I have a 1947 59ab flathead stock dual point distributor with crab style cap, Thickstun pm7 with 2 97s .043 jets and #67 power valves rebuilt with Stromberg kits. It runs great, super strong all the way through the rpm band till I go to full throttle. No matter the rpm I am at as soon as I go full throttle it has multiple cylinder misfires. Falls flat on its face! Back off to half throttle takes off!!! It has brand new Drake cap and rotor. I have changed the coil, and the condenser... I even changed the distributor to one off of another running flathead. Changed nothing at all. Doesnt matter if the car is warmed up or cold. Surges on the highway too... it started all of a sudden on a long trip from Kansas City to Des Moines. Never got worse or better all weekend. Will run 70 mph all day with a surge but not bad enough to make it not run. Doesnt heat up either. Any help would be great! Thanks guys!
I forgot to add it starts and idles great too. Will rev sitting just fine even at full throttle. Only misses under a load.
That I haven't done but the filter is full and runs really good all the way through the rpm at anything under full throttle... wouldn't low pressure cause it to miss at higher rpm despite how open the throttle is?
had issues with new plug wires on two motors (cars) recently.....one I saw at night....the other I just kept after.....
Check for the spark arcing through the cap to the spring clips that hold it in place. I had a cap fail recently. It breaks down at wider throttle due to the cylinder pressure putting the required spark voltage up. You might see a little witness mark inside the cap right opposite where the clips go. In my case it deteriorated until I couldn't get up a hill even in first. I let it cool down for 20 minutes and was able to limp home. It could be other things but check this first. Mart.
I forgot to add. If you have a manual choke, get it surging on the highway and slowly pull choke out. If that smooths the Engine out, you know you are lean. Check jet (jets) for blockage.
Thanks for the suggestions guys. I am off work tomorrow so I will start with a full electrical once through then move on to fuel. Change the filter and pump to start with.
yup, sounds lean, it doesnt like all the air at WOT so give it some fuel, if it used to work fine, I would check for main jet obstruction
Coil polarity must match battery ground. If you are grounding positive on the battery you need to have a positive ground coil. A miss match will cause the problem you are experiencing.
The second way of the above happening is if a battery gets completely discharged, it is possible to connect the charger leads backwards with no arcing and the battery gets charged with reverse polarity, thus creating the miss match with the coil. Knowledge comes from experience, and experience comes from lack of knowledge. You have to have made these mistakes to remember them.
The coil is wired properly and the battery is a brand new dynabatt. It ran great for days and over 100 miles that day... then it just developed this issue.
If it ran good then start by checking the basics...fuel pump giving enough flow/pressure. Crud in suction line gives this type of behavior. Stick an electrical fuel pump in line and check that you have a nice good flow...then out and run it. If it runs good now rebuild pump. If you got fuelfilters installed carbs shouldnt get clogged over night... So if you have fuel and it still misbehaves i would start looking at the ignition.
Having over 50 years of hot rod flathead experience,let me put my impressions in.I run my FLATMOTORS hard and fast; I usually run big inch strokers,so I want them to perform great all the time.Years ago when experimenting for max spark,I ran the crab type dizzy with dual points.Even with all new innards,you could push the rpm to about 5 grand,and then misfire.I would readjust the dist. And even brand new points would rev to about 4800-5000 and then float(I guess due to inferior spring tension). I have had magnetos,Mallory dual pts,etc and they needed constant adjustment to perform adequately.Years ago,I bought a new rt angle drive at a swap meet manufactured in Colorado by Frick.I stuffed a cut down small block Chevy dist with an advance curve for the flathead and a Petronix hot spark with a 50,000 volt coil and since then,my flathead will rev to the moon and never miss a lick.I also rid myself of those weak 39 ford trans and weak closed drive rear ends.For over 25 years and counting,I can pop the clutch,spin the tires,and not have to worry about spark and weak Ford drive lines.I prefer the Muncie 4 spds with 8-9 inch Ford rearend.All units are capable of handling 400 hp and the hottest FLATMOTORS don't even make the drive lines sweat.
My issue has nothing to do with rpm. As soon as I go full throttle it starts missing like crazy. Falls on its face. And with all do respect I am way too much of a Ford man to put all of that Chevy stuff on a flathead!! But thank you for the input!!
I had a similar problem that I chased for a long time. Easy engine start, good idle, good revs in the garage, good cruise on the highway, but full throttle caused it to fall on its face. Tried everything imaginable and finally fixed it by changing the spark plugs. The old plugs weren't dirty or fouled and they weren't even that old, they just crapped out under heavy throttle.
At part throttle, a stocj '49-53 has its advance worked by ported vacuum which works pretty well. At full throttle, very little vac is available downstairs so the thing works on the increasing but very small pressure drop created by air flow through the venturis. This is feeble at beat, and often at full throttle advance drops off to near zero. The first rodders experimenting with late type flatheads noted that many '49 Fords would accelerate 0-60 faster at 3/4 throttle than full because of this. Lack of advance is a performance problem in itself but also tends to load the sparkplugs like a rich condition...
Bubba can go through the distributor and check the point bounce. He can also supply a Vertex Magneto condenser. The .043 may be too small, how does it pull away from a stop?
Fuel delivery and float level. If the float level is too ow it will not have time to fill the bowl before it runs out of fuel (full throttle).
It's a 59ab so it has the crab distributor not the 49-53 style bruce. I changed the fuel filter... no change. Checked the pump and it was pumping fine. Changed my back carburetor and it cleared up completely. But now I have an absolutely horrible flats pot just off idle. Like wants to kill the car between gears! If I roll into the throttle as slowly as possible it's fine. Accelerator pump?!
2 or 3 things! Crab has excellent advance if all is well. Flat spot could be off-idle, accel pump, or slow advance... 1. Look at brake puck adjustment on crab...loosen lock nut, screw it out to limit BY HAND so you don't pop off the spring seat, back in maybe 1-2 turns. This thing slows advance adjustably and is largely unneeded with modern gas, but the slight drag adjusted most of way out stabilizes the disk on front of advance. Now put a timing light on it and see that advance starts moving right off idle. 2. Make sure pump shot is strong and even to both sides...pump it with engine off, several times to see that aall works. There are adjustment balls for pump at linkage, try rich and lean to see if any change. 3. Next, worry about idle--off idle circuits in carbs. Make sure idle jets are clean, and all passages down to the discharge holes in base. Though I don't think its the problem here, I'd look into slightly bigger jets like 045 stockers. Run engine in the dark and eyeball the wiring for flashes to ground. Make sure that slot indexing cap isn't broken...that's about the only place you could get phasing problems on a crab.
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