90 Johnson 120hp V4 Loses Power at full throttle

Kmoonster · Jan 16, 2002

Help! I've been on the lake 3 times tring to figure it out. Last time this happened it was a power pack. Idles fine and revs up great. When it's cold it runs down the lake fine about 5-7 minutes at full throttle (5800 rpms) it dies down to about 3200 rpms. If you stop rev it up w/o a load on it it revs fine. I've checked the fuel line bulb and tired to pump it while experencing the problem but no change. just bought the manual today but where do I start. is there any way to check the power pack w/o a spark tester for all 4 cylinders at the same time? any ideas or help will be appreciated. Thanks for any help. Keith

Jack B · Jan 16, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

Keith, see the posts 2 up from yours "88hp loses power at full throttle. It is my post and I think I am close to solving the problem. I will let you know in a day or two Jack

Kmoonster · Jan 16, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

Hey Jack any info would be appreciated, I've had these symtoms before and it just feels the same. I'm going to do some of the test in the manual to check out the ignition sytem 1st where is your problem area? I'll definately post my results from what I here Johnson 120's are famous for eating power packs and it was about 3-4 yrs. ago when I replace the power pack. So I'm starting there 1st I guess, then to the fuel thing next after a compression check. I'll let you know how I come out

Jack B · Jan 16, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

Keith, I am pretty good with the fuel systems but the electrical is a different world to me. Please let me know if you come up with a solution and I will do the same. Thanks, Jack jbud@knology.net

petryshyn · Jan 16, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

Hi gents Use the search feature above and do a search on S.L.O.W. then do one on SLOW good luck...

JB · Jan 17, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

Ahoy, Moonster. After you've done the research that Schematic suggests: If you suspect that you are getting partial ignition failure due to heat, try heating the powerpack with a hair dryer while under test on the "ears". Revving an engine while not under load means nothing. Those V4s will rev beautifully on 2 cylinders and sound like 4. Red sky at night. . . JB

Philby · Jan 17, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

Moonster Try the hairdryer on the powerpack trick You may look like a complete idiot, but get the powerpack good and hot and see if it fails. If not, try lipstick on the stator. Won't do any good but it will look nicer !

Kmoonster · Jan 17, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

Update!!! Got home late to night so I was only able to conduct a few resistance checks so I just started with a couple short ones. The Senor Coil Resistance check is good w/ no grounds. so I moved on to the Charge Coil resistance check it was 348 ohms and the tollerance is 480 (+/- 25 ohms), I ran out of light before I got to check the do the ouput test. I've got a couple of theory's. I found out that if you have a bad coil or a stator it can send spike voltage back to the power pac and blow it. Yep, schematic that's my next test after this one, I have a question? is there any way i can get a DVA or peak reading here at home, all I have is a spark gap checker and a 500 Volt multimeter any suggestions. I'll update again this weekend, hope the weather holds out. Thanks Keith

petryshyn · Jan 17, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

To answer your question,.....gotta have a DVA adapter, Peak to Peak meter or an oscilloscope to measure charge coil spikes... I don't believe you have a problem caused by spikes. Spikes cause terminal problems (unit dies and does not recover). You have an intermittent problem possibly heat related. 95% of electronic failures are solder joint failure. I would do JB's hair dryer suggestion to see if you can aggravate the problem with either the pack or spark coils (heat up both, one at a time) If I was a bettin' man I'd bet on a failure of the pack you haven't changed yet. I'm not a bettin' man, so I have to see it fail first before I replaced it! Good luck!

Kmoonster · Jan 17, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

can the low ohms on the charge coil test cause excessive charging voltage and cause the power pack to fail, like I said If the pac is the problem this will be the 3rd replacement. the other 2 lasted about 4 yrs each or is that normal replacement time of a pac for this model?

petryshyn · Jan 17, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

>what kind of ohmmeter are you using? >have you tried another ohmmeter? >whats the temperature of the coil when you are testing it? (approx.) >is it the same side pack each time?

