KIKIRIKI

Alright, so I have a misfire. Sounds like on 3 cylinders, and it's fairly random (all speeds including idle (except around 2000rpm it *seems* like it goes away), and it comes and goes... usually it comes).

Using a timing light, I've noticed that cylinders 7, 4 and 6 are missing. The light will come on, but erratically. Here's what I've done over the course of the past 6 months:

-New distributor (checked it yesterday with ohm meter, coil and pickup appear to be good, and mechanically everything looks as it should).

-New cables

-New spark plugs







In both images top one is cylinder 1 which fires properly, and in both images bottom one is cylinder 7, which misses like a blind guy during target practice.

Can anyone read these plugs for me and tell me what they say? Does it match with what my timing light is indicating? Also, can anyone tell me what my next troubleshooting step is? I'm at a loss here.

KIKIRIKI

Oh, and it's an '81 with the computer ripped out (ie, an '80).

KIKIRIKI

Update: went out and got me a proper spark plug tester (one that you put inline and has a bulb)... and it indicates that every single sparkplug is good. So now what? Which instrument do I trust? Does this indicate a strong spark or does it matter? I'm back to thinking it's my carb now, which is also effectively brand new.

There needs to be a new "ripping hair out" emoticon.

Jud Chapin

The center electrodes don't look too good to me. I'd try replacing the plugs. They're cheap enough.

Douglas Mariani

First things first, I would check the contacts on the inside of the distributor cap. Make sure the contacts look good and that the rotor is secured to the distributor shaft. I have seen loose distributor rotors cause your exact problem.

Hope this helps

doorgunner

Also.......connect voltmeter test lead to the Distributor battery + connection and negative to a good ground on the engine.

Turn on the ignition switch to verify 12V........

While cranking engine watch test meter to verify voltage isn't dropping below 11 Volts..............

With engine running slowly rev engine while watching test meter to see if voltage fluctuates.......

If possible mount the test meter where you can easily see it while driving the car to watch for voltage drop.

I'm not saying this is your problem, but it's a fast easy way to eliminate voltage supply problems.

My car gave me trouble for six months as the problem got worse until members encouraged me to do the test.

chstitans42

Vacuum leaks can cause a miss. Too much air with not enough gas, even with spark, can cause a miss. BUT I would pull each spark plug wire one at a time on the engine end, put in a new plug that you know is good. Then ground that plug on the engine somewhere while you have a helper spin the motor. Do this on every cylinder and verify you have continual spark at every cylinder.

DUB

I AGREE with Douglas Marini 100%. I DO NOT trust new distributors. i have seen many new coils be painted BLACK..and thus...the ground strap UNDER the coil can not make good contact due to the paint....and also....the black wire that is screwed to the top of the coil...also does not make good contact.

Hopefully you actually ran the test in the GM book to make sure the coil and pick up are good as you wrote. And I would do a visual check to make sure the module is protected my the special grease between it and the distributor housing.

I use a Snap-On Kv tester when I am checking to see how many Kilovolts the spark plugs are actually getting.

If I have a cylinder that has 14 Kv...and the next cylinder has 2 KV....I swap out spark plug wires an check again.

Also...I make sure that the plug wires are routed correctly away from heat and NOT zip tied tightly together.

DUB

Paul L

You might want to check the gap on the spark plugs. My old eyes see about 0.060.

terry82

did the misfire happen over time or did it just start ?

7T1vette

Plug gaps look too big (as mentioned above) and there aren't any crush-ring washers on the plugs. Note the soot at the chamfer near the threads on one of your photos. Not sure the plugs are sealing well in the heads.

Whether you have points type ignition system or HEI, you can set the plug gaps at .035" just to see if that eliminates the 'miss' problem.

Also, be aware that the ground path for the distributor is through the hold-down clamp and bolt that retains the dizzy. If there is not a non-painted contact path from the dizzy body through that bolt and clamp and into the engine block, your ignition system has a bad ground path.
(unless the ground path is separately wired, as with some aftermarket ignition boxes)

LS4 PILOT

Way to much gap.
Yup ....with stuff changed from stock, experimentation maybe in order.

Need to,start off with new AC plugs ...those electrodes do look worn ..

People neglect and fiddle with stuff they don't understand . You may have start from scratch and go over the whole motor with known good parts , that fit the application .

speedreed8

gap and heat range of plug...

DUB

An 81 Corvette does not use spark plugs with the compression ring. The taper seat design is correct.

AND...KNOWING how to PROPERLY GAP spark plugs is also a benefit.

These are Platinum plugs....not knowing anything more and getting a response...I guess we are all in a 'holding pattern'.

DUB

7T1vette

Sooo, you can determine the angle on those plugs is correct from that photo? great eyes... [wonder where that carbon came from]

Yes, I see the shiny line around the chamfer...which shows that some of the plug is seating. If it looks that way all the way around on all of them, sealing plugs is not likely a problem.

DUB

Not knowing if you were commenting on my reply.

BUT...if you were....the reason I know these plugs are the correct taper design is that the compression ring design spark plug was not used on a 1981 Corvette.

Where the carbon is coming form is not a concern of mine at his point. It could have been there from the previous spark plug that was removed and the spark plug hole not inspected and cleaned before the new plug went in.

DUB