1991 175 hp johnson

"I have a 175 johnson that idles fine and reaches 6500-7000 rpms, when I run it in the it wont go higher than 3.5 rpms. Now someone told me i had the wrong prop on it The engine is 175 the prop is from a 150.. true or false? Also while i ran the boat i was using an external tank not the one in the boat it would also stop runnig and when I looked at the primer bulb it was completely flat."

"If the bulb is flat you have

"If the bulb is flat you have a fuel restriction between the tank and bulb. Possibly a failed anti-siphon valve. Your first sentance seems to be missing some words, but I am gathering that you can rev it to the moon on muffs (bad idea), but only hit 3.5k while underway.

The prop is the match between the boat and the motor. They are availible in a wide range of diameter and pitch to suit everything from a 10,000lb work barge to a 500lb tunnel hull. The right prop is the one that gets you near the top of your full throttle rpm range (5500) with a light load in the boat. There isn't a specific prop for a specific horsepower engine once you get above 10-15hp. In your case, if the engine is operating properly, you are seriously over propped (too high of a pitch)

An interesting note on the 1991 version of the 60 degree Eagle V6. OMC built that motor hot! it sits right on the top edge of the 10% rule for horsepower while in the 4500-5500 recommended operating range. However....if you run it a little over 5500, it can make close to 200 HP!!! That is until Merc started whining in 1992 and the motor got tamed a bit. Don't go nuts with it, but 5700 with a light load in the boat isn't going to hurt anything."

"That is great info, and I tha

"That is great info, and I thank you very much. So just to be clear I might have the wrong prop? I guess I sould have mentioned it before but it is a 20 Proline center console."

"Correct, IF.......IF the engi

"Correct, IF.......IF the engine is running properly you have the wrong prop. What prop do you have on there right now?"

I borrowed one from a friend

I borrowed one from a friend he had it on a 15ft bass boat 150 johnson. I want to say its a 15 3/4 but not sure. Someone is selling me a stainless prop 17 3/4 x 14 if that sounds correct.

"14 3/4 X 17 most likely. Prop

"14 3/4 X 17 most likely. Props are marked diameter X pitch. A 17" diameter prop won't fit on there. That size would be about right to a bit small for a 20' bass boat. If you center console isn't substantially heavier you're probably in the ballpark prop wise. If the one you have on there is a 15", going to the 17 will hurt instead of help. For that engine, most everything will be around 15" in diameter. It's the pitch size, the second number that you need to look at.

Did you just get this boat, do you have any history as to what might have been on it before? You're sure the engine is operating properly?"

"yeah got it a few months ago

"yeah got it a few months ago and repowered it. It never had a prop. The lower is a 150 the top is a 175. Carbs cleaned, new spark plugs and so on. Im thinking it will be the fuel valve. I will check it in the morning. Would the lack of fuel keep it from reaching top?"

"while running on the water, y

"while running on the water, you might also find that your boat runs a little higher rpm if you trim up the motor a bit, which will lift the front of the boat slightly."

"Props are very complicated I

"Props are very complicated I would think you should be around 23 degrees the rule of thumb is for every 2 degrees you + or - 4 hundred rpm's
that prop would be a good power prop for say towing a boat back to dock. In my bass boat my spare is low pitch like that so i can bring back a stranded boat. It is possible that he does that too. I have attached a prop calc below which will help you see the relation but 5500 rpm's is what you should top out at with the correct prop.
Prop Calc."

"A lack of fuel will definatel

"A lack of fuel will definately slow you down. A lean mix can burn holes in pistons and do all sorts of nasty stuff. Generally, the engine will have a "rap" or "knock" while running that lean."