running with no airbox?

[mpsyamaha]

i was working on my 30 yamaha last weekend and got it up and running perfectly. i decided to try running it with no air box in front of the carbs just to see what happened. it was a slightly lean at idle so i adjusted the mix screw a hair. the sucker ran like a bat out of hell. noticeably faster top end. dont have a speedometer but it had to be 5 mph, it was very obvious. it was a little sluggish on acceleration at times but damn did it scream up high. i know this is not necessarily good for the motor because it is making it run leaner. but i was wondering if anyone has ever tried running bigger main jets in order to compensate for the additional air provided by removing the air box?? seems like if you could get it right, it would be a nice reliable horsepower bump. any opinions on this?

 

[LORDY611]

Re: running with no airbox?

We used to do that all the time on motorcycles. I had 3 different Yamaha 4 strokes where I put a high-flow air filter on, open the jet, tweak the mix, and ride hard. Did that for years and nearly 40K miles with no ill effects.

 

[BillP]

Re: running with no airbox?

I've never noticed a change when running without airboxes. All I see them do is make it quieter and catch all the fuel that spits out of 2 stroke carbs when they idle.

 

[Rancherlee]

Re: running with no airbox?

I've seen many people do this on snowmobiles, which turn into melted pistons. 2 strokes are very touchy on what you do and even a little bit lean and a piston is gone. Its evident your running lean due to the "sluggishness" which means you are overly lean. There might be richer jets avalible but you really need to use a EGT (exhaust gas temperature) gage to help you tune it with different jets to get it right.

 

[jtexas]

Re: running with no airbox?

Man o man, I got a 3-cyl rude that will scream & fly with the cowling & airbox off. As I found out once when test-driving some carb repairs. Makes you wanna run it like that all the time. But don't. [sigh]

 

[Joey One]

Re: running with no airbox?

I didn't realize that removing the intake silencer raises the intake pressure and lowers the vacuum in the venturi, and thus less fuel is sucked into the engine.
I forgot to install th silencer on my 6 HP Johnson after the last annual. It will be very difficult to install the silencer while it is on the boat. While I never run at WOT, I may have to in an emergency. Is this a serious enough problem, or can it wait until next winter?
There is the normal amount of black gunk coming out of the exhaust, so I guess that lubrication is OK.