Johnson V4 120Hp -86 Bogs down/ loses power at RPM above 2500

forsete

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I have been using my Johnson now for many trips this summer, it´s been running flawless until yesterday..
On the way home I cruised at 3000rpm for a while and when I started to throttle up it suddenly started hesitating and lost power, it newer reached 4000rpm and just boged down to even less than 3k. There are some very short bursts whit normal operating but they last for less than two seconds and then bogs down again.

From what I can hear, it sounds "normal" at idle and runs ok at less than 2500rpms.

I did a compression test with the following results: STBD: 2x115 PSI PORT: 2x125 PSI
I have also tested with different tanks wih fuel.
The spark plugs look equal, they are all new for this season.
I tried to run it whilst pumping on the primer bulb, but the issue remained, how stiff should the bulb be when running? I felt it went quite soft for a short period when I was running and pumping at the same time..

What different issues could cause this behavior and what is the most likely cause?
 
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Joe Reeves

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Feb 24, 2002
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1986 120hp Johnson:
PM Problem.... Yes, iBoats did an upgrade of their site program... big mistake in my humble opinion, lots of problems. I gave up on the PMs. Hopefully they get it all straightened out soon.

The compression sounds okay but I have no idea why there's a 10 psi difference between the banks unless you have had a slight bit of milling done on one side.

You don't mention the spark. With all plugs removed, you should have spark that will jump a 7/16" gap with a strong blue lightning like flame... a real SNAP! Does it? Note that the 7/16" gap is important. While checking the spark, move the throttle slowly from idle to full throttle to advance the timer base under the flywheel, if the spark varies while the timer base moves, there may be a frayed wire touching the powerhead somewhere.

Have "all" spark plugs removed when doing the spark test so as to obtain the highest cranking speed.

If you're using surface gap plugs... don't! Use Champion QL77JC4 plugs gaped at either .030 (revised gap) or .040 (original gap).

The fuel primer bulb: The bulb should be able to be pumped up hard and hold that hardness while you hold pressure on it. If it goes down while you're holding pressure on it, either a fuel primer bulb valve is leaking somewhat (not a big deal)... OR... a diaphragm within the fuel pump is leaking, a fuel hose has broke, carburetors are flooding, something of that nature (Yes... a BIG deal!).

The fuel primer bulb is full and hard ONLY when primed. When the engine is running, the fuel level in the bulb drops down to the halfway mark. This is due to the usual position of the bulb which it's simply laying horizontal on the deck or motor well somewhere... the nature of the beast so to say and this does not interfere with the engin's operation. The only way the bulb stays full is if it were secured in an upright position with the outlet being on top.

Let us know what you find.
 

forsete

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Hi Joe, I agree, the update of the site was not an improvement..

I belive it was NGK plugs (not surface gap), recommended for the engine.
Is there any risk of doing damage to any electric components cranking the engine w/o plugs? Some people claim this on forums..
Is the correct way to have one plug connected and just hold it 7/16" away from the powerhead and watch for the sparks jumping from the ground electrode and power head?

Would you think its the powerpack? What is the easiest way to rule the powerpack out?

I will also disassamble the fuel pump tomorrow if sparks look ok on all plugs.
 

Joe Reeves

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I don't like those Jap NKG plugs. OMC recommended Champion plugs.

DO NOT test the spark by using spark plugs in any manner. If you're not using a spark tester, see the following:

(Spark Tester - Home Made)
(J. Reeves)

You can use a medium size philips screwdriver (#2 I believe) inserted into the spark plug boot spring connector, then hold the screwdriver shank approximately 7/16" away from the block to check the spark or build the following:

A spark tester can be made with a piece of 1x4 or 1x6, drive a few finishing nails through it, then bend the pointed ends at a right angle. You can then adjust the gap by simply twisting the nail(s). Solder a spark plug wire to one which you can connect to the spark plug boots, and a ground wire of some kind to the other to connect to the powerhead somewhere. Use small alligator clips on the other end of the wires to connect to ground and to the spark plug connector that exists inside of the rubber plug boot.

