BF75 bogs out in the water in its down position, but runs in the midway position

Hello,

I was wondering if anyone has had this problem before. I recently purchased a
BF75L outboard for my boat. If i run it out of the water with a garden hose hooked up it will run and idle fine in the down position. As soon as I get in the water and try to run it in the down position, it will idle in neutral, but once in gear it will die when accelerated.
If I move the motor to the midway position on its bracket, " so it is still in the water but angled" it runs like a champ. I have check the hoses to make sure they are not pinched, but at a lose to understand why it is taunting me....

It might have something to do with the "L". If you are trying to use it on a low transom boat, the exhaust may be experiencing too much back pressure from being too deep in the water. Especially if there is an exhaust leak under the hood. Try running it with the cover off and see if that makes a difference. If it runs better with the hood off, then you are probably backing up exhaust gasses into the engine compartment and starving the engine of oxygen.

If the lower end is just too deep, you could try fabricating a "jack plate" to mount it a bit higher. The rule of thumb is that the cavitation plate of the motor be only 2 or 3 inches lower than the bottom of the transom. You can usually fudge this quite a bit but a long shaft motor doesn't do well on a low transom boat.

One other possibility would be a carburetor float problem. It could just be out of adjustment.

I've also seen cases where water is in the float bowl and the engine runs better when tilted because the gas that was floating on the water is shifted so that the jet can pick up more gas and less water. But, in those cases, the engines wouldn't even start unless tilted up. Wouldn't hurt to drain the bowl though and see if that makes it run better.

I agree with JGMO's analysis, and he certainly has more experience than I do. But I have always followed the rule that the anti-cavitation plate should be even with the bottom-most point on the hull unless there is an offset with a motor extension. In that case, the anti-cavitation plate should be raised one inch for every 12 inches of offset off the transom. Ultimately, at WOT on plane, the anti-cavitation plate should be just skimming on top of the water coming off the hull.

Another possibility is a loose or broken wire that is connecting with the engine raised and disconnecting when the engine is lowered. I would especially check all the ground wire connections.

Well, we took it out again yesterday and I removed the cover and it ran in the down position with no problems, I will check the level of the cavitation plate today and see how low it is sitting. Thanks for the pointers!

Great! Let us know what you find.

well, I checked the cavitation plate and it is about 2-3 inches below the hull. We took the boat out again yesterday, and with the motor in the down position and the cover removed it ran fine. If I put the cover on while running, as soon as it was closed the motor would either die or it would drop to half the rpms it was running at. So now I am thinking it must be a problem with the exhaust? I blew out the vents in the cover and they are not blocked. I'm kinda wondering if its a bad gasket somewhere, all the hoses look good.

Ok! The good news is that your motor fits your boat! The bad news is that you are correct, you have an exhaust flow problem and it is most likely the exhaust pipe seal between the oil case and the extension case. The better news is that those aren't too difficult to replace if you have a modest amount of mechanical ability.

However, I would advise you to buy the shop manual before attempting the job as you will need to take off the extension case to get to the seal and that involves disconnecting the shift shaft and then getting it readjusted properly when you go back together.

One thing to check first though is to make sure the small vent hole in the top of the extension case is not plugged. It is located at the back of the motor where the extension bolts up to the oil (or upper) case. In layman's terms where the motor goes from fat to skinny. It is a small hole about 1/4" in diameter. The inside of that entire extension case fills with exhaust and can become pressurized and get backed up if that hole is plugged. Not real likely but check it first.

Learning how to take that extension case off also offers a bonus for later on as it is part of the procedure for replacing your water pump. That's a chore that comes around every other season or so if you want trouble free operation and, if you don't know when the last pump impeller was done, this might be the time to knock it out and be done with it for a couple of years or three. Again, get the manual and it you will find it's well worth the $35 or so.

JGMO is likely right on with his diagnosis. However, My nephew had a very similar problem - when the cover was on, it forced one of the wires against the block and was causing an intermittent short. You might want to also check out your exposed wiring that runs near the cover.

Very common to find a corroded leak in the exhaust pipe, forward side and difficult to see without removal of power head.

A replacement will be of the newer style that has a water jacket, instead of the water just squirting into pipe on older style, it is a bolt-on replacement.

Another change is replacing bolts with studs, but I would not use their 'cap' nuts, but rather ss nuts with locking washers.

If you go into Boats.net site, you can see blowup parts diagram for this.