engine missing under load

[fearnatine13]

So driving home from work the other night, my engine started missing under load. Quite strange i thought, since i did a complete tune up oh, about 3-4 weeks ago...new rotor, cap, plugs and plug wires. at first i thought it was a loose wire, but to no avail, they were all tight. then i suspected a loose plug, but they were all pretty tight, meaning that i couldnt loosen them with my vice-grips for hands. thought it might be a vacuum leak, but the idle is pretty good, and i checked all my vacuum lines and they were fine. its pretty strange, since the misses are fairly erratic. any ideas?
350 sbc
edelbrock carb (1406)
edelbrock performer intake manifold
dynomax ceramic coated headers
flowmaster super 40's

[1BadSS]

Could have a flat cam lobe.

[Sport/Truck]

Your wires could be buttco, what brand are they?

[fearnatine13]

i have borg warner wires on, and just regular autolite spark plugs.
as for the flat cam lobe, that sounds like it will suck, especially since the engine isnt even a year old yet

[Sport/Truck]

Not sure about the spark plug wires (I hear they make good gages though) I don’t like the Autolite plugs, but lets see what others say. If it were mine I would try my old wires and new plugs. If your old wires are no good, just try new plugs.
Are you using HEI? What gap are you running?
Couldn’t be a combination of bad gas/too much timing? I tend to lean towards the wires/ plugs though.
s/t

[fearnatine13]

i agree, i just need the time to be able to pull the plugs. this is the second set of borg-warner wires ive had, and they're pretty good. im lookin at trying the ngk spark plugs this time around though.

[cdowns]

have you changed fuel filter?? could be clogged or the filter in the tank could be clogged/ have you checked the rubber hose under the cab it could be dryrotted. or if your using rubber hose from the pump to the carb(very unsafe) it could be kinked or touching the block or manifold .. have you changed air filter lately. did you gap the plugs when you installed them// is it points or hei ignition??

[67chevemall]

AC Delco Plugs. always

Always change one thing at a time and then you know the problem.

[st1x10]

Might check the plugs for cracks. Should be black from the sparks. Also I would get some good ole AC Delco plugs, best you can get.

[Sport/Truck]

I say NGK or AC Delco. Leave Autolite for Fords!

[regal8r]

sounds like fouled out plugs....how old are they? replace them with some ac delco's and see if that makes a difference

[Steelawork'n]

I agree with the fuel filter. Big intakes require clean filters. Another idea is the hot lead going to the distributor. Make sure that wire is in good condition. I've got a 12g wire running through the ignition switch to the distributor.

Oh, you could also have...dare I say it???...a leaky head gasket. Do a compression check.

Simple things first though.

[WorkinLonghorn]

You're not saying what you have for a distributer. If they're points then maybe re-check the dwell.

[hvychve]

well, the best way to find a missfire , is to have your buddy hook up his ignition scope and have someone powerbrake it and watch the burn time on the osciliscope, oh wait a minute you dont have a buddy w/ a scope right?
The backyard method works too. get you some insulated pliers and have someone you can trust to powerbrake it.(dont powerbrake too long or you will cook the tranny) load the engine in gear to see if you can get the miss to show up, gradually pull a wire one at a time, if the rpm doesnt drop on the cylinder thats the culprit, you found it, now you have a starting point, check to see if plug wire is leaking, you can do this by spraying water on the wire from plug to cap, you better not see any arcing, if you do , wire is junk, or boot needs replaced, if ok, remove plug on that cylinder and verify gap isnt bridge or porcelin isnt cracked, if all this is ok, you may have moisture in cap or carbon track , vacuum leak will NOT cause a miss under load, your vacuum decreases under load . you could have a massive leak on the intake and it wont cause a missfire......rough idle yes . throw a vacuum gauge on a manifold vacuum sourse and see what it looks like, should see around 17-20 inches and steady, if needle is bouncy , you may have valve spring issues, although this would give you a rought idle and not necessarly a miss under load only....... points wacked out , wont give a miss under load only. sounds like you have a secondary ignition issue to me.... plug/wire or cap....good luck

[mrein3]

Check compression. You may have a bad valve or two.

