Pulsating idle help

[ed cougar]

Hello, and thanks to all who respond, my 428 SCJ has this pusating idle, the air adustments on the 735cfm are responsive when I turn them in the idle roughens then back a little an it smooths right out but still has that pulsating, the timing mark also hunts a little with the pulse, what would be the most probable cause- valve adjustment-worn distributer shaft? Could the manifold be leaking vac. and whats the best way to check, it pulls about 14Hg at idle and it also hunts a little with the pulsing idle. The cam is a crower nothing ratical 530in 531ex 110* overlap. I can enter more specs but I think I'm missing something simple and hopeing for a good tip. Thanks alot.

[mr0077]

Ed, sometimes the idle will "hunt" if the vacuum advance is hooked up to a port with vacuum at idle. It should be connected to a timed vacuum port, where you have no vacuum at idle. If you have a vacuum advance, disconnect it and see if the hunting goes way...it may be hooked up to the wrong port.

[ed cougar]

advance is hooked up to ported vac off carb pulsing still there when disconected and plugged. RPM 800 drops to 750 and back to 800 in a constant pusating.

[trularin]

Depending on your cam duration, carb settings and engine configuration, you may just need to set the idle at 800 and see how it goes.

Had a BB that refused to idle below 1000 RPM. Sounded like a would die at any second if I took it lower.

Just a thought.

[mr0077]

Next place I would look is the mechanical advance...you said the advance changes with the rpm, but the mechanical advance shouldn't start until just above the idle RPM. If this problem just started, maybe an advance spring broke or came off? What are your ignition timing settings (idle and total mechanical advance)?

[ted]

Don't know the history on your carb but if the throttle blades have been removed from the shafts in the past, then you may want to pull the carb and check the following.

Looking at the carb while it's upside down, look through the carb bores at a light source with the throttle in the idle position and secondaries closed and insure that each pair of blades are open the same. If the secondary blades are unequal and likewise, if the primary blades are not open equal, then the blade screws will need to be loosened and the blades likewise shifted around so that equal air flow through each pair will be made.

Also insure that the idle transfer slots in the primary side are exposed with the carb in the idle position. If the primary transfer slots are not exposed or just barely (0.005" or less), then check to see if the secondary blades are possibly open too much. The secondary blades can be closed by adjusting them at the small slotted screw that is accessible only from the bottom of the carb. This will force the primary blades to open up more thus exposing more of the idle transfer slot and possibly better fuel delivery at idle. Be sure to leave some amount of load or adjustment against the secondary air gap adjustment as taking it all out can cause the secondaries to stick closed.

If the carb checks out, look for a vacuum leak. The thin paper gaskets under carbs can be misleading at times and my require replacement although they visually look okay.

[Jack21]

The "hunting" sounds more like distributor than carb. You've checked the vacuum advance which is where I would have looked first. Next, check the mechanical advance. Is the timing moving when this idle speed changes? Disconnect vacuum line from carb to dist, plug carb port, and check with timing light. If timing moves as idle RPM changes, distributor is causing this. First place to look is advance weight retention and advance springs.

If dist is OK, look for something changing your A/F ratio. Crack in power brake line, worn, or wrong PCV valve would be my next suspects.

One thing I always do to insure a good vacuum seal between carb and intake is apply some wheel bearing grease to both sides of the carb to manifold gasket.