uneven spark plug condition

[laverdaas1]

hi there, on my 124AS, i have kinda of a "hole" once pushing the throttle from low rev or idling while idling and acceleration are good after that . Looking at possible causes i found that plug 1 and 2 were tan and clean while 3 and 4 had black sooty deposit. How come they are different? Can anyone help in understanding the cause and find a fix? thanks

[TX82FIAT]

The hole in acceleration could be related to the plugs or it might not. The plugs are most likely a symptom of something taking place upstream. Typically a black sooty deposit is the result of a rich fuel mixture. Rich Idle mix will run it smooth at Idle but foul a plug and could show up in other areas of RPM band. What carburetor do you have on the car? Dual carb with adjustments to air/fuel different for front two plugs 1 & 2 than back two plugs 3 & 4. A carb that sits for some time may need to be rebuilt or have some other tweaks. Not really a carb guy but I would suspect a carb adjustment.

[laverdaas1]

The hole in acceleration could be related to the plugs or it might not. The plugs are most likely a symptom of something taking place upstream. Typically a black sooty deposit is the result of a rich fuel mixture. Rich Idle mix will run it smooth at Idle but foul a plug and could show up in other areas of RPM band. What carburetor do you have on the car? Dual carb with adjustments to air/fuel different for front two plugs 1 & 2 than back two plugs 3 & 4. A carb that sits for some time may need to be rebuilt or have some other tweaks. Not really a carb guy but I would suspect a carb adjustment.

thanks for your time. I actually have a single Weber 34DHS, and that's why i can't understand how come cylinders 3 and 4 be could be served with a richer mix or having a specific issue with ignition (stock ignition) vs cylinder 1 and 2.

[laverdaas1]

guys really need help here or gonna get mad: to recap, having a single barrel WEBER (DFH) can't really understand how come spark 1&2 are ok while 3&$ shows clear signs of rich mixture. I swapped ht cables (to me the only thing they don't share) between 2&3 with no effect. Can wrong valve clearance cause this?

[So Cal Mark]

it may be oil fouling the plugs, you need to assess the condition of each cylinder. You might also have a vacuum leak affecting two cylinders that appear clean. If you want a detailed analysis you really need to post a lot more information about the condition.

[sptcoupe]

I agree with Mark. DO a compression and leak down test. You likely have some oil getting into those cylinders Worn valve seals, rings) or a vacuum leak, making them run lean. Are the cylinders next to each other? Does the car run ok?

[laverdaas1]

thanks guys. Previous owner had done a compression test and all cylinders where (nearly) equal. I will arrange for a leak down test. Meanwhile, to provide more hints: yes cylinder 1&2 (the one with nice spark plug) are close one the other. Similarly for cyl 3&4 (black sooty deposit) these two are really dry, cannot see any oil leak trace there. The cars starts fine at both cold and hot. Idles fine (at a stable 800rpm) if you give it enough time to warm-up (...takes a little time indeed). If you kick the accelerator quickly it shows kind of a hole and then it start revving up. At high rpm it runs OK but it looks like at WOT it starts to "choke". At high elevation things (choke at WOT) get worse similarly if i pull the choke lever (it's an european model with a simple weber DFH carb). thanks again

[RoyBatty]

Possibly, you have a vacuum, (induction), leak on the two cylinders for which the plugs look good.
If this is the case, the mixture may have been richened to make those two cylinders run acceptably while making the other two a bit rich.
Possibly.