1987 Johnson 120 Looper, problem at WOT

[rapier469]

I recently picked up a used 1987 Johnson 120 looper V4 that was supposed to be in good running condition. Ran great on the stand when demonstrated for me, but now on the water is giving me some issues.

The problem occurs mainly when above 3500 RPM or so and consists of the motor constantly dropping several hundred RPM and then picking back up. At first I thought it might be a failing coil, so I replaced all four with four known good ones I had lying around. Did not change a thing. I checked the fuel line (3/8) for kinks, and the bulb pressure was firm while the problem was occurring. Pumping the bulb did not change the issue.

After pulling the boat out, I checked all four plugs and they were wet. In addition there was quite a bit of unburnt fuel/oil residue around the exhaust ports and all on the motor mount/back of the boat. I performed a compression check and the left bank was 120/120 psi. The right bank 115/115 psi. Anyways, so now I am thinking it is a fueling issue even though the carbs were supposedly rebuilt according to the previous owner.

The following are videos of the engine along with some pics of the plugs and fuel residue.

 

[ezeke]

Re: 1987 Johnson 120 Looper, problem at WOT

Are you using the VRO?
What oil are you using?
Champion QL77JC4?
Thermostats good? (Loopers hate running cold)
Spark test good 7/16 snappy blue spark?

Check your gasoline line for leaks.

De-carbon treatment (looks like it needs 3)?

Surging is often caused by air leaks in the fuel system.

 

[HighTrim]

Re: 1987 Johnson 120 Looper, problem at WOT

I agree with a possible air leak into the fuel system.

Install a fuel pressure and vaccuum gauge along with 8 in. of clear, fuel resistant vinyl hose before the fuel pump. Take it for a spin and monitor the gauge and clear hose.

-Vacuum of less than 2 inches and air bubbles in the clear hose indicates an air leak into the fuel system. Tighten all clamps, check them for corrosion, try to find any pin holes.

-No air bubbles and less than 2 inches indicates a bad fuel pump usually, and should be investigated further.

-Vacuum over 4 inches indicates a restriction. Do you have a built in tank? Vent open? Anti syphon valve inspected if applicable? Pickup tube inspected? Clogged fuel filter?

 

[James R]

Re: 1987 Johnson 120 Looper, problem at WOT

Looks and sounds like she is running rich. Carbs could be flooding. When you pump the bulb do you get any fuel leakage from the carbs. If not the floats could still be set too high and that will cause flooding.

 

[rapier469]

Re: 1987 Johnson 120 Looper, problem at WOT

VRO is disabled. I am premixing instead. Oil is Pennzoil premium 2 Stroke?? do not really remember... Spark plugs are brand new QL77JC4. There are no leaks in the fuel line. I run a fuel/water separator and to eliminate that from the possibilities, I ran a direct line from the tank, completely separate and nothing changed.

I run portable 6 gallons, and have tested two different fuel tanks with the same result. I will have to check for air leaks as suggested.

 

[ezeke]

Re: 1987 Johnson 120 Looper, problem at WOT

I think you have a bad adapter gasket; and possibly other bad exhaust system gaskets such as the megaphone to adapter gasket. See your mid-section parts diagram.

http://shop.evinrude.com/

 

[rapier469]

Re: 1987 Johnson 120 Looper, problem at WOT

Well to update this thread, it appears I am chasing several problems. I did a good looking over of the engine, and it turns out that the previous owner never capped the oil inlet side of the VRO oil/fuel pump. I assume this was most likely allowing extra air into the fuel mixture causing a lean condition. I capped that and the surging problem is gone and the plugs now look good.

Now the motor seems to run great up to about ~4500 rpm consistently. Above ~4500 rpm will hold for about a minute or so and then bog down considerably unless I throttle back. If I hold the throttle when the engine starts to bog down, it will stall. The fuel bulb will not stay firm so I am thinking an air leak in the fuel supply somewhere. The problem seems to happen with all three of my portable 6 gallon tanks, so that rules out the fuel tank pickups. I am going to try the fuel pressure gauge T idea mentioned previously, but does anyone have any other suggestions?

Also, any chance of it being the fuel pump?

 

[Daviet]

Re: 1987 Johnson 120 Looper, problem at WOT

The primer bulb will not be firm when the engine is running. Try pumping the primer bulb when it starts to act up, you will be manually bypassing the fuel pump. If it picks up, I would question the fuel pump.
Have you check the high speed jets for any blockage?

 

[kenmyfam]

Re: 1987 Johnson 120 Looper, problem at WOT

Could be the pump but before you go spending money on it squeeze the bulb when it acts up and see if it picks up again. If it does then you either still have an air intrusion somewhere or indeed the pump is starting to fail. Is it the original VRO pump with the oil side blocked off or has it been replaced with a regular pre-mix pump ??

 

[rapier469]

Re: 1987 Johnson 120 Looper, problem at WOT

Its the original VRO pump with the oil side blocked off.

 

[kenmyfam]

Re: 1987 Johnson 120 Looper, problem at WOT

Check for air intrusions and if all is well there give it a squeeze and see what happens.

