Thread: back fire ford flathead

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36 flathead starts fine ,runs fair while under load,but when eng starts to rev ,backfires a lot,any one have any ideas

Well I really don't know but I'll keep your thread going until Richard-Tech1 or Bob Parmenter gets back to you. Unless your block has been rodded with adjustable lifters you can rule out adjusting the valves since they are ground in at the factory. Also I don't know whether you have a '36 distributor or a later model. It basically sounds like a timing problem so try using a timing light and check the static advance and also check to see if the automatic advance works. My V8 book says the inititial advance should be 4 degrees with an automatic additional advance of 16 degrees (crankshaft) between 400-3000 rpm and the stock setting for the spark plug gap should be only 0.014"-0.016". Thats assuming you have a stock distributor. If it runs well at low rpm and backfires at higher rpm maybe the advance is not working? Back when I was fooling around with flatheads I used the system of replacing parts until it got better and I recall that a rebuilt distributor and a new or rebuilt carbuerator with new plugs really made a difference, but today the parts are harder to find so you have to pinpoint the problem(s). The stock '34 distributor had more total advance of 4 + 22 degrees at 3000 rpm so maybe you have an earlier distributor rather than a later one? Maybe you have too much advance? I don't know but there were several types of distributors as well as carbs so you need to identify them before you can get the right settings. In this day and age it would not be unusual for such an older engine to have "any parts that fit" from various years. Lost in the modern SBC-350 plain vanilla track is the fact that there used to be the magic numbers "Ford '32-'48" with almost complete interchageability and believe me a whole lot of swapping occurred!

Don Shillady
Retired SCientist/teen rodder

Well Denny is no slouch and Tech1 is usually totally correct, but I remember a little more. If you can find a '47-'48 distributor they had dual points and a flatter cap rather than the two bulbous round caps on the earlier distributors, but do you have a later block with 24 studs or the earlier 21 stud heads? I am not sure the later distributor fits the 21 stud block, in spite of a lot of interchangeable parts. If you have a 24 stud block that is better. If you have the later block it might actually be cheaper to buy a 4-barrel intake and a small 4-barrel (about 400 cfm) than to buy a rebuilt Stromberg 97? The last time I checked on the price of the 97 it was out of sight (I think I paid $17 for a rebuilt one from Pep Boys in 1954!). I think the first simple thing I would do in your situation is to just clean the plugs using a small blade of a penknife and regap them. That fix is much poorer than using new plugs but if the engine runs better with just the "penknife-plug-scrape" you know to get new plugs while you worry about the worn Stromberg 97. In the '50s it was common to "sandblast" the inside of used plugs, but I never found that to be any better than the penknife-scrape and new plugs are about five times better than either method. Just reminiscing back to when gas was $0.24/gallon and Pep Boys had just about any Ford part you needed ('32-'48 of course).

Don Shillady
Retired Scientist/teen rodder

In concert with Richard's comment about lean condition, I had a similar situation with a '36 Ford (coincidence I'm sure, but ironic nonetheless). Drove me crazy trying to track it down, at first I was convinced I had a bad valve/spring/guide.

Turned out it was a hairline crack in the intake manifold that grew when the engine warmed up and the aluminum expanded. New intake manifold solved that one. So, something more for you to check out in your trouble shooting.

Tech1, I did check the site you gave and you are correct for a single Stromberg 97 rebuild kit. I note the corresponding kit for a Rochester Quadrajet is still less than $20 but the 97 is clearly simpler than the Quadrajet and should be easier to rebuild. So for a single 97 a rebuild is not expensive. My kneejerk response was based on comparisons of two- or three-pot setups for flatheads. Even if a small 4-barrel is $300 and a 4-barrel intake is $200 that is only half of the multi-97/94 setups at roughly $1000. So to get the '36 running, rebuild the 97, check the timing, change plugs to new, add new points, maybe new plug wires and look for leaks as Bob mentioned, BUT if there is a temptation to add multi-carbs, headers and maybe aluminum heads it would clearly be cheaper to use a 4-barrel carb and intake AND probably easier to tune. I knew you guys would have good answers if I just kept the thread near the top for a few minutes and hopefully this flathead will be purring/roaring along soon.

