Motor bogging down shortly after running full throttle.

[canadianlakes]

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

[Frosty]

Is the breather right open on the fuel tank?

[canadianlakes]

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

[marshmat]

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.

[canadianlakes]

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

[marshmat]

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....

[canadianlakes]

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.

[marshmat]

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.

[canadianlakes]

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

[marshmat]

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.

[canadianlakes]

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.

[mazsalleh]

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.

[longliner45]

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

[marshmat]

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.....

[mazsalleh]

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.

[marshmat]

mazsalleh- the Evinrude 30 is a two-cylinder two-stroke and will not run on a single cylinder, so that test wouldn't be of any use here. On a multi-cylinder block it could prove more useful.

[Frosty]

A carb delivers roughly 14.7 air to fuel ratio,It should be no more no less. Restricting RPM by restricting fuel would mean weak mixtures this would be bad for the motor and cause damage. The only overheats I have heard of is alarms that give you the warning and lts up to you to decide what to do. To have an automatic shut down might be dangerous in some situations.

I have never heard of an over heat cutting down RPM iether ,--simply if its overheating it needs to be shut off and investigate the overheat problem not just slowed down.

Im pretty sure you got fuel starvation,--- tank, pipe, bulb, conector valve, pump filters, carbs.

[marshmat]

Fuel starvation is a possibility.... again, how quickly the problem manifests itself will be a clue. A fuel flow problem would normally manifest itself as a gradual slowing of the engine, or a sputter-and-stall. From the first post I gather the problem occurs as an almost instantaneous drop to idle?
OMC's S.L.O.W. interlock does indeed deal with excessive temperature by limiting the RPM; the idea is that even if the motor is overheated you can still limp to a safe area at a high idle if you're caught in something nasty, but they do advise you shut it off for repairs if the system trips.

[canadianlakes]

Wells here where I stand after being up at the cottage this wkend working on the boat. I took off the prop 10.5 x 11 and had it inspected and cleaned up. I cleaned out the fuel filter and flushed out the water intake. Made sure the tank was venting properly, replaced the plugs, fired it up at the dock with prop disengaged and reved her up to 5200 rpms. Everything seemed good with engine sound, water flow from pisser and exhaust so I took her out for a run. When I got her out and opened it up to WOT it ran for a couple minutes than dropped right off but what seems different and maybe was always teh problem and I didnt notice before was that it seems like the the rpms stay strong and high but the drive just disengages . What I mean is that I was back watching the motor while my wife ran the throttle and at that point on WOT after a couple minutes the water quit coming from the pisser hose, the speed dropped right off, but the motor sounded like it was ready to over-run.
Anybody have any ideas on where to go next? Thanks

[mazsalleh]

This are good and bad news?

Good news are that you know now that it's not the ignition or carburators.

For me it sounds like that once your engine runs warm, there is no power getting to your Driveshaft. Maybe a mechanical problem at your engine Driveshaft conection.

[Frosty]

Well now it sounds like you spun a prop!

[canadianlakes]

I'f its the engine getting warm thats the problem, how come I can back the throttle down and it will run fine all the way to around 3/4-7/8 of WOT? But if I bring it to WOT the problem returns. What do you think could be causing no power getting to the driveshaft only on WOT? The engine rpm sounds strong when it happens. By "spun a prop" does that mean that the spline is stripped allowing the prop to slip? This problem was happening even before I took the prop off for inspection and at that time everything seemed to be OK but I'm not an expert in this. Thanks

[Frosty]

If you have the kind of prop that has a rubber centre bush, this rubber bush can detetriate and get old,what happens is it then slips and can not drive the outer portion of the prop. The relults is that Max torque can not be transmitted infact once it breaks free it transmits less until you slow down and build up the torque again.

[canadianlakes]

I'm not sure if my prop has what you are refering to or not with the rubber bushing but I'll look into that. If that was the problem and it was the prop slipping wouldnt I still have water coming from the pisser hose though? It's almost like the motor just quits driving anything. Is there something inside like a clutch pack or something that may be slipping under the higher torque demand at WOT?

[Frosty]

Well, I was dis-regarding that as your original post said that you dropped a couple of hundred RPM and now it does'nt. Its quite likely you have 2 problems here.

I'm sure we will all stick with you to get you through but some proper information would help.

[canadianlakes]

Sorry for your confusion Jack. I have only had this boat for a couple weeks and last wkend was only the second time out with her. During that time I tried to get more and better info on what was actually happening which I tried to explain in my post #19. If I am lacking any important details or more facts I apoligize and will happy to find them out if I know what it is your looking for. Thanks

[Frosty]

Yes its ok--- Its just that we have all been trying to think of why your RPM was dropping,---# 24 pages of it. Dignosis is difficult at the best of times.

