If someone out there can help with a frustrating problem of a rough idling 302 it would be well appreciated. I have a 302 in a '73 ford that runs rough at idle and up to about 30 mph. The vacuum gage fluctuates rapidly between 15 to 18 in. at idle and 10 to 13 in. at 25 to 30 rpm, but steadies out at 17 in. at a constant cruising speed 0f 50 -65 mph. I have replaced the coil and ignition wiring as it tested faulty on an analyzer. The spark plugs are good and the timing is set at 10 BTC.
I have an RV cam with an Edlebrock perfomer carb and manifold and a Jacobs ignition module. A compression test registered between 125 and 135 psi for all cylinders and I have checked for vacuum leaks but haven't found any.

 

Why don't you try another ignition module?

www.PerformanceDistributors.com

 

It didn't occur to me to try another module as I thought they either worked or they didn't.

 

Plug wires crossed up?
Flat cam lobe, cam timing off a couple of teeth?

 

Does the truck have header leaks or exhaust manifold leaks?
Check for a leaky intake manifold gaskets or carburator gaskets.

 

Check out how to read a vacuum gage:
http://www.users.bigpond.com/ergoff/vac1.htm

 

The ignition wiring is correct, 15426378, and I pulled the carb and intake manifold to see if the gasket was leaking. When I replaced it there was no difference in engine operation. I am at about 2600 ft elevation and the vacuum gauge readings are about right for the altitude. Rapid fluctuation at normal vacuum supposedly indicates worn valve guides but the engine doesn't use oil and there is zero smoke in the exhaust when I back off the throttle.

 

Does the engine have points, make sure the points are not burned and are properly gapped. Then set ignition timing with the vacuum disconnect and plugged on vacuum timing advance. Make sure the vacuum timing advance does not have vacuum during engine idle and is set on the ported vacuum port. What is the setting of the timing and point gap?

Try adjusting the fuel mixture screws .
You will be suprised how easy it is. Start with the engine off. Hook vacuum guage to a constant vacuum port. Turn both air fuel mixture screws in (clockwise rotation) and do not tighten the screws, just to were you barely feel the screws bottom out (overtighten these screws can ruin them). Then back out each screw 1 1/2 turn (counter clockwise rotation). The vacuum gauge will have the highest reading when the air / fuel mixture screws are set correctly. Start the engine. Adjust your engine idle to about 600 rpm's. Then at 1/4 turn intervals set to both air screws in a counter clockwise rotation. Listen to the engine till the engine idles smoothly and does not idle roughly. Might need to keep setting the carb to 600 rpm's. If you notice the engine smooth out and then begins to idle rought as you adjust the air mixture screws, you have set the air screws to far out. When the engine idle will smooth out, then Gas and Rev the engine and it should not have a noticable dead spot on the carb when the air mixture is set properly. Example: if the engine idle and the maximum vacuum reading is set at 2 1/2 turns on air screws then both screws should be set at the same turns. I was really suprised you had replaced the gaskets when all you need was to check for vacuum leaks.

 

Consistent fluctuations usually indicate a burned valve, but that would have shown up on compression test. If all the ignition parts are good to go, look further into the carb. ANY trash in the metering block will make it hard to tune and run terribly rich at idle. You can also try adding a little more timing to see if that helps, some engines like more than others, my '73 302 runs and idles best at around 16 deg.


Greg

 

When a module is "on the blink" it can act very strange before it finally goes out. One common symptom of a module on the blink is a very erratic timing mark, jumping around 6-10 degrees.

 

These are all great suggestions and I will go through all of them. The distributor is a Ford magnetic reluctor type so there is no points. I have adjusted the carb (its new) but haven't tried using a vacuum gage with the engine off for initial setting of the mixture screws. The distributor vacuum is connected to the timed port on the carb and I set the timing at idle with the vacuum line disconnected and plugged. The module has a light that indicates its working but thats all it has.