Posted by Sherman in Idaho [108.162.246.140] on Friday, June 02, 2017 at 01:25:11 :
Once again it's summer in Wallace and once again I'm working on our 1974 Minibus trolley powered by the 225 cu in leaning tower of power. Last spring it ran fine. Sometime in the middle of the summer it started acting up and they dead-lined it and used the other trolley (1985 Chance with a Cat 3208). It has its own problems and I'm trying to get the /6 unit into reliable condition.
I found the usual accumulation of small things (headlight circuit shorted out, fuel line leaking, vacuum advance non-op), but the big problem is an irregular stutter at modest (~2K by ear) RPM under load. I'm suspicious that it's running rich because last year somebody put in an electric fuel pump that puts out 7 psi, whereas I've read that Weber carbs want no more than 3.5. I don't quite trust my fuel pressure gauge though because it's a high-pressure gauge for fuel-injection systems and this is at the low end of its range. The spark plugs (RC12ECC) are all pretty sooty. That alone should not account for the mid-range miss, though, and blowing propane into the intake while it's missing doesn't change anything at all. At this point I'm thinking the issue is spark rather than fuel. I do intend to check the fuel pressure with a better gauge.
Compression is 90 PSI dry on all cylinders. I tried 2 cylinders wet and it didn't bring it up much. Disappointing because I had the valves ground 10 years ago, but I suspect they're getting sooty too.
The main clue I've found so far is that #6 spark plug is apparently not firing when the engine is sputtering. Went through all the plugs with the timing light clamped on each one and with the occasional exception they all made it flash smoothly even when the engine was sputtering, except #6. I swapped plugs 1 & 6 and also their wires and it made no difference. Distributor cap and wires look pristine (they were new last spring, as were the plugs). Compression on #6 is 90 dry and 105 wet. Electronic ignition module was a new "premium" one last year and I replaced the ballast resistors at the same time. Can't remember if I replaced the "reluctor" or not.
What could possibly be making #6 miss far worse than any other cylinder, even though compression is the same as the others and even when swapping plugs and wires with #1?
I hate to "shotgun" things but at this point I'm thinking of putting a Pertronix ignition in it, since they are supposed to be good. Any recommendations/advice there? I'll need to get a new vacuum advance mechanism too, assuming the Pertronix uses it. Centrifugal advance is working.
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