advanace problem in a flathead...

i have a problem with my vaccume advance on an 8ba flat head. i have the advance comming from the venturi port on the carb, but it doesnt have any vaccume. what are my options when it comes to mechanical advances?? are there any kits to switch the vaccume advance to a mechanical?? or is there something i can do to to fix the vaccume problem with the carb? any help would be great.

 

With a stock 8ba,the dizzy is a vacuum only advance unit. Assuming you are using a single '94' type carb,the signals to the vacuum cannister are'ported' vacuum. This is an internal balance in the carb between manifold,and venturi vac. signals. Ford called it 'loadamatic". It is a delicate balance between vac. signals,and calibrated springs,in the dizzy. It works OK with a stock engine,and carb. Make sure you are using a new vac. canister,and hope nobody messed with the springs. I converted a delco window to use in my flathead,to gain centrifical advance. I have even seen GM HEI dizzys for a flathead. Do a search there are lots of ways to use diffrent ingnition systems,only to improve on the Loadamatic.Sparky

 

Sounds like you've a pugged passageway in the carb... You might be able to blow it out with some spray carb cleaner. Are you running one carb or a multiple setup? Is the diaphragm in the distributer good?

Flatman

 

well, the engine is running off of the single carb, and yeah, the diaphram on the dizzy is good, and the vaccume port on the backside of the bowl of the carb is clear, but it has no (measured a hair above 0 on my vaccume guage) suction...


i think i want to change to a mechanical advance, so in the future i can change to a multi-carb set-up with out messing with the advance. also, i want to correct the advance issue now and not have to deal with the carb venturi port system...

any good places to look for a mechanical advance(new dizzy/ conversion kit)? i've done a few serches and havent really found anything.

thanks,
dave

 

How did you determine you have no vacuum?

Some rebuilds that mix up parts between 8BA and pre-8BA can also cause problems.

The proper term for the signal is venturi vacuum as defined by Holley who designed the Lodomatic and the carb.

 

heres another question.... how many passasge ways are there feeding the vaccume port on the carb?? the only one i know is clean is the one that feeds from the single barrel that goes straight from the venturi opening directly to the barrel. are there more that could be plugged??

 

how i measured the vaccume from the venturi port-


i have a hard rubber line that connects the metal line from the venturi port and the metal line to the advance on the dizzy. i removed the ruber line from the metal line and connected a vaccume guage to the metal line running out of the venturi port.

 

There are 2 vacuum sources. Air flow down the venturi, thus the name. The other is from a very tiny hole in the carb base which is engine (manifold) derived.

There were several mechanical only dizzys in the era, Mallory being the most common. Be careful you dont select a racing only dizzy which is always full advance.

MSD makes a drop in for 6 or 12V or you convert a Mopar or Chevy electronic or Chevy window type. Daves HEI Ignitions also provides one using Mopar guts I believe. Going to any of the new type requires you modify the carb for a ported vacuum source in the base of the carb; easy to do.

 

Did you do it at idle, or at speed?

I'm pretty sure you won't get jack squat at idle with venturi vacuum....the signal is basically reversed from manifold vacuum.

 

Ernie is correct, you will not read much venturi signal at idle through the port, it is measured as intake but is actually a measuremant of flow.

If you are measuring inlet manifold vacuum and have a low reading, it could be bad timing , air leaks , hot cam.

The only way to test it properly is with a T into the signal line for the guage and a timing light on the engine.
Now I know there are no timing marks as such, you just have to set the motor at TDC on No 1 and create some on the front hub and cover ,use a white corrector pen ( twink). Then watch the marks move as you bring the revs up, ( or not).
If you see no movement, remove the guage and T and replace with a vacuum pump ( mytivac type) run the engine again with the light and apply vacuum , see if the marks move , if not the advance is jammed or the diaphram is faulty.

If you have set TDC marks and have a timing light with adjustable timing function you can accurately measure/set timing.

 

Just to add to 97's comments, you should already have a timing mark on the 8BA pulley since they came with one from the factory, so no need to find TDC.

 

I got lucky. I found an early Mallory mech. adv. distributor for my engine (because it looks cool) when I got into flat heads late in my hot rod life.

The only thing that I have truely learned about the Ford flathead distributor is that it doesn't work like the vacuum advance dizzys that I and most of us grew up with. Everything that I learned about vacuum advance dizzys just confuses the issue when it's applied to the Ford dizzy. It's hard to "un-learn" info that has been gospel for years and years.

 

Go to search button--search loadamatic and loadomatic--no one can ever remember which it is...there are a number of discussions covering this thing.
Don't forget the flotation test.

 

Ported (or venturi) and manifold (or full time) vacuum taken from my mildly cammed 462" Buick engine:

Ported vacuum indicated by PV.
Manifold vacuum indicated by MV.

Two fairly matched vacuum gauges connected.



Firing the car from dead cold and on the elec choke, MV reads 18-19" and idle is around 900-1000 rpm.
Ported Vacuum (PV) read 12" on startup.
Once the engine warmed up, MV reads 19" and PV reads zero at about 500-600 rpm.

