Diagnosing misfire - Forums - Official C3 Vette Registry

SPONSOR AD

Support our Vendors! | Advertise Here

Topic: Diagnosing misfire

in Forum: C3 Engines

Already a Member?
Login
Not yet a Member?
Register for Free!

Diagnosing misfire (1/12)
11/10/15 11:48am

DarinM
Former Member
Send Private Message

Caddo Mills, TX - USA

Vette(s):
1980 Black on Black 4speed 383 Stroker 17x9 rear, 17x8 front with 255-50R-17 Nitto tires OBX Stainless Steel sidepipes DeWitts radiator with dual electric fans

Joined: 9/15/2015
Posts: 38

Hello all, i'm trying to diagnose a misfire that i think i know where it's coming from but wanted to confirm my thoughts with the masses first.

1980 C3 with L-48 350 and 4 speed manual trans. Stock engine with 101k miles. Mods include, Weiand Street warrior intake, Holley 600 cfm carb with mechanical secondaries. DUI street/Strip HEI distributor with Taylor plug wires and Accel shorty plugs. Exhaust is Stainless headers with side pipes.

Issue presented itself this past weekend. This isnt a daily driver so it sits for a quite a while between use.

At idle (cold or hot) i get a consistent miss on the drivers side of the sidepipes. I pulled all spark plugs to check the gap which all checked out fine with exception of some had fuel and oil (which i know i have an issue with valve stem seals leaking on the drivers side). I then checked all plug sires for cuts and chafing which checked out fine. Next i pulled the cap and rotor to inspect for wear, that also checked out fine.

This miss does get worse when you accellerate to the point of seeing a backfire through the carb. I watched a youtube video where a guy shows putting a piece of paper against the exhaust to show a proper working engine versus a burnt/stuck valve. On the bad engine, it sucks the paper into the pipe versus a good one only pushing out. I did this last night and it definately sucks the paper in and has a very rich smell with black soot in the pipe.

at this point i'm feeling i have a problem with the drivers side head and plan to pull the valve cover this weekend for visual inspection and try a cylinder leak down test on the drivers side. I think this should validate my thinking.

my two questions for the group.

First, do you think on on the right track ?

Second, if this is the case, next will be a set of heads will run around $1k and i would assume i should replace the cam and lifters while it's all apart which is another $300. So at this point i'm at $1300 plus miscellaneous gaskets etc, so i'll round it to $1500. Based on a compression test i did a while back where all cylinders are around 125psi i would assume i still have a few miles left in the motor, or would it be worth my while to wait a bit longer, add another $1000 or so for a crate motor ?

This could go either direction... if i do the heads and cam now and get another year or more out of the motor, i could re-use the heads, intake and carb on a new motor so they wouldn't go to waste, or just bite the bullet now and do the whole thing.

Thanks in advance

______________

Quote

Our Sponsors help support C3VR

Re: Diagnosing misfire (2/12)
11/10/15 1:38pm

eldredjames

Lifetime Member
Send Private Message

Temple City, CA - USA

Vette(s):
1969 Daytona Yellow. 350 / Automatic.

Joined: 10/3/2010
Posts: 365

DarinM said: ...Based on a compression test i did a while back where all cylinders are around 125psi..

I would redo the compression test and also verify a spark at each plug. After that, test each cylinder head. I saw Edd China do this on an old Wheeler Dealers: remove the head and rocker arm assembly so all the valves are pulled tight into the head, turn it over with the head level and fill each combustion chamber with gasoline. If any of the valves are bad, the gas in that chamber will trickle out.

Crate engine? I have a ZZ4 engine. It runs great and gives me no problems. But it would have been nice to have the original engine due to the numbers matching craze.

Jim

Quote

Re: Diagnosing misfire (3/12)
11/10/15 2:20pm

Adams' Apple

Lifetime Member
Moderator
Send Private Message

Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

Joined: 11/8/2003
Posts: 20129

Before you start ripping into this thing, let's do some basic checks.
Did you Ohm out the plug wires? Just looking at them(or the plugs) will not tell you if they are capable of carrying the spark. Use an ohm meter, and check the resistance of all of the wires. You're looking for consistency here, not specifics. If you have one(or two) wires that ohm far differently than the others, those wires are suspect.
Use a spark tester if you have one....test each wire to see if they will all fire across the same gap.
If you have a dead miss on one cylinder, check the wires first, then move the spark plug to another cylinder and see if the miss moves with it, or just install a new(or different) plug in that hole to see if the miss goes away. If it doesn't, check compression again on all cylinders, ignition unplugged, throttle held wide open. If that one cylinder is down on compression more than 10-15% of the others, then it's time to dig into it deeper. Thumbs Up

