Could this be a ventilation problem?

At close to WOT on my 20' pontoon with a 30hp evinrude the boat will lose speed and the motor will actually rock back and forth (chuggging) as if it is sliping in and out of gear or something. The rpms will go up and down in sync with this. I can back the throttle off and all is fine until I bring it back up to that point again. I thought it may be prop slip but I put a new one on this wkend and have the same problem. The boat is fairly new to us but it seemd to run fine in WOT on the test run before we bought it. ANy ideas or input would be great.

 

How pronounced and rythmic is this chugging? Is the engine running smoothly, otherwise, not missing? Could it be a fuel restriction of some sort, perhaps?

 

The chugging would be similiar to a car engine sitting idle and you punched the gas on and off, the engine would rock side to side due to the torgue. with the boat it is much quicker (on/off time) as if the engine is going side to side in harmony with the rpms as if it was engaging and disengaging a load. This is why the prop slipping seemed possible due to that at a certain speed and load on the prop the hub would break free than quickly catch again which also would effect the motor rpms.

 

OK, but have you considered that this may be a power issue and not a mechanical linkage issue? Fuel or air starvation, or possibly a spark/ignition issue, could also be the culprit.

Give me an idea of how many times per minute the engine's rpms are swinging up and down. And is the engine running rough or sputtering when it seems to disengage?

 

I am not ruling out anything at this point. The funny thing is that in neutral I can open her up and not have any problem with fuel delivery or spark but it seems consistant when it happens while driving at that certain throttle/rpm position. Times per minute would be like every second when it gets to that point , and the fluxuation would be about 300 rpms up and down like it catches or engages than slips or disengages.

 

I could be wrong, but to me, that suggests it's not getting quite enough fuel, or that the fuel delivery is erratic. How old is the motor? When's the last time you changed the fuel filter? Is the engine carbed or fuel injected?

It wouldn't hurt to check your ignition system, either. An engine under load requires more spark energy than revving it in neutral. Start by checking your plug gaps.

Note: I'm no expert on 2-stroke engines. These suggestions are merely what I'd do if I ran into the same problem you're describing. It could be something as simple as changing the fuel filter or checking the spark plugs.

 

It could be ventilation. Have you tried lowering the engine a few inches. When you get up to speed the prop could be so close to the surface it's sucking air.

 

How many people were aboard for the test run?

How many people are aboard when it acts up?

If there were more people and other gear on the boat when you tested it, and fewer people and/or less gear on the boat now, I suspect it is ventilation. The boat was probably just enough deeper in the water when it is loaded with more people/gear to prevent the prop ventilating.

 

The engine is a 1993 evinrude with carb. I replaced the plugs and cleaned the fuel filter screen ( I dont have a manual yet so I dont know if there is another filter besides this inline screen). When we test drove the boat it had 3 people on board but it acts this way now whether there is 2 or 8 people on it. I'm also trying to make a connection as to what could be differnt now vs when we test ran it. The only thing that really stands out as different to me is that our lake is very weedy and shallow compared to the lake we tested it on so I was wondering if that was messing up the water flow causing ventialtion or something. There is adjustments for the tilt with a pin but I dont know that the motor can be lowered as it appears to be bolted securing onto the back. Thanks

 

Is this a long shaft motor or a short shaft (20 inch vs 15 inch)? To lower the motor you would either have to make a cutout where the motor is mounted or get a mount that bolts to the stern that can be adjusted, sinilar to a jack plate, but you probably don't need something as expensive as a jack plate.

 

The model is a 30ELET which I believe is a 20 inch shaft. My A/V plate ( The fin above the prop if thats the correct name for it) is about 3 inches above the water line when sitting still. Does this sound like a good height for my motor? I will be going up this wkend and am thinking of running it on the water with the hood off and checking the fuel spray into the carb and also try and see if it may be sucking air into the fuel line from the primer ball or a cracked line. Than I was thinking of setting the manual trim so that the prop is more forward and towards the front of the boat with the thinking that when I get going close to WOT and the boat is planing up that I will still keep the prop from coming up to high out of the water. Any thoughts or ideas on what else I can try to help narrow this down?

 

No. The plate above the prop should be right at the water line or just below it when running. Defeinitely below the water when sitting still.

 

lakes, if the plate is 3" out of the water at idle, then the prop's coming out of the water at times when running. Hence the symptoms you've described.
Aha! Look at your lower unit... see the cooling intake screens between the plate and gearcase? I bet those are coming out of the water too! This explains everything you talked about in your other thread on (presumably the same boat) http://www.boatdesign.net/forums/showthread.php?t=13263&page=3
Adjust the engine height as Ike suggests, and then see how she goes....
Start by setting the plate level with the hull bottom immediately ahead of it.

 

I will know more when we go up this wkend but I dont recall seeing any way to actually lower my motor. It is sitting onto the back and bolted down. What do you guys suggest on getting it lower short of buying a jackplate? If it is that the motor is up to high, why do you think it ran fine on our test run before we bought it? Perhaps the weeds in our lake are enough to cause the added restriction. Do you have any advice on where to set my manual trim or is it more of a trail and error thing? Thanks to all and have a great wkend.

 

Lakes- if you have a camera, and can take a few shots of the tail of your boat while it's sitting at the dock, that would be great- it'd make it easier for us to recommend changes that would suit your particular setup.
EDIT- attached picture of your motor (different colour but same engine) with the approx. waterline marked