Question on carb tuning for the track

[Tommy]

My 351W has a Holley DP 600 CFM. It's tuned to run slightly rich when driving on the street. I recently took it for a day at the Gainesville track with the Deep South Cobra Club. We were running counter-clockwise on the track and I noted occasional black smoke from the right exhaust. When I got home, I pulled the plugs. The four plugs on the driver's side looked just about right. The four plugs on the passenger side looked rich with the two in the center looking especially rich (black soot).

I assume the carb is running rich with the excess fuel going to the outside of the turns. To lean it out, I can lower the float level (primary or secondary or both) or go to smaller jets (primary or secondary or both). Where would you experienced track guys start to address this problem without leaning it out too much?

[Tommy]

I take it my question was the equivalent to a Google search with no hits.

[Curt C.]

I have been thinking about your problem and while I am certainly no carb expert, I will try and comment. It seems very weird that one bank is rich and not the other. The fuel gets atomized and then sucked into the cylinders. As a result, I would think that lateral G's would not have too much effect on the A/F mixture. Have you checked your fuel mixture screws? Maybe the passenger side screws are too far out - fat causing the right bank plugs to run black. Does the plugs exhibit the same condition just driving on the street? Interesting problem.

[Tommy]

Curt -
Given that some people who post to this forum seem to take a lot of heat, I appreciate you taking the time to respond to my question. I originally suspected the carburetor because I had read the plugs before going to the track and they all seemed about the same. While at the track, someone pointed out some smoke from the passenger's side exhaust. When I got home, I pulled all the plugs with the results I described earlier. A local mechanic suggested the carb was dumping extra fuel during hard cornering with the unaerosolized fuel flowing to the outside of the turns. That's what led to my original post.

Since then, I installed all new spark plugs, slightly lowered both the primary and secondary float levels, and dropped the primary and secondard jets by four sizes. I then drove the car about 8 miles including one full throttle pull through second gear. I pulled the #2 and #6 plugs and found the #2 blackened and #6 clean. As I did no hard cornering during this time, I concluded that the soot was likely due to oil leakage rather than fuel. Consequently, I had a compression leak-down test done. All eight cylinders tested normally.

At this point, I don't know what is causing the soot buildup on the #2 and #3 plugs, but I have one more idea. The engine was rebuilt about 3,000 miles ago. Shortly before my last visit to the track, I switched from conventional to synthetic oil. It is possible that the synthetic oil is leaking past some of the rings to a greater degree than the conventional oil. My next step is to change back to conventional oil and drive it a while further. If the soot goes away, I'll know where it came from.

[Curt C.]

If the compression checks out the rings should be ok. However, oil could be getting in there via the valve guide seals in the heads. You should move this thread to Shop Talk which gets a lot more attention. Mr. Fixit will likely respond and he knows carbs very well. Your idea on the carb just dumping fuel makes sense. Could be that the carb just needs a rebuild

[Bonos]

Tommy,
unburnt fuel = black exhuast
burned oil = blue exhuast