Kmoonster · Jan 17, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

Cheap Fluke, I don't have another one trying to borrow one. The temperture was cold iron, hadn't been started in a few days. and my understanding my model only has one pack for all 4 cyls. If I'm reading the manual correct this Charge coil test is testing back toward the stator itself. I figured with a degrade ohms value would allow less voltage drop therefore allowing more voltage to the pwr pac and possiblity burning it up. Going to test the charging coil output tomorrow, it states the output should be 175 Volts while craking over the engine.

petryshyn · Jan 17, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

Is there such thing as a cheap fluke!....LOL >One possiblity for your low resistance reading is the cold temp of the coil. Its not that far out of spec. >Sorry, Kmoonster, a partial short (as you suspect) would cause low voltage due to less 'effective' conductor exposed to the rotating flywheel. >would be nice to know if the same cylinder failed in each pack. Then we'd change that coil for fear of a primary short. >just remember, if you test the charge coil with a DMM without an adapter, it will read low and mislead you..... good luck!

Kmoonster · Jan 17, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

If that's the case then sounds like my charging coil is ok, does that mean the stator and sensor coil are also good or are they 2 seperate things? I think I'm heading in the wrong direction, my gut tells me it's a coil or power pack or both that breaks down after it get warm, is there anyway to test the coils other than with a spark gap tester and a resistance check? The only thing is it'll run fine all day at idle will I be able to find the problem without the moter under load? tomorrow I'm going to do the SLOW test and resistance checks on the coil, alone with putting the spark gap tester on each cyl, and just see if I can find a weak firing cylinder, what do you think any recommendations? Thanks Keith Now I see why they pay the Mechanics so much,,LOL

petryshyn · Jan 17, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

The 'stator' refers to an electrical winding which resides under the flywheel. There can be many... >battery charging system stator >charges the battery >CDI charge coil >charges up the capacitor for ignition >trigger coil >tell the CDI module when to dump >Secondary trigger coil >used for timing advance >field control coil >used for regulation of the charging system >probably a few more on 4-strokes used for crank position Each can be its own individual coil with its own potential problems... 1) at idle, aggrivate coils and pack with hair dryer. if it starts missing, you are in the right direction 2)when engine warm, check coil resistances 3)take for ride until fails. check to see if engine overheating or hot. feel heads. 4)disconnect SLOW feature and re-test(unplug the 2 sensors, one on each bank) 5)if still missing, unplug each coil wire primary, one at a time, retest to see which one is not contributing. Good luck...

Kmoonster · Jan 18, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

Man this thing is kicking my but!!! LOL 1)Tried to aggrivate the problem on the coils and pack with hair dryer. it had no effect. 2)After it was warm i checked the coil resistances, all are within specs. 3)felt the heads and they seem to be cool, doesn't seem to be over heating but that's just here in the yard, I'd have to drive a ways to put it in the water. 4)conducted the SLOW checks as per the manual and it checks out to be good, reduces RPM's when I ground each tan wire coming from the pac (the one that goes to the temp sensor one each head) reduced RPM's down to 2500. also did assoc. resistance checks for it and they were within spec's. Any more suggestions? I'm really confussed. Where can I buy a DVA adapter for my multimeter, guess that's my next step, that or just buy the power pac and put it in a try it. Help

Jack B · Jan 18, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

break down the fuel pump it is simple and fast. Also, check the tank vent and the check valve on the fuel line. Jack

petryshyn · Jan 18, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

take it for a buzz, when it acts up, disconnect sensors to be sure SLOW is not kicking in with warmer engine.........

Terry H · Jan 18, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

Kmoonster...I had a similar problem last year with the same motor...I tried everything to get it to run properly...someone said I should replace the old vro pump with a new one...I did and it fixed the problem...by the way that's a great motor...just a Thought

Jack B · Jan 19, 2002

Re: 90 Johnson 120hp V4 Loses Power at full throttle

I had the same symtoms as you and today I fixed it by rebuilding the carbs. I found a piece of the fuel pump diaphram in the needle seat. This was allowing to runup to about 2/3 speed and then it would starve itself for fuel. Runs like a champ. Jack

90 Johnson 120hp V4 Loses Power at full throttle

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