that really needs power brakes to begin with.

I personally think they were put in because of the concept of marketing the car as a "luxury sports car" through Lincoln-Mercury?

You can chase this low vacuum phantom all you want, but it's the overlap of the cam that you have that is causing it.

There is no other way to bring up idle vacuum then to reduce the camshaft overlap.

you could install a SSBC 28146 or similar

The downside to not having a roller.
A 244 duration @50 on a hydraulic lifter cam
should give you about 12 lbs of vacuum.
A 304 HR with the same duration but a roller
gives 13 lbs.easy.
Kick you timing to at least 16 at idle, don't be afraid to go to as much as 20.

Besides all that, since most everyone uses a Holley 4bbl of some kind, low vacuum affects what power-valve you can safely run without idling problems.

quote:

Originally posted by Bosswrench:
Besides all that, since most everyone uses a Holley 4bbl of some kind, low vacuum affects what power-valve you can safely run without idling problems.

"Webers" don't care. That is a Holley issue. Since 48IDA's were initially intended as "racing" carbs to begin with, high or low idle vacuum is of little concern with them as well.

They idle as well at 8 inches as 17.

Initial advance was 10 degrees. Increased it to 17 to get an idle vacuum around 15". Much better.

After I've been out driving the car it diesels when I turn it off. Doesn't do this when its just idling in the drive way and I turn it off. Could this be a spark plug issue?

No it is because of the total advance you have dialed in and you probably have a vacuum advance distributor?

Look for a vacuum advance limit switch. It is used in line on many Ford products since about 1975 or so.

It is a simple device that you just install in line in the vacuum hose to the distributor.

This will reduce the strength of the signal to the vacuum advance in the distributor and should limit or eliminate the dieseling.

Other than that you would need an electronic ignition system that retards the ignition when you shut the ignition off. I don't know of any that exist right now for that though?

Fuel injection will also stop this from happening by shutting off the fuel supply to the engine.

You are caught in the middle between the old technology and the new ones that fixed these issues as a result of dealing with "emissions requirements".

17 degrees is also a little on the high side. You may need to slowly dial it down until you stop the dieseling. I would guess 15 degrees will do it.

Another consideration if this doesn't work is that it many not be the total advance, it may be the rate of advance in the distributor.

In my experience you need to use the heavier mechanical advance springs so that total advance is not in until a minimum of 4500 rpm if not 5000rpm.

The light springs cut that in half, or advance twice as fast. They would be all in by 2,500rpm.

Todays pump gas will not permit that. Faster advance creates higher cylinder pressures just like a higher static compression ratio would and the gas can not take the added pressure without detonating, aka, dieseling.

High octane racing gas such as Sunoco 106 will have greater resistance to being compressed into firing without a spark, again, aka, diesleing.

Mark,

What is the idle RPM with the initial timing advanced to 17 deg?

Ron

Ron,
I bumped it down to 1100. I think that's a bit high. I'm thinking 900 is more reasonable.

Doug,
I have a Unilite distributor with mechanical advance.

800. 850. Is what you want.

You might need to look into a throttle solenoid as well?

I have mine idling with 16 degrees, 650 rpm, with the Webers and a 294S Compcams solid lifter cam. No solenoid. No vacuum delay. Vacuum can added to the A341 "Motorsport" distributor.

Shuts right off. 12 inches of vacuum, which is enough for the brakes and in this car all the vacuum level is a concern at all for. The Webers don't care how much idle is there...if any?

The 85 Mustang GT 302 uses a "Motorcraft" carb, which is actually a Holley and it uses the throttle solenoid.

I think I just saw one at low tide in my shop yesterday? IF it's there and not a mirage, you can have it to try and keep.

The 69-70 Boss 302 uses an earlier version of it. It might be easier to adapt to the Holley throttle plate than the "Motorcraft" version is?

They both have "unique" mounting brackets. The Boss set up since it is now a "rare antique" is expensive to come up with. The "Motorcraft" is available used on ebay but if you buy a new one is about $125.

The Boss version is easier to use and more compact.

I don't remember all of the solenoid set up details. I gave up on single Holleys sometime in the last century?

quote:

Originally posted by Husker:
Ron,
I bumped it down to 1100. I think that's a bit high. I'm thinking 900 is more reasonable.

Bring it down to 900 and see if the dieseling goes away.


Ron

Husker, not sure with your Holley distributor, but Ford distributors and those aftermarkets that use Ford's vacuum advance apparatus have adjustable vacuum. You stick an allen wrench (3/32?) in the hose bib and turn it clockwise or counterclockwise to adjust the vacuum level applied to the advance plate. Some engines like more than others- cam, compression etc.

Ron,
Will do.

Bosswrench,
Unilite distributor. Mechanical advance.

If this problem is recent, maybe you've gotten a tank of fuel not quite up to your usual preferred octane rating? I ran very low once and had to put in five gallons of 87 regular. Had to put into gear, let out the clutch and stall it to stop the dieseling. I dumped in five gallons of race gas and problem stopped. Never used regular again.
The other common problem, which you are addressing by reducing the idle speed, is excess throttle plate opening. Many carburated cars will diesel if you shut off a hot engine with your foot on the gas.
Smells really nasty too.