 

[rapier469]

Re: 1987 Johnson 120 Looper, problem at WOT

Well I had the opportunity to get out on the water for a little bit today. Forgot the fuel pressure gauge, but since the last time I had the boat out, I eliminated my fuel water separator and checked all fittings and such. So now my fuel supply setup is as follows -> 6 g. portable with OMC fitting-> moeller 3/8 fuel line with primer bulb-> antisiphon valve below fuel pump -> 3/8 fuel line from the valve to the fuel pump inlet. The valves are definitely oriented correctly.

Got in the water, ran it up to about 5600-5700 rpm and ran fine for a couple of minutes. Then it proceeded to die back as it has been. I throttled back and had my dad take the helm while I held the primer bulb in my hand. I had him gun it again and the engine ran great for another minute or so, but with the bulb in my hand I could feel a pulsing inside the bulb. What ended up happening was the primer bulb went flat under full throttle, and if the throttle was brought back, the bulb would slowly return to its normal shape.

So clearly there appears to be a restriction in my fuel supply. I know the motor needs 3/8 line, but I was taking a look at those OMC tank quick disconnect fittings, and I cannot imagine that enough fuel gets through that fitting to properly supply 3/8 fuel line. I did not have an issue with this on my previous 100 hp crossflow motor, but I have heard most people say that the fueling requirements on the loopers is significantly greater. Any suggestions?

 

[emckelvy]

Re: 1987 Johnson 120 Looper, problem at WOT

Maybe it's a bad primer bulb. I've had trouble in the past with bad bulbs, the check valves either leak by or stay shut and mess with fuel flow.

You might try a brand new OEM (not aftermarket) bulb and see what happens. Merc had a big problem with bulbs due to the alcohol-laden fuel eating up the check valves, they resdesigned the whole mess to improve check valve action and give better resistance against the Evil Gasohol. I wouldn't be surprised if OMC and others have had similar issues thru the years and improved theirs as well.

HTH..........ed

 

[ezeke]

Re: 1987 Johnson 120 Looper, problem at WOT

The small quick disconnects were not used on the built-in 120 and 140 looper installations. They are OK for portable tanks for trolling, but are too restrictive for all around use. You might consider a petcock or other larger fitting.

The water separating filter is a practical necessity with ethanol and should be the first thing after the tank, mounted in the most stable position possible - no other gasoline filter is needed.

The anti-siphon valve (if any) should normally be at the top of the tank. They are not required on portable tanks, but are used to prevent the fuel from being siphoned out of the tank by a leaking hose or fitting.

 

[rapier469]

Re: 1987 Johnson 120 Looper, problem at WOT

The small quick disconnects were not used on the built-in 120 and 140 looper installations. They are OK for portable tanks for trolling, but are too restrictive for all around use. You might consider a petcock or other larger fitting.

The water separating filter is a practical necessity with ethanol and should be the first thing after the tank, mounted in the most stable position possible - no other gasoline filter is needed.

The anti-siphon valve (if any) should normally be at the top of the tank. They are not required on portable tanks, but are used to prevent the fuel from being siphoned out of the tank by a leaking hose or fitting.

Are there any quick disconnects (not necessarily marine) that might work? I realize that I could just use a barb fitting, but that would be a pain for swapping tanks while underway...

 

[ezeke]

Re: 1987 Johnson 120 Looper, problem at WOT

The larger, 5/16 quick disconnect fittings will probably work.

 

[rapier469]

Re: 1987 Johnson 120 Looper, problem at WOT

The larger, 5/16 quick disconnect fittings will probably work.

Are you referring to 5/16 fittings for another application (mercury, chrysler, honda etc)? Or do they make the male OMC tank fittings in more than one size? I looked around quite a bit and while the female fittings appear to come in 1/4, 5/16, and 3/8 (sized for the appropriate fuel line that you are using), the male tank fittings are all the same. I currently am using the 3/8 fuel line sized female fittings in combination with the standard OMC male tank fittings.

 

[James R]

Re: 1987 Johnson 120 Looper, problem at WOT

I recently had a problem when recommissioning a rebuilt 150 Fast Strike. Tried running it on a standard 6gal tank. fuel alarm went off and the motor died. Switched to the on board tank without any quick disconnect fittings, only a big water trap filter in line and no problem since.

 

[tal]

Re: 1987 Johnson 120 Looper, problem at WOT

I have an 85 120 with moeller plastic tanks and fittings like you describe. I have not had a problem with it leaning out on the top end or starving for fuel for that matter. I do NOT have the vro pump because it was removed when I bought the motor.

I have run mine at WOT and can feel the pulse from the fuel pump in the bulb too but it's not supposed to go flat. I'd suspect the bulb. The aftermarket ones have a reputation for being problematic. I've only had one go bad on me in 7 years.

 

[rapier469]

Re: 1987 Johnson 120 Looper, problem at WOT

I have an 85 120 with moeller plastic tanks and fittings like you describe. I have not had a problem with it leaning out on the top end or starving for fuel for that matter. I do NOT have the vro pump because it was removed when I bought the motor.

I have run mine at WOT and can feel the pulse from the fuel pump in the bulb too but it's not supposed to go flat. I'd suspect the bulb. The aftermarket ones have a reputation for being problematic. I've only had one go bad on me in 7 years.

Are you using the OMC fittings on the tanks or another design? I have another fuel line I can swap in to check that out, just waiting on some of the reservoirs around here to come back up with this rain on the east coast.