Cruise at 40 mph with a light throttle setting on a flat road gives you 18.5 - 19" MV and just about the same on PV.

Rolling the throttle in about half way shows 8-10" of vacuum on both MV and PV during light acceleration.

Once at 60 mph MV read 18 - 18.5" vacuum (keep in mind this is a very light car) and PV read 10".

Flooring the throttle at 40 mph or 60 mph brought the MV down to 1" or so and PV to zero.

The key thing is, at idle with a fully warm engine, MV reads 18.5 - 19" and PV reads zero.


Your flathead - depending on cam - should read about the same.


Fwiw - stock or very mild cams idle @ 17-19" vacuum, the closer to 19" means better ring seal etc.

A 280 degrees advertised duration cam idles 10-12" vacuum.
Idle speed in this case = 600 rpm.
You can run the engine up to 1000 rpm with no load and the vacuum reading in a well sealed engine will go to 18-19".


If you're running dual carbs keep in mind that air flow will be halved in each carb so you won't pull the venturi vacuum levels you did with one carb.

It shouldn't be a problem at idle, but as the rpms come up the advance will not follow rpms like it did with a single carb.

All of which makes me think running a distributor with both centrifugal and vacuum advance on a built - or at the least multi-carb - flathead is a good idea.

Since stock flatheads run about 21 degrees advance - measured at the crank - that figure should be easy to reach with an overhead type distributor utilizing limiting devices for the centrifugal advance.
Vacuum advance will be a bit of an experiment, but I'd start with 10 degrees additional - again, measured at the crank - and see how it did.
Keep in mind the overhead distributors vacuum advance retards timing when manifold vacuum drops.

Basics are: centrifugal advance reflects engine rpm.
Vacuum advance reflects engine load.

 

See if you can find the article where Barney Navarro talks about timing for a flathead.

He is one of if not the flathead guru and knows his stuff.

He explains where people were getting confused and how to set things up.

I'm fairly sure the Navarro article is on the net . . . somewhere.

 

Here tis:


Motor Life Feb 1954



Spark Advance Hop-up Trouble Spot By Barney Navarro

Among the mistakes made in hopping up engines, few exceed in number the misapplication of spark advancing principles. The chief source of error is the limited information available on the subject of spark lead. That which is distributed, unfortunately, fails to cover some essential factors and very often is no more than a comment to the effect that fuel charges take a certain amount of time to burn so spark must be advanced enough to compensate for the time lapse.

Well informed engineers wish that the problem really was that simple. Most ignition system purchasers overlook every factor except the amount of spark produced. The wrong system can cause plenty of trouble: plug fouling, poor gas mileage (even though the engine has no tendency to misfire), overheating in slow traffic, and other maladies. Basically, engines require some means of advancing spark timing as rpm increases since the pistons, in effect, try to get ahead of the burning speed of fuel charges. Combustion, witch takes a definite length of time, must occur when pistons are at the top dead center before the start of the downward power stroke. If burning finishes too early, energy is wasted because the resultant pressure rise produces a force opposition to rotation. This is readily apparent when starting an engine that has too much spark lead; it will actually kick back against the starter’s efforts. Modern high compression engines, while under full load, audibly indicate spark that is too far advanced by pinging. So the popular method of setting spark timing for maximum horsepower is to set it just below the ping point under full throttle operation. Distributors that employ flyweight governor advance mechanisms use a spark advance curve that conforms to the engine’s requirements under full throttle at any point within the rpm range. At low rpm a lesser spark lead is required so the governor advances a small amount. As speed picks up it advances more and more, always conforming to the full throttle full load requirements. On a drag machine, where full throttle and full load conditions are maintained, the flyweight governor is required. But for ordinary driving, which consists mainly of partial throttle operations with Very light loads, it is not enough. Some other means of compensating for varying loads must be provided. The load compensator is necessary because a light fuel mixture burns more slowly than a heavy charge since the concentration is less and flame takes longer to travel from one fuel particle to the other. If the utmost energy is to be obtained from light charges, their burning should be completed at the same point that the heavy charges finish. So if they take longer, the only way to make them finish at the same point is to start them earlier. Consequently, partial throttle partial load operation requires more spark lead at any given speed than is required at full throttle full load. Load compensation is the most commonly achieved by using intake manifold vacuum to actuate a diaphragm. This diaphragm advances and retards the distributor breaker plate and in some cases the whole distributor case. When the engine is operated with Very light throttle pressure, the manifold vacuum is high, so the diaphragm advances the spark timing to produce the most efficient combustion possible. As the throttle is depressed, the vacuum drops of and the diaphragm produces less advance until it reaches a point of being completely ineffective at wide open throttle. Thus the ideal load compensation is always maintained and results in more power from every drop of fuel.

