engine miss

[Okiedokie]

What am I missing? 84 302 Ford HO motor. Edelbrock intake with 500 cfm Edelbrock carb. HEI distributor. Motor has developed a dead miss on # 5 cylinder. Compression is 120 to 130 in all cylinders. Idle vacuum is 20 inches. Following the chart for vacuum gauge reading, there is no indication of any related problem areas. Once I isolated the cylinder by removing plug wires untill one had no effect, I installed an ingition test device that goes between plug and wire end cap. It was firing strong, so I replaced the plug, no change. Thinking I might have a cam problem I pulled the vc and found that both valves are opening and closing. I am just a hobbiest and have no training in diagnosing this stuff, have I overlooked something? Joe

[kb426]

Is there any chance it has the wrong firing order used? The ho's use the same as the 351w not the older 289?

[wayne petty]

i know it may be a pain....  but can you pull the intake manifold and look down the intake port... and see if the back of the intake valve is coated with carbon build up from oil leaking through the valve guide...


that is one thing that can happen and not really affect the compression too much...   the fuel soaks into the carbon and prevents it from entering the chamber to be burned...

this might also happen on the exhaust side... but it is rare...

be sure that you swap wires first..       examine the inside of the dist cap...

you said it has HEI?   you are talking about the ford duraspark  with a remote module    or a ford TFI with the module on the side of the distributer housing....    

or  a real GM large cap HEI unit from an aftermarket manufacturer...

[Leon]

I had a motor a few years back that whenever it would warm up it would miss on the #1 cylinder.  I found a crack in the intake runner of the head and when warm it would leak into the valve area, and that cylinder would miss bad.  Valve was clean and top of piston was squeeky clean, looks like it was steam cleaned from the water.

[Okiedokie]

Thanks for your replys guys. Leon, it misses hot or cold. Wayne, it is an aftermarket GM style HEI. It is firing the plug though, so I don't think it is involved. The carbon may well be a suspect. It is a used motor that came out of a running, driving car. I will look at that if nothing else seems apparent. kb426, I chased down the firing order thing when I first removed the CFI and Ford electronic dizzy. It just ran lousy and idled rough. Joe

[kb426]

If you have a leak down meter, that would be my next choice. By leaking the motor, you will eliminate anything inside the cylinder or related to the valves sealing at tdc. Whenever you have something weird happen, you have to check everything about 3 times to make sure you didn't mess up. Good luck and keep us posted. I used one of those motors but I never had any troubles. It had about 140,000 miles on it and ran great. I used the earlier duraspark ignition and a holley 2 barrel on the original intake.

[Okiedokie]

I was visiting with an experienced wrench today and explained my delema. He said I bet your intake is leaking at that hole. Suggested I spray some carb cleaner in that area and see what happened. Bingo. That was exactly it. I was amazed that it would not show in the idle vacuum reading. Looks like I get to pull the intake, yeah I tried tightening the manifold bolts. Joe