1600DP mystery problem SOLVED!!

[prom8n]

Posting this in case anybody else runs across this:

Tuned up the 1600DP in the Bug Saturday, taught my son how to set the valve clearance, points, timing, etc. It ran great for about 6 hours, but then it started running rough, then back-firing thru the carb real bad. Pulled over and looked, and the oil-bath air cleaner had popped off the carb, spilled oil all over everything, and the 12v wire to the coil was almost melted thru. I figured the oil had shorted out the wiring and the distributor, causing it to crossfire and run awful.

Towed it home, cleaned up the oil, pulled the distributor and used dielectric spray to clean it. Replaced the melted wires. Still ran bad, popping back thru the carb. Compression check was good and consistant on all 4 cylinders. Hmmmm....

Found that if I pulled the plug wire off #4 cylinder, it stopped backfiring, but still ran bad. Replaced distributor, coil, ignition wires (I had extras laying around), no better. Pulled the carb and cleaned it all out, as I was running out of ideas. Nothing helped.

I was trying to figure out why #4 was backfiring, so I pulled the cap off the distributor to watch the rotor. Had my son crank the starter, and the darn thing was still popping back thru the carb. What the *%#$? If the cap is off the distributor, how the hell can the engine be backfiring!?!?

By now I've been working on this for two days and can't find the problem, so I'm ticked off. Finally, the answer came to me. Anybody want to guess?

I looked at my son, and said "You forgot to tighten the lock-nut of #4 exhaust valve, didn't you." Sure enough, we pulled the left valve cover, and the nut fell out.

The adj. screw had backed all the way out. It wasn't backfiring thru the carb, it was the ignited gas in #4 escaping out thru the intake valve when it opened. I had good compression because both valves were still closed, it was just that the exhaust valve was never opening! When it popped back, that's what blew the air cleaner off the carb and spilled oil all over. That was an effect, not the cause.

It took 10 minutes to fix, and the blasted thing has never run better.
Moral: Never assume you know what you're doing. And always double-check what your teenage son does to the car.

[V_DUB_YOU1972]

[Rome]

To your credit, you used logic and knowledge of internal combustion engines to come up with the ultimate reason- this is something you must also instill in your son to diagnose any mechanical object. Glad to hear it was a simple repair.

[berniedd1]

Good for you! Just a minor correction - oil does not conduct electricity and cannot short out your distributor wiring.

[thetravman]

Just a word of wisdom from travman the nobody. You should always recheck your valve clearance AFTER you tighten the locknut. Do this and that'll never happen again.