I have this problem that I can not correct, theres a random miss at idle that comes and goes. If I load the engine up it wont miss, it also doesnt miss at idle or under heavy acceleration, just idle.

The miss is a hiccup and I checked timing after I did my water pump, its set at 8* BTDC as per the decal. I have the idle mixture on the carburetor set at 4 turns out on driver side and 5 turns out on passenger side.

If I drive the truck hard as in hitting passing gear and going up till she shifts out of passing gear at 4,000 RPM, if I come to a stop and idle the engine idles smoothly and if I idle for about 2 minutes then it starts with a slight miss then some times it gets like a cough where it misses multiple times before smoothing out again.

The dist was replaced not that long ago, the carb thats on the engine is the 4th reman that we put on it and its been on the truck for a good six years.

My question is, could a coil over heating act like this? I honestly dont think so as its random.

Our engine builder that we use at work, he worked for Watco who built this engine and he said that they bought reman short blocks then they did the heads themself and put it together. He said they did not replace the valve guides and thats why this engine from day one would consume oil at a rate of 1 quart every 80 to 100 miles. Could this cause a random miss at idle like this?

I thought about using my old repair manual that has a troubleshooting flow chart that covers a miss at idle only. But I think that is more a miss that wont go away and mine will come and go.

I am honestly at a loss and I am very close to junking the engine out and getting something new even though I dont have the money to do it.

If anyone has any ideas let me know cause I am fresh out of them. Ive idle the motor up and it helped some (I did have it idled too low running at 500 rpm) and I can play with the mixture screws and it will have a slight miss that you barely feel but in three or four days it starts to get worse when the miss does happen. With it coming and going, I dont think its a compression issue as that would be a steady miss reguardless and that would also cause a miss under load as well. Only thing I am thinking about is the ignition switch (flow chart says to check that first) but the ignition switch works every time so I cant blame that. The Ignition coil because it has never been replaced but the miss doesnt seem to be ignition related as theres no miss under load. Final suspect that I am leaning at is the carb. Being a reman, maybe they arent building them right cause we got one for a '80 F150 Ranger with a 400 in it and that thing doesnt miss but the thing runs like junk and its the carb.

 

How do the spark plugs look? If its pushing that much oil through you should be able to see it on the plugs. I've seen people make that mistake before and it almost always comes back to bite them putting worn out heads on a fresh short block. I've always had new guides and a 3 angle done no matter what. Could be a cracked valve seat. I had a 351m that would fowl the #4 plug in a week. Some times a couple of days. It was worn out guides. Rework the heads and you should be fine. A compression check wouldn't hurt. If its a cracked seat that cylinder will be lower. Also is it a 2v or 4v? It may just be me but I always use a vacuum gage when tuning a carb and I take the fuel and air out evenly.

 

I havent pulled the plugs but I can pull them all out today one at a time and check them out.

Its a 2v at the moment, if I can get the engine square away to idle fine I want to swap the heads out for some aftermarket stock style replacements with better flow and switch over to a 4v carb down the road.

I did set the idle mixture using a vacuum gauge. I was at I think 17 or 18 inches when I set the driver side at 4 turns out and the passenger side at 5 turns out. If I set them both the same to me it just seems like the engine is sluggish which is odd but maybe that is just me and its not really doing it.

When I did change the plugs out the plugs that were replaced have been in the truck since the engine was rebuilt back in 93/94 and I didnt see any that was fouled out. But I cant explain where a quart of oil every 80 miles or so is going considering it doesnt leave puddles of oil on the ground and it doesnt smoke. Its smoking now when you rev the engine but I did add quite a bit of MMO to the fuel to see if that could clean out the fuel system just incase it was the carb giving me this idle issue. So I cant say that smoke now is from oil or from the MMO I have to wait till I burn through this tank and then I will know by next tank full.

 

Ok I pulled all eight plugs out and this is what I found.

#1 : Ash deposit heavily, about thickness of paper gap.

#2, 3, 4, and 6 : Ash deposit slight, no decrease in gap.

#5, 7, and 8 : clean burning, white color to electrode.

I did put some MMO in but if that was the cause then it should have had ash build up on all cylinders not just some of them. I do know after I moved the accelerator pump up one notch to spray more fuel the truck ran great. I set the timing and reset the idle mixture and for like a month or two the truck ran great. These plugs have been in the truck for about 2 years now I would estimate and after I got the truck running great about a month or two I noticed the slight miss came back that would come and go randomly and not happen every day. Then it go to the point that it would miss hard and I could correct it playing with the fuel mixture but it would come back after a couple days.

I think cylinder 1 is the one that was causing my miss. I dont know how long it will last since I cleaned the plugs and reset the gap from .043-.044 to .045. But I think cylinders #1, 2, 3, 4, and 6 are the ones the valve guides are leaking oil considering my book for this truck says ash deposits are caused by burning fuel or oil additives. I have no additives in the oil but I am thinking maybe the oil is just enough to cause an ash deposit and with the number of cylinders doing this is why I am using as much oil in a short period of time.

But then what confuses me is the plugs when I changed them out two or three years ago the plugs I took out was in the truck since the engine was rebuilt back in the early '90s and the thing has always done this with the oil. But yet the plugs werent ash fouled especially #1 so I am unsure as of now if I want to say its this or that.

 

Update, I drove the truck a little bit today, ran great, never missed once at idle. I will see how long it lasts till it starts missing again then I will check cylinder #1 to see if it has ash deposits as bad as it did before. I just hope it doesnt do it again cause that ash deposit is what caused the missing to begin with I realize. Or just drive till it start missing then pull #1 and clean it off and put it back in and drive again.