My 300 i6 has a small miss in it, that drives me crazy, it makes a bumb sound when it misses. I have changed the rotor, cap and all the spark plugs. I also fine tuned the timing.

Is there anything else I can do???

 

spark plug wires

 

Need to know more. Is this fuel injected or Carb? What year?

 

It's a carbed 86 ford f-250, with a 4 speed manual. I have brand new wires too

 

Is it only at idle or all the time?

 

It only misses, when it idles.

 

I had the same thing. Funny how these 300's have some of the same quirks. Me and my buddies tried to troubleshoot it but couldn't nail it down. Once you open the throttle though, miss would go away.

 

At idle, you have maximum vacuum in the manifold system and minimum fuel consumption (that is, minimum mixed fuel/air consumption by the engine). ANY minor vacuum leak will lean the mixture so eventually one or more cylinders will miss.

I suggest you very carefully check for ANY vacuum leak. A dried or cracked line is a likely culprit. If the '86 is like my '87' "fuel-rejected" truck, pay particular attention to the molded rubber line that runs between the separate vacuum port down under the manifold between #3/4(?) cylinders and the PCV valve...especially where it bends at the rear of the engine up to the valve cover. The excessive heat from the riser tube going to the EGR valve may have dried it out and it has developed pin-hole cracks at the inside of the bend radius. Completely remove the tube from the engine, and flex it at the inside of the corner ... I'll bet you find hairline cracks.

Also, when was the last time you changed the PCV valve? If the check valve in it isn't seating 100%, that can also have the same effect as a vacuum leak.

One last thing to do. With the engine hot & running, remove each line connected to the vacuum tree (at the tree, not the other end of each hose!) on the manifold and cap the port on the tree. Then wait to see if the engine still misses. Do it one line at a time and see if the miss goes away. If it does, that's the line that has a leak in it somewhere along its length.

Please post back as to what you find.

 

I had a similar miss on my 93 EFI and it was caused by the ignition timing being set to 20 odd degrees btdc. Once I set it to 14deg the miss ( and wandering idle speed) corrected itself. Worth a check

 

I replaced my pcv and it is running like a champ.

Thanks for all the Help!!!!!!!!

Cracked Head every valve rusty oil leaks every time
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