Diagnosing a misfire
 

Well I've been chasing a misfire for a week now, and it's really putting a damper on the very limited time I even get to drive the truck in between work. I just changed all the spark plugs, checked valve lash on all valves, checked spark plug wires, checked my timing curve, all looks great, so I'm stumped.

It starts up on the first bump of the key, idles like a champ, but when I put it in gear and accelerate, slow or mildly, I can feel a misfire.

Re: Diagnosing a misfire
 

Spark Tester? Make sure you have strong spark at each hole.

Compression Test? Make sure it's even.

Leakdown Test? Should be even and low.

If you don't have a leakdown tester, today's a great day to learn - presuming you have a source (even a small one) of compressed air.

Re: Diagnosing a misfire
 

Check for P0301-P0308 to determine the exact cylinder. P0300 means its random and you may have to drive it harder and make it misfire more so the computer can determine which one is misfiring... unless you can watch misfire counts on your scan tool to se which cylinder.

Just Kidding!

In the dark, spray the wires and harness down with a spray bottle of water while the engine is misfiring (in gear like you say). Look for sparks (hence doing this at night).
Maybe motor mounts are allowing a harness to move just enough to short or even shift linkage is touching something.

Just had a newer G van where when sitting on the street in front of guys house (it was nose down on a hill), would pop starter fuse intermittent, but only there. It was a harness and only when the truck was really hot when parked, the harness would be warm enough to sag that direction and touch the frame.

I also had a old hot rod cal-bug that would only misfire in the dark under a hard launch, full throttle condition. Turned out the taillight harness would be pulled back just enough from the Gs of the launch, to come in contact carb linkage at full throttle.

Wasn't till someone was behind me and asked what the sparks were when i took off...

Re: Diagnosing a misfire
 

Well I went through the whole truck inside and out, and I fixed two spots that I thought might be something, but it's still the same. It's a little hard to start, not real hard but it's not on the first bump of the key now. I have to let it crank a little and give it a few pumps on the gas pedal.

I don't have the tools for a leak down test but I suppose I could go get them. Could that actually be the cause of a misfire though?

Re: Diagnosing a misfire
 

Did you pull the plugs and check for cracked porcelain ? What do the plugs look like ?

Re: Diagnosing a misfire
 

Not sure what you mean by cause - but yes, if you've got a valve lashed incorrectly, it'll show up on a leakdown test. Any mechanical issue in the valvetrain, rings, and so on will show up. Basically anything besides ignition and fuel.

Ignition is much easier to fix, so start there, which was my first thing on the list! Check for arcing wires, broken porcelain, boots shorting to header tubes, all of the regular suspects.

Engines, as you know, need spark, fuel, and compression. I check them in that order when something is amiss (no pun!)

Re: Diagnosing a misfire
 

One other thing to try is the old remove one plug wire at a time (remember to insulate your hand) and see which, if any cylinder, does not influence the idle. That would be the one with a wiring or plug problem.

Having said that, if the problem (misfire) only occurs under load, this test might not be very helpful. Have you checked the spark at one of the plugs to see that it is a good, hot spark, with a nice snap noise?

Re: Diagnosing a misfire
 

First go get you one of those infared laser thermometers then Let it run and get up to temp then shut it off and check each exhaust tube they should all be fairly close in temp if you have one that is way off from the others that will be your cylinder that's not right, then from their your only having to diagnose one cylinder instead of 8

Re: Diagnosing a misfire
 

He took mine but before you go get a infra red get like a little squirt[spray] bottle and just squirt a drop or two on each tube of the header. the one that DOESN'T fry off the water right away is the missing cyl

Re: Diagnosing a misfire
 

OK I just got a compression tester and a spark tester, I have a bad feeling I'm not gonna like the results of the compression tester but will know in about an hour how it looks

Re: Diagnosing a misfire
 

Here's one of my spark plugs. Why is it so sooty

Re: Diagnosing a misfire
 

Also when I took the little rubber cap off the manifold vacuum port on my carb, there was raw fuel in it.

Re: Diagnosing a misfire
 

I started fishing around under the dash and found that the power to my electric fan manual switch had a splice that was totally bare, not sure how that happened but I took the splice apart and put a connector and a shrink tube/heat gun to it. That didn't change anything. Those plugs look really sooty, and the bit of raw fuel coming from the plug the goes to manifold vacuum on the carb makes me wonder if my carb is ok?


Another thing to note, I have a solid cam now and I tightened up the lash a couple thousandths while tuning the motor. Could the misfire be coming from lash being to tight?

Re: Diagnosing a misfire
 

I vote carb problems even before sooty plugs. Unless its only one plug then might be plug or plug wire.

Re: Diagnosing a misfire
 

Nope it's all 8 plugs, and I wouldn't be surprised if you were right about the carb. You can't stand within 30ft of the truck while its idling or your eyes will start watering it's so rich

Re: Diagnosing a misfire
 

Just to be clear, the carb is a brand new Holley 750 dp which I am still very uneducated about. I'm learning, but by no means have the thing figured out

Re: Diagnosing a misfire
 

Check the float level, and make sure the idle adjustment screw isn't turned all the way/too far in. That is way rich. Tighten until just barely seated and try 2.5 turns out.

Re: Diagnosing a misfire
 

I know you don't want to hear it and I said months ago ...Too much carb .:smoke:

Re: Diagnosing a misfire
 

Unburned fuel .

Re: Diagnosing a misfire
 

You mean turn the a/f mixture screws out 2.5 turns?

Re: Diagnosing a misfire
 

Ya I think you're right. I had the 650 dp on the sales counter too and intended to buy it, but the sales guys were pushin for me to buy a 750, and when I made a desicion the two must have gotten mixed up. The speed shop where I got it won't take stuff back unless it's broken or will benefit them somehow. The good news is I am learning a lot about these Holleys and I'm pretty confident I can tune it down to work well with my combo.

Re: Diagnosing a misfire
 

Odds are your compression test will be fine then, but still worth doing. You've been washing the cylinder walls for a while with extra gasoline, so it's worth checking just to know.

Remember to disconnect the coil and block the throttle open to get accurate readings.

My big block puts out 160psi, so somewhere in that 140-175 range is probably right. Be better if you had a fresh baseline from when it was new to compare to, but at least now you'll have some kind of baseline (don't throw away the results, because you might want to compare to them in 3 years).

Re: Diagnosing a misfire
 

you are getting sound advice A 750 carb is big. My truck had a carter 750 i swapped an edelbrock 600 it feels faster. however your carb has as a stuck float or something. ive never had fuel come out of vacuum port. Never owned a holley carb though. Im no carbexpert. Ive had few problems maybe because I change my fuel filters often. I would check floats first

Re: Diagnosing a misfire
 

Yes but lightly seat them first. You can jet it down later, I missed that you had a 750. You can make it work but it will cost you a little torque.

Sounds like that thing needs opened up for sure. You might have some trash stuck in your needle & seat.

Re: Diagnosing a misfire
 

If all checks good on the floats and idle mixture screws, jet it down a couple of sizes. Holleys are really easy to tune, if you do jet it down you will need to readjust the mixture screws.