Hi all, a few months after I started driving my 1923 touring / pickup conversion (I bought it in June of 2013), I pulled the spark plugs and they were dry but black, a sign that I had the mixture adjusted too rich.
Since then, I have been progressively leaning the mixture on each drive and have noticed that the engine runs crisper and I am getting better mileage.
At some point, I will go a bit too far and adjust to the lean side of the optimum, what symptoms can I expect to see from a slightly too lean mixture?
Based on prior experience with '60s muscle cars, I suspect that I'll encounter light surging / hiccups during cruise and possible light backfire (popping) through the carb on rapid accelerations, am I right?
If the mixture gets really lean I would expect more vigorous backfiring, along with overheating, but I hope to detect the mild symptoms first and never let it get to that point.
Mark, You can generally find the sweet spot between too rich and too lean very quickly. After the engine is warmed up cruise along at 25-30 mph. Enrich the fuel until the engine starts to stumble and run slightly rough. Then lean the fuel mixture until the engine starts to stumble and runs slightly rough. Then set the mixture right between the first two settings. Also keep in mind that the plugs may be off colored from running with the spark advance in a not so optimum range or if the engine is consistently running cold.
Mark, I wonder if things are too lean just before the mild symptoms show up? My car runs it's best at that level except that it takes a mere five minute cool down before the symptoms start to show during start up and running which mean I often have to enrichen the mixture a bit for start up and the first minute of running. It seems that the thermo siphon cooling doe a good job after shutdown which could be the cause. I hope this makes sense to you.
Running too lean can cause your exhaust manifold to glow, warp and burn valves. Part of fuels job is also to cool not just run the motor.
Good point there Mark G. I find I get a bit of a stumble on acceleration unless I run my car that lean. What I worry about is that I might be running too lean during cruise. Henry should have put that mixture control right on the center of the dash.
The more likely cause of a red hot exhaust manifold is a retarded spark.
Too rich will carbon up the engine and too lean will destroy your valves. After the engine is warmed up, turn the needle to right until the engine begins to lope, then turn to left until it begins to slow down and set it half way between the two points. Note the position of the adjuster. Next time you can just set it at that point and it will run fine. Note, when changing altitude, or when driving in very cold weather, you might need a fine adjustment but otherwise, just leave it as is. If it won't keep running right after you start it, you might need to make it a quarter turn richer to start and then after a few minutes turn it back to the "sweet spot" and continue driving.
Lean will cause the engine to run hotter but you get a bit more power than running on the rich side. In my Kart racing days, I had an adjustable main jet and ran it lean through turns and breaking into the straight. The head temp would hit my preset high and I'd open it up (richen) to cool it down just in time for turns again. It was a busy cockpit in those days.