Don Shillady
Retired Scientist/teen rodder

Originally posted by techinspector1
If you're using an electric fuel pump, limit your pressure at the carb entry to 2 1/2 to 3 lbs max. with a regulator. [/B]

If he's using a stock pump I would also look there. Always carried a spare in the '40

thanks for all the help,I will begin to check all the possibiltys I have been given,it is a 21 stud ,and does have a elect. fuel pump,has org. distribitor,with the adapter to use regular 6 volt coil,I under stood that this eng has no timeing marks and it timed by special machine,and that advance is controled by machanical weights , that are not ajustable ,is that correct ,as you can tell just bought this truck,and not up to speed on flathead yet. Thanks to all Dave

has it sat very long could be a sticking valve how are the plug wires are the oilly could be crosfire through wires is it back fireing through carb or exhaust.

yes ,it had been setting a while,it backfires through the exhaust,a contenueing backfire as long as you keep feeding it gas, plugs and plug wires all look good,plan to check intake manifold and carb,for any signs of leakage today,it has a O6H single carb. Dave

Another cautionary note related to the distributor (beside memories of skinned knuckles on radiator fins) is that if you take the distributor off, note that the key slot is offset from center and if you bolt it up 180 degrees off, the first time you turn it over the distributor housing will likely crack. On the one hand that makes it likely you will only be able to reinstall it with the correct firing order, but if you do manage to get it put on 180 degrees off, the wobble will probably destroy the distributor housing. 1936 saw some mid-year changes around April '36. The later '36 carb was indeed a Stromberg 97 and the earlier one looks similar so it is likely the carb has been replaced sometime with a 97. The earlier distributor had a three bolt mount for the coil on top while the later one had only two bolt holes for a coil but they function the same as far as I know. One other factoid I discovered is that the flatter pancake style distributor came out as early as 1942 and that had the dual points so '42-'48 distributors were the dual point type. As I recall (from 1954) one of my high school buddies rebuilt my pancake distributor for his '35 when I got a new one, so maybe the later distributor will fit the earlier block, but of course the distributor cap is quite different. Maybe you could take off the side wire "bulbs" and try a shot or two of WD40 inside the distributor without taking it off and maybe check the vacuum advance for carbon clogging or corrosion. The advance mechanism uses pressure changes from the intake manifold so it may be possible that something is clogging that mechanism and you may have to take the distributor all the way off to clear the advance mechanism. If that vacuum line is clogged that would explain the late firing due to insufficient advance. Anyhow you can see that I sure would like to mess around with this piece of nostalgic hardware, but instead I am trying to understand a much "newer" '76 SBC.

Don Shillady
Retired Scientist/teen rodder

thanks for the tips,going to try the wd40 first,then go from there,sometimes the easy way is best way to start

I put a gage on fuel line,and got 5 psi,and I think that is too much for that little carb,could that be part of my problem,and where can I get a good deal on a regulator and gage Dave

I re read all the help so far and see that techinspector 1 had said to check the pressure a long time ago,I was not reading carefully enough,a lesson learned

Try Auto Zone or Advanced Auto locally, if not there surely Speedway can supply one in a day or two.

Don Shillady
Retired Scientist/teen rodder

I had an Autopulse type pump on my Strombergs, they kept flooding, put a Kragen regulator on it, it wouldn't pass enough fuel to keep the little V8-60 running all the way up Kellog Hill. I finally ended up with the little 3 psi $35 "square box" pump from Pep Boys. End of problems.
Charlie Price's Vintage Speed has good Stromberg rebuild kits with viton tipped needles.
If you're using the stock looms with all the spark wires crammed inside, it could be crossfire (not the new Chrysler! what a stupid name)