1989 Johnson 90hp running rough

[spdjnky_42]

Just wanted to say thank you guys for the help previously I recently bought a 90 hp Johnson titled in 1989. Compression tested good, motor ran good the first day. On day one it would gurgle at 1/4 throttle then come out of it with more throttle and run great. I assumed either just a 2 stroke thing with a v4 or its a little cold here right now (50*F) Then after it warmed up it still did it so I figured normal or low throttle jet issue. Yesterday I was running it and after about 5 minutes WOT it lost 500 rpm and kinda sounded like it was out of fuel then continued to run at 5000 rpm, 1st day it ran 5400rpm. After checking ball was pumped and tank still had new fuel. Now keep in mind this is a jet drive now. So maybe it had something in the shoe. IDK havent drove it again. After backing up and pulling on the boat seemed like maybe it was just something on the shoe. Ill let you know in a few days of the issue condition. I did notice that the left intake hose was spraying a fine mist so is it possible it was getting water in the cylinder? Ill get that fixed and report back. My main question right now is if I need to pull the carbs what does it take to bump this thing to a 115 or 140? Just 115/140 carbs and exhaust manifold along with maybe honing out the intake slightly? Is this a fairly easy job for someone with mechanical skills? Finally what year parts are interchangeable? Sorry for the long post TIA

 

[emdsapmgr]

Re: 1989 Johnson 90hp running rough

The 90 is the same size block as the 115, but it has different porting. You'd need to tear the block down completely and have to get a skilled/knowlegeable machinist report the block. You'll need locate/purchase the two exhaust manifolds and the exhaust filler blocks and have that same machinist install the blocks in the exhaust chest, then deck the exhaust face on a mill. Different heads, 1 5/16" big bore carbs and you are all set. You can do your own parts comparisons at the factory website: epc.brp.com. It's a big ordeal to do all the above work. May be cheaper overall just to purchase a locate a good-running 115. You may find that just the bolt-on additions will provide hp improvements. Additional nice additions are the older (NLA) intake filler blocks from the early 115/140's and a set of composite reeds.

 

[kenmyfam]

Re: 1989 Johnson 90hp running rough

Give it a basic health check.
Compression
Spark
Fuel delivery
Go from there.

Best way to get a 115hp or a 140hp is to buy one in my opinion.

 

[spdjnky_42]

Re: 1989 Johnson 90hp running rough

Thanks guys for the response. I took it out again today. It has an intermittent loss of 800 rpm. I wanted to double check it wasnt the jet getting clogged with debris. It does that when debris gets in there but it also does it when its clean. It will run fine for 20 minutes then act like its running out of fuel then drop the rpms to 4500 from 5300 and thats how it will run. If I back the throotle off a hair it will try to jump back up but immediately goes back to 4500 rpm. This boat had another motor on it before but was only a 50 hp. It ran fine and pumping the ball doesnt help. As stated before it gurgles at low rpms 2-3k. Sounds like someone gargling salt water. As soon as you bump the rpms up above the 2500 mark give or take, it runs smooth. Any ideas? Carb?

 

[emdsapmgr]

Re: 1989 Johnson 90hp running rough

Possible the engine is losing spark to one cylinder. Ignition problems tend to show up once the engine heats up to normal operating temps. You can do a simple test: Next time boating, take an inductive timing light with you. When the engine starts to act up, check the spark on each plugwire.