So to start again I would suggest to your pisser is also suffering the same effect as your prop. By that I mean that the impellor in the pump is very similar in construction to the prop, ie the rubber can spin in the brass inner. This will cause it to work ok at low RPM and then loose its grip at high.

Im not saying it is or isnt the problem but it would give you those symptoms you describe.

[marshmat]

Hmm... well, the prop hub slipping would explain the tendency to over-rev, which would of course cause an overheat... for some reason I thought the 30's water pump had a keyway though (I've never actually dismantled the lower unit on mine). Why the cooling flow stops seems a bit of a mystery. If anything, it usually intensifies as you rev higher.
Whatever is causing the cooling issue is a big priority for you right now. Since we've now traced two possible causes to the prop assembly (overloading or slipping, which could also be related), I'd be really tempted to test a known good prop. Keep a real close eye on that cooling stream until we figure this out....

[Frosty]

Marshmat, I have to admitt to not being familiar with this engine to the point of knowing what the method of attaching the prop is. If it is a shear pin(large engine for this) then I could be incorrect at this diagnosis. Are you sure its not an exhaust hub.

The impellor can suffer the same fate as a 'spun prop' Yes it may be keyed but the rubber comes away from the keyed inner bush.

Or the impellor wears to the point that at higher RPM the rubber blades can not touch the sides of the pump and loses its pumping effect.

If it were mine I would fit another prop--any prop-- and fit a new impellor. Gotta start some where!!!

We'll get there in the end!! At least things are making more sence now!!

[marshmat]

Jack, you are correct that the prop has a rubber-slip exhaust hub as you describe. The rubber-vane water pump, I believe, has a keyway and has no weak-link failure mechanism (if it goes, it simply breaks a vane). Impeller wear is a definite possibility as these pumps are supposed to be rebuilt every few hundred hours (can't remember the exact interval). Lakes, I'd fit a new prop and have the impeller checked (if you have the shop manual you can do this yourself, but be careful with the shift linkages and cooling line gasket).

[canadianlakes]

Thanks for the help, I wish I would have put on that new prop I had on hold but after the guy said mine looked fine I figured I'd save the money. Do you think I should go with a direct replacement of the 10.5 x 11 or go to a 10.5 x 9 ? Do you think I should replace just the impellor or buy the hole kit? Any manuals in particuliar you suggest? When I took the prop off last wkend I dont recall seeing anything that resembled a "thrust washer" that I read about in my small engine book that came with the boat. Any thoughts on this? Thanks

[marshmat]

Visual inspection doesn't tell you much about the rubber slip hub on the prop. If it's failed, it will still look fine until you actually dismantle the prop hub.
From your earlier posts it looks like you're over-propped anyway and so replacing with the same 11" pitch probably wouldn't be a good idea; I'd try the 9".
The only parts in the water pump that tend to go are the impeller and gasket. The housing will be fine unless you regularly run it up on sandbars.

[Frosty]

A thrust washer is a stepped washer ,quite thick say 4mm. As the prop is rubber bushed ( you now know that) the prop will give slightly under thrust and stop on the washer. The washer will then ,---take the thrust.

If you dont have this it will mean that the prop thrusts on its rubber cush drive and could touch the drive housing.

The idea of the cush drive is shock absorbing and forgiving gear selection. Once selected its job is done. Appart from driving the prop!!

[canadianlakes]

My main interest when I get through this is to hopefully gain some top end speed. How much speed would you guess I'd lose if I go with a lower pitch ( 9" ). Or should I be more concerned about getting the right prop (sized right to my WOT rpms) regardless of my speed loss ? I will check for the thrust washer but I dont remember seeing it when I took the prop off. Is this something I can probable get from the same marina store that has my prop or should I look into a special order now? I will also go ahead and get the water pump kit but I want to get a manual that is more specific to my engine instead of a generac one that covers many like they seem to have in the supply stores. I have read that Ken Cook Company is a good source for this but I'd like to have one sooner than a mail order, do you suggest any other sources? Thanks again.

[marshmat]

On this boat you should be worried mainly about getting your engine properly loaded, and running in its correct RPM range. An overloaded engine is always a bad thing.
High speed and low-powered pontoon boats don't usually mix.

[ron17571]

Im reading the outboard posts to tryin learn,so what was the problem?impeller?