Merc Cowling off, Idle high...
 

Hey guys, I was out doing a de-carb on my motor yesterday and noticed something strange. I was idling at 750, when I opened the clamshell covers on my 86 blackmax 150, the idle slowly crepped up to about 1100. What the hell is that about? I closed it up and back to 750 it went. I checked for anything that may be hitting the cables or the linkage, nothing. I tried it a few more times about 20 mins apart with the same result. It seems like whe I open the cover it lets the carbs breath better. Should I install vents and adjust my idle speed accordingly or is something wrong here?

Sounds normal to me. These engines are designed and jetted for the vents in the cowling and restrictors in the airbox, any changes will change the mixture. This is why it is not suggested to run without the cowling on except in emergencies.

I was just thinking of general performance theory, engines need to breath easy. Is this not true with 2 strokes? When I pulled that cowling, the engine woke up like a breath of fresh air... Literally

You puled the cowling off and it leaned out at idle is why it woke up. And 2 strokes do NOT like to be run lean at ALL! you can open up the cowling, remove the air box, and install airhorns on it for a performance gain. BUT you will also have to rejet the carbs, and be prepared for a melted piston or 2 in the process if your not careful. 2 strokes are VERY sensitive to any airflow change, even the slightest airflow change can make the difference between one living for 5000 hours or grenading shooting a rod through the block.

Well that settles it... No vents then. Thanks

I hope the difference in elevation and air pressure from the Foothills of SC to the coast of NC didn't cause mine to scorch a cylinder...I'd hate to think they are THAT sensitive...naaaa