Using the above, one could easily build a spark tester whereas they could connect 2, 4, 6, or 8 cylinders all at one time. The ground nail being straight up, the others being bent, aimed at the ground nail. A typical 4 cylinder tester follows:

..........X1..........X2

.................X..(grd)

..........X3..........X4

Best way to rule out the powerpack is to replace it.
 

forsete

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Hi Joe

I couldnt test the spark when throttle is out of neutral since the crank is disabled then.. I guess there is a way around this safety feature?

All ingition cables where tested with the home made variant (but single spark).
All spark plugs where replaced with new and tested to produce a spark.
I ran the boat with a timing light gun connected to each ignition cable and the light was flashing equally between the cables from what I could see.
I disasmbled all carbs and found and cleaned out a small piece of thin black membrane inside the high speed screw in one carb.

The boat still lacks lots of power and barely plane out and can max reach 4k rpms..

What do you think should be next step?
 

forsete

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Update

I checked the new plugs again today and found that the one on STBD bottom stood out from the rest in terms of soot, see comparison picture.

I also disassambled the carbs once more just to blow them with some compressed air as well and actually saw that the floater pin(?) (carb. upper port side) was nicked quite much, It looks like someone really scratched it deep, I can´t see how that would occur due to anything but man-made. This carb does not sit in the oposite side of the spark plug that is fouled. See picture
 
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SilverSS07

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Honestly I doubt that would be enough but I'm not certain on that. I'm chasing down something similar (loss of power) with my motor right now lol. Leaking spark could definitely contribute though. I would check the rest of the wires carefully also. If you haven't tested the spark to jump the 7/16's gap that's definitely the next step. I ordered a power pack last week for mine so am hoping that solves my issue. I tested it with a multimeter (ohmmeter) that showed it had a bad diode. Factory manual says a bad diode can cause it to run rough but doesn't mention anything about loss of power. I'll let you know if that fixes mine as soon as I get it.
 

forsete

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Honestly I doubt that would be enough but I'm not certain on that. I'm chasing down something similar (loss of power) with my motor right now lol. Leaking spark could definitely contribute though. I would check the rest of the wires carefully also. If you haven't tested the spark to jump the 7/16's gap that's definitely the next step. I ordered a power pack last week for mine so am hoping that solves my issue. I tested it with a multimeter (ohmmeter) that showed it had a bad diode. Factory manual says a bad diode can cause it to run rough but doesn't mention anything about loss of power. I'll let you know if that fixes mine as soon as I get it.

Thanks Silver! I will wait for the result!
Hope it solves your issues.
 

forsete

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OK, so the problem is now solved! Engine runs like a crazy cheetah again.

This is what I did, unsure what it was that fixed it.
- Cleaned all carbs again with compressed air through all holes.
- Changed to Champion plugs.
- Changed the ignition cap that gave me a shock when touched (just new rubber cap, re-used the contact inside), suspected a "electric leak" between cap and head.
 

SilverSS07

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Awesome! I just got my power pack and put it on today but didn't have time to take her out.
 

SilverSS07

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Wasn't the power pack. It's odd that it ohm'd bad when on the motor and when I ohm'd it after I took it off it showed good... Oh well guess I have a spare. What I DID find out is that if I run the motor with the cowl off it runs/idles great. Almost threw me out of the boat lol. I think the plug wire is touching the metal latch and grounding it out. Trying to decide how I want to tackle that. I'm going to try and find my thread so I can update it.
 

forsete

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Wasn't the power pack. It's odd that it ohm'd bad when on the motor and when I ohm'd it after I took it off it showed good... Oh well guess I have a spare. What I DID find out is that if I run the motor with the cowl off it runs/idles great. Almost threw me out of the boat lol. I think the plug wire is touching the metal latch and grounding it out. Trying to decide how I want to tackle that. I'm going to try and find my thread so I can update it.

If you are correct in the guess, then it´s nearly identical to my problem.. if it was the ignition hat update that really fixed it for me =)
 
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