______________

Joel Adams
C3VR Lifetime Member #56
My Link

(click for Texas-sized view!) NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

Quote

Re: Diagnosing misfire (4/12)
11/10/15 3:28pm

Adams' Apple said: Before you start ripping into this thing, let's do some basic checks.
Did you Ohm out the plug wires? Just looking at them(or the plugs) will not tell you if they are capable of carrying the spark. Use an ohm meter, and check the resistance of all of the wires. You're looking for consistency here, not specifics. If you have one(or two) wires that ohm far differently than the others, those wires are suspect.
Use a spark tester if you have one....test each wire to see if they will all fire across the same gap.
If you have a dead miss on one cylinder, check the wires first, then move the spark plug to another cylinder and see if the miss moves with it, or just install a new(or different) plug in that hole to see if the miss goes away. If it doesn't, check compression again on all cylinders, ignition unplugged, throttle held wide open. If that one cylinder is down on compression more than 10-15% of the others, then it's time to dig into it deeper. Thumbs Up

On the plug wires, yes it was visual only, no VOM yet, but that's easily done and i'll do that next. I dont have a spark checker but was planning a trip to tool store and will get one (good to have for many other reasons). I also saw a digital inspection camera that will fit in the spark plug hole on sale and with a 20% off coupon puts it under $70 so i'll get one of those at the same time. On the compression test, they all came back at 125psi except cylinder 1 and 3 at 120psi, but i can certainly perform that again as well.

______________

Quote

Re: Diagnosing misfire (5/12)
11/10/15 4:19pm

jim in oregon
Former Member
Send Private Message

Milwaukie, OR - USA

Vette(s):
'78 Silver Anniversary T Top coupe,58k orig miles. THM350 auto, 350 L48, JBM headers,Oyster White interior,.as of Nov '15 in process of painting.

Joined: 10/19/2015
Posts: 71

I'd definately FIRST verify that plug wires are NOT reversed on 1&3 plug connections..

Those two wires are routed beneath the L48 motor and it's easy to get them cris-crossed.

Happened to me that's how I know..Jim

______________

Quote

Re: Diagnosing misfire (6/12)
11/10/15 5:37pm

Adams' Apple

DarinM said: On the compression test, they all came back at 125psi except cylinder 1 and 3 at 120psi, but i can certainly perform that again as well.

Those readings are fine, but...was it missing when you took those readings? I think you're really looking at an ignition related issue here, not a mechanical one. I've had new plug wires fail within months of installing them, so new(or new-ish...or even high dollar) don't mean diddly. Tongue

______________

Quote

Re: Diagnosing misfire (7/12)
11/10/15 5:52pm

Adams' Apple said:

DarinM said: On the compression test, they all came back at 125psi except cylinder 1 and 3 at 120psi, but i can certainly perform that again as well.

You know, that's a great point. No it wasn't missing when i ran compression test. This did come on very quickly so i certainly could be ignition related !

See this is what i love about these forums. You can stare at an issue for hours and not see something in front of your face. It takes a new set of eyes to view from another angle !

______________

Quote

Re: Diagnosing misfire (8/12)
11/10/15 7:51pm

johnu
Standard Member
Send Private Message

Scottsdale, AZ - USA

Vette(s):
1981 Corvette Two-Tone Claret color, 4 spd, C7 Vert

Joined: 5/23/2007
Posts: 614

I purchased a carb once to fix a miss. Turned out to be a spark plug with "crud" in the gap!!!!

______________

Only 32 years together

Quote

Re: Diagnosing misfire (9/12)
11/10/15 8:11pm

Yep..ALWAYS check-doublecheck the most likely offenders first....:)Jim

______________

Quote

Re: Diagnosing misfire (10/12)
11/11/15 2:56pm

jim in oregon said: Yep..ALWAYS check-doublecheck the most likely offenders first....:)Jim

Thanks All, i'll have a new set of wires, cap/rotor and more diagnostic tools which i'll do this weekend. I'll report back my findings !

______________

Quote

Re: Diagnosing misfire (11/12)
11/16/15 11:53am

Well, good news. Good news is a new set of plug wires, distributor cap and rotor fixed the problem ! Runs very smooth now. The set of taylor wires that was on it was routed under the headers and around the motor mount etc. I put a new sire of DUI Firewires over the valve covers and all is good now !

So i guess next will be electric fan then new heads and cam/lifter combo.

Thanks again for everyones help and suggestions !

______________

Quote

Re: Diagnosing misfire (12/12)
11/16/15 2:15pm

Adams' Apple

Glad to hear that fixed it, but sure sorry you weren't able to spend all that money on new heads and stuff.... LOL

______________

Quote

Our Sponsors help support C3VR