1989 johnson 120 hp wot problem

[Mustidog]

Hello.
I lost some wot rpm lately so I tried to find proplem.Compression is good- 135 + or - 3 psi on all 4 cylinders. Spark jumps 7/16 inches on all coils. Installed new plugs and carb kits. When I test drive on lake I get up to 5500 rpm with air silencer cover off but when I bolt cover back on I can't get over 5000 rpm. Can anyone help me out?
Thanks.

 

[ezeke]

Re: 1989 johnson 120 hp wot problem

With the cover off you will run lean with engine damage very possible.

Fuel obstruction is a possible cause of your loss of power. You might want to check everything on the way to the fuel pump, especially the anti-siphon valve and fuel filter.

The possibilty of ignition components breaking down under load when heated is high. You may want to use an inductive timing light to check spark at high RPM when the problem occurs; it's a two man job.

 

[Mustidog]

Re: 1989 johnson 120 hp wot problem

Thanks for reply EZEKE. I did replace anti syphon valve and filter earlier but didn't help. I have noticed carb float bowl flanges are a bit warped. Could a gasket leak there cause loss of wot rpm? Also I am original owner of motor and have never touched any ignition parts except for replacing plugs and wires. I bought seafoam today and will try run that in gas to see if that helps. I'm sure it can't hurt. I guess to check high speed timing I will have to run on lake wide open and check with timing light. Is that the only way? It is not a problem because I live right on the lake. I forgot to mention that I replaced vro pump for regular pump a few years ago and mix gas 50 to 1 but that set up worked fine until now. Can a weak fuel pump cause loss of wot rpm? I will check all fuel lines again tomorrow to look for air leak. Does anyone have any other ideas?

Thanks.

 

[ezeke]

Re: 1989 johnson 120 hp wot problem

I would not worry about the timing unless someone has changed it; The inductive timing light is just a good way to see if there is an erratic pattern to the firing. If the float bowls leak, that will be a problem.

If you are running on a single conventional lift pump, that can be a problem because they are not very strong to begin with and if they weaken they will not be able to deliver enough fuel. We recommend using a pair of them on the big bore loopers, set up like the earlier V6 motors and some larger V4 motors.

 

[Mustidog]

Re: 1989 johnson 120 hp wot problem

HI Ezeke.

I tried a few more things with motor and no change. I ran a mix of 16 oz. seafoam to 3/4 gal. of gas through it as per forum post. I also checked wot timing using Joe Reeves method and it was good. I bypassed boat gas tank fuel line and used portable tank. No change. I tried my original vro fuel pump. No change. I pulled the carbs off again and everything looks good except for warps on bowl flanges. Is there some way to seal bowl flanges like maybe thicker gaskets or some compound? I hate to buy new float bowls if this is not even the problem. I have never checked carb base gaskets or intake gaskets. Do you think these could cause lower rpm? I am running out of options.

Thanks.