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Technical Flathead spark plug questions

Discussion in 'The Hokey Ass Message Board' started by dmoz, Feb 23, 2020.

  1. dmoz
    Joined: Sep 8, 2019
    Posts: 29

    dmoz
    ALLIANCE MEMBER
    from Texas

    I'm about to change the sparkplugs on my 1950 mercury. It has the 8CM flathead v8. I decided to go with the autolite216s. I recently did the pertronix 12v ignition and coil upgrade. She currently has a stock 94 carb but will be going to a dual 94 set up with progressive linkage soon. What should I gap these plugs at and should I change that once I switch to dual carbs?

    Thanks in advance for your responses.

    If there is a thread on this already somewhere that I have missed; I would appreciate a link

    :)
     
  2. I would gap them stock and would not change it for dual carbs. I use Pertronix on all of my Farmalls and run the stock gaps on them.


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  3. big duece
    Joined: Jul 28, 2008
    Posts: 6,152

    big duece
    Member
    from kansas

    You might consider a distributor upgrade from Bubba's Ignition when going to dual carbs.
     
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  4. theHIGHLANDER
    Joined: Jun 3, 2005
    Posts: 8,210

    theHIGHLANDER
    Member

    The famous Bill "Grumpy" Jenkins: "Always run the widest gaps you can without misfiring for the highest [efficiency]." That said I never see issues adding up to .005 or more to the stock spec. With newer or upgraded ignitions gaps that were once stock at .018 can safely go .030 without fail.
     
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  5. Truck64
    Joined: Oct 18, 2015
    Posts: 4,631

    Truck64
    Member
    from Ioway

    Thing is, it isn't going to misfire at idle, the only way to tell is when under load, maybe a long uphill pull on the highway. Internal/external crossfire inside/outside the distributor cap. The points style distributors and caps weren't made with more than about 20k ignition volts or so in mind, so they can be picky about excessive gaps.
     
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  6. tubman
    Joined: May 16, 2007
    Posts: 5,641

    tubman
    ALLIANCE MEMBER

    I totally agree "Big Deuce" and "Truck64". I would also suggest giving some thought about running a progressive linkage. It is certainly a bad idea if the manifold you are planning to use is a "Super" and may or may not work with a "Regular". I would try it both ways and go with what works best. You have to take fuel distribution into consideration.
     
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  7. Jim Linder"BUBBA" is not taking any new work due to health reasons.
     
  8. dmoz
    Joined: Sep 8, 2019
    Posts: 29

    dmoz
    ALLIANCE MEMBER
    from Texas

    I will be running them on an Offenhauser 1075 that I got a good deal on. After speaking with Charlie at Vintage speed, he recommended going with progressive linkage. Not building a hot rod, just want a little more umph. Any further thoughts? Thanks for the feedback!

    Offy.jpg
     
  9. Mike VV
    Joined: Sep 28, 2010
    Posts: 2,354

    Mike VV
    Member
    from SoCal

    What a JOKE...
    Six "answers"...and NO ONE has answered the original posters question..!??? Pretty weak.

    Ok...a quick look into the world of...Google..! It provided me a great tune-up sheet of stock specs. for most flat head engines.

    Seems that the factory used .025".
    And why would you think that adding carburetors would require a different plug gap. No...no change with any amount of additional carburetors.

    As for the "Bill Jenkins" comment. This is true, ONLY of the coil and the distributor components are in good condition. Especially the plug wires themselves. A weak coil will/may give you trouble, weak or used up plug wires will cause arcing. Do NOT bundle or tie the plug wire together. Even a wimpy stock assembly will arc and cause misfires with bad wires.

    With the Pertronix parts, you should be able to go to .030" gap without trouble. But again, with good, "unbundled" plug wires.

    Mike
     
  10. chicken
    Joined: Aug 15, 2004
    Posts: 164

    chicken
    Member
    from Kansas

    With the new Pertronix stuff I'd gap the plugs at .030". If the Autolites give you any problems (they aren't as good as they once were) I'd suggest NGK plugs...B6L work nicely in engines like yours and mine.

    Terry
     
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  11. tubman
    Joined: May 16, 2007
    Posts: 5,641

    tubman
    ALLIANCE MEMBER

    "Mike VV" has a point, but there are more important considerations in play here than "spark plug gap". Certainly, if the Pertronix is in a stock "Load-a-Matic" distributor (that application is available) and the O/P wants to add a dual carb setup, plug gap will be the least of his problems. As to plug gap and carb linkages, both are super easy to experiment with. My recommendation as to plug gap? Start at .030 and see how it runs; if there is a high speed miss? Maybe close it up a bit. Runs fine? Leave it alone or maybe open up a bit more and see what happens.

