Engine: why is the lark running so rough?

[straycatsue62]

My 62 lark idles fine but once its in gear it feels like its going to die. Adjusted the Carb, new fuel filter, oil filter, water pump, plug wires and plugs. Also it gets much worse the warmer it gets. Any ideas?

[Gunslinger]

Does the carburetor have an anti-stall dashpot? If it does, it sounds like it's either in bad need of adjustment or it's bad. It keeps the the idle up when put in gear.

[Pat Dilling]

Welcome to the Forum.

Some questions to help you troubleshoot. Is this a V8 or a 6 cylinder? Did this just start happening or has it been doing it for a long time? Is the choke opening all the way? Are you getting any black smoke from the tail pipe or does it have a "rich" smell to the exhaust? What does it do when you are driving down the road? Can you feel a miss then or does it smooth out? I would check for vacuum leaks. Make sure there are not any cracked hoses. Have a friend put it in drive while you observe the engine compartment watch and listen what happens.

Pat

[Dick Steinkamp]

Does the carburetor have an anti-stall dashpot? If it does, it sounds like it's either in bad need of adjustment or it's bad. It keeps the the idle up when put in gear.

Actually, the dash pot's function is to slow the throttle's return to idle in the last bit to prevent the engine from dying. It isn't connected to the shift linkage and doesn't increase the idle speed when the car is put in gear.

I'm not sure what you mean by "adjusted the carb?" Did you set the idle mixture and idle speed according to the manual? If not, that's the first place I'd start.

After that it could be worn/dirty carb, auto choke not working correctly, carb float level set incorrectly, points gapped wrong (dwell), timing set wrong, plugs gapped wrong, valve(s) adjusted too tight, worn engine (low compression), vacuum leak, plugged air filter.

[PackardV8]

More suggestions:

1. The exhaust heat riser valve below the exhaust manifold. Confirm it is fully opening.

2. If it is a V8, the intake manifold gaskets may have rotted out. An intake leak makes for a rough idle.

3. How recent are the coil, distributor cap and rotor. An idle which is rougher under load can indicate weak spark.

4. Did you set the ignition initial timing and point dwell?

jack vines

[JoeHall]

Not enough valve clearance?

[Gunslinger]

Actually, the dash pot's function is to slow the throttle's return to idle in the last bit to prevent the engine from dying. It isn't connected to the shift linkage and doesn't increase the idle speed when the car is put in gear.

I never said the dashpot (or as sometimes called...throttle kicker) was connected to the the shift linkage. Every dashpot I've seen from that time period was vacuum operated and adjustable as it was mounted on a threaded bolt with a locknut. I incorrectly stated it keeps the idle up when put in gear (or a/c turned on)...it keeps the rpm's from dropping down. Maybe a distinction without a difference, but it keeps the engine from stalling suddenly.

It should be able to be adjusted by loosening the locknut and running the rod out. If that can't be done the dashpot is bad.

[E. Davis]

Hook a vacuum guage to it. It will give you a lot of information and help pinpoint the problem.

[maxpower1954]

A worn out distributor will often run smoothly at idle but terrible under a load. Common with the Prestolite especially, as found on your '62. Check the advance plates/pivots/springs for excessive play.

Russ Farris

[ROADRACELARK]

Back in the days of "mechanical" distributors, as opposed to the modern "electronic", using a Tach/Dwell meter could very easily verify the operation and condition of the internal moving parts. It will give you the dwell (point gap) reading and if the distributor shaft bushings are worn, or the advance weights are worn or inop, it will show with erratic movement of the needle. Can also show the operation, or lack there of, of the vacuum advance assembly. Very useful tool on the older cars, 6 or 12 volt, V8 or 6 cyl. Hope this helps.

Dan Miller
Auburn, GA