90HP V4 tune-up/ignition questions

boomtime · May 7, 2008

I recently picked up a small CC with a 1990 V4 90HP Johnson 2-stroke outboard on it. Im in the process of "improving" the boat with things like paint, new throttle/shift cable and new side mount BRP controls (these items are already replaced).

The other day i was out runnin around and noticed the motor started missing while i was cruising along at about half throttle. It was audible that the motor was not 100% there so i brought it down and let it idle a little and then hit it again and it started missing again. I drove it back to my dock this way without a problem and at this point im assuming its ignition related based on how the problem manifests itself. To that end i decided i would just replace the plugs and wires anyway to see if that remedies it and if that doesnt work i will probably buy 1 new coil and swap one side first to see if that remedies the issue.

Am i on the right track with my troubleshooting?
Anyone have any tricks or tips to offer on this particular motor for performance issues.

jtexas · May 8, 2008

Re: 90HP V4 tune-up/ignition questions

welcome to iboats!

Back to basics.

First, compression test.
Second, spark test. Set your tester to 7/16" and look for strong blue spark.

Spark plugs should be champion QL77JC4 at .030

Third, fuel. Should be *fresh* 87-octane gasoline with no water or debris in the tank. The fuel line should hold pressure. The tank vent must be open and clear. Try pumping the primer bulb when it's acting up; if the bulb collapses, you have a restriction in the tank. If it stops missing, rebuild the fuel pump.

If all the above checks out, rebuild carburetors.

In addition, highly recommend you test your overtemp horn: find the tan wire coming out the cylinder head (you may have one on each side), peel back the sleeve and touch the bare wire to the head with the key "on" - the horn should sound a continuous tone.

Also, if you don't know the age of the water pump impeller, this would be a good time to replace it.

tashasdaddy · May 8, 2008

Re: 90HP V4 tune-up/ignition questions

totally agree, trouble shooting time. don't go throwing parts at it. get an OEM manual at outboardbooks.com or ebay.

boomtime · May 8, 2008

Re: 90HP V4 tune-up/ignition questions

ok well i was planning to do plugs and wires anyway so they are already on the way. when changing plugs i plan to compression test each bore so i will post the results this weekend.

boomtime · May 9, 2008

Re: 90HP V4 tune-up/ignition questions

here are some pictures of the tuneup i did 20 mins ago. the compression test wasnt perfect but it isnt terrible i realize all bores should be within a certain percentage or at least close. i dont know the cylinder numbers but i drew a little picture of the motor and used the psi results as markers. here are some pictures of the plugs and wires. anyone know the cylinder order? im gonna make a small diagram in paintshop and post it up later but im late for the bar right now gotta run.
TIA

iwombat · May 9, 2008

Re: 90HP V4 tune-up/ignition questions

Cylinder numbers are (as looking from the back at the heads)

2 1

4 3

Did you do the compression test with all the plugs out? It's always suspicious when numbers creep up or down as the test proceeds.

HighTrim · May 9, 2008

Re: 90HP V4 tune-up/ignition questions

Also, throw out those NGKs and get Champion QL77JC4's. They are what your outboard was designed to use. Before chucking the NGK's, use them to do a decarb (see the FAQ). Then replace them with the Champs.

jtexas · May 9, 2008

Re: 90HP V4 tune-up/ignition questions

NGK makes a great plug, but never in a johnnyrude - nothing but champs.

You want to see all cylinders within 5% of the median, with no more than 10% variance from highest to lowest. Yours are -8, -4, +4 and +16, with the worst being 21% below the best. Wouldn't hurt to repeat the test, just to be sure, but I wouldn't spend any more money on it before finding the cause of the poor compression.

Besides having the plugs out, make sure your battery is fully charged.

ledgefinder · May 9, 2008

Re: 90HP V4 tune-up/ignition questions

Run the starter through several complete revolutions (5 or so typically) until the pressure reading maxes out.

If you were already doing that - bad news - sounds like you have scored cylinder walls (or at least sticking rings).

All of which may have nothing to do with the miss you noticed.

The only way to really diagnose the miss is on a dyno or test prop, so you may have to bring the boat in to the shop. Could be one of several things, or a combination of them. I'm leaning (guessing) ignition, but have to ask, does the CC have an in-hull fuel tank that may have sat for a while with dirty, old, gummy fuel in it? That may be your problem right there. If so, probably want to clean those carbs, through an in-line filter in, then make sure you're running fresh fuel in the future.

