'92 johnson 8 hp won't plane off 12' jon boat

[wesman]

Man, I read the thread posted in May '06 by silverwater, and darned if it isn't EXACTLY the same as my issue. She runs like a top at idle, but at high speed it just doesn't quite get to full rpm. It seems to me it's running too rich, but there is no high speed jet adjustment on the carb. The carb is rebuilt, it has a new fuel pump, new plugs and wires, and I've adjusted the throttle cam and butterfly valve stop to the best that it'll run, but it still seems way too rich. Any suggestions? Unlike silverwater, I can't take it back, so I need to make it go.

 

[R.Johnson]

Re: '92 johnson 8 hp won't plane off 12' jon boat

I would not expect a 8 horse to plane that boat, how much do you weigh, and how much of a load are you carrying?

 

[LORDY611]

Re: '92 johnson 8 hp won't plane off 12' jon boat

I have a 14 foot semi-vee with a 9.9 which planes immediately. I taught my kids to ski behind it. I would think an 8 should do the job on the 12 footer, just maybe a bit less zip. Payload will definately play a role, though.

 

[R.Johnson]

Re: '92 johnson 8 hp won't plane off 12' jon boat

You can't compare a 9.9 to an 8, all together different engines. That 9.9 is closer to 15.

 

[steelespike]

Re: '92 johnson 8 hp won't plane off 12' jon boat

Unless Wesman is a ton that motor should plane easily.Maybe a little weight distribution or possibly a tiller extension to help.
Check the plugs,Check the compression.
Pull one plug wire at a time and see how it runs.
If it still has its original coils one or both could be weak.
Let us know what you find.

 

[itstippy]

Re: '92 johnson 8 hp won't plane off 12' jon boat

When the motor's tuned to specs and you can't get the right RPM's it's time to look at "setup". If the boat's not set up right your motor will work like He** trying to get up to peak RPM's. It'll burn gas like crazy, carbon up like crazy, and age like crazy. It may seem like it's running too rich. Weight distribution in the boat is your first step. You need to get weight toward the front enough that it's evenly distributed. Get a longer fuel hose and move the gas tank forward. Move the anchor and cooler forward. Etc.
Also make sure your motor's anti-cavitation plate is within an inch of the keel. Jon boats often have low transoms and the motor sits too low. If this is the case you have to add stout lumber to the top of the transom to get more motor height.
Traveling light, I plane out a 12' Ouchita Jon with my 1975 4hp Evinrude. I move right along. If I add a buddy in the boat we go nowhere fast, she works like a demon just churning water, and will not get to top RPM's. She won't plane. She turns into a barge. I just throttle down and let the boat go at her displacement hull speed of about a walking pace. Why burn up my prize 4hp trying to plane an overweight barge?

 

[Scaaty]

Re: '92 johnson 8 hp won't plane off 12' jon boat

Sold my neighbor a 4hp for a 12 foot aluminum rowboat and I get it on plane with 2 cinder blocks in the bow. 8 should have zero problem

 

[iwombat]

Re: '92 johnson 8 hp won't plane off 12' jon boat

Is it a long shaft motor on a short transom by any chance?

 

[wesman]

Re: '92 johnson 8 hp won't plane off 12' jon boat

Yep, no one can agree, but I used to plane it just fine with a '77 6 hp, so the weight dist., etc. can't be the problem. It's a short shaft. Both cylinders are firing and have good compression. The coils are the only thing I haven't changed. S'pose I'll dump that much more $$ in and see if it helps, but from what I've been seeing I'm not hopeful. Thanks all.

 

[tashasdaddy]

Re: '92 johnson 8 hp won't plane off 12' jon boat

have you check compression. i had to put remote controls at the middle seat on my 14' w/ 15 rude. could not plane with tiller control, all it did was stand on the stern.

 

[steelespike]

Re: '92 johnson 8 hp won't plane off 12' jon boat

What makes you think it is too rich?
If it was rich it would be smokey and it probably would four stroke.(sort of a stutter or shudder)
If it was lean it would tend to stumble or bog and feel weak.It would usually pick up with a little choke.
Usually you can tell how it is burning by looking at the plugs.Black and furry or looking to oily if too rich,nice clean black/brown if right .If it has a nice blue spark you can hear snap then the coils are probably good.
By the way You could have water intrusion and still have good compression.If one plug seems extra clean or maybe a little rust you may have a little water getting in that cylinder.
Does it start with one or two pulls when cold?
Does it stay running or require repeated choke to stay up till fully warm?
How does it respond to a little choke at speed?

 

[wesman]

Re: '92 johnson 8 hp won't plane off 12' jon boat

Spike, it is a touch smokey and shudders sometimes (but then it will smooth out). Both plugs look a little wet. There isn't any sign of water or rust in the cylinders. She starts on the 1st or 2nd pull every time and idles great w/o repeated choking. The plot does thicken a little bit here. I neglected to mention that it was running horribly, and when I took the air filter off to get to the adjustment screws it ran 100% better (but stil not right). That's was made me think it's too rich. Choking at speed only makes it worse. The coils may be weak, but would that cause such a problem?

Thanks for all the help so far.

 

[Chinewalker]

Re: '92 johnson 8 hp won't plane off 12' jon boat

Is it running on both cylinders? With the motor running, pull one of the plug leads and note what happens. Nothing - the motor is running on the one still attached. Motor stops - the motor was running on the one you pulled. Motor drops RPM, then you were running on both...

- Scott

 

[steelespike]

Re: '92 johnson 8 hp won't plane off 12' jon boat

If it starts good cold than it isn't likely the spark is weak.
The spark should be blue and you should here it snap.
Idle speed is 650 to 700 rpm.Mix screw should be adjusted warm in gear unrestrained.
Get a manual and check the ignition testing procedures.It seems that a good spark at cranking speed doesn't mean it is good at speed.
Check the flywheel key if the key is damaged or the keyway damaged it will hinder proper spark developement.There is a fairly complicated spark output test procedure.
I assume this has some sort of recirculation set up. Perhaps it is dumping too much waste fuel back into the system.I don't have high speed jet specs for the 92s but the 89s have a # 40 high speed jet.Maybe someone thought a bigger jet would make a 9 hp.
Is that an air filter or a silencer if it is a filter it may be damp with oil or some sort of moisture if it is a silencer perhaps someone is living in there.
I think I would go through the ignition testing procedures as best you can and check the flywheel key.
let us know what you find.

 

[tashasdaddy]

Re: '92 johnson 8 hp won't plane off 12' jon boat

http://www.iboats.com/bbboard/bbBoard.cgi?a=viewthread;fid=28;gtid=818798;gpid=818802