carb breather cover

[offshore100]

Hi all, this is my first post. I've searched like crazy through these posts but have come up pretty dry. So I've registered in the hope that maybe somebody here can help.

I own a number of outboards (OMC pre Bomby), here's my question:

Why do some of 'em have breather covers, (my '86 90hp and my '97 8hp), and some don't, (my '85 30hp and my '81 4.5hp).
The reason I ask is that I don't like them. I like being able to see the mouth of the carb, and carb adjustments are much easier without the bulky boxes in the way.
So I want to remove them from my 90hp and my 8hp.
What little I could find in my search mentioned either they are absolutely necessary or your motor will run too lean and burn up, or that they're put on only for noise control.
Anybody have any real-world experience they'd like to contribute?
I'd be much obliged.

 

[iwombat]

Re: carb breather cover

You could probably run w/o them, but you'd likely have to rejet. They will run leaner w/o the boxes.

 

[hoeser]

Re: carb breather cover

I suspect you'll get a lot of opinion on this, but I personally don't believe they will impact your mixture enough to actually cause any issues. They allow flow at basically the same rate as without. I've run with and without and have noticed little to no difference. Mine however is still in place and I don't mind having it there... it's not that big of a deal. It's also the mount for my VRO and my primer system.

 

[iwombat]

Re: carb breather cover

If your 8 is like mine, that box is right in the way of the idle mixture screw. I drilled a hole through the open end of the box to I could pass a screwdriver through. End of problem.

 

[offshore100]

Re: carb breather cover

Thanks, guys; yeah, wombat, the cover on the 8 is a particular nuisance. I just finished rebuilding the carb.
The mouting screws are a pain to get to, and doing sync and link is a real pain too with the box on.
As to the notion of re-jetting, wouldn't that be a sort of hit or miss kind of scenario? I mean, there are no published guides (to my knowledge) as to just how big to go.



Actually, though, the idle mix screw is the easiest one to get to; it's the cam follower ball hex screw and the throttle "cracking" screws that are the tough ones.

 

[iwombat]

Re: carb breather cover

If you had to rejet at all, it'd be to the next size up I would think. Probably not on the 8, but maybe on the bigger engine. A set of jet drills would come in handy for that kind of operation. Just make 'em a tiny bit bigger. Your plugs should tell the story there though.

If you've got a tach you could just set the motor at 1000rpm (just to pick a number), remove the airbox and see if the rpms change. If it doesn't you're getting roughly the same air into the carb body.

 

[offshore100]

Re: carb breather cover

As to running with the box on and then off and monitoring rpm's, I kind of did that while running in the tank (barrel, actually) after the carb rebuild.
I adjusted everything: idle mix, cracking screw, cam follower with the breather off, let her run for a while, then placed the breather on and held it in place by hand, and listened for any discernable change. Admittedly very imprecise, but certainly no cough or stalling.
Like I said earlier, those breather mounting screws are a true engineer's revenge on somebody, I'd rather not have the box.
Reckon I'll run it awhile and as you suggested, check the plugs as high temp indicators.

 

[iwombat]

Re: carb breather cover

I speculate that a lot of what they're designed to do (at least on the 8) is maintain the same airflow characteristics whether the cowling is on or off. On the 8 all adjustments have to be made with the cover off. You'll notice that motors w/o boxes perk right up with the cover off. That'd make adjustments on that little guy a real pain. Adjust, put cover on, listen, take cover off, adjust, put cover on, listen, take cover off . . .

 

[Chinewalker]

Re: carb breather cover

There is, in fact, somewhat of a tuning factor to running with the cover on, on SOME models. There is a sonic wave at the mouth of the carb that varies in length with RPM. You can tune the motor to use that wave to act as a ram of sorts, pushing a bit more air into the carb. A friend of mine who races APBA A-Stock Runabout did some controlled tests and found that a plate placed a specific distance from the carb mouth made a small, but not insignificant RPM difference - about 100 RPM if I remember correctly. May not sound like much, but 100 revs here and a 100 revs there and pretty soon we're talking real horsepower! He must be doing something right - he's running over 55mph with a 15hp Evinrude...
- Scott