90 Johnson surging at WOT Runs great with air silencer off

[thebobcat]

2000 Johnson 90 PLSSE. Just finished replacing powerhead gasket due to what I though was an exhaust leak. Get everything put back together only to find it's doing the same thing. At WOT the engine will turn 4000-4200 rpm and surge and lurch like it wants to turn up more but something is holding it back. If I run the engine with the air silencer off ( cowling on) it will turn up to 4900 rpm and runs great. If I leave the air box on the poor running conditions continue. It should run as designed with the airbox but can not figure it out.

 

[ondarvr]

It's not running correctly even with it off, it should be much closer to 5,500 RPMs when running wide open. You may have the wrong prop on it though.

The cover will restrict the flow slightly, so if runs better with out it, it means it's running rich.

Did you clean the carbs and all related parts?

 

[thebobcat]

Yes the carbs have been disassembled and cleaned. Im not so sure the tach is set correctly. Just listening to the sound of the engine, i couldn't imagine it turning 500 more rpms. It sounds wound pretty tight at Wot with the air box off.

 

[thebobcat]

What could be making me run rich with the air box on other than the high speed jets?