Tonight I had a defining "moment" in my quest. Last weekend I replaced the batteries and the Scott really liked the extra juice, but just about every other time I went to start it the motor would turn 1/2 - 3/4's of a turn and then just stop. The starter would disengage and spin out. If I'd hit the key again and hold it on the motor would usually turn over and fire up. But it would take everything the two batteries had to give just to get it started. This made me wonder how many starts I could get in a day of harbor cruising before my batteries would be dead.
Putting a timing light on it revealed that the motor was running at about 10-12° of advance at idle. When the throttle was at the start position the mechanical advance pushed this to 18-20° of advance. At WOT it was at something like 40°!
Basic combustion motor troubleshooting says hard starting is a symptom of too much advance. The idea is if the spark ignites the fuel when the piston is on the way up it may stop the piston dead in its tracks not allowing the motor to turn over. So it was time to see how it got so far advanced and if I could dial it back.
This got me to thinking about the the timing belt that drives the distributor. This motor uses a notched belt that rides in teeth on the distributor pulley and in teeth on the crank shaft under the flywheel. The notches prevent the timing belt from slipping.
When I bought the boat I was given a brand new timing belt but it hadn't been installed. I hypothesized that if someone had removed the distributor trying to install the new timing belt but realized they needed a gear puller to remove the fly wheel... so they gave up. Maybe they could have re-installed the distributor one or two notches advanced.
I figured if I removed the distributor and rotated the pulley one or two notches counter-clockwise on the belt. This would retard the timing back to a more normal position.
So I removed the four machine screws on the distributor and slid the belt off the pulley. I rotated the pulley one notch counter-clockwise and slid the timing belt back on to the pulley. I re-tightened the screws and hoped for the best.
Much to my surprise and amazement, when I put the controls in the start position and turned the key, the motor fired right up! No stopping, no pausing, no struggling, it just started. I turned off the key and tried it again and she fired just like it was 1960. So I put the controls in the idle position and put my timing light on the fly wheel. At idle I was running at 5° retarded, at the start position I was at 10° and at WOT I was at maybe 25°. So I fine-tuned the timing with the Spark Advance Adjustment and tested the starting several more times. Now the motor starts effortlessly!
When I bought the boat the former owner said "the starter was not strong enough". One notch on the timing belt, is this why the former owner gave up on it? One notch on the timing belt, is this why Advanced Marine destroyed two starters and had it all last summer?
Granted there was a metering rod in one of the carbs that was jammed (so the fuel delivery would have been questionable), the carbon brush on the distributor cap was not making contact with the rotor (so the spark would have been weak) and the timing belt was one notch off (so it was too advanced and would be hard to start). If you add that up you have a motor that won't run. A really good mechanic would have found all three of these things in one afternoon of tuning. For me it has been a journey.
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