Flatty not running right

C

cdan

Senior Member
I got a 36 flathead with a helmet style distributor with a speedway coil relocation cap and a 94 Holley carb,runs beautiful at idle and starts with just a snap of the switch.problem is that at cruising speed it sputters and at high rpm it acts like it flattens out and starts sputtering and popping.carb is good and it has new ngk plugs, I'm pretty sure its ignition related I just can't put a finger on it.maybe a coil trying to go bad? Advance weights not moving enough? Any ideas would be greatly appreciated.
 
Maybe not enough fuel, check fuel pump, filter , lines, etc. could be float level as well.
I had a similar problem with a 47 sedan and it turned out the fuel tank was full of crap.
 
Drive it and when the problem presents itself, try introducing a little choke to see if there is any improvement.
 
If it idles fine, then float level is probably fine. It may well not be getting enough fuel though. Does it run the mechanical pump?
I have often suffered tunnel vision and tried chasing down electrical faults only to find it was a fuel issue.
Your mechanical pump must deliver pressure (dictated by the spring) AS WELL AS volume. I'd look there.
If this is where it takes you, it might be the pump stroke or a blockage.
See how you go.
 
Dirt clogging the powervalve circuit in the carb ?

The new fuel makes rubberhoses swell.

Dirt in the fuel line or tank.

If it starts up and holds fuel over night the pump seems to be in good shape.

Ignition timing being stuck could be another thing.

That what came to mind...
 
Ya it fires right up over night,in fact it can sit a month and it will fire up with just a snap of the switch,I just had the carb apart two days ago to check the power valve as that was my first suspect and the carb was clean and good,no popped power valve and proper float level.that's what's got me stuck on ignition but I have been wrong before,o and gas tank is new,it does have a mechanical pump and it is a year old.
 
If it runs fine on idle even when warmed up and after running a while the condensor should be fine.

Check distributor for wear and that vacuum advance works.

If you have a sparecoil swap it and try.
 
Don't think it's vapor locking but I'm not positive, I thought they wouldn't start if it vapor locked,is that correct? The distributor is two year old rebuild with new parts so I hope it's still good
 
I would suspect the remote coil adapter. I have never seen one of these work better than the original to the distributor coils especially a Skip rebuilt coil. I bet the ones now are China made if Speedway is selling them.
 
After it starts pull a spark plug wire and the tip about a 1/2 inch from a ground. It should have a strong blue spark. Then increase engine speed and see if it still gives good spark. That should eliminate the ignition if the spark stays right.

If not then try the crank test without engine running but with the starter cranking. See if the spark at the plug wire is the same intensity as the spark right out of the coil. If noticeably different, check the condition of the distributor caps and rotor.
 
Ya but mine is 12v so I have to run a newer style coil unless someone converts the old coils to 12v
 
That's good to know kahuna thanks,I think I got it,I set the point gap to .011 instead of .015 and it runs way better,have a little sputter at the top end still but at least it's driveable now
 
Point gap is just a way to set the dwell approximately. If you have someone close by with a Sun or Allen machine, have them set the dwell exactly. Remember, those 2 sets of points work as one. One side "makes" the other "breaks" the primary voltage and it is critical they work together properly. Also, if you can find them, NOS points work best.
 
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