Honda 15 running problem very unusual

[dbajaer]

I have a 1992 Honda 15, it runs fine out of the water, however, launch the boat, and when under power, misfires, runs at almost 0 rpm at full throttle. Now here is the strange part, if I remove the motor cover the motor runs great, in fact if the throttle is advaced when the cover is removed, look out as the boat instantly accelerates to the throttle setting. Drop the cover back into place and it almost shuts the engine off! Sounds simple, but, I removed the air baffle at the top of the cover, so no restriction, and same thing. Now, when the cover is lifted about 3 or so inches the motor begings to run normaly, I have used a metal object grounded to the motor and held it next to the spark plugs, nothing happens, no sparks, I have taken my hand and touched the wires, moved all of the wires and can not get any of these things to affect the problem. I have changed the spark plugs, rebuilt the carb, checked the carb air intake, choke is free and does not close, this is a truly strange problem! It must be related to load on the engine as it does not occur out of the water, I am sure it must be related to the ignition system, but, I can not see any problem with a plastic engine cover that is not touching any of the wires making this running problem occur! Has anyone ever seen this problem? I have the motor with a outboard mechanic and he put it into his tank and ran the motor, I got a phone call last night, he has never seen anything like this, ever....

[Fouled Plug]

It is possible you have an exhaust leak at the base of the powerhead....could be enough to choke the engine under load. I'm not too familiar with the cowlings on those motors but you need some type of opening to let fresh air in. Perhaps yours is somehow blocked?

[dbajaer]

Your idea is good, this would expalin whats happening. I will check it out, did just get off the phone to the Outboard Mechanic, he still can not believe what he is seeing with this motor! He is talking about trying to call Honda, he has twenty years of experience and lots of factory schools to his credit, at least I do not feel as bad for not being able to figure this one out myself. I will head over there and see if we can find an exhust leak, it would account for our problem as the motor does need air to breath. There was not any obvious noise with the cover off. Thanks..

[Bear]

Check these: http://www.iboats.com/cgi-bin/ubb/ul...;f=17;t=025109 http://www.iboats.com/cgi-bin/ubb/ul...;f=17;t=001848 Good luck!

[jprasisto]

sounds like when you tighten the cover down you're crimping a wire somewhere or ading pressure to a hose? You say if you run with the cover a open a few inches it's fine. Does it do the same if you put the cover on and not tighten it down? If so I'd look for a crimped control line or maybe a fuel line near the control.

[dbajaer]

What a great resource you all are, Fouled Plug and Bear right on target! I just ran the motor in the mechanics test tank and I can see water and of course exhust comming out below near the exhust pipe inside the upper motor area, this is just recirculating the exhust back into the motor and when the cover is lifted only 3 inches the fresh air hits the carb intake and wham it takes off. This spot is impossible to really see as it is buried below the motor, as soon as he tears down the motor and we can see which parts I am off the the USA for replacements, err its 1200 miles of bad road.....It was time to get back to the good old USA! Thanks a lot to you guys.