I'm really stuck with this! I'm running out of idea's to try so ANY help would be REALLY appreciated!

Full engine spec:

Scat 383 Bottom End / AFR 195 Heads / Felpro 1010 Head Gaskets / 9.5:1 Compression / LT1 Intake & Fuel Rails / 30lb Injectors / Stock TB / Comp Cams 306 / 74deg Overlap / 230/244@.050 / ZF-6 Manual / Singlemass Flywheel / Hedman Headers / LT1 Exhaust / Small Distributor & External Coil

Now I've had a pretty major time trying to DIY tune this on a daily driver basis - WOT it pulls like a train and always has.

As the tune is getting closer I'm realising that I've actally got a misfire at light throttle/cruise. Its most noticeable as I get above 2000 rpm. It sounds like its just a single cylinder and its rather random, almost like morse code. Its not enough to cause the engine to stumble, but I can hear it and it just kinda "tugs" like lope.

I have just pulled the plugs (well 7 as the last is a PITA) and they are all so mixed!

Driver Bank: Porcelain / Strap
Plug 1: Light Tan / Tan
Plug 3: Brilliant Clean White / Tan
Plug 5: Tan / Tan
Plug 7: Black / Black

Passenger Bank: Porcelain / Strap
Plug 2: Dark Tan / Tan
Plug 4: NA / NA
Plug 6: Light Tan / Tan
Plug 8: Black / Black

I know that plug reading is more involved than this but I was rather shocked to see such varrying results - especially plug 3 in which the pircelain looked brand new, not a singe mark on it.

I will try some new leads, the Cap and Rotor are new. Its had a fresh set of plugs now. The only other thing I can think of is the injectors are not flowing equally? How can I test this? Any other ideas???

PLEASE HELP BEFORE I GO MAD!!!

 

ALl I can add right now is on my 388 ltx stroker I had almost the EXACT same reading issues. the plugs 7 & 8 were black as can be. leads me to believe there is an air starvation problem at the back cylinders no doubt because of the manifold design. I ended up cutting a few % of fuel from the rear cylinders and they started to run cleaner

 

Your back plugs are black from too rich a mixture. Send you injectors to FIC injectors for balancing. Rear injectors might also be defective.
All plugs should have white insulators. More power comes from rich mixtures which darken plugs, but they should all be about the same color.

 

I am thinking that you've probably nailed this one with regards the last two cylinders - I'm not able to tune fuel by cylinder, only bank so I guess just tune for the leanest!

Being in England sending the Injectors back over to FIC would be a fairly lenthy process and one I would rather do without at the moment.

Under mediuim/heavy load and the engine seems to run smooth, its just the very light "just pulling / cruise" that seems to be the problem.

Whats your thoughts on trying a Hotter plug in the back two cylinders? Could these be fouling under cruise but when loaded up clearing themselves?

 

What year is the car and what year is the engine management computer.
Stock computers can handle mods to a certain point then ?????
I have a detailed plug reading thing I'll gladly pass on to you though it is really for Sportsman Drag Racing.
With a beast engine like yours it should work. LOL

Hello I just thought I knew how to read plugs.
Now if it was just EFI .
Fuel and air is fuel and air Roger !
I know there are as many ways to read plugs as there are racers.
God why does everything have to be so complicated !
Back to school again.

Plugs can only be correctly read when the car has been shut down immediately at the end of a run without driving it back to the pits. Get off the track and coast to a place you are safely out of the way and either read the plug there or change one with one you have in your pocket so you can read it when back in the pits. It may take a couple of runs to get the correct color.
The porcelain around the center electrode is divided into three main area for reading. The third that is closest to the tip is reflective of the idle and transition circuits and is of no concern to a racer. The middle thirds is only colored when you drive down a road at around 30-40 mph and normally reflects the primary jets with the power valve closed. The part you are interested in is that third all the way up inside the plug where the sun don't shine. It will take a magnifying plug reading flashlight or to hold the plug so the sun will shine up inside. Normally aspirated cars should read a light gray or tan ring all the way up inside with the color depending on type of fuel. Most fuels will be light gray/tan. For nitrous cars it will normally just be a faint gray coloring or just turned completely white (off white maybe).

The ignition timing can be checked by looking at the color of the ground strap and looking for the blue line on the ground strap which is really the annealing point of the metal. If you are using a gold colored ground strap like with an NGK then not enough timing will show the ground strap as still gold or going light gray. As you advance the ignition and put heat in the plug the strap will turn darker gray as well as the metal at the end of the threaded area. When it turns medium to dark gray you start looking for the blue line (band) around the ground strap. You want this blue line to be where the ground strap makes the 90 degree bend. If you go too far the strap will pick up rainbow colors and the next step beyond that is to start melting the strap. When you are close then only change the timing by one degree at a time. Also remember that 10 degrees air temp change is the same as changing the timing by one degree. So at night when it has cooled down you can add some timing. If your ignition has the capability of adjusting the timing of each cylinder independently then you can use that feature to have the blue line in the same position on all the plugs.

The round flat area of the plug at the end the threads should be dark gray or flat black and should not be sooty. If it is sooty then it can mean that your plug has not been tightened enough and you are sucking fuel and air past the threads of the plug.

I've posted this for years and can't even remember who did it .
Warren Johnson says plug reading is in the racers bible.

 

Also, missing under light load can be caused by timing too far advanced. I had that problem and found my timing too far advanced.

 

The missing could be caused by timing or the rear plugs getting fouled.

I did pull fuel from the rear cylinders BUT I did not pull much at WOT. Reason being at wot the throttle blade is open so it might be easier for them to get some air.

I like the idea of using a hotter plug on the rear cyls. I would try it. If you have headers you can always pull the plugs and recheck them. I went through this process and went through like 10 sets of plugs over the course of a week. My favourite plugs were the nology silvers. They were in the stock heat range and none ever fouled (although the rears 7&8 were dirtier) and they were a good color all the time. I had fouling issues with the NGK tr6s.

try the hotter plug. Should keep it hotter so it can self clean the extra fuel. I don't think 1 heat range will be a problem on a NA car.



*also don't tune for fuel to get them clean. the front plugs will be super white. I tried that. If you lean it out for the rears the fronts get the most air and they will run hotter. I doubt it's injectors if its fairly new because mine were brand new and flow tested with my build.

 

Having re-read this I too agree that it sounds like you have too much timing at part throttle. If it's not too much timing it's too lean around this part of your tuning.

 

I have tried a slightly richer mixture and the misfire did get quieter but it was just as noticable as before. I think the extra fuel was just drowning out the sound slightly? I could try a LOT richer but I can't see this helping either.

Timing isn't something I would have thought would give me a misfire but you are right, my spark advance in the light throttle area is high and varies from around 36-43* (42 being max limit) - why would this sound like one cylinder misfiring though?

With the overlap on the cam its very hard to tell if this is happening at idle. The noise is very similar to that of a "burble" of snapping the throttle closed and the rpm's dropping in a 4cylinder car - think rally car burble? Not a BANG more of a "putt putt" sound.

Another thing I have been thinking about is a header leak. Both rear sets of ports are showing a slight black mark on the gasket. Obviously these are the hardest to get to and the hardest to get tight, and re-tighten. Should I be able to easily hear a header leak or could it be just leaking enough to not scavenge without me noticing?

 

Something else I have been reading about is rocker arm adjustment - if I had a rocker on the tight side and floating the valve slightly would this happen at ALL loads? Might I not notice it when the other 7 are pulling strong?