The Stovebolt Site Forums The Shop Area Engine Shop hesitation

Have almost finished my 53 3100. Been driveing around a little and have noticed the longer i drive it the worse it runs. Here's what I have. Complete rebuild on a 261 bored 60 over, Langdon's distributor, round coil, plugs and wires. I rebuilt the zenith carb from the correct kit from carb king. New gas tank, fuel pump and filter. It starts great, idles great but when you give it the gas it wants to hesitate in spots. I can go through the gears but not always smooth. I can pull a grade but i have to ease down slowly on the pedal. Today i checked the plugs and they looked fine, dry and light tan. I check the vacuume advance by sucking on the hose and i could see it move. I'm leaning toward the carb but not sure. What else could i check or check differently.

I would start with rechecking the timing. If it is within specs, try retarding it a few degrees and test drive. Keep doing this until it improves. If it only makes it worse, try advancing it a few degrees until it improves the test drive. You can also adjust timing to maximum vacuum, but with all of the modifications that you have made, it might not be the best setting for your application. It is important to get the timing right before you start messing with the carb.
Carl

An engine running worse after driving it awhile is normally either the fuel delivery system or heat breaking down something electrical.

Fuel delivery could be a plugged fuel tank vent, plugged fuel filter, etc.

Electrical could be electronic ignition conversion, or a coil where the secondary resistance is going non-linear with heat.

The Zenith carb has no personality. If it works cold, it will work hot (providing of course that fuel is coming in from the fuel delivery system).

Jon.

Check to see if the accelerator pump is working correctly.
Easy test. Take off the air filter, open the choke. shine a light into the carb and open the throttle linkage all the way. There should be a good steady stream of gas squirting into the manifold. Then close the throttle and reopen it about 1/4. Should get another squirt only smaller.

Not much of a squirt on the first test, fix the pump.

Good first test but little or none on the second test, too much slop in the linkage or adjusted wrong.

IME electrical problems are load-related - it always gets better when you back off, worst at high speed and load, and worse in high gear than 1st gear.
Your mechanical advance may be sticking?
1 valve too tight, and not fully closed when hot?
The accelerator pump only works for the first small throttle travel distance, and then only for a few seconds. It will never go on and off, or occur at part throttle.

Panic: Are you suggesting a condenser? Or some other thing like a voltage regulator? I have the same issue and the only thing that makes it go away is a bit higher idle speed. Around 650 RPM. I would like to not have to do that. Good cap, rotor, points, coil, Accelerator pump, and timing set with just a bit of "ping" at heavy load. Dwell is 28-30 degrees and valves are adjusted to a slight tick fully warmed up. Only thing that I can't measure is the condenser. Head was off and valves, guides, and seats were inspected.

If this is tip-hesitation, and not load-related, the most likely reasons (IME) are:
1. not enough initial spark advance
2. bad accel pump
3. vacuum leak

Yes and those are things I can check. I suspect a leaky vacuum advance. Not that it doesn't work, but I think it leaks a bit. But to be through I will look at the accel pump again. And because I have more than one dizzy I can take the time to make sure the mechanical advance is working as well. Thanks, this has been bugging me.

Finally got a chance to work on the truck. I checked the accelerator pump, works fine. I stared moving the distributor around and I have it running better. Not perfect but better. Put 18 miles on it just driving around the neighborhood stopping to adjust then driving on. After playing with the timeing today I'm hoping i can get it just a little closer next time. Thanks for all the answers. You guys are a big asset. I'll keep you posted.

one more thing to check is the carb a correct carb for your 261? Light tan color is very close to a lean setting. Is the main jet correct for a 261. OK that's two things.

Basham, once you get the best results possible from playing with the timing (even if it still isn't running perfectly), then it becomes time to adjust the idle mixture. I would adjust it to the highest reading on a vacuum gauge, then test drive it.
Carl

whar are we calling idle mixture. I have a idle screw and air screw.

Different terms are used in the Owner's Manual and in the Shop Manual

idle screw = throttle stop screw = idling speed screw for idle RPM (450-500RPM)

air screw = idle mixture (screw) = mixture screw = idling mixture adjusting screw
(adjust to maximum vacuum at idle)

just took it out and drove around for a while. seems to be running good but i'll check the idle screws, maybe i can improve it even more. One thing i did notice is that it pops a little out the tail pipe, just every so often. thanks.

You might have a sticky valve, usually caused by a valve guide being incorrectly sized for the valve, or gunk in the fuel which ends up on the valve stems. Hold a piece of paper against the exhaust pipe and see if it occasionally gets sucked tight to the pipe. If it does, you likely have a valve issue.

Think i'll readjust the valves tomorrow.