Slight miss mystery

[rangerin]

For the whole time I've owned my 2.8, about 2 years, it's always had a slight (but constant) miss at idle when warm. It's more pronounced on a hot day after I've driven it some. Since getting the truck, I've put all new seals on the engine (they all leaked), rebuilt the carb (also leaked), and durasparked it. It also has fairly new plugs, wires, and coil too. Vacuum is a steady 20 inHg. I have not compression tested it yet, as I have no tester. I think I will soon though. Other than the miss, the engine is healthy for as old as it is. It does not burn oil or coolant.

Question: Any ideas? I'm about to tranny swap and while I was at it I was gonna pull the engine and de-grease it and the bay as they are very filthy. Do you think lapping the valves would might get rid of the miss? I'm pretty much at a loss right now. When I just got the truck, it was very rough, but each time I worked on it, it gradually ran smoother. Eventhough the miss is barely there, it still drives me crazy haha

 

[Andy D]

I've no experience with the 2.8 Could be something in the valve train. But if the vacuum gauge needle is steady, then that indicates good valves. Does it consume oil between changes? It might be out of adjustment idle mixture screws , assuming you have a 2 barrel carb like a 2150 or something.

 

[rangerin]

I've no experience with the 2.8 Could be something in the valve train. But if the vacuum gauge needle is steady, then that indicates good valves. Does it consume oil between changes? It might be out of adjustment idle mixture screws , assuming you have a 2 barrel carb like a 2150 or something.

Oil consumption is nothing out of the norm. I think it's pretty low actually for this old of an engine. It's running a 2 barrel holley carb. Admittedly, I wish I knew a lot more about carb tuning, but "I think" I've got it setup pretty good (rock steady vacuum, mixture screws adjusted for most vacuum, etc.). When I've had the valve covers off in the past, I never noticed anything out of the norm other than worn valve guides, but nothing serious. Top end is clean and valve lash has recently been adjusted also. Guess I should mention that the timing is spot on also (using a vacuum advance distributor also). As far as I know, there aren't any vacuum leaks from the carb/hoses as I've been over that thoroughly. Really, this has mystified me for some time now.

 

[a31ford]

I had a "miss" it would only pull "not bad" under load, the hotter the engine compartment got the stronger the miss......

Solution >? coil was "arching" could only see it at night or in a dark (black) shop. the coil looked like something from star wars... call it a "plasma generator"

Greg

 

[rangerin]

Hmmm. I'll have to check on that. Not to sure though as the entire ignition system is new since the Duraspark. It had the miss before the Duraspark too.

 

[tmcalavy]

Test the coil and wires, distributor pickup and ignition control module...just guessing your truck still has these after the Duraspark conversion since you don't mention what year it is. Look for post on here called sporadic hard miss that I did recently that tells how I chased my mystery/aggravating miss down and got rid of it...the post has a link for the ignition tests...takes some time and patience but very low cost to do. Mine turned out to be the distributor/ignition control module.

 

[rangerin]

Test the coil and wires, distributor pickup and ignition control module...just guessing your truck still has these after the Duraspark conversion since you don't mention what year it is. Look for post on here called sporadic hard miss that I did recently that tells how I chased my mystery/aggravating miss down and got rid of it...the post has a link for the ignition tests...takes some time and patience but very low cost to do. Mine turned out to be the distributor/ignition control module.

Thanks, I'll check that stuff out and will test the stuff also. But unfortunately, it had the miss before the Duraspark (I'm actually using a GM HEI ignition control module now) and all the ignition/distributor stuff was changed when I did that. I even ran new wires to replace the old OEM ones. BTW, It's an '84 with the 2.8.

I appreciate all the help as I'd really love to get rid of this!