This is mainly to help anyone searching in the future for sources of popping out of the exhaust. I'm going to run through my trouble shooting procedure, and findings. I had been suffering from an exhaust that sounded weird (like a leak), and popping from the exhaust for several weeks of racing. Aside from those oddities, the car seemed to be running fine (this is my first year in legends racing). Finally got around to troubleshooting it after some comments from other racers this past weekend.
- Noticed bad popping at idle and low throttle inputs, flames coming out of muffler
- Warmed car up, checked spark plug condition/gap, swapped carbs, swapped red box, swapped coils, popping was still present
- Next step was to take an infrared temperature gun and shoot each header pipe to determine what cylinder the issue was coming from. Cylinders 1-3 were consistent at around 300 degrees, cylinder 4 was only reading 150 degrees (problem with cylinder 4)
- This led me to check compression in each cylinder to see if it was a problem there, cylinders 1 through 3 were consistent at 150-155 psi (1250 sealed motor), but cylinder 4 was 0 (couldn't get any reading whatsoever!)
- Next I went into the valve cover to start checking valves. Seeing as how the popping was unburnt fuel coming from the exhaust, I started with the exhaust valves on cylinder 4. Immediately I found the problem. Exhaust valve 1 on that cylinder was set at .008, valve 2 was so tight a .0015 wouldn't go in. That exhaust valve was never closing!
The shim in that valve looks to be much thicker than those in the other valves. I'm going to be re-shimming that valve at the track this weekend and I am fully expecting this to fix the problems I have been having. I have raced 5 heat races, and 3 features since I paid to have the valves checked/adjusted. The exhaust leak noise was present the whole time, but the popping only started a few weeks ago, so it seems like it may have been a mistake during the adjustment at that time. I never really noticed anything too major at the track because I was consistently keeping up with mid field cars at Atlanta Motor Speedway.
On a side note, anyone got an idea how much power I have been down with no compression in that cylinder?