Engine popping from exhaust-exhaust valve adjustment incorrect

[Jason Treschl Racing]

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?

[justfreaky]

Losing a ton with 0 compression. Usually, you should be within 10% on any given cylinder. Think about it this way; Running on 3 cylinders on a 4 cylinder engine.
You are missing out on a bunch of power.

Another symptom of popping out the muffler could be realated to a rich condition on your carbs. Unspent fuel will burn due to heat in the exhaust system as raw fuel pours into the header. Have seen this happen a lot in the past when backing off the throttle. Watch a NASCAR race at night. Notice the flames and popping that comes out of the exhaust exit.

Steve

[Legends57x]

I assumed you rechecked compression on cyl. #4 Hot after valve was readjusted?  If you run too much with any valve hanging open you may overheat and burn the valve as the valve seat acts as a heat sink when valve is in contact(closed) with valve seat.   You may be able to check this visually using a boroscope tool if you have one by removing spark plug and looking into cylinder .  A leak down test may help you diagnose this also, but I never had much faith in cylinder leak down tests as we often have seen mixed results here at  work. 

Good luck! 

[Jason Treschl Racing]

I assumed you rechecked compression on cyl. #4 Hot after valve was readjusted?  If you run too much with any valve hanging open you may overheat and burn the valve as the valve seat acts as a heat sink when valve is in contact(closed) with valve seat.   You may be able to check this visually using a boroscope tool if you have one by removing spark plug and looking into cylinder .  A leak down test may help you diagnose this also, but I never had much faith in cylinder leak down tests as we often have seen mixed results here at  work. 

Good luck! 

I haven't been able to readjust yet. I don't have shims or the tool (I ordered some though!). Will have the adjustment done at the racetrack on Saturday and will recheck afterwards. I assumed even with a valve open some that I would at least get some sort of compression, but I couldn't get any reading whatsoever. Fingers are crossed that there is no damage done!

[Jason Treschl Racing]

Put a small valve shim in that exhaust valve (went from 2.88 to 2.40). Still couldn't get a gap, but it closed the valve enough that the popping stopped for the race. However, after the race, the popping had started coming back very slightly. Motor is out of the car and on its way up to charlotte now.

[ZForceRacing01]

I'm curious what was your end result? I'm having the same issue and plan to take the car to US Legends tomorrow. Luckily they are up the road.

[Jason Treschl Racing]

I didn't realize I never finished this one out. The valve seat on cylinder 4 was bad. Had to have it replaced at US legends (1250 sealed).

[ZForceRacing01]

What did that cost you minus the shipping?

[Jason Treschl Racing]

My dealer actually drove it up to charlotte during a parts run, so no shipping cost. I don't remember exactly on price but I believe it cost about $90 for parts, plus labor (maybe like $300 total). They threw in some used parts cus I had 2 bent valves also and recommended that I get the engine rebuilt at the end of the season.

[ZForceRacing01]

If I can get out of there for $500 or less I will be a Happy Racer! Thanks for the info.