proper prop size for johnson 150 hp and rpm loss after one year of use

[gw gary]

My boat is a 2000 Alumacraft Tour Pro 175 and has a Johnson 150, currently has a 14 3/4 X 23 aluminum prop, and I can only get 4200 rpms at wot and 45 mph loaded w 4 people. I switch to a 14 3/4 x 21 and I get 4400 rpms and no change in speed with 4 people. Model#J150 PLSIG ser#G04939705. The motor seems to want to run faster with either prop, it kicks in for a few seconds and then drops back, it kind of surges forward and back down. Had it rejetted and it didn't help and run seafoam though and put in new plugs with no help. This motor should run at 5500rpm. I didn't pay attention when it was new, what the rpms were but by sound I think they were higher. It use to go 55mph the first year loaded with 4 people and 58 mph when I was alone.

 

[hondon]

Re: proper prop size for johnson 150 hp and rpm loss after one year of use

Sounds like you are dropping a cylinder to me.Why the carb rejet?Seafoam is nice maintenance but usually solves few problems.Fuel restiriction? Crappy fuel?Dropping spark or intermittant spark from anything from a crappy plug boot to a faulty power pack or timer base.That ought to get you started.

 

[gw gary]

Re: proper prop size for johnson 150 hp and rpm loss after one year of use

New fuel and all hoses seem fine no pinched lines and all vents seam clear. New plugs didn't help and the spark plug wires and boots look fine. Do I take it to the dealer to have the power pack and timer base checked? Had it out this weekend with new plugs and still only 4300 rpm's. If I over trim I can get 4600 rpm's with loss of speed.

 

[hondon]

Re: proper prop size for johnson 150 hp and rpm loss after one year of use

You did'nt answer my question.Why the carb rejet?Are you trailering this engine to Tahoe from Redwing Minnesota?Why ,why why?

 

[gw gary]

Re: proper prop size for johnson 150 hp and rpm loss after one year of use

Before I had this problem the motor would not start easy when warmed up and shut off for awhile, the dealer said this would fix the problem and it did, but did it create this one?? Embarrassing with a new boat and motor that you had to crank and crank to get going. Cold start was fine. What is the proper prop size for this motor?

 

[gss036]

Re: proper prop size for johnson 150 hp and rpm loss after one year of use

I am no mechanic, but if your problem started after rejetting, I would bet your mechanic put in too small of jets and you are starving for fuel. Otherwise, like Hondon said, you are losing a cylinder at higher rpms. Could be a power pack, I am not a mechanic. Just my thoughts. I would check the jets. Sounds like lack of fuel.<br />I say that because I put too small of a 4 barreled carb on a Dodge engine and it lost the top end because of not enough fuel getting into the intake manifold.

 

[Hooty]

Re: proper prop size for johnson 150 hp and rpm loss after one year of use

Get yourself a spray bottle and put some premix in it. Next time out, when you're at wfo and at 4200rpm, give each carburetor a spray or two of fuel and see if, when you spray one of the carbs, it picks up revs for a minute. If it does, that's the carb with the stopped up high speed jet. Also, if the revs pick up, take it back and have your old jets reinstalled. I'm with hondon, I don't think they needed rejetting. About the only time ya gotta do that is when making a drastic change in altitude.<br /><br />g'luck & c/6<br /><br />Hooty

 

[gw gary]

Re: proper prop size for johnson 150 hp and rpm loss after one year of use

found a loose ground wire now I have the boat that I bought.