outboard Idle

[asmw25]

I have a 1988 Johnson 150 on a 17' checkmate. If I put my trim all the way down the engine will not idle. If I put the trim 1/4 up the engine idles perfect. By the way with trim down I can warm up for ten minutes and still won't idle.
So at 1/4 trim I idle away, start to accelerate and trim down to get boat on plane and all is well, boat runs perfect. Can run at 55mph no problem. Talked to a few dealers and they say that's not normal but are puzzled as to how trim effects it. Just looking for ideas or should I not worry and just continue what I am doing.

 

[NoNotMe]

carb floats?

 

[ForbesZ]

carb floats?

I agree that's definitely something to check. I'm not that familiar with those Johnsons but if the carbs have pilot jets on them they could be set too rich also (in which case you could try closing them a bit to see what happens). I would start by getting the carbs cleaned though, because if there's any dirt or varnish in them it could be causing the problem you're having.

In general with carb motors the force of gravity affects the fuel flow into the motor so when you change trim angle it can change the amount of fuel that "falls" into the motor if the carbs aren't tuned right or if excess gas is getting past the seats of the jets. In your case it sounds like when the trim is down there is too much fuel getting through the carbs and falling into the motor but when the trim is up the excess fuel has a harder time going uphill to get into the motor so it tends to idle better.