Motor bogging down shortly after running full throttle.

[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).