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I am getting a misfire and loss of power when accelerating. The engine idles nicely revs fine parked but once I am on the road and accelerating I get the misfire. All of the plugs look good but there are cheap wires on the car. Could this be the wires,vacuum advance or something more serious? I thought maybe a burnt exhaust valve but I get good consistent vacuum on the tale pipe. Any ideas would be appreciated. Compression test is next on my list.
Ted
Ted,
This sounds like a weak coil problem. Usually it acts like a lean fuel mixture problem created by a bad accelerator pump in the carburetor.
There are a great number of test procedures that you can Google, but for me, it's faster and easier just to try a new coil.
Keep us informed as to your progress.
Geoff Overley, Editor
DeSoto Adventures Magazine
I had insufficient fuel supply in the back of my head..... enough to idle, but not enough to go down the road?
It's supposed to be fun!
1949 De Soto Custom Convertible (project)
I will go along with Tim but will specifically call out the accelerator pump. Also, the spark plug connections will cause this but you should be able to tell if a plug is not connected well by listening at the tailpipe or just checking for an engine shake. Yes, you should get the bakelite or ceramic spark plug wire insulators. The ones on my 53 seem to hold the wire connectors in place well. Good luck Ted.
All good ideas, thank you. Time to order some parts and see what happens. I will get back to you with the results.
Ted
Hi guys,
While waiting on parts I have been trying to do more diagnostics. With the engine idling I try pushing on the accelerator pump and it is putting fuel into the carberator and I get a steady acceleration. It springs back smoothly. When I press on the throttle abruptly for a quick rev the carberator back fires at me and a couple of times the car just died. If I press on the accelerator slowly I get a steady acceleration but the fuel seems to be draining in rather than spraying in. Could I be looking at a bad fuel pump rather than accelerator pump? Is there a way to identify if one is not working properly? The Carb looks relatively fresh like it was rebuilt not that long ago although I could be wrong.
Thanks,
Ted
Have you tried a known good coil yet?
bad condenser
Backfiring through the carb? Could be timing is off or spark advance isn't working properly. I would start with the basics. New points correctly adjusted, new condenser and timing correct. New spark plugs if there is any doubt about them. Make sure there is vacuum at the spark advance and it is working properly. A new coil was a good suggestion but I have seldom had to replace a coil. I would do the other stuff first unless you have a spare coil on hand. If you have had spark plug wires off check for correct firing order but if it idles smoothly probably not the problem. Also if it idles smoothly probably not a burnt valve. Just some thoughts.
Hi everyone,
Looks like I have solved the problem. I checked compression, 2 cylinders were 110 and the rest were 120. I cleaned up all the plugs and re-gapped them noting some were pretty black (to rich). I replaced all the cheap plug wires with quality wires and proper hemi tube insulators which made all the difference in making sure the wires were well connected to the plug. I am still waiting on a coil I ordered but in the mean time I cleaned the contacts and replaced all the wires to the current coil. It finally dawned on me that when I goosed the accelerator hard and the car would either backfire from the carb or die along with the black plugs that it was a case of too much fuel and not enough. I readjusted the carb to lean it up. Between these two items the problem as pretty much gone away. There is still a hint of roughness under hard acceleration but I figure a new coil and a little more tweeking and I will be a happy camper.
Thank you everyone for providing all of the ideas. Each day I learn a little more.
Cheers,
Ted
wash, rinse, repeat.
Black plugs = too rich. Backfiring through the carb = too lean. But backfiring from the carb + black plugs = weak spark. of course, it could be slipped timing chain etc.
A rich mixture will mask weak ignition, until the plugs foul out.
Coil? Maybe. Usually fails completely or fails when hot
Wax paper wrapped tinfoil is a real problem when both the wax paper and tinfoil is made in China.
Have a nice day
Steve
Thanks Steve.
Timing is on my to do list as well. I just got this car and the punch list continues to grow.
Ted
Oh, just one more thing sir. '56 (and up) have ballast resistors famous for failing. Look for about 9V on the coil side of the ballast, if it is OK, it could still be faulty when it gets hot.
Have a nice day
Steve