[mnunn454]

I'd check these things in the following order
Fuel Filter(s)
Plugs
Points/Ignition dwell,
Distrib cap
Coil
Plug wires
Compression test

The very last thing I'd ever do (alive at least) is be hanging over the fender testing and pulling plug wires while some buddy of mine powerbraked my engine. Sorry, but that's just plain dangerous.

Should you decide to do it though, may I nominate you for the Darwin awards?

[dailydragger03]

AC plugs (R45ts) Belden wires (federal mogul) all can be found at napa. Also could be points or coil. If your motor is only a year old its probably ignition, hopefully.

[hvychve]

Leaning over the hood yanking plug wires while a coworker powerbrakes the engine is a common practice with proffessionals. if you have a dyno, that would be the most preffered. if you follow a certain order you could get into trouble,, compression in all reality should be checked first. not last. if the compression is good , than check plug, wire, coil and dwell is not going to give you a missfire underload, a missfire is going to be something that is affecting only a cylinder, fuel filter, coil , dwell will affect all cylinders

[gator2511]

When I had my 64 vette, the only plugs that would stand up for a solid year were Autolites. It would beat ther Delcos to death in 6 months. Don't know why but it ran like a scalded dog with the autolites. Now I'm back to Delcos in my truck. Unless I find out they are made in Mexico like so many of their other products.

[dmec]

if a points distributor, could be a bad condenser.

[68speedalert]

Do what hvyche says, he is dead on it. I bet he does this crap (or has) for a living.

Since it's been a few weeks since the wires have been put on, I bet that those lovely headers, that everyone thinks they need, have burned thru a plug wire or boot. Check each one CLOSELY.

A cracked porcelain is is also very likely. Headers don't always give enough room for the socket to stay straight. It gets in a bind and the porcelain gives. If that has happened, you need to change that plug wire or boot too, because a carbon track will be made inside the boot and will allow the spark to eventually bypass the plug again.

If you still have points in it, check where the plug wire is attached to the dist cap. Often times the wires are not seated in the cap. You push the wire on the cap.....the terminal doesn't go into it and the wire just pushes out. The boot is on tight, but the wire is not seated. When ever you put on these type of wires, slide the boot up, push the wire terminal into the cap COMPLETELY, then slide the boot back down over the dist cap.

[fearnatine13]

In conclusion...
I changed the spark plugs yesterday morning. My truck subsequently idled even worse than before, and kinda backfiring. So i finally crumbled and took it to my mechanic. They checked everything but the spark plugs, so i asked them to check the plugs. As it turns out, when i changed the spark plugs i put a little too much muscle into it and broke the ceramic. On two plugs. Go me. So in any event ive learned a few lessons here. Well...i learned one...dont over-torque the spark plugs.

[67ChevyRedneck]

Don't know what plugs you bought, but I've had great results with NGK's in my silverado and C-10. They're dirt cheap too (comparatively.)

[68speedalert]

Originally Posted by fearnatine13

In conclusion...
...dont over-torque the spark plugs.

AND don't let the socket get crooked on the plug. If you used a spark socket, that is a must. If you didn't use one, that's why they broke.

[hvychve]

Fearnatine13, you stated you changed the plugs on a whim and actually made it worse by cracking the porcelin, I can relate, but what was your original miss before you changed plugs? did your mechanic find that? was it the plugs? did fixing the broken plugs fix both your original problem and the one you caused? BTW, I would only use A/c plugs in GM, Motorcraft or Autolites in a ford product, Champions in a Chrysler Product, NGK or Nippendenso in an import, I hate Bosch, I have had many customers change their own plugs and bringing to me with a problem , only to find out that the plug was a cheap bosch and created a problem, hey before all you yell at me and swear that bosch rules and I stand corrected, this is my experience and my Opinion, I am entitled , I am not telling anyone to do anything, just take it with a grain of salt.......