Amongst a range of teething problems on my 35 cabriolet, I have problems with it running rough. Here is the story: French Flathead (rebuilt) Thickstun hi-rise, new Strombergs, Mallory dual point. Got it running, took it on maiden voyage, did over 500 miles, with the odd misfire but it went well. It then was difficult to start and would cut out, misfiring like hell, had a spare condenser, still the same. Did not have spare points but had just bought a petronix elec conversion. Fitted it, started on the button and seemed fine. So now have new ignition, Taylor plug leads and flame thrower coil, carbs seemed fine, fuel pressure 2lbs and began my 500 mile journey home, 10 miles into it, running like shit, called it a day and got home on a flatbed. On my return, have adjusted timing, it was slightly out, started fine but after a few miles the same, checked cap, new rotor, still the same. It has a Columbia, so thinking could be a vacuum issue, blocked take off, tricky to start, but fine for first few miles then putting it under any load it starts to bog and run rough, foot off the gas, it is running sweet. I have standard power valves and jets in carbs. Possibly related, after a bit of running, some blow by from the oil breather. Did a compression test, 5 cylinders on 100, 3 on 98, not perfect but ok. So I am in a mess, changed probably too many things and still have same problems. Any advice most welcome, thanks in advance. Gordon.
How hot is your ignition coil getting? If the coil is going south, it will misfire under load. What do the plugs tell you? Any sooting or fouling? What plugs are you running and what gap? Flatman
Flatman, Will check coil, I had wondered about it as being a possible cause but as it had only done a few hundred miles, put it out of my mind, will maybe try borrowing another one. Plugs are new NGK B6-L, when I checked them last night, dark round the rim, biscuit colour in the centre and the tip was clean, blue / gray, I can't remember gap but pretty sure they were set as per instructions. Thanks for your comments.
"fuel pressure 2lbs" Is that enough to feed two stroms? I was going to say coil too, but your using a flame throwers and I don't know if those have any problems. Have one in a box but never used it.
"Is that enough to feed two stroms?" Hmm, I sort of thought pressure was different from volume, if that makes sense. Got a friend that runs 2 lbs with 3 strombergs on a SBC, neighbour runs 2 lbs with 2 carbs on a flathead. Interesting point though, need a bit more research on that. Equally I had thought that if the presuure was there then the line must be ok but again I suppose it could be a blockage which has no impact on pressure, ie kinked hose, blocked filter, would maintain pressure but under load reduce volume. Jeez my brain hurts! Thanks for input.
You are correct. Volume and pressure are different. Low pressure with high volume. I'm assuming, with the same pump, a large fuel line will up the volume and lower the pressure. Small line more pressure/less volume.
No. Pressure is independent of line size. Problems at operating temperature is a classic symptom of bad electronics. You can check the coil while cold and it will test fine. Unless you can test it while hot, the best test is to swap the coil for another. Bill
Yea I'm not so sure about that. Plug read sounds right or a little lean. I does appear that the problem is electrical. I had a similar problem that was fixed with a skip coil. I just run stock dizzy setups, so no help here.
I rather have to agree with B Blue on the coil being suspect. And one needs to remember that just because it is new, name brand, no reported trouble with that model doesn't automatically mean that it is in perfect working condition. Years ago I found out that a coil that works great running back and forth to work and on Saturday runs to the wrecking yards could and did break down the first time I took the truck on a serious road trip.
I had a similar issue with my Flattie on the '48. I had to tow it home from Goodguys in Columbus. (Embarrassing!). I am thinking that it's my points. What is weird is that I had taken it into the shop simply to get it aligned after putting some new radials on and I think the guys at the shop (unfamiliar with Flatheads) left the ignition key turned on. I was thinking about doing the Petronix conversion anyway. I have also added a new fuel filter and regulator (as I was getting like 8lbs for 2 Strombergs on a tallboy intake - way to much!). Additionally, I went ahead and purchased two new Strombergs, just to be safe. Now it's a matter of finding the time of putting everything together - or actually having to pay someone to do it for me. And hopefully, this will fix my problem. I have gone the majority of yet another summer without the ride and that sucks.
I had the same problem with a '36 Ford I had. It would run fine for a few miles, and then start sputtering and missing. Turned out to be crap in the gas tank. I found it would run fine until it got below a half a tank, and then the problem would show up. I figured something was floating around in the tank, and if the level got low enough, it would clog the pickup. I am embarrassed to say that I just changed the filter a couple of times and kept the tank almost full all of the time, and the problem finally cleared itself up.
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