The NGKs are fine (as are AC, Bosch, etc) - there's nothing magic about Champions and OMC does not "design" the motor for Champions.

boomtime · May 10, 2008

Re: 90HP V4 tune-up/ignition questions

iwombat said:
Cylinder numbers are (as looking from the back at the heads)

2 1

4 3

Did you do the compression test with all the plugs out? It's always suspicious when numbers creep up or down as the test proceeds.

i re-screwed the old plugs in by hand after labeling them in to maintain equal test results.

HighTrim said:
Also, throw out those NGKs and get Champion QL77JC4's. They are what your outboard was designed to use. Before chucking the NGK's, use them to do a decarb (see the FAQ). Then replace them with the Champs.

i already ordered and installed those copper champs your talking about.

ledgefinder said:
Run the starter through several complete revolutions (5 or so typically) until the pressure reading maxes out.

If you were already doing that - bad news - sounds like you have scored cylinder walls (or at least sticking rings).

All of which may have nothing to do with the miss you noticed.

The only way to really diagnose the miss is on a dyno or test prop, so you may have to bring the boat in to the shop. Could be one of several things, or a combination of them. I'm leaning (guessing) ignition, but have to ask, does the CC have an in-hull fuel tank that may have sat for a while with dirty, old, gummy fuel in it? That may be your problem right there. If so, probably want to clean those carbs, through an in-line filter in, then make sure you're running fresh fuel in the future.

The NGKs are fine (as are AC, Bosch, etc) - there's nothing magic about Champions and OMC does not "design" the motor for Champions.

yes im familiar with how to use a compression gauge...basically rotate the engine until the gauge stops going up. as far as the "bad news" what is the typical psi for a fresh bore on this motor? im not all that concerned about these readings yea they arent that close but i think if a particular bore was toast it would be reading much lower from my experience. Judging by the condition of the plugs and wires id say that was the cause but we will find out for sure tomorrow since the wind is supposed to lay down and ima take a crack at the reef.

The-Machinist · May 10, 2008

Re: 90HP V4 tune-up/ignition questions

The NGKs are fine (as are AC said:
Woo boy, Thats a bold statement around this forum You may want to invest in a pair of chain-mail undershorts

tashasdaddy · May 10, 2008

Re: 90HP V4 tune-up/ignition questions

Hang MOM from the highest tree, use champions. others are known to cause missfiring problems. if you go to NAPA, and look up the other brands, there is a note or their computer, that says this.

iwombat · May 10, 2008

Re: 90HP V4 tune-up/ignition questions

"i re-screwed the old plugs in by hand after labeling them in to maintain equal test results."

Well, that explains the numbers. Take all the plugs out and leave them out while checking compression.

boomtime · May 10, 2008

Re: 90HP V4 tune-up/ignition questions

heading to the reef...hopefully the problem is cleared up.
high tide - 1:11pm
low tide - 8:44pm

here is a diagram ive fashioned to show the results anyone who likes coloring books or just wants to help can edit it to show proper cyl #
TIA

edit: yea i realize its a flywheel not a crank pulley so spare me that fire.

jtexas · May 10, 2008

Re: 90HP V4 tune-up/ignition questions

my own experience with NGKs - the center electrode wore down I mean fast - replaced 'em in 6 months. I get 2 or 3 years per set of champs. I'm not saying NGKs are bad, just bad for my motor.

The spark test I described above - spark plugs can fire outside the cylinder then fail under compression - but a spark across a 7/16" gap is a good indication that's its strong enough.

boomtime · May 10, 2008

Re: 90HP V4 tune-up/ignition questions

can the water pump/impeller be changed with the boat in the water and engine raised?

jtexas · May 10, 2008

Re: 90HP V4 tune-up/ignition questions

it's possible - if it's where you can stand behind it to remove the lower unit. I wouldn't do it - too prone to dropping things.

boomtime · May 11, 2008

Re: 90HP V4 tune-up/ignition questions

it has now become a question of identifying the motor's year and exact type. I know its a 90hp, a V4 and i was told its a 1990 model. from what ive seen online the 1990 model has a slightly different cowling than my motor.

any ID inside the cowling like location of a serial or block/engine code?

HighTrim · May 11, 2008

Re: 90HP V4 tune-up/ignition questions

You will need the model number. It will be on the ID tag on the stern bracket. If the tag is removed, look for the welch plug, it is about the size of a quarter and usually on the top rear of the motor.

boomtime · May 11, 2008

Re: 90HP V4 tune-up/ignition questions

what is the welch plug? there is no tag on the stern bracket.

90HP V4 tune-up/ignition questions

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