120 Johnson high rpm cutting out problem

[Nodakr]

After a cold startup and at 5000 rpm running for a short time my engine starts shuttering and will only throttle to about 2000 rpm.I found that if I shut off the engine and restart it the problem goes away and may not return at all the rest of the day.I've tried to return back to idle and then to full rpm but it still shutters at about 2000rpm every time.the only thing that makes it stop is when I shut off the key and restart the engine.I will also add that it starts very hard after restarting after the shutting happens.I've done a search and don't seem to see my problem listed.Any help will be greatly appreciated!!!!

 

[OBJ]

Re: 120 Johnson high rpm cutting out problem

Hi nodakr...welcome to the forums...<br /><br />Sounds like (possibly) a problem with the S.L.O.W. system. This system automatically slows the engine down in case of overheat....if the engine is equipped with the system.....<br /><br />Could you post the year or model number please?<br /><br />Thanks.

 

[rickdb1boat]

Re: 120 Johnson high rpm cutting out problem

overheating per obj....water pump time and maybe t-stat

 

[Nodakr]

Re: 120 Johnson high rpm cutting out problem

Sorry it took me so long to respond,I took another trip to the river Sat. This trip,after going about 5 miles down river and moving around several times,then heading back to the boat ramp it happened again,running at 5000rpm the motor starts shuttering.I shut off the key,this time the motor started right up like it does most of the time when this happens,sometimes it seems to restart hard,sometimes not.OBJ,I think I do have the S.L.O.W. system,the motor is an 1989 120.When you guys mentioned it pos.being overheating,I paid particularly close watch on weather or not I was getting a normal amount of return watercooling and had my passanger watch for it on the trip out and back and it seems to be squirting out return water just fine.Don't know alot about it but the system does have a beeping system for overheatingand low oil warning,which seems to work ok cause it has let me know when I was low on oil before.A friend of mine at work said he had this happen with his motor a long time ago and he said his neutral switch was sticking,any ideas of that pos.being one option??Thanks in advance for your help in any recommendations,Bruce...

 

[Dhadley]

Re: 120 Johnson high rpm cutting out problem

I would agree that its going into SLOW. Perhaps a faulty heat sensor, perhaps a buildup of scale or corrision in the head. My Intruder did exactly what your motor is doing (went into SLOW without the alarm sounding). We verified that it was not overheating. I happen to have another sensor but that was not it. I removed the heads and cleaned them out and all is well.<br /><br />Just a thought -- if 5000 is top rpm thats putting quite a strain on the motor which causes heat. That combined with something like the wrong spark plugs might possibly be enough to trigger the one side of the sensor.<br /><br />Good luck!

 

[tony120]

Re: 120 Johnson high rpm cutting out problem

i have that same year 120.runs super, my s.l.o.w. works also. but with a big bore looper like we have,theirs less block material around the sleeve.mine wore away right to the sleeve.marine tex'ed it and it got me through the season.and still runs strong.I would check to see how much is left around your sleeves.the gaskets are cheap.maybe thermos are stuck with crud.just some ideas..keep us posted

 

[Nodakr]

Re: 120 Johnson high rpm cutting out problem

Just got back to my post,sorry it took so long,Dhadley,now that I think back,it seems that I didn't have this problem until I changed the plugs last fall.What brand and type would you guys suggest?? When I bought this boat and motor about a yr ago from a neighbor down the street,he told me that it had been overhauled about a 100hrs before I bought it.The guy seems very onist(sorry for the spelling)besides knowing that I lived just a couple houses down from him,I don't think he was lying to me.If it is corrosion in the head,could I run an additive to see if it makes any difference,I should add that I'm running the highest grade unleaded that the station has and that I'm running arco fuel,Maybr I should try a diff.Brand??? My plugs do seem to fowl Quit often,but I thought maybe that was because I fish sometimes pretty close to the boat ramp and don't always get a chance to run at higher speeds for any length of time???

 

[Dhadley]

Re: 120 Johnson high rpm cutting out problem

Ah, high test fuel. Todays high test burns hotter than the recommended 87 octane. Regardless if thats the cause or not we need to get back to the 87 that OMC/Bombardier recommends. I use the Champion QL77JC4 plugs in mine. When alls well it should idle for hours without fouling and perform flawlessly. <br /><br />That may not cure the motor going into SLOW but it'll run better. We may still have to soak the heads to clean out the water passages. <br /><br />Good luck!

 

[Nodakr]

Re: 120 Johnson high rpm cutting out problem

D,thanks for your help,I just got out of the shower and am on my way to work,get back with you later,BIG THANKS AGAIN FOR YOUR HELP!!!!!

 

[Dhadley]

Re: 120 Johnson high rpm cutting out problem

Work??? Wait -- we gotta go boating!! Er....uh.... I mean testing.<br /><br />Keep us posted!

 

[Nodakr]

Re: 120 Johnson high rpm cutting out problem

Hi,Guys,sorry again for being so slow to reply.I bought a set of plugs yesterday(QL77JC4) I'll stick them in before I head out pos. tomorrow on a fishing adventure.I'll also change back to 77oct.fuel and see what happens.If you guys ck my profile you can see I have mech.skills but have not done much outboard repairs,I tore down an old 40hp evinrude several yrs ago to change to over to a long shaft and did a few other things,like w/pump,but that my extent as far as working on outboards.Do you guys think maybe I should do a test on the slow censor or is it cleanable?I need to get out the shopmanual as soon as I can locate one I guess!!Thanks again for your HELP!!! I'm on my way to a parrot show so I'll get back here asap when I get home...

 

[Nodakr]

Re: 120 Johnson high rpm cutting out problem

Ops!!!I mean 87 octane!!! Just a note to add..When a bought the boat two yrs ago,I could swear I read a post on the local fish site here in Portland that on the bigger motors you should run a higher octane fuel??Not that i'm doubting any of guys posting!!Beleive me any help is greatly appreiciated!!!!Also,if I were to take my lazer temp gun with me on my next trip out,could I be fast enough removing the cover and taking a temp reading on the heads to tell for shur if I am truly running hot or not???May sound stupid??? Give me your opinions!!!

 

[Solittle]

Re: 120 Johnson high rpm cutting out problem

Ckeck here for manuals<br /><br /> http://www.theoutboardwizard.bizhosting.com/parts_service_and_owners_manuals.html <br /><br /> - The OMCs are the best - get Parts Manual while you are at it.