The second most popular method of obtaining load compensation, though further from perfection is that employed in Ford V-8 distributors from 1932 trough 1948. Instead of a diaphragm, there is piston brake actuated by manifold vacuum, The flyweight governor mechanism is equipped with a breaking disk which cancels five degrees of the governor’s advance when pressure is brought to bear on its edge. At this edge a spring-loaded piston is located in a small cylinder. The spring is on the side of the piston opposite the disc so it causes the piston to be pushed against the disc. Vacuum is introduced on the spring side to oppose its action and lift the piston off the disc. In action, the high vacuum produced by operation with small throttle openings lifts the piston of the disk, allows the full action of governor weights to take effect and gives the Ford engine five degrees more spark advance. By depressing the throttle further, the manifold vacuum drops off and the spring again pushes the piston against the disc to retard the spark. The flaw in the operation of this mechanism lies in the fact that it is either “full on or full off” and permits no gradual compensation like the diaphragm.

Ford’s latest method of controlling spark advance employees an ingenious system utilizing manifold vacuum and venturi vacuum. With this system the flyweight governor is eliminated and in its place is nothing but a diaphragm. This diaphragm not only advances the spark to conform to rpm changes but is also makes load compensation adjustments. All ´49 through´54 Ford and Mercury carburetors have in addition to the conventional manifold vacuum takeoff, such as is found in the throttle body of most passenger car carburetors, a connecting venturi vacuum passage. The manifold vacuum, as usual, is obtained from a small port in the throttle body located slightly above the butterfly’s closed, position, on the side where the butterfly swings upward to open. When the throttle is closed at idling, the vacuum port does not receive vacuum because it is on the opposite side of the butterfly. As the throttle is opened slightly, this port is uncovered and a vacuum is applied to the distributor diaphragm to advance the spark. If the throttle is fully depressed, the manifold vacuum is destroyed and no advance takes place. As speed increases, however, the venturi vacuum increases gradually and advances the spark to conform to the rpm. Letting up on the throttle increases the manifold vacuum (Provided it isn’t let up all the way) and the spark receives load compensation. A balance is always maintained so that the correct amount of spark advance is supplied for all speed and load conditions.

The greatest installation errors center around the misunderstanding of the late Ford distributors. A distressingly large number of mechanics are unaware of the difference between manifold vacuum and venturi vacuum. In fact many attempt to operate Ford and Mercury distributors by connecting the vacuum line to the windshield wiper connection on dual intake manifolds. This sometimes results from a desire to use the old Stromberg carburetors, which are not equipped with vacuum takeoff. So the simple solution seems to connecting the distributor vacuum line to the handiest apparent source of vacuum. Such practice is worse than having no spark control at all for when the engines idles the spark advances fully and retards as throttle is depressed. There is no venturi vacuum available to advance the spark as the speed picks up and it remains retarded until the throttle is let up. So if the old style carburetors are preferred, the stock Ford distributors must be discarded on the late models. However, Stromberg has resumed production of the old “97” and is now fitting it with a venturi vacuum takeoff to make its use feasible.

Four throat carburetion installations also have had their share of improper distributors. Early articles in certain publications gave the impression that no vacuum control whatsoever could be tolerated. It wasn’t pointed out that the only forbidden type is that of the stock `49 through `54 Ford and Mercury distributor. This caused many to purchase distributors and magnetos that were equipped with flyweight governors only. Such installations get very poor gas mileage, so the car owners blame the four-throat carburetor. Even more irritating, is the tendency for spark plugs to foul. Having no load compensation, the spark is never far enough advanced under partial throttle to fire the fuel mixture charges at the most opportune time. In effect, the engine is being operated with a lower effective compression ratio because burning is completed as the pistons travel down the cylinder bores. And since the plugs never receive a hot flame, soot collects on them. Furthermore, the condition cannot be remedied by using hotter plugs because they will burn up under full throttle operation of flyweight governor distributor with vacuum-operated load compensation device.

In practice, the installation of dual intake manifold on Fords and Mercury’s of the ’49 trough ’54 series should be accompanied by a change in distributors such as prescribed in the preceding paragraph. The addition of two carburetors divides the airflow so only half as much airflows through one carburetor as previously at normal operating speeds. Venturi vacuum is dependent upon the air velocity through the venturi so any reduction in velocity will result in less spark advance. And connecting a line to each venturi vacuum takeoff of a dual set up will not increase the vacuum---such a practice is just a waste of copper tubing. The best advice to keep in mind when purchasing a distributor is not to pinch pennies. An inexpensive unit, if it doesn’t do the job correctly, can prove to be the most costly. The best way to avoid mistakes is to study the problems involved and learn enough about them so that you can select a distributor that matches your engine requirements.

 

thanks for all the help.

well, when i tested the vaccume at the venturi port, it was at idle, and also while revving the motor. it was not under load, though... maybe its different under load?? i want to eventually change to a multi carb set-up, so i figure i should might as well change the dizzy now.

i'm trying to find a mechanical advance unit, but i dont have 200 bucks for one. does anyone know of a good used one for sale??