Non firing cylinder, do have compression & spark

[Greg Greer]

I have compression, spark, and gas squirts from the accelerator pump injection nozzles. However, cylinder 4 does not fire. It sometimes coughs flame/sparks up through carburetor throat on this cylinder too, what next? All other cylinders are fine.

[gus91326]

Try swapping out a known good wire/spark plug from another cylinder to rule out any intermittent problems with these parts. I’d also check distributor cap and rotor for cracks and corrosion.

[wkrtsm]

The plug might be fouled, spark plug wire might be bad, or the valve lash could be off. I would first pull the spark plug for cyl #4 to see if it's gunked up too much and not able to fire cleanly. If it's ok, then swap the spark plug wire (like mentioned before) for #4 with another cylinder. If that's not it, since it's spewing flames and coughing, check the intake and exhaust valve clearance ("lash") for #4.

You didn't say much about all the necessary parameters, but for these 912 engines, you have to have everything in good order all the time if you expect to crank them right up and have them run well. This really equates to:

- carbs recently adjusted
- timing recently adjusted
- mixture adjustment screws on carbs good
- spark plug wires almost always news
- plugs in good condition
- breaker points (use Pertronix) recently replaced
- distributor cap and rotor good (recently replaced)

It's also amazing how something else can be the culprit A LOT OF TIMES. In other words, quite often, a problem can be due to something else you don't expect -- until you replace and then everything seems solved. Thus, your carbs could need adjustment, or your timing is off, your points are bad, etc.

Pic below is for inspiration!

[jaybrame7]

Anyone knows what is the unit on the left with the cooling fins on it connected to a metal pipe?

[wkrtsm]

Looks like there's a pulley with a v-belt on it, since it's bolted down pretty heavily. Was thinking of a fuel pump, or a small AC compressor. Hard to guess without looking at all the lines to and from. It's definitely not a turbo-charger. I am not a fan of those air filters or Solex carbs, but do see a 031 or 022 distributor, which is good.

[John2244]

1968 912 smog pump.

[John2244]

Also, I like the factory solex carbs and air cleaners.