View single post by Mark Rosenbaum
 Posted: 03-07-2007 03:13 am
PM Quote Reply Full Topic
Mark Rosenbaum



Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
Status: 
Offline
 For an engine to run, you need:
  • Sufficient compression
  • Sufficient spark, at the right time
  • Proper fuel-air mix
Since the engine was running, lack of compression is not very likely to be the cause of your problem, and I'll ignore this area for the moment.  Which brings us to the following possible problem causes:

Sufficient spark, at the right time.
Verify that the spark occurs at the right time -- ideally about 8° to 12° BTDC though the engine should run (perhaps very poorly) if it's anywhere between 0° BTDC and 28°BTDC.   The distributor mounting can sometime get loose and allow the distributor to jump free for half a turn before it re-engages.  Also, certain distributor internal faults can give sudden timing changes.

Try a replacement distributor cap.  Try a replacement rotor -- and if you have one of the rpm-limiting rotors, make sure its sliding weight isn't sticking in or near the spark-disable position.  Check the points gap and ensure it's about 0.015" -- too small, or too wide, and you end up with a weak spark that may light up a plug in free air, but which will not fire a compressed charge in the cylinder.  Try a replacement condenser -- again, a fault here gives a spark that won't fire a charge in the cylinder.

Proper fuel-air mix.
You mentioned that the rear carb had a "sticky needle."  If its piston-like vacuum valve isn't free to rise and fall, you'll have an uncontrolled fuel-air mix which is unlikely to support combustion.  If you can see fuel dribbling into the carb throat, you have a leaking fuel inlet valve or sinking float, which in addition to messing up the fuel-air mix is also a major safety hazard.  Both of these conditions, if they exist, would have to be fixed before you could expect the engine to run properly.  A severe vacuum leak could also result in a bad fuel-air mix.

From what you've described, it would be prudent to presume that you have several problems at once.  If so, you will probably have to go through the whole troubleshooting process several times in order to be certain you've found them all.  Please do let us know your results.  Further advice and suggestions are available if needed.