    I would be more concerned with the choice of carb linkage. From the looks of that manifold, my initial thought is that it work better all the way around (including gas mileage) with a straight linkage. Look how far from one carb base the mixture has to travel to the far intake ports. It looks to me like a sure case of 4 fuel starved cylinders when running on one carburetor. Also, which carb is the primary; the front? the rear? Progressive linkage works best on those manifolds that have the carburetors biased towards the rear with the front carb as the primary, as it is closer to being centered between the two sets of intake ports. If it were mine I'd try it all three ways : straight, progressive with the front as primary and progressive with the rear as primary. See what works best. (Actually, if it WERE mine, I'd just go with straight linkage)

    Those two factors should be settled before you should start worrying about "fine tuning" the plug gap.
     
  12. tubman
    Joined: May 16, 2007
    Posts: 5,641

    tubman
    ALLIANCE MEMBER

    A question for the O/P : what kind of distributor are you running? A converted stocker or an aftermarket unit? If the latter, mechanical advance only, or vacuum over mechanical. It's something we need to know to go very far with this.
     
  13. dmoz
    Joined: Sep 8, 2019
    Posts: 29

    dmoz
    ALLIANCE MEMBER
    from Texas

    Converted Stocker
     
  14. tubman
    Joined: May 16, 2007
    Posts: 5,641

    tubman
    ALLIANCE MEMBER

    There are folks that say a stock distributor will work OK with a dual setup, but if you want to get the most out of this setup, you should upgrade the distributor. It's also another reason why a progressive linkage might have some advantage. Try it, but if it doesn't live up to your expectations, consider a distributor upgrade. I really can't say, as the only "Load-a-Matics" I have run were on bone-stock engines.

    On the slight chance you are unaware of it, Fords of this era used a completely different ignition advance mechanism than just about anything else. I'm sure you are aware of the vacuum line going from the distributor to the center back of the carburetor. It carries a combination of venturi vacuum and ported vacuum that you can only get from a carburetor originally used with this type of distributor. If you change carbs (or even add one), the vacuum signal isn't the same and the advance mechanism in the distributor won't work properly. It's also useful to know that the stock distributor is vacuum only and has NO mechanical advance built in.
     
    Last edited: Feb 23, 2020
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  15. dmoz
    Joined: Sep 8, 2019
    Posts: 29

    dmoz
    ALLIANCE MEMBER
    from Texas

    Thank you very much Mr. tubman for the pointers. I was not aware of the difference in vacuum signal. I must confess that I have only recently gotten into old cars. I have been reading and trying to learn as as much as I go. Your advice is very much appreciated. Can I bother you with any recommendations on a distributor upgrade for my current set- up?

    Thanks again!
     
  16. 19Fordy
    Joined: May 17, 2003
    Posts: 7,196

    19Fordy
    Member

    Does that OFFY intake require the generator to be offset towards driver side?
     
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  17. dmoz
    Joined: Sep 8, 2019
    Posts: 29

    dmoz
    ALLIANCE MEMBER
    from Texas

    "The generator may not fit into the stock location due to the clearance issue with the carburetor. The generator can be relocated to the passenger side using bracket # 325-1146. If using a GM alternator, use our chrome 916-67909 or plain 916-67910 bracket."
    I have the GM alternator and bracket installed as listed above
     
  18. big duece
    Joined: Jul 28, 2008
    Posts: 6,152

    big duece
    Member
    from kansas

    Can't go wrong with a GM points converted to 8ba, or Mallory flat top for 8ba.
     
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  19. tubman
    Joined: May 16, 2007
    Posts: 5,641

    tubman
    ALLIANCE MEMBER

    The most common way to go is with a distributor for a small block Chevrolet converted for use in a flathead. There are several people who do the conversions, the best one, in my opinion, is now a guy known as "Charlie ny". He is a regular on "The Ford Barn" forum and can be found there by that name. If this is your first and only old car, that forum might be a better place for you to get information. That way, you don't have to wade through all kinds of Chevrolet, hemi, and nailhead stuff to find what you need.

    The other way to go is to find an aftermarket distributor manufactured specifically designed for use in flathead Fords. They are available at swap meets and on eBay. I personally am hung up on old Mallory dual point units called "flattops" that have a two-piece distributor cap and just look neat. They are kind of expensive and are getting hard to find, though. Or you can have the best of both worlds and get a "flattop" for an SBC of of eBay for a lot less than one for a flathead, and have "Charlie ny" convert it. I have seen them for less than $100 and (don't quote me on this) I think he gets in the neighborhood of $300 for the conversion. The Mallory's are usually mechanical advance only, while a converted GM distributor will have both vacuum and mechanical. Although I personally prefer the mechanical-only version, many people like the vacuum advance unit better. If you do buy one of off eBay, make sure it is complete, with a good cap and rotor.

