Dead PTO cylinder

[jim downing]

Issue: '97XP 787 engine not running right, when under load in water it bogs down and dies after slowly increasing RPM above idle. If you hit throttle quick, it might sometimes opens up and runs but not at mid range. Grandson was riding machine. He said it started to slow and finally just quite. He was unable to get it to start, he said starter just clicked, needed towing back to landing. He said no beep from overheating, just slowed and died.

Engine appeared to be ceased up, could not rotate drive tube with pliers. Suspected the worst - a ceased up engine. However, discovered starter bendix was lodged against flywheel where teeth were badly worn. Replaced flywheel and starter bendix. Starter cranks engine without problems. Took XP back to the lake and had above described bogging and dying.
Tests performed:
0) rebuilt carbs!!!
1) with engine idling, pulled plug wire off PTO plug, no evident change in idle RPM
2) engine will start with MAG wire on, PTO wire off (that is, engine starts and runs on MAG cylinder only)
3) engine will NOT start or run with PTO wire on and MAG wire off even if I give PTO carb a squirt of starter fluid
4) compression 150 in both cylinders
5) the above is true even if I squirt starter fluid directly into carbs
6) both plugs (new plugs!) show nice blue spark when cranking engine
7) to test crankcase seals I made a plate to cover carb inlet, attached vacuum gauge to plate to measure "sucking" Both carbs showed vacuum pulsing from 0 to -9" mercury.

I have two of these machines. Out of desperation I pulled carbs off the other XP, put them on the above described XP engine and got the same results. That is, carbs do not seem to be the issue. I put the carbs from above described engine on the other good working XP and discovered basically the same things. That is, neither engine will run on the PTO cylinder alone. I am going to take both machines to lake to see if swapping carbs changes anything. But, I don't understand why engines will not run on PTO cylinder alone.

Has anyone else observed this problem? Jim

 

[burtshaver2021]

I would check the voltage regulator

 

[jim downing]

Problem solved ---
I discovered that when I put the PTO plug wire on the MAG plug the engine would not start. That directed me to a problem with the ignition system.

Even though both plugs showed a blue spark apparently the PTO spark was not hot enough to fire under compression. I pulled the electrical box, cleaned all the connections and measured the primary and secondary resistances on the coil. The secondary was in spec but the primary resistance was about 10% too high. I pulled the plug wires out of the coil, trimmed about 1/4 off both and had to glue them back into the coil (you have to break a retainer to get them out). Then I trimmed about 1/2 inch off the plug ends of both wires. Had plugs out and visually checked spark. Maybe looked a little better than before. However, now the PTO wire on the MAG plug started the engine. Put wires in proper order and engine ran on both cylinders. FIN except for putting everything back together.

Lesson: Visual check on plugs firing is not an adequate test of the ignition system. Have read about trimming plug wires but never thought it was my problem until today.

Recommendation: The plug wires are cut to length so the MAG wire will not reach back to the PTO plug. Make a plug wire extension so you can reverse the wires to the plugs. This would have revealed the problem almost immediately.

Heading to the lake for an end of year test before winterizing. The kids are all busy and grandkids are all in college and school so these machines will have to do without human companionship for about 8 months.

Jim

 

[jim downing]

I would check the voltage regulator

 

[burtshaver2021]

Nice work Jim

 

[jim downing]

Burt I solved my problem as described below (maybe above). However, I am interested in why you suggested the voltage regulator. Were you thinking the running voltage from the alternator was too low and hence the plugs were not firing hot enough? Please explain. Jim

 

[burtshaver2021]

Burt I solved my problem as described below (maybe above). However, I am interested in why you suggested the voltage regulator. Were you thinking the running voltage from the alternator was too low and hence the plugs were not firing hot enough? Please explain. Jim

I had suggested the voltage regulator only because it is a known issue that when they go bad they cause a variety of issues like not reaching higher rpms and I specifically remembered Mikidymac mentioning on a post at one time a machine he had that would only run on one cylinder, voltage regulator was the issue. I really don’t have a great grasp on the inner workings of a lot of components, I know the regulator/rectifier converts AC voltage to DC and that it regulates the amount of voltage leaving it and that’s about it.

 

[burtshaver2021]

I guess maybe when the regulator/rectifier begins to fail, maybe it doesn’t convert the AC to DC as well and that maybe causes a weak spark? Interesting though, something to look into further