1989 Johnson 20hp surging at mid-high rpms help!

[hbryant123]

My fathers 20 hp john done something similar it was the fuel line fitting drawing air in the carb.

 

[Abassed]

Got off the water about an hour ago and sadly, still surging. I noticed some fuel leaking from one of the elbows while I was out there, and the bulb wouldn't stay tight. I could watch the fuel level drop in the fuel filter as I was putting along. Squeezing it didn't really help anything and it would leak out the spot more.

So when i got home i fixed the leak and checked for others, which there were none. Pumped the bulb up tight and it stayed tight (I watched it for a couple of mins). When I cranked the motor though, the bulb wouldn't stay tight anymore. I have videos of it in and out of the water surging. Hopefully getting to see and hear it happen will help. Thanks!

 

[racerone]

Fact----Once a motor starts the bulb DOES NOT stay tight.---And it does not need to.-----

 

[Abassed]

Oh okay thanks didn't know that. Having trouble posting the videos. I'll post them to youtube I guess and post the link to them

 

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[oldboat1]

Link and sync? Throttle plate opening needs to coincide with timing advance.

 

[Abassed]

Never tried to do it but I'll try. How do I know when it's correct though?

 

[Abassed]

Does seem like the plate is off. When i turn the throttle all the way it goes way past horizontal.

 

[oldboat1]

The roller has to engage the mark on the cam plate as the throttle plate first begins to open. No further adjustment is necessary provided the cam is attached correctly to the timing plate. Do a search or two here for this procedure, but may need a model specific manual for your motor for adjustments.

 

[Abassed]

I have the manual for it but it doesn't say anything about when the throttle plate starts to open. When you say engage, should it be touching the cam plate?

 

[oldboat1]

It should touch the scribed line on the cam at just the point the throttle plate (back of the carb throat) begins to open. Prior to that, the roller would not/should not be touching the cam when properly adjusted. With the motor in gear, the roller will advance to the other end of the cam when the throttle is advanced, fully opening the throttle plate. Get the idle contact point right, and the rest should follow.

[edit. I'll guess that the adjustment is made by bending the link (#24) as needed: http://ww2.iboats.com/Intake-Manifold/dm/cart_id.879320373--session_id.040215493--view_id.1537686 ]

 

[Abassed]

Well there's a screw on the carb you can see in one of the pictures i posted that you have to adjust. But I noticed when I turn the grip all the way open, the throttle plate goes wayyyy past horizontal. That's why I was wondering if the cam follower roller should be touching the cam (figured it maybe should since it has a roller on it). This is what OMC service manual says to do for linkage.

Set the throttle grip to the STOP position.

Slowly rotate the throttle grip toward the ADVANCE position until the throttle cam timing marks align with the cam follower roller. On 1980-1982 25 hp models, the roller should align with the second cam mark; on all other models, the roller should be centered between the marks.

If the throttle valve is not closed when the cam roller and throttle cam marks align in Step 5, loosen the cam adjustment screw ( Figure 30 ). Adjust the cam up or down as required to align the marks and tighten the screw.

With the shift lever in FORWARD gear, the throttle lever should touch its stop.
If it does not, loosen the adjustment collar screw

 

[racerone]

Throttle valve MUST be closed at idle.-----And no the throttle cam does NOT always contact the roller at idle !

 

[oldboat1]

On many of the motors I have, the motor must be choked to be stopped (or a stop button engaged). That is, the motor will idle low and slow with the grip fully turned counterclockwise (full stop). The throttle plate is fully closed and the roller is well away from the cam at this point. I’ve assumed that’s why there are holes in throttle plates, but don’t know if that is correct. I know it’s great fun to get a motor to idle so slowly that it barely makes a ripple.

 

[oldboat1]

edit... Sorry. That's obviously clockwise to idle. Not something you want to confuse when docking.

 

[Abassed]

Haha very good point. I took the boat out again today after the link and sync. Started up first pull and sounded fine. I did have to hold the throttle ever so slightly open to keep it running though. Let it warm up and started adjusting the carb, which didn't really respond too much when adjusting it in or out (needle valve). Finally opened it up out of the launch and motor ran just fine at WOT..... for about 100 yards. Then the motor suddenly had a loss of power and started that dreaded surge again.

After playing around with it for a while, I looked at the fuel filter to see if it was low, like last time. Come to see that the fuel filter was bone dry! The motor was literally running on the small amount of gas in the carb, so I would pump the bulb here and there to keep gas going to the fuel pump.

The fact that there was zero gas in the filter is making me think that air is coming out of the system somewhere. Just don't know where. Could be that fitting that connects to the motor. Just an FYI though, I do have one of those attwood "self venting" gas tanks that balloons up when it gets hot. Haven't heard too many great things about these tanks, but the fact that the motor starts easily, but starts surging after 5mph or so, BUT ALSO will run fine right out the gate (100 yards haha) makes me think there has to be some sort of vacuum leak in the line. I'm definitely not as good with motors as yall though so I could very well be wrong.

P.S. Keep in mind this isn't some surge from say 2000 rpms to 1700 rpms. more of like a WOT rpms (idk what that would be at WOT) to like maybe idle speed. It's like it's picks up every time it gets a little gas or something

 

[oldboat1]

Let’s see. If the tank isn’t venting, the bulb will collapse. — can run with the cap loosened and see if you get improvement.

If you have an old hose connector for the motor, you might try that. I’ve had some problems trying to fit new connectors to old motors — probably just that the new O-ring is a lot tighter, but I’ve had success just using the old connector instead. (Need to be sure you have an OMC connector — some seem close but won’t fit (Yamaha, maybe)).

I’m not sure what filter you have. Sounds like you added an inline filter(?) If so, you might try running without the filter and see if you get a constant feed, particularly if it’s ahead of the fuel pump. Note that some filters can be incorrectly installed backwards — look for an arrow indicating flow direction.

I know you tried pumping the primer bulb, but there still might be a fuel pump issue. There is a screen on the face of the pump, and that could have a blockage. And it might be worth disassembling the pump and checking the diaphragm. If its torn, you would obviously want to replace it (kits available), but if it’s lost its elasticity (taken a set) you may want to replace it as well. Old ones can have the texture of cardboard. New ones are thin and flexible. If you have an inline filter between the pump and the carburetor, a dry filter might indicate a bad fuel pump.