Engine miss firing
 

Ok well as some of you may know I recently got my emissions and safety inspection done on my truck..the one in my sig. The first time I took it, it failed because of idle emissions and suspension. I fixed the suspension and raised my idle because I found out it was to low anyways. When I was driving it home the truck just didn't sound as solid as before I got it tested.

The day I was going to take it to get retested I came home from school and found out my dad had taken it down to the inspection shop while I was at school. When he got home hes tells me it passed...but instead of just seeing if it would pass with the idle speed raised he paid some guy there 30-40 bucks to do a "carb adjust".

Now let me explain a little something. My engine was running smooth as silk the day I started getting it tested. It was only after these guys had tested it that it started running bad. after the first time I took it in it just didn't sound near as good but after the second time..well now its just garbage.

When my dad got home from getting it tested again, he said to me "Kevin...the truck is running horrible!"..needless to say I was a little peeved. So I go out there and cranked her up only to find that it runs like garbage. The same engine that was running flawlessly before is now missing on one cylinder. I thought..well maybe they changed the timing so I checked that..nope still missing. This is not just a little missfire every now and again like a little stutter because of bad timing, it's an obvious consistant missfire thats just making the engine run horribly.

I don't get what happened.

Any help on this would be greatly appreciated.

P.S. So you guys know where I stand it's a 350, quadjet(I recently rebuilt)4 barrel, brand spanking new 8.8 Accel spark plug wires, fairly new spark plugs(I pulled them and they are all tan) oh and I have HEI ignition...I'm just at a loss.

P.P.S. This missfire happens all the way from idle to reved up.

As a start I would recheck plug wires and firing order in case something got changed in the process and take a vacuum test. John

Wait until it's dark out, pop the hood and check for sparks. They may have cracked a plug wire. Check your cap and rotar for "lighning marks." Also, make sure the ground strap in the cap that goes from the cap to the coil is still in there. Also, are you SURE it's a miss? They may have turned the idle mixture screws way down. That would cause it to run like crap too. Can you call or go back to the place and have them put it back to whatever it was before, or at least ask them what they did to it?

Ok I've been working on it for a little while and heres what I just checked.

I took of the coil cover on my distributor and the ground from the coil to the cap seems to be in fine condition.

I also bottomed out the idle mixture screws to see if they got changed. They had the left one at 1 6/8 turns out, and the right is 2 1/8 out. I don't exactly remember but I think I had them set at 2 and 1/2 turns out but yeah don't quote me on that.

67chevy I'm 95% possitive its missing. My dad just told me a while ago that the crack mechanic even said "seems to be missing on 1 cylinder." This quote came from him apparently right after he got done "working" on it. I was just informed of this quote a while ago. Your idea about cracking a spark plug..that very well could be...I'll let the engine cool down and pull the plugs and get a look at them. I just finished all my finals for this semester so I've got alot of free time to get this thing fixed...again.

EDIT: Oh and yes I did try putting the mixture screws back to 2 1/2..didn't seem to fix it.

Man I wish I could record the way this engine sounds...it would probably make diagnosing whats wrong alot easier.

Ignition System
 

It sounds like your ignition system:

1. Rotor
2. Cap
3. Wires
4. Plugs

If its missing, you have a open or shoted to ground problem. Basically, the plug is not firing for the discribed reason. I have a glow light that I plug onto the plug head and insert the other end into the SP wire. The light glows if you have fire to that point.

Other things to check,
Internal HEI componets
SP Gap / .045
Power to the HEI.

Remember, look for the easy / small stuff first.

Ok thanks, I guess I'll take out the plugs first and check them all for cracks and make sure they are connected good to the wires...then I'll take out the distributor and make sure all things are a okay there.

Now you say the gap on spark plug should be .45, everything I've ever read said .35. Was that just a mistype or is that the gap for certain applications?

HEI / .045
Points Dist. / .035

Hmmm...well they are at .35...but it ran real good before.

I'm not exactly sure what year the engine is. I need to go searching for the engine number. I think it does depend on what year the engine is right? Or does the HEI, points gap always hold true?

My truck had a difficult time passing the idle speed emissions test because I had one bad spark plug that was causing the cylinder to miss about once every three or four revolutions of the engine. I figured out which spark plug was bad by hooking up my timing light to each spark plug wire one at a time until I found the one that wasn't making the timing light fire at a nice constant rhythm. Once I replaced the spark plugs, my truck passed the emissions test. Of course it didn't help much that several lobes on my cam shaft were severely worn.

pjmoreland, thats an interesting idea hooking up the timing light to each spark plug. I'll have to give that a try.

UPDATE: Well I took out all my plugs regapped them to .45 and I even found a spark plug that had 3 cracks in the ceramic. The cracks extended from the top to the bottom of the plug. I replaced it with an older spark plug and started the truck up. It actually seems to be running great.

