Hello I am new to the forum and still a novice mechanic when it come to the inner workings. Any help would be appreciated. Ok, I have an 85 w-150. It has an older 400 put in by the previous owner. Ran real rough on me and barely got it a few miles back to the house. First couple miles seemed to run a little better at high RPM but wanted to nearly stall at lights. By the time I got into the neighborhood it did not seem to want to shift into 2nd, so I limped it the rest of the way in first. So here's what I found. Dead miss in 1,4,6,7 determind by pulling plug wires while running. Is not jumping around..only these 4. Have changed fuel pump, plugs, fuel filter and intake manifold gaskets/valley pan. (as I thought I detected a leak using carb cleaner around cylinder ports 4,6.)Cannot find vacuum leak anywhere else. Distributor is a Mallory electronic ditributor with Mallory promaster coil and has spark at the cap and at the plugs. Cap looks to be good with no crack or crusty contacts. Compression test shows 1@125, 3@120, 5@120, 7@125, 2@140, 4@140, 6@120, 8@125. Also has edelbrock intake with holley 2 barrel. I noticed cyl 1-4 and 6-7 are on same intake runners...could a head gasket break between 4-6 cause 1-7 to misfire? Thanks for looking at this!

 

Not if the compression is what you say...Could it be one side of the Carb is not working. spray carb cleaner into one side or the other of the carb to see if it changes anything...Bill

 

Check the Brake booster for leaks also....

 

Thanks Bill...wanted to rule that HG out if I could. The discharge nozzle seems even on both sides when I give it throttle....not a super strong "jet" but steady streams. When I first got it home I tried spraying a stream of carb cleaner in while running (and having to rev the engine enough to keep from killing it) kind of just moving side to side slowly for about 10 seconds.


I just tried spraying the intake again around the bad cylinders and I am still getting some rev with the carb spray. Think I need to play with this intake somemore. So same question...different gasket: If I get a leak on an intake gasket in one spot...will it affect all other cylinders on that intake runner? Or would the compression test results rule that out too? Thanks for the help.

 

I think you have a carb problem......................


Please read this carefully. I am NOT trying to make fun here

DO NOT pull plug wires (without grounding) and here is why

No1-------On any electronic ignition, pulling wires loose creates an OPEN. The spark is looking for a place to go, and this causes REFLECTED PULSES back in the electronics, which can "kill" your ECU.

2--When you pull a wire loose, this open is "looking" for a place to ground, just like lightning. This often causes CROSSFIRING which can and will lead to improper conclusions.

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The best way I've found to look for non-firing cylinders is to do one of two things

A--Pull the dist. boots so the wires are loose, and use a grounded clip lead and probe or screwdriver. Hold the probe near the entrance to the dist. tower, and pull the wire up out of the tower, and as it comes up, slide the probe down in to ground that spark tower.

B--You can also slide very small thin brads down beside all 8 wires after pulling the boots up. Now all you have to do is go 'round the cap, shorting each one to ground with your probe.

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Depending on compression / leakdown tests, don't forget to cast a CRITICAL eye on plug wires, plugs, and dirt/ etc in the cap and rotor.

Also, I've seen some "pretty flat cams" that would mysteriously generate what at first glance seemed to be passable compression

 

Ok...facing motor from front I sprayed carb cleaner in right side of carb and got a strong rev. When I spray the other side it's a very mild rev.

 

At the base of the carb.? you might have a bad carb base gasket. I just bought a new 1 " spacer for under the carb. and the thing had a terrible leak and I could hear it hissing pulled it back off and installed another 1" spacer used the same gaskets and it ran fine. I still dont know what is wrong with that one spacer but it is scrap metal as far as Im concerned.. Check the base gasket it mighr have fell apart causing all your problem. I have seen it happen many time over the years...Bill

 

No, this was spraying in the carb. I had already replaced carb gasket too when I did intake gaskets (forgot to mention that) and did not help. Thanks again.

 

There has to be a vacuum leak somewhere... Bill

 

I had a Duster with a smooth running 360 2 barrel. I switched it to an iron 4 barrel intake with an old ThermoQuad. It ran like crap afterwards. I thought I screwed something up! I pulled the carb and looked closely for defects. Apparantly it had been through a few fiery backfires or something because some stuff was melted in there. I switched to a Holley 600 on an aluminum intake and it idles smooth again.

 

With 4 plugs miss firing I would suspect a miss match on the firing order..

 

Thanks for your willingness to help Bill. I reinstalled the intake manifold again and am detecting no leaks around it,the base of the carb,vacuum port or any of the lines. The side of the carb that revs strong with the carb spray is the side corresponding with runner for 1,4,6,7 which are misfiring. Does that pretty much conclude vacuum leak still? Thanks again!

Also, I have always had an oil leak..which I assume to be from rear main seal, but since all of this began it seems the oil leak all of a suddden has greatly increased...would this be relevant to a vacuum problem? Checked PCV valve and it seems ok...rattles and can blow air in one way and not the other.

 

You could just plug the PCV valve and if it smooths out then thats the problem. What happens in the other side of the carb when you spray carb cleaner in it does it bog down if it does and it races on the other then its not getting fuel on the 1 4 6 7 side and the carb will have to be replaced or rebuilt. The oil leak I would check real close make sure its not the intake or oil pressure switch ETC.. Bill

 

The one side races that goes to the misfiring cylinders. The other side revs slightly but not near as much as the other.

Idle mixture screws are at about 1 1/2 turns out. If I turn the screw all the way in on the 1/4/6/7 side (the side that raced strongly with the spray) it has no effect at all on the idle. The other screw has an obvious effect if turned only 1/4.

 

I suppose a misfiring engine could increase crankcase pressure, making an existing leak even worse. Do yourself a favor and either disassemble that carb and clean it or borrow one from a buddy and try it out, gotta rule out the carb.

 

Excellent diagnostic skill. Looks like you found the problem, lack of fuel on the "bad" side. Time to look inside or try another carb.

 

The carb needs to be fixed one way or the other BUT before you pull it off try this... With the car running air cleaner off Rev the car up and manually close the choke tight and let it off and clear it out. Some times this will pull the dirt or blockage out and run fine. Its worth a try you have nothing to loose... Bill

 

Cleaned out the carb and all 8 are firing again. Thanks for your help folks!!

 

Thats what we are all here for.. Glad it all worked out... Bill