Vacuum Advance on Flathead?

Been fighting a stumble problem need some advice.

This is on a 52' 255 Mercury Flathead.
+1.0 Compression
Triple Strom 97's on Offy Intake
Mild Cam

I keep getting stumble at idle-1/4 throttle, checked static timing (fine), carbs are set correctly on fixed linkage. New plugs ~100 miles ago. Car runs great just about everywhere else.

Pulled plugs show clean burn except for a bit of black ash, so standard guess would be vacuum advance.

1. Any idea what a Flathead with the above parts should pull vacuum.

2. Sucking on the end of the vacuum line gets me ~0.5-1mm of movement and vacuum advance unit does not hold a vacuum, it leaks. (Is this normal).

3. How much advance should I see when a flathead vacuum advance is working properly. How much movement on the rod?

 

You need mech advance with multi carbs!

 

Yep,, if you got the coin contact GMCBubba here on the Hamb and he can hook you up with the cures for your ails

 

the load a matic dist has no centrifugal advance it's vacuum only, GMCBUBBA makes a neat sbc converted dist for flatheads.

http://www.lindertech.com/bhrs/flathead_chev.htm

 

Can he recurve the stock unit to my spec's as well?

I will try and send him a PM later today.

 

with vaccuum advance only theres not really a curve, you get what you get till it hits the stops. Mechanical advance is what you need, and the stocker has none.

By far the best modification you can make to your flatty. It is hard to describe what a difference it will make across the board.

 

They're right........Gotta either go mechanical or make two block off plates.

 

Stock distributors were barely adequate for a stock flathead on the later motors. Get rid of it for starters.

 

Ford used that vacuum-only distributor until the mid-'60's, my 240 pickup had it. You wouldn't believe the difference when I swapped in an electronic mechanical advance dist. from a newer truck! Quicker starts, more power, and 3 MPG more, at least. Plus no more low speed stumble, I can turn a corner and mash the gas in high gear and she just pulls away, no fuss. I changed the carb too, the old one has the vacuum sourced from the venturi, it won't work with the newer setup. You can just tap manifold vacuum, I suppose.

 

I used the MSD Ready-to-Run dizzy. A lot of folks don't like the big cap, but in most respects it's a quick drop-in, just a change of springs on the centrifugal advance, a swap to an Accel adjustable vacuum can, and go. Like said above, even on a totally stock engine the difference is immediate and unbelievable!

 

Horrible design from Holley, rarely adequate even when brand new. A 99 cent replacement for the good early distributor. Only works well at part throttle when there's manifold vac available...California Bill reported in about 1950 that most '49 Fords would go 0-60 faster at part throttle than full because of this!
The curve specs, which it could usually not meet even wthout the torn diaphragm yours has, are in the shop manual and service bulletins. It was to be tuned on a distributor machine, which in this case gives highly misleading results because it is fed vac from a pump.
Finding its actual full throttle curve in use requires a lot of ingenuity...revving up the engine with a timing light will tell you nada.
Other than that, though, it's a great distributor.

 

Sure the chevrolet conversions work very well to replace the old vacuum unit. Kinda like pulling a bad tooth , get it out of there and no more pain.
The conversions are $150 with free shipping to barners and hambers.....
We will hit over 400 units shipped to happy flatheaders this year......

 

You do not have the correct carbs for that Loadamatic Distributor.
You must have Holley 94's. The holley carbs incorporate a venturi vacuum whereas hooking vaccuum to strombergs or to the intake will in fact make the advance work backwards. Get rid of that distributor and install one of GMCbubbas setups.

 

I have a Bubba converted Chevy dizzy in my flathead and it is great I love it.
I also got him to modify an old school Chevy duel point mallory that is currently sitting on my mantique shelf.
I will put it in this summer because it looks way more authentic.

​

 

Explains a bit of my headache, BUBBA's going to get my cash.

As soon as my membership goes through your the first person on my list. Do I need to send you a CORE or do you have them?

That and those cool little Strombreg throttle springs that fit on the butterfly shaft.

OK this is a bit odd for me to follow. Say I hook the new vacuum diaphragm up the manifold vacuum. I would get idle advance off idle and on low RPM while cruising due to the high vacuum. Them nothing as I move up the RPM range correct?

Just trying to understand. The vacuum idle pump is supposed to increase timing not retard timing right? Then it levels out off idle on medium to large load. Which if I had the 94's or the stock carb the Ventri vacuum would kick back in and bring it up for more power.

 

The thing is hooked up to ported (off-idle manifold) vac and venturi depression via internal passages on original carb. The part throttle pull from the vac works pretty well (if you fix broken diaphragm) and gives OK part throttle advance. Full throttle advance is handled via the venturi drop, which is small. Ford somewhat reduced the advance curve spec for these but that signal is seldom adequate on a stocker for a good curve. It is based on the amount of air going through the venturi to produce the drop...run the air through 4 venturis instead of one, signal drops and also comes in later...advance typically becomes feeble-to-none.
If hooked up to manifold vac directly, you would have full advance at idle and then advance would fall off to nothing or close to it as throttle opened...
the major problem is with the high throttle opening curve which is generally way off spec even with stock carb...
measuring actual advance conditions on the road is a whole science project in itself! Advance depends on everyvariable of speed and load, and so must be measured in action. Stationary readings are meaningless, setting on a machine with a vac pump does not tell you what is happening with actual depression from your engine at X road speed and Y throttle opening.

 

Bruce thank you for the clarification. Cleared a lot up.

How anybody tuned these things on car back in the day... must have been almost all trial and error and improved trap speed's.

The scientist in me wants to throw a vacuum gauge on the car and fiddle, but I am going to go with the prevailing wisdom and get the mechanical advance setup and spend my time getting it to work exactly the way I want.

 

If you want to fiddle...first arrange for venturi and ported vac. Strombergs with '49 Ford fitting were nade, but were always scarce...So...tap into one of the little plugs above throttles on the lower casting with a tube on your middle carb.
Drill into a venturi from behind...vey small breakthrough, larger entry of some sort for a small tube. Scrutinize a Ford 94 for location tips...
Then hook up a very sensitive vac gauge into a T on the combined line into distrib...
Record drop under lots of combinations of load and speed, probably focusing first on full throttle accel and high speed cruise where the shortcomings are worst...
Then using SAME gauge pump up those readings with a hand vac pump with car idling and see what advance you have at that level. I suspect that full throttle with three carbs is not going to move the needle on the gauge much...
There were later additions to this system in various Fomoco's, like a shuttle ball in the joint passage and an off valve in carb base...and I think some HD trucks may have used a double diaphragm with split hookup...but I am not very familiar with these later kludges. You would need lots of service manuals and bulletins to research them.
Or find a brave friend who will lie on the front fender holding a timing light...
If you have enough vac to do anything at full throttle, fiddling with springs and their adjustable posts may get results, but may also allow timing to over advance at part throttle.
Definitely an interesting science project!

 

The '57-72 Chevy type is an ideal choice for this because plenty of hardware is readily available to adjust centrifugal and limit the vac to flathead needs, and of course the architecture of those allows changes to be made quickly and easily.

 

Well I tried a bit of fiddling today like you discussed and only managed to screw almost everything up. You could hear the backfiring two counties over.

Stripped out one of my 97 bowls, lost my original timing settings and in general just had a crappy day. I would have been better off just flushing my head down the toilet. Will have to

So going to order the correct dizzy, reset my Stormbergs to basic setting and pray that I can find the right timing marks.

I am seriously thinking of going down to one carb for the time being

 

When it comes to flatheads, "Been there, done that, wore out the damn Tee shirt 50 years ago".
But "back in the day", you could differentiate betwween a real hot rod mechanic and a "wanna bee" on this issue alone. A good mech. would know to avoid that Loadamatic dist like the plague, and the usual solution was a Mallory dual point, which had centrifugal, mechanical advance.
The knowledgeable, but penny pinching guy would use the correct venturi vacuum carbs and spend untold hours trying to get a system of lines, restrictors, diaphrams, and springs to work, so as to avoid the distributor purchase and use that abomination OEM Loadamatic. One guy I knew then and he still jury rigs stuff today, even removed the diaphram and hooked a choke cable to the advance plate for manual advance!
The clueless relied on a knowledge of only part of actual correct setups, and used Stromberg 97s, those illustrious paragons of performance lore, along with a Loadamatic dist.

 

Couple hours of trying to get it to work is more then enough for me to buy the Chevy piece. Might not be original but at least the car will work.

I love the way the 97's look but in the long run I am going to have to really get mine dialed in.

Everything is working but thinks like the accelerator pumps are not matched properly but after a bunch of fiddling I cannot find a way to balance them out.

 

I run a Mallory Double Life flathead dizzy with points & 2 Strombergs, it runs great!

 

Talked to GMC Bubba and ordered one up for the motor said it should go out sometime this week. Easiest 3 minutes on the phone that I have spent with a parts seller.

Cannot wait.

Also I am down to a single carb and no vacuum right now so that I can at least drive to a show this weekend ~10 miles away. But if I can get the distributor in fast enough I would love to have it in by then. Still running a little cold on temp but much better low speed throttle response until I really start needing the advance.

 

Please, when you get the dist from GMC Bubba dialed in, come back on this thread and tell us how you like it!
Dave

 

I am seriously thinking of going down to one carb for the time being "

If you are having multiple tuning issues, this is a good idea. Get a distributor with probable good curve...Bubba, old flattop Mallory...or rebuild a modern Mallory with NAPA tuneup parts and get it recurved, mostly pretty bad from factory.
Tune in each carb as a single WITH a distributor that has reasonable curve, then start figgering out the multiple setup. Trying to tune when you may have three unrelated issues leads to madness rapidly.

 

Been thinking bout the timing on my flathead and this post popped into view. I,m thinking of using a chevy distributor conversion using the chevy mech/vacuum advance setup. Now my question is what do I havto do to setting it up with the GM type of advance, not the ford type. Like what type of setup does the old flathead Caddy/Lasalle,s use? Or any flathead, wouldnt a Dodge or Plymouth flathead have a conventional distributor, with mechanical and vac advance? wouldnt a ford flatty work with GM style distributor advance? To me a flathead is a flathead, maybe I' missed it? ??

 

The ford factory unit is vacuum only and worked very well when used with a stock engine and gear ratio etc.
However when modified the engine needs a rpm based mechanical advance and the Chevrolet has the best to offer.
We have shipped over 400 in the past couple years and they solve too many problems to mention.....
Glad to help anyway I can......

 

Just got my distributor back from them last week and it works great. You can't go wrong with Bubba's and Peggy is fun to harass.

 

ok is it hooked up to manifold vacuum like the chevy, I guess is what i,m asking??