Wet spark plug. Carb problem?

[Miguel P]

Spark plug of cylinder nr 2 is always a little bit wet. I don’t think it smells like fuel (or maybe it is but very subtile...). No matter which distributor cap I use, spark plug wire or spark plug. Valves are also adjusted. Any suggestions? thanks!

[Huib]

As you suggest yourself probably a carb issue? Did you already measure compression? If it is a series 1 engine, it may be the result of what we call a "vastloper" in Dutch. I don't know how that translates into english. It means that the piston touched the cilinder wall.

[Miguel P]

Hi Huib, thanks for your reply. It's a S2, I should had mention it. Compression (please see attach. -cold measured'- ) shows nothing specials in comparison with, let's say, last year when the problem still didn't show. Thanks

[Andrea Nistri]

Compression data look good. Perhaps the carbs need careful tuning and balancing. Have you tried to increase the supply to cylinder 2 at idle? Sometimes there is insufficient mixture to fire regularly, yet enough to wet the plug. Gently and very carefully undo the adjusting screw for cylinder 2, Andrea

[Miguel P]

Andrea, thanks very much for your suggestion. I'm going to try it this weekend!

[marnix]

I am not convinced this is a carb problem, but anyway. Is the plug wet with the engine warm? Wet plugs from carb problems are normally only there when trying to start the engine stone cold, and mostly because engine gets flooded by too much fuel. Andrea's expalnation is another possiblity, the engine misfiring, but normally you should experience irregular running. Does the wetness evaporate rather quickly (in a few minutes)? If not, it is most likely oil, not fuel, and that means a cylinder problem (like broken piston ring or something). Btw, compression is best measured with the engine warm, may give more accurate results. Do not forget to give full throttle when measuring. To set the mixture, the procedure is as follows, assuming the balance (both between the blocks and internally between barrels) is set: 1. connect a reliable rev counter, many on board counters are way off. 2. set the mixture rich. On Solex PHH/DDHF carbs, this is about half a turn out from fully in. 3. start with a rather high idle setting (turn the RPM screw in a few turns) and start the engine 4. try to get the idle down to an acceptable level (a bit of personal taste, normally 800-900 RPM) 5 turn each mixture screw in VERY slowly (let us say 1minute turns = 1 turn / minute). The RPM will normally start to increase slightly at first. Continue turning, the RPM will very slightly go down again after a peak RPM, almost imperceptible. Continue turning, and at a certain moment, the engine will rather abruptly fall down in RPM. This is slightly too lean. Turn very, very slowly out again, till the RPM picks up again and turns smoothly, but do not turn further till the previous peak RPM, which is slightly too rich. With some experience, when leaning, just before the abrupt drop in RPM, you can feel the engine starting to vibrate very slightly. If you are able to recognize this, stop there and very, very slightly turn out again, just enough till that vibration stops. 6. all the mixture settings will have an effect on RPM, so you have to make an iteration: after setting a mixture screw, RPM most likely increases, turn down RPM again, which in turn calls for a slight reset of the mixture settings you have set previously. If you are measuring the balance too, then the mixture setting also influences the balance. On Solex PPH and DDHF carbs, the correct mixture setting is really almost completely turned in, about a quarter turn out, or even less. It is something to take your time for, there are (balance, mixture, RPM) a total of 10 setting screws to deal with. It takes time with subsequent iterations to get it right, but with some practice, it is not that hard to do.

[Miguel P]

Dear Marnix, what an impressive and valuable explanation! many, many thanks for taking the time to do it, you and the rest of the guys. I ll try it this weekend with the recently bought digital RPM meter.

[NeilS]

I would recommend the use of an accurate rpm meter, my S2 with Dellortos has been on a rolling road this week and the rev counter overheads by 200 rpm, now nicely set up and smooth with 91 bhp after some recent engine work.

[Ed Levin]

The usual great explanation by Marnix. My DDHFs are really off after the engine rebuild, so this is just what I need. My own suggestion only attacks the symptom, not the cause. But I've had almost no fouling issues, and much better starting since I changed to iridium, thin-wire plugs. Get the carbs dialed in, but run iridiums, too.

[Kaha]

Ed, what is the make and model of the iridium plugs that you use?