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Strange Power loss Issue help needed.

H

Hammer58

New member
I have a 140HP 1979 mercruiser 4 cylinder stern drive.
It runs fine with people in the boat. Had 7 people in it.
I hooked up a 3 person tube to the boat and started to pull 3 riders around on the lake.
Note it ran fine for the first 10 minutes at full power with 3 riders on the tube and 4 in the boat.
After about 10 minutes the engine bogged down and I could hear the engine missing.
I could only get about 1/2 throttle out of it. If I opened the throttle any more the boat would slow down even more and miss fire badly.
Tried only one rider on the tube and I could get about 3/4 throttle before it would begin to miss fire.
Unhooked the tube and put 7 people in the boat and it sputtered a bit as it planed out then cleared up and ran fine at full throttle.
Tried to pull the tube again with one rider on the tube and it miss fired at 3/4 throttle. Could not get any faster out of it.
Anyone have ideas on what the problem is?
It has an after market electronic ignition and has a 2 barrel carb.
Again it runs fine until I try to pull a tube with it then it has a power loss and miss fire.
 
I was thinking that as well. But it did run fine for ten minutes pulling the tube at full throttle.
It almost acts like it was overheating. Which I fine odd for a boat in a lake full of water.
The longer I went trying to pull 3 riders the slower the boat went.
Over time it was slowing down and the problem was getting worse. Which makes me think over heating.
Yet after I unhooked the tube and went around the lake with 7 people in the boat without a cooling off period
the boat ran fine. Water temp did drop from 140 to 120 degrees during that run if that means anything.
P.S. I do have a water temp gauge. Not sure if it is even close to accurate. But it never went over 140 degrees.
Oil pressure was good the entire time.
 
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Ayuh,..... By the sounds of it, o2batsea has it right, ya got a fuel delivery issue,.....

Could be anything from the tank to the carb,.....

I'd start at the fuel filters, 'n the fuel itself,....
 
Good Lord! You ran that overloaded boat with 3 people on a tube at FULL throttle for ten minutes! You probably burnt the rings and valves out of the poor thing.

Get a compression test done, then start looking for a rebuilt long block 'cause you're probably gonna need one.

Jeff
 
What rpm was it running with the fullload on the boat and pulling the tubes? You can try using a prop with less pitch. Sounds to me like your beating that engine to death also.
 
The tach does not work. So far, compression test is good. Rear two cylinders spark plugs show signs of engine running rich. Carb choke may be not functioning properly.
Fuel line is clean, filters are clean. I am suspecting carb issues mainly the choke. High speed jet screw was also turned out 4 1/2 turns. Have not checked fuel line pressure yet. Also thinking it may be the engine is not getting enough air flow. The engine compartment has been significantly modified by previous owner. I can test this by leaving the engine cover off on a test run. Keep in mind it runs fine with people on board, only bogs down when pulling a tube. Boat is ratted for 7 adult passengers.
 
It never hurts to have a spare prop and hardware just get a prop with two inches less pitch and see how she performs under a full load pulling water sports. I can guarantee your over propped with the heavy loads.
 
High speed jet screw was also turned out 4 1/2 turns.

there is no such thing, idle mix maybe. 7 people is overloaded not to mention have 3 in a tube also.
 
The 7 personrating is the USCG rating for safety purposes, i.e. won't sink! NOTHING at all to do with what the engine can do.. The total life of a marine engine at full wide open throttle is measures in 10s of hours...less if you're pulling a load that will not let the engine get up to its specified maximum wide open throttle RPMs. Then ????
Notoriously, back in the day, a brand new 327 Corvette engine catastrophically failed on a dyno test after 15 mins at wide open, full load (Curtis Wright circa 1964 while doing comparative tests of US engines vs the Wankel Rotary... the engineer that did that test was a good friend of my Dad.)
 
Bt Doctur said:
High speed jet screw was also turned out 4 1/2 turns.

there is no such thing, idle mix maybe. 7 people is overloaded not to mention have 3 in a tube also.
Click to expand...

When I pull a tube I only have 2 in the boat and up to 3 on the tube. That is not overloaded.
But that is when I have had a power loss. When I unhook the tube and have 7 in the boat it runs fine.
Explain that? It even ran bad when I had only 1 person on the tube and 2 in the boat.

Then I saw a jet ski on the lake with 2 people on it pulling a 4 person tube and it was traveling at twice my speed.
That 4 person tube was huge with a chair built in it, much larger than my tube. 4 people sitting up against a back rest on that tube being pulled by a jet ski with 2 people on it. My engine, a 4 cylinder industrial engine has a lot more torque than a jet ski 2 stroke has.

Note when I had 7 people in my boat without a tube I was slightly faster than that jet ski was that was pulling a 4 person tube with two on the jet ski.
My speedometer said 40MPH. Not sure how accurate that is. It does 43 MPH with just 2 people in my boat. And when I pull a tube with 2 in the boat and 1 on the tube it said 21MPH. When I had 3 on the tube and 2 in the boat it said 17 MPH. Something is very wrong and that is why I am looking for help. I would think 7 people in the boat is a bigger load than 2 in the boat and 1 on the tube. But my boat does not seem to agree with that.

P.S. I need to add that when I first started to pull a tube I had 4 in the boat and 3 on the tube and it ran about 37MPH for maybe 10 minutes before it started to slow down. The engine was cool at that time as well. That was when I started to experiment with different loading's to try to figure out what was going on. After i got done with that I went back to 7 people in the boat and went out for a joy ride and it ran fine back to 40MPH with no miss firing and no cool down period. When I pull a tube the engine ran rough and miss fired and I could not advance the throttle past 1/2 or 3/4. If I did the miss fire got worse and the boat slowed down. As I closed the throttle slowly the miss firing went away and the engine smoothed out and the speed would pick up a bit. Try to go faster again and the miss firing would happen and the boat slowed down. So as I said above the max speed I could get was 21 or 17 depending on load setup.
 
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Cannot compare jet ski and a boat. They (jet skis) weight next to nothing and have much less hull drag... and on the subject of drag, towing a tube is like towing a sea anchor.

Boats need TORQUE to spin props... HP is related to torque x rpms. Small displacement engines (4 bangers) get their rated HP by spinning faster. Pulling a load requires a large diameter prop and 4 bangers can't swing them, so they compensate with a larger numeric gear reduction ratio and more RPMS... As a result, when the going gets tough for the engine, it can't spin the prop and the speed (and RPMS) drop faster under load than with a V8... Additionally, as the load increases and you try to go faster, the prop slips more in the water, dropping boat speed even more. As the dragsters say, "there is no substitute for cubic inches."

Just concern yourself with your engine's RPMs as other posters have suggested. It is the BIGGEST indicator as to what is really going on.
 
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Hammer58 said:
When I pull a tube I only have 2 in the boat and up to 3 on the tube. That is not overloaded.
But that is when I have had a power loss. When I unhook the tube and have 7 in the boat it runs fine.
Explain that? It even ran bad when I had only 1 person on the tube and 2 in the boat.

Then I saw a jet ski on the lake with 2 people on it pulling a 4 person tube and it was traveling at twice my speed.
That 4 person tube was huge with a chair built in it, much larger than my tube. 4 people sitting up against a back rest on that tube being pulled by a jet ski with 2 people on it.
Click to expand...
First off, the jet ski is a really bad point of reference. Not because he passed you, but because he was breaking USCG rules. You are not supposed to have more passengers in the boat plus towed device than the watercraft is designed to hold. The absolute largest jet ski out there used to be a Polaris Genesis which was rated for 4 passengers. Most likely you saw a 3 passenger ski which means he should have had no more than 2 people on the ski and one on the tube. Also, many of the newer skis are supercharged so their engines make crazy amounts of power compared to your 4-cylinder engine.

Your problem sounds like ignition or fuel delivery related....possibly both. The reason your boat can go fast without the tube but bogs with someone on it is because those tubes create a HUGE amount of drag. Your poorly running engine can make enough power to run the boat without the tube, but when that extra load is applied it becomes outmatched.

Why did it run 10 minutes then start acting up? Because that's when the piece of gunk clogged your fuel line or the ignition component heated up and failed. These things happen with mechanical parts. Have you replaced spark plugs? Maybe they are weak and create just enough spark in light load conditions but fail under heavy loads.
 
OK Nag mode on: 7 people in your boat at 40 mph is just wrong. How many were actually WEARING PFDs? None, right? You may have been having a blast but the chances of disaster increase exponentially when you get that much weight into one of these little runabouts. The boat can't plane properly and you could have handling problems ( look up Texas Flats boat safety ) that can happen unpredictably. The spectacle of 7 bodies flying through the air at 40 miles per hour being chased by a spinning prop is just too horrifying. Take fewer, switch passengers more often, and keep the safety margin wider. Nag off.
 
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