    There are other kinds of aftermarket distributors for later flatheads (MSD, Accel, and I think some electronic units from China), but I would avoid those unless the price is really right. There is a thread on "The Ford Barn" currently about "HEI Ignitions - Yes or No" that mentions a complete electronic unit (Chinese) available on eBay for a reasonable price. A couple of the guys over there seemed to think it was OK. I have no idea about the quality and much prefer points ignitions over electronic units, so I have no dog in the fight. The one thing that did turn me off about is that it has a BLUE distributor cap. Like "Finger-Dippin" and "Carpet Showers", "that just ain't right".:D
     
    Last edited: Feb 23, 2020
  20. dmoz
    Joined: Sep 8, 2019
    Posts: 29

    dmoz
    ALLIANCE MEMBER
    from Texas

    Thank you very much!
     
  21. I run Pertronix on two hot rods.......0.o35 inch plug gap, Taylor wires, 93 octane fuel with no issues for over 25 years.
     
  22. tubman
    Joined: May 16, 2007
    Posts: 5,641

    tubman
    ALLIANCE MEMBER

    To the O/P. If you're interested, here is the type of dual intake manifold you need if you want to run progressive linkage. You can see that the front carb is mounted just about where the carburetor goes on a single carb manifold. You run the front carb as the primary, and you still have decent fuel distribution, front to back. When you kick in the secondary carb, it feeds the back 4 cylinders and the majority of the flow from the front carb goes to the front four cylinders. It has to go a bit further, so fuel distribution is not ideal, but it should be satisfactory. I think you can see why this manifold would work better when running on one carburetor (which you will be doing almost all of the time) than the one you have.
    Flathead dual.jpg
    This particular picture is of a manifold for the earlier engines (pre-48), but they make them for the later engines as well. Also with this kind, the stock generator can be used in the stock location.
     
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  23. dmoz
    Joined: Sep 8, 2019
    Posts: 29

    dmoz
    ALLIANCE MEMBER
    from Texas

    Thank you very much
     
  24. clem
    Joined: Dec 20, 2006
    Posts: 3,360

    clem
    Member

    I agree, straight linkage only.
     
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  25. Ziggster
    Joined: Aug 27, 2018
    Posts: 896

    Ziggster
    Member

    I was reading up on the Holley 94's and watched the Vintage Speed vids on YT where he rebuilds them. My understanding is that with multiple carb setups, due to the reduction in vacuum, the power/economize valve must be changed from the stock 7.5" Hg to one with a lower setting. Yes?
     
  26. tubman
    Joined: May 16, 2007
    Posts: 5,641

    tubman
    ALLIANCE MEMBER

    Quite frankly, if "Vintage speed" recommended running progressive linkage on the Offenhauser 1075 manifold shown in an earlier post, I wouldn't put much faith in anything they say.

    What you are saying is generally true about changing power valves. There are some who recommend removing them and replacing them with plugs. There is a huge selection of different kinds of dual carb manifolds for flatheads, (Hi-Rises, Supers, Regulars, Biased Regular, and on and on). I believe that power valve selection will depend on the setup you are running. Unless you find someone who has the exact setup you are running that performs the way it should, you are in for some experimenting. Oh yeah, it's called "Hot Rodding".
     
  27. Ziggster
    Joined: Aug 27, 2018
    Posts: 896

    Ziggster
    Member

    Yep for experimenting. I know you you are not a fan of Vintage Speed. Lol...
     
  28. BoogittyShoe
    Joined: Feb 18, 2020
    Posts: 330

    BoogittyShoe

    Yeah. I put 2 deuces on a straight six once upon a memory. (Solid lifter cam). At the time Mallory had a dual-point conversion kit. Before the upgrade I had to wait for second gear. After it I had to be ready for second. It literally lit th' fire.
     
  29. tubman
    Joined: May 16, 2007
    Posts: 5,641

    tubman
    ALLIANCE MEMBER

    I will give you an example. I am currently finishing a 258" 8BA with a MAX-1 cam, Edmunds heads, and a Mallory "flattop". I got the engine running and the cam broken in using a single 94 on a stock manifold. I have a Navarro "Universal" dual manifold I will be running on it with two 94's with straight linkage. A "Universal" has the carbs spaced like a "Super", one right over each set of intake ports, but with exhaust heat. After conferring with "Charlie ny", and on his advice, will be starting with plugged power valves and the carbs jetted up two steps. We will see how it runs and go from there. Just before I left Minnesota for the winter, I finished running both carbs on the single manifold to make sure they were right on. When I get back this spring, I will change the jets and install the power valve plugs and install the dual manifold and carbs. After that, it's road testing. Hopefully it will be correct and will need no changes, but things don't usually go that way for me.:( I'm sure that I will get it, though.

    I could also tell you the story about running around one whole weekend with a vacuum gauge taped to the cowl of my Corvette getting that carb tuned the way I wanted it, but I think you get the idea.
     
  30. Boden
    Joined: Oct 10, 2018
    Posts: 747

    Boden

    What does a larger gap do?


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