1986 90HP Crossflow Johnson - RPMs increase at WOT when steering

[smak90]

I've been running this engine for 3 years without a bit of trouble. It currently seems to be running well. Currently when I'm traveling at 3/4 throttle or higher when I make slight adjustments to steering the RPMs jump between 200-300 RPM causing boat to oversteer feeling like you're about to lose control of steering. First I thought it was prop issue (spun hub) or steering issue but last trip out I confirmed RPM jumps and power/speed increases.

I changed props earlier when I was thinking it might be spun hub but that made no difference.
I put new plugs in yesterday but haven't run it yet, I'm doubtful that will make a difference.
Throttle cable seems ok, it doesn't seem to be binding and RPMs don't fluctuate at low speed when turning. I'm not sure if there is something else with throttle cable to check.

Any suggestions on where to begin in troubleshooting this? When I put the engine on 3 years ago I went through it and replaced the power packs and converted it to pre-mix. I'm still running original stator and trigger. I haven't checked compression lately but it was fine last time I checked. The motor has been run about once a week for the last 3 years straight so it hasn't been sitting.

 

[racerone]

Water in the fuel ?----Drain float bowls and have a look.----Just 1 screw on each carburetor.----Costs nothing to check.

 

[Chris1956]

Turns at high speed tend to cause the prop to ventilate. Some props ventilate more than others. I suspect that when you changed props, the new prop ventilates a bit more than the old prop.

 

[F_R]

Sounds like a ventilation issue to me. If it isn't the prop, perhaps something about the boat changed? Or load? Do you have it jacked up on the transom?

 

[oldboat1]

Perhaps changing the trim. Might also be a good time to be sure the transom is solid.

 

[smak90]

I may not have worded swapping the prop very well. I started having the issue in the last month and the prop has been on for years. I then switched props to see if the issue occurred with other props and it occurs on all of them. I have multiple props since I've had several of these V4 Johnsons over the years.

I hadn't thought of water in the carb so I will drain them and make sure that isn't the issue. I always add stabil with each fill up and keep the boat in my garage but you never know.

I just checked the transom and can jump and down on the engine with no flex so that isn't it but that was a great suggestion.

I forgot to mention the issue doesn't seem to occur when I first launch and make my first couple runs early in the morning. It seems to happen once the engine is warmed up for the day. I did test the overheat horn and it it working. Engine does not seem to be running hot at all. Since it doesn't seem to occur when cool I was leaning toward an electronics issue but I don't want to get tunnel vision and head down the wrong path.

I may take it out tomorrow morning to do more troubleshooting. I'll watch the trim because that might explain it if the engine shifts a little. Thanks for all the suggestions and I'll report back what I discover.

 

[racerone]

Run with a timing light one cylinder at a time.-----See if there is a spark issue in the turns.

 

[smak90]

I took the boat out yesterday and I'm fairly certain it's a cavitation issue. I was able to get it to occur in all situations like going straight or slight turns. Engine being hot or cold had no impact, I was able get it to occur in all scenarios.

I inspected the bottom of the boat and transom and can find no issues. I can stand on the engine and rock it up and down and see no flex. That leaves the tilt/trim and transducer. With the tilt/trim, it seems to be operating fine, if I rock it up and down there is a little play but I thought that was normal and expected. Is there something in particular I should be looking for?

Next trip out I will also adjust the transducer in different positions. It's been on there for at least 5 years and has been bumped into many different positions but never had any issues, so it's odd that it would start creating problems now.

The engine can be mounted so it sits lower on the boat, would this be worth trying and would you expect it to make a difference? I would prefer to figure out what changed before resorting to this but if this is what it takes I will gladly change the mounting.

Thanks in advance for your suggestions and recommendations.

 

[racerone]

Lower the engine as needed and test run.

 

[smak90]

Lowering the engine stopped the cavitation/ventilation issue so I no longer see the RPM changes. I still have the feel of loose steering at higher speeds. The steering cable/helm are approximately 5 years old. When I look back at the engine while running, the steering linkage appears to be moving accurately to the steering wheel but the engine appears to have some play in relation to the linkage.

The only thing I can think of that would cause this would be the main rubber engine mount in the mid-section. I pulled the covers on the side mounts and visually those looked normal. Is it very likely that it's the main mount? I haven't seen or heard of this being a common problem when I look at the message boards or google it so I don't want to pull the powerhead to do it and find the mount is fine inside.