I have a '81 Ford 302, 2 BBL carb(Ford), engine stock except for Pertronix ign. This fall it started "missing" at idle, I thought. I changed the plugs(Bosch-I know some hate then but I have great luck) still missed at idle. Swapped carbs(went from a 1.14/.050" mains to a 1.23/.050" mains and same results. Can find no vac. leaks. Timing correct. Changed coil. Each time I try something I road test on same stretch of road, well warmed up and I get the exact same results. 60-65 MPH runs good but a bit flat on acceleration, 35 MPH seems normal but at a light it starts "missing" Last time I was at a light and the "missing" started, it seemed to be in a specific pattern(like every 2-3 seconds). My milage(22 MPG before) has dropped . Vac has dropped from 18" to 15"(rock steady). Have a slight rattling sound from the rocker covers at idle in the garage(everything is tight when engine is off) and exhaust has a distinctly different sound and the air going into the carb sounds a bit odd. My guess is the cam has gone south. Any takers?
It could be as simple as a bad plug wire, I dont care for the Bosch plugs much, seems that everything I put them in runs worse afterword, NGK is what I usualy run. Alot of the time you can hear a plug wire that is shorting out kinda a snapping sound, run the engine at night with the lights out somtimes you can see them arcing. Next I would start pulling the wires one at a time while the engine is running till you find the one that dosn't change change the way the engine sounds. I that dosn't find a problem run a compression test. Also might want to take a look at the distributor cap, cracks or carbon tracks on the inside
I had a similar problem with a 460 Ford.It turned out to be a worn exhaust valve guide.The valve would move around and not seat at times.It made the exhaust sound odd at idle and rocker noise
Dist. cap looks good, no cracks. It won't "miss" in park, sitting. Will idle all day fine, can run RPMs up in the garage and runs perfect.
I'd look at the plug wires again. Had close to the same issue a few months ago on my SBC. Ran great idling, but when I took it out and got it warm, it started running rough. Thought it had to be carb/cam/valve train as it was only after running for a while. Ended up a plug boot was split and when it got hot it flexed and shorted. Did all new wires, new cap/rotor to be safe, and all was well with the world.
Sounds plausable but it happens EXACTLY at the same point in my trip. Idles in neutral OK, at any RPM & good in drive with the E-brake on.
I installed the Bosch plugs in 1995 and ran them until about 2005 and changed them basically for drill. They looked just like new axcept of the gap off a tad. Got 22MPG on a trip to Florida 3 yrs. ago. Checked all plug wires for any arcing plus have had them off multiple times and I can't believe i've replaced them in exactly the same position and the arcing continues. Not a snapping sound as in an arcing wire but a definite rattlle from the valve covers. Also the steady low vac at idle....
My vote is spark plug wires. Most of the time when issues with spark plug wires start they are not noticed until everything is up to tempurature and under a load. I had a haul truck years ago that ran fine driving around by itself, yet put a trailer behind it with some equipment on it and it would start to bang, pop and fart when it started to rev over 3000 rpm. Sometimes you can see wires starting to break down if you look unsed the hood in the dark. If it is dark enough you will see little arcs of electricity dancing around and yet the engine will be purring like a kitten.
take the egr off make a gasket out of a pop/beer can that blocks off the hole in the egr and try that if the egr is leaking its a vac leak u cant hear or test for cuz its interal
Also I've noticed the PCV rattles at idle(buzzes-can feel it openning and closing rapidily). Increase the RPMs a couple hundred and it goes away. Installed new PCV and same results.
I think this is the right track as well. Also if it is a petronix conversion is it the one with the flying magnets? IE little magnets on the rotor? If it is make sure that the magnets are tight and all of them are present. Nowe on to the rattling valves in the garage. I have an engine here that I chased and fought a noisey tappet for several months, I would run the valves then drive it and get the rattle and then I would adjust the valves again. I actually thought that i may have a loose lifter bore. Then one day I was leaning over the engine while it eass running and felt a little hot air. I replaced a bad exhaust gasket (not a major leak) and my noisey tappet went away.
EGR was removed 17 yrs ago. Went all over the intake, base of carb, all vac lines & EGR block-off plate with propane and starting fluid. No change in engine RPMs so I'm ruling out vac leak. Like I said, vac is low but steady.
That's the problem. It idles perfect, will got up the hi-way at 60-65 and acts normally except is a bit lazy on accel. At 35 seems to be OK but at a light it sits there and acts like a miss every 2-3 sec. Take off and it stumbles then catches the gas and is OK.
Checked the magnets and they looked OK. Had a replacement on hand and swapped the mag. ring and same results. no exhaust gasket leaks. Can feel the rattling under the valve covers,with my hand
You have not mentioned a compression check. It sounds like a bad valve or seat. A compression check should prove on way or the other. Then ckeck plug wires with the ohm meter.
I have not done a comp. test because the vac. is steady but low with absolutely no pulses as I would expect with a bad valve/seat.
Do you have power brakes? If you get a hole in the power brake booster diaphragm it'll do this too. When you apply brakes (i.e. stopped at a light) it'll be more of a vac leak than sitting in park.
the gasket on the block off plate between the 2 holes might have a leak it would still be internal and starting fluid would not find it it is a 17 year old gasket
If the vac has dropped from 18 to 15 and is holding steady,, it would seem to me that the timing is retarding,, you have also mentioned that it is lazy on acceleration. all this points to retarted timing,, or restricted exhaust. There is my 2 cents worth.
Do you have a oil preasure guage? If the lifters / rockers are starting to rattle when it warms up are you losing oil preasure to the lifters assuming their hydraulic. Had a Ranger pick up that would start and idle fine, drive it for a mile or so and it would start ratteling like it was gona come apart, shut it off let it sit a few minutes it would start and run fine. The oil drain back holes were plugged with sludge so it would run till all the oil was all pumped up to the top end. Also oil thins out as it warms up if the engine is worn might be a cause also.
Thanks for all the info. I'll keep it in mind. Am in the process of replacing the cam as it was 14 yrs old when I got the engine and I've put 100K + miles on it. If the problem is still there I'll start checking your solutions. Thanks for no responses that make me sound like a dumb ass!!!!!!!
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