My 1925 Touring runs good at road speed, pulls good, and idles ok at no load. The problem exists when trying to run at slow speed. It bucks and "lurches" (my word) It has just recently started doing this. I have changed plugs twice and now have the "X" plugs installed. It has an anderson timer which I have cleaned and checked. I have run the fuel out and replaced with high grade--no ethynol, drained the carb, and no change in performance. I have also switched all the coils to known good ones. I'm inclined to blame the carburetor even though it is a professionally rebuilt carb. Any words of wisdom from you experts will be greately appreciated. Thanks. Dewey
When I switched over from Motorcraft F11 plugs to a set of new old stock Champion X plugs, my car ran badly. Then I switched back to the F11's, it ran great.
Have you done a compression test?
Mine did the same thing, diagnosed low compression in #1... still haven't found the cause.
John Regan just mentioned to me that this can also be a coil problem, mine also is doing this - at 25 miles an hour or so on a flat road, mine wants to buck or surge slightly. I've changed carbs with a proven good one, JOhn says to check my coils closely. Under load or power, my car runs great, but back off on her on a flat road and she wants to buck a bit - almost like slop in the drivetrain.
Have you checked your fuel mixture? Maybe even the float level. If the float level is too high, it will run fine when pulling, but when going slow with little throttle, it will run rich. Try leaning it down first and see if that fixes it, then if it still lurches, check the float level. Could be an ignition problem, however, ignition problems usually present themselves when you are pulling under load.
Norm
Compression test!!!
On mine, I dont think its a fuel problem. My 25 truck runs great, so I switched the carbs between the two - problem still there. I'll compression test it Wednesday - what should the range be?
Around 50 psi and all 4 should be even. It can vary depending on what head you are using and the altitude. Lower at higher altitudes. Most important for smooth running is even compression on all cylinders.
Norm
Go ahead and do a compression test but what you describe sounds "fuelish" to me. If you had low comp. you'd have no power, hard starting and slow speed operation would not cause lurching. Besides the change of gears only two things alter when shifting: spark lever position and throttle position. You've gone over the ignition system. Unless you have a timer wire touching/shorting in the retarded position (which clears up when it's advanced) I guessing fuel. Professional re-build or otherwise you're saying when you open the throttle it runs OK. Carb.
Have you tried backing off on the spark as well as the gas when you slow down?
Thanks guys for all the help and guidance. I checked compression and had 45-50 on all 4 cylinders. I checked the new X plugs--gap was a tight .o25 and I opened them up to .030 and only had time for a short run but it appears much improved--would that little difference in plug gap make the difference?? Tomorrow will give it a more thorough run. Thanks for your help. Dewey