I'm not going to get my hopes up yet, but so far so good. Tomorrow I'm gonna let it fully heat up and see whats up.

I guess my next move will be to check the distributor if it's still not running good. Since it was dark when I regapped all the plugs and replaced the cracked one I turned off all my work lights when I was finished and looked all around the distributor and the spark plugs for any leaks...there wasn't any.

Sounds like you found it! I'd go ahead and check your cap one last time and look for odd discolorations on the electrodes and "lightning marks." The misfire may have messed the cap up a little.

Well I'm glad I didn't get my hopes up. I just went outside and cranked her up...same old deal :rolleyes: .

I guess I'll be taking off the distributor cap after all.

Pull a wire and start it up see if its not worse if its not then that plug or wire is your problem if it worse shut it down put the plug back one and move to the next one. I will be willing to bet you the guy pulled a wire off wrong an broke it where you cant see it. Check the number 1 firts

Ok I have lots of pics of my distributor.
Heres the cap off
http://i4.photobucket.com/albums/y13...r/7c76a06a.jpg

Heres the distributor with the rotor off.
http://i4.photobucket.com/albums/y13...r/IMG_0017.jpg

Underside of cap...there seems to be some small carbon marks in the middle
http://i4.photobucket.com/albums/y13...r/16c6a463.jpg

Better view of the center.
http://i4.photobucket.com/albums/y13...r/IMG_0014.jpg

Notice the carbon mark...it was the only one on the side of the cap
http://i4.photobucket.com/albums/y13...r/IMG_0013.jpg

Rotor
http://i4.photobucket.com/albums/y13...r/IMG_0016.jpg

Underside of rotor :/
http://i4.photobucket.com/albums/y13...r/IMG_0015.jpg

The underside of the rotor as you can see has tons of little carbon lightning.
Could this be my problem?

looks like the cap and rotor could be replaced

I was looking in my Haynes manual and they show a huge carbon mark on their distributor to show what to look for...now is it always big and charred like that or can it be little ones like the one on my cap?

That stuff could probably be replaced, but it doesn't look bad enough to be causing a miss.... I'd try what flip said about pulling wires one at a time and pick up a new cap and rotor anyway. I'd also try to get in touch with the shop that did what work and ask them what the heck they did to your truck. Have you checked the timing with a timing light? A lot of people retard the timing of a vehicle so that it will pass, you may need to adjust your distributor.

Been watching this thread. Finding the cause of a miss can be very frustrating. Hoping to learn something from your misery. I'll try to contribute: Can you narrow it down as to wich cylinder is missing? Someone mentioned hitting each one with a timing light. That sounded good. You can ground out each plug wire too. If you are brave you can just start pulling 1 plug wire off at a time. The getting shocked part is kind of a drag though. ;)

:lol:

I did that once, hurt like hell. Your supposed to shut the motor off, pull it, then start it and see if it runs different. Shut it off, plug it back in, pull the next one, start it, etc.... :) :lol:

Yeah, I get impatient and start 2nd guessing myself: "Is it running the same or worse?" *plug it back in, start it back up* "Was it running the same or worse?" *repeat*. Screw it. ZZZZTTT "YEOW!"

Haha well I'm not big on getting shocked so I think I'll do them one at a time. I tried the timing light and all the wires seemed to give the light a good consistant rhythm.

I don't know if this means anything but when I rest both my hands on the valve covers I notice that the drivers side is very rough...its feels like thats the side thats missfiring. The passenger side however seems to be as smooth as ever.

I checked the timing and its the same...I even played around with it a bit but no matter where I go with it you can still hear it missing.

I'll get myself a new cap and rotor like you guys said...that way I know its just one less thing that could be wrong.

Chickenwing, yes this is pretty annoying hehe...thanks so much all of you for your help. I know with your guys help this truck will run right once again.

Yes be sure to turn it off when you pull the wire had a friend do that walked around with his arm twitching for 20 min

Funny, I've pulled the wires with it running and never got zapped. Maybe I'm just lucky.
Or maybe I'm just wearing the right shoes. I never have a problem with static electricity either, but my g/f gets zapped all the time getting in and out of her car.

I pulled the wires off at the cap, of course. Pulled one off, if it got worse I'd put it back on. No change I would leave that one off and pull the next one off. I once found that I had a blown head gasket between two cylinders that way, had two plug wires disconnected and the motor was running the same as with them hooked up.

Todd

I don't have to pull plug wires anymore. ;) I have a snap-on scope on a rope. Kinda like a cheap o-scope. Tells you what KV a plug is firing at. Wish you were local. I could definately tell you wich one is missing.

hmmm...so where do I get myself one of these ropes?