Bonneville America Forums Forums Technical Forums Performance Mods Drilled out airbox and Unifilter

Ok...so I've been promising pics for a year. Beat me with sticks. Here's a few photos of the airbox mod Gary James and I did adding, more or less, 9 HP and saving a whaleload of cash and hassle. I didn't dyno mine, neither did Gary, but the guy Gary spoke to that gave him the idea did. So he may be lying, but I'm not making the 9HP up for sh!ts and giggles.

BB


Bottom of Airbox

Bottom of Unifilter

Pretty cool Tad. I've been tossing around the notion of doing something similar - completely cutting the filter bottom out and the airbox to match. 'Poor guys Freak'. Got to build up my resolve to try my first rejet however....

Hey, bluebass -- where'd you get the crash bars? (I'm also curious about the