1991 johnson 90hp

[thosch]

Recently bought a used boat with a 1991 Johnson 90hp outboard. The motors runs pretty good, but I have a few general questions. This is my first Johnson.<br /><br />The VRO was disconnected by the previous owner and a regular fuel pump installed. I took the cover off the fuel pump to check the screen, and noticed that the screw was not tight when I started to back it out. Is it supposed to be tight, or is that screw for fuel pressure adjustment?<br /><br />Checked compression before I bought it. The cylinders in one bank are at 125lb (cold) and the other bank is at 120lb. Possibly a leaky gasket somewhere, or shouldn't I worry about it?<br /><br />Seems to spray alot of water out of the exhaust bypass holes just below the powerhead. Is this normal? My last outboard was a Yamaha and didn't spray any water at all.<br /><br />Any general things I should watch out for with this motor?<br /><br />Thanks for your time in answering some questions for a newbie.

 

[Dhadley]

Re: 1991 johnson 90hp

The screw should be fairly snug. Be careful not to overtighten and crack the plastic. Other than that it sounds like all is well. Compression is OK and it's pumping good water. Run it on the boat and let us know.<br /><br />Good luck!

 

[thosch]

Re: 1991 johnson 90hp

Gave it a good run in the water. Has plenty of power through the whole range.<br /><br />Seems a little cold-blooded. Had to give it the choke button for the first 20 seconds it was running.<br /><br />When warm, it popped and died a couple of times when the throttle was pulled back rapidly. Restarted immediately.<br /><br />4800rpm and 43mph at w.o.t. and best plane, but a different pitch prop would help there. It's pushing a Tracker deep-V hull.

 

[Dhadley]

Re: 1991 johnson 90hp

Sounds good so far, but you are about 800 to 1000 RPM short so a different prop and / or set up change is certianly in order. Don't run it any more without doing something to get the RPM up. Make sure the tach is set on 6.<br /><br />The lean condition at idle doesn't surprise or concern me. Remember, the carbs were designed and jetted around 1991 fuel. It's not that unusual to have to rejet the idle circuit for today's fuel. You are using FRESH 87 octane I hope. And Champion plugs.<br /><br />Keep us informed. Good luck!

 

[thosch]

Re: 1991 johnson 90hp

Thanks for the response. This is great info.<br /><br />The current prop is 13.25x17. What would be a good pitch to get the rpm's up in the proper range.<br /><br />Yes, I have fresh 87 fuel, and Champion L77JC4 plugs were in the motor when I bought it. When I pulled them to check compression, they look pretty new.

 

[Dhadley]

Re: 1991 johnson 90hp

What size boat is it? Even if it's a 21 or 22 I would think you should be getting more RPM's than 4800 with a 17. Are you sure the tach is set on 6? Is it an OMC tach? Are the carbs opening all the way?

 

[thosch]

Re: 1991 johnson 90hp

The boat is a Tracker 18ft.<br /><br />Please explain about the "tach set on 6".<br /><br />The tach was installed at the Tracker factory. Will it say OMC on it somewhere?<br /><br />I'll check on the carbs opening.

 

[Dhadley]

Re: 1991 johnson 90hp

The tach should have a dial on the back that can be set to several different settings. Make sure it's set on position number 6. Unless your boat has water trapped in the hull somewhere you should be getting more than 4800 RPM with your existing prop providing the linkage is set correctly.<br /><br />When you are running can you use all of the trim range? Is 4800 what you get at WOT and fuul trim?

 

[thosch]

Re: 1991 johnson 90hp

When the throttle lever is pulled fully forward, the carb butterflys are almost horizontal. Very slight tilt.<br /><br />I pulled the tach out of the dash. No brand on the case (light green case). There is a black plastic adjustment piece with the remnants of a paper label around it. The pointer on the plastic piece is pointing to "12 pole". A 1/4 turn from that position is "10 pole", another 1/4 turn has another position, but is unreadable. The label is torn, so I don't know if a fourth position is indicated.<br /><br />Regarding the trim, I adjusted the trim through the point where the rpm's starting falling, and then went back up to max rpm. The trim indicator on the dash is slightly above the midpoint.<br /><br />I can't see or hear any trapped water through the floor access panels. The hull looks pretty day on the inside.

 

[thosch]

Re: 1991 johnson 90hp

Back to the boat again. I've done a couple of things.<br /><br />Checked the tach function at different settings. The "12-pole" setting seems to be the only one that gives normal readings at idle. Does this tach seem compatible with the engine? The engine sounds pretty wound up at WOT, and I'm wondering if the WOT tach readings are valid.<br /><br />I noticed that the bulb does not seem very firm when running, and the fuel filter did not have very much gas in it. Temporarily bypassed the fuel filter, and I'm up to 5000rpm at WOT. Also idled a little smoother. I assume that the screen in the fuel pump isn't a good enough filter, and I should check connections for air leaks?<br /><br />I also noticed that grind marks under the prop seem to indicate that I have a rebuilt prop. How would this affect rpm's at WOT?