Motor bogging down shortly after running full throttle.

I have a 1993 30hp evinrude on my 20' pontoon. When I try to run the boat full throttle it will bog right down after a few mintues of running. I tach out at around 4200 rpm and about 12mph. If I keep the throttle backed off a little from full it will run fine but I would like to get as much speed as I can out of her. My owners manual says I can run this motor full at 5400 rpm max. I am not sure if the motor might be starting to overheat and a temperature limit is coming into play or what. But after I pull back on the throttle I can repeat this problem right away. Any help for this novice is greatly appreciated. Thanks

 

Is the breather right open on the fuel tank?

 

The only thing I see on my remote 14 gallon gas tank is the line feeding the motor and the cap. So I guess I should loosen the cap to allow air in when I run it full ? What are your thoughts on it overheating and a temperature limit coming into play that drops the rpms ? It just seems strange to me how it wont hesistate or fluctuate when it gets to that point, just like a finger snap it will drop the rpms right down. I can than bring the throttle back to neutral and take right off again to repeat the same problem unless I stay short of full throttle. My main interest is a little more speed so is it possible to add a different prop and just not utilize full throttle ? Thanks

 

I have the white-painted version of the same motor on my runabout. While it can run full-out for a while, that's really not too nice on the poor thing and I'd advise an 80% throttle setting as the maximum for continuous cruising.
The S.L.O.W. overheat protector on this motor only kicks in if it actually does overheat, and it restricts it to typically around 1200 rpm. If it ever trips, make sure you get the cooling system inspected and the water pump dismantled and checked. Check the pisser hole on the starboard rear of the motor, and the cooling spray out the two exhaust bypass holes on the centre of the shaft, to make sure the flow is good. If the flow is not good, then cooling is your problem and you should not run the motor until it's been fixed. This could prove as simple as having a mechanic back-flush the cooling lines to clean them.
If you have a stock OMC fuel tank, with the gas gauge and fuel line fitting integrated into a little square on the tank top, then no, you don't need to open or loosen anything on the tank as it does that by itself. Although you could try loosening the gas cap a bit, if that solves it your fuel tank pickup might be faulty.
What prop are you running now for your pontoon? The stock one that came with the motor is meant for lighter, faster boats than you have. You probably need a wider, shallower prop to make the most of this rig.

 

Thanks Matt. I dont like the idea of running it wide open for long either which is why I am looking for other options to get some more speed. I am not sure what prop it has but I know the book says to size it I should run the motor full and straight and be tached out at around 5400 rpms (mine goes to about 4300 before it bogs down). Any suggestions on a different prop ? Higher or lower pitch ? 3 or 4 blade ?. The lake we are on is pretty shallow and weedy. The water that is coming from the "pisser" hole would probably fill a 16 ounce glass in about 20 seconds. Does this seem weak ? I am not sure about the "2 exhaust bypass holes" and to be honest I am not sure where to look or how to check (sorry for being an idiot). So that I could rule out the SLOW protection, is it possible to bypass or jump this out and see if it doesnt bog down ? ( Dont worry, I'm not that much of an idiot to leave it this way). Thanks

 

Your RPM is too low at top speed. You probably have a 10x13 prop right now (diameter x pitch), this is stamped on the prop hub. For a big pontoon you want a bit more diameter and a fair bit less pitch. Right now you are overloading the motor which could be the cause of your overheating problem.
If you look on the back of the shaft, about 8" below the powerhead, there are two downward-pointing holes with exhaust stains around them. Water should spray out of these holes when the engine is running at speeds above a fast idle for more than ten seconds. More water sprays out as you go faster. Since the pilot spray on your motor is fine, I think cooling flow is not your problem, but do take a look at this to make sure.
The cue that S.L.O.W. protection has tripped is the motor suddenly slowing to around 1200 rpm. What rpm do you show after it bogs down? As far as I know you can't disable the system without risking serious internal damage.
Has the boat run fine at WOT in the past, or did you just buy it and so don't know? I'm almost certain it's overloaded with the current prop and so that might be the cause of an overheating issue....

 

I feel pretty certain it is indeed a SLOW issue and after you explained the prop sizing it seems even more likely to me (thanks). We just bought this so I am not sure if the problem existed before but it would help explain the "good deal" we felt we got. Is changing out the prop a fairly simple task? Do you suggest anything in particular for our shallow and weedy lake? A 3 or 4 blade? What would you expect a cost to be on a new prop? Thanks so much for your help Matt.

 

Changing the prop requres a deep socket wrench and a pair of pliers, it takes maybe 5 minutes at most, read your manual for directions. A good prop dealer will recommend a starting point and will have you try a couple of props to find the best setup for your boat.
Running at full throttle at a thousand revs too low could easily be the cause of overheating on this motor. The cooling water pump turns at the same speed as the powerhead and its flow rate depends on the motor speed. The S.L.O.W. mechanism trips to make sure the engine isn't damaged by this kind of overloading/overheating. Try again with a shallower prop. I might start with an 11x10 or so for this boat, but your prop dealer will know better than I as he can see the actual boat.

 

I'm eager to try the shallower prop now. Do you have any input on me relying solely on my 1993 tach to judge my rpms that will determine my new prop? Is there a easy way to check to make sure it is at least close to accurate without having a GPS or running along side another boat? Thanks again Matt

 

A tach is a very simple device and pretty reliable, but the only way to get an rpm pickup is via the engine itself of course- so if in doubt, the only way to check the tach is to connect a mechanic's portable dwell/tach meter to the motor. The tach's probably fine, the only thing that commonly goes wrong with them is rusty electrical plugs, indicated by the tach needle wavering while the engine sound is constant. Speedometers on the other hand are notoriously unreliable as they usually have a pitot tube on the transom that clogs up all the time, these you can check against a GPS as long as you don't have a significant current in your lake.

 

We'll be heading up to the cottage for a long wkend and a local shop there has a 10.5 x 9 prop I will be trying out ( he would need to order the 11" ). If I understand this correctly now; if this 9" is to shallow than I could excede the max rpms of the motor (5400) on WOT. Should I than go to the 11" or can I leave the 9" on and just never let the rpms get that high by holding back the throttle from full ? I can't tell you how much we appreciate your advice and time in helping us with our new "toy". Thanks Matt.

 

Did you checked your float in the Carburators?

If your engines slows down after a short while using full throttle there is the possibility that one of your carburators is not getting enough fuel.

Try the next time when this problem occurs to use the hand pump conecting your tank with the engine. If then everything goes back to normal you will know for sure it's the float and maybe needle.

 

dont know for sure but it could be a little rubber O ring in the carb ,,,,,,let us know

 

I wouldn't rule out carb, for sure.... but carb trouble doesn't usually manifest itself as a split-second drop to idle; I'd expect a bad carb to take several seconds to gradually slow down. Good luck, Lakes.....

 

What is about Ignition? Maybe a Spark plug, coil or Powerpack. Do a cylinder drop test to find out if there is a cylinder causing the problem.

Cylinder drop test is done this way.

When the symptoms show up on your engine.

1. Turn it off.
2. Remove ONE spark plug lead and ground it. <---Place bolt into it and allow it to touch the engine block.
3. Restart engine and see if the engine runs any different. If it does run differently, that cyl is most likely working properly and is not the problem.
4. Turn it off.
5. Replace the spark plug lead and remove a different one.
6. Follow the steps above until you isolate the cyl causing the problem.