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1992 351 pro boss HO help.

I

indyskischool

New member
Since Monday, June 7th, my boat has ran great for a few hours until she heats up (never above 150degrees) and then cannot stay in idle without revving and dying. We have replaced the THERMOSTAT, FUEL PUMP, 14FT OF FEUL LINE, BYPASSED THE WATER/FUEL FLITER, REPLACED THE IMPELLER, THE PVC VALVE AND HOSE, CHANGED ALL SPARK PLUGS, AND CHANGED ALL NEW SPARK PLUG CABLES. Every time we change something the boat runs like new for a few hours and then the problem starts back up again.

LAST NIGHT WE RIPPED OUT THE PCM/PROTEC COMPUTER SYSTEM AND REPLACED IT WITH A PCM/MALLORY DISTRIBUTOR SYSTEM KIT DIRECT FROM THE MANUFACTUER. This morning she ran great for about an hour before the problem started right back up again. WTF?

The carb was rebuilt last August and when the boat is running, shows no sign of trouble. The boat purrrrs like a kitten for 2 hours, can pull a barefooter at 42mph with not even a sputter. However, when this problem rears it’s head, she starts running rough before, finally revving and dying. Turning off the key and letting it sit for a moment clears the problem up enough to get the boat moving again and a skier out of the water. I just cannot idle back to pick up the skier when they fall…the boat will die unless I am throttling up.

I have spent a small fortune trying to diagnose this problem. What else could it possibly be?




 
Bought a vaccum pressure tester gauge. When boat runs fine (for an hour or two) the needle is steady at 20@idle. Drops to 16@gear and low RPMS, and drops to 13@3000rpms and higher. Steady and concise at these numbers almost two hours.

Then the problem starts. When I put is back in idle it runs rough at first then it is followed by the engine revving and the vac pressure fluctuating with the revving. First from 20 down to 15, then 15 down to 10 and then dead.

No problem keeping the engine going with higher throttle or RPMs. While I have the boat running at the dock at 3000rpms I sprayed carb cleaner all around the intake manifold gaskets and the carb....nothing, no sputter or engine increase.

Now what???? Leaking valves?
 
Intermittent fuel issues are usually located in the tank, at the pickup. Did you put any ethanol fuel in? Could be bad gas. Try running from a spare tank with known good gas. Also look at anti siphon fitting, and your "rebuit" carb. 99 times out of 100 a rebuilt carb will not work as well as a new replacement.
 
We pull the tank and check the filter. We even knocked the ball bearing out of the anti-siphon valve to remove that from the equation. My mechanic's theory is that it is not the CARB simply because if it was the carb the problem would be there all of the time and not just when the engine got hot....agree? I bought another carb-rebuild kit and plan on rebuilding it again just to be sure, but I am leaning toward a new carb. I run SKI SCHOOL so I go through a tank every other day, so it is not the gas. I am so out of ideas.....
 
Did it overheat in May or the first week in June?

The vacuum fluctuating when the problem starts makes me think there is a vacuum leak that is only present when hot. Could be as simple as loose carb mounting bolts.

May also want to do a leakdown test as that will give you much more insight into potential trouble areas.
 
After spending close to 2k on trying to fix the boat’s heating-up then dying in idle problem, it came down to a stripped and cross-threaded “carb-stud” that, when a wrench was put on it felt tight, but could never fully be tightened down enough to seal the carb-plate to the manifold. When it got hot, it sucked oxygen and killed the motor. I replaced the gaskets between the carb and plate and added a layer of “form-a gasket” for good measure. I also replaced the bad carb-stud. The boat runs like a dream.

Cost of new carb-stud that fixed the problem…..$1.80 at NAPA AUTO PARTS.

Kill me now.
 
Sorry to have specifically ID'd loose carb mounting after the $2K investment.....look at the bright side - at least you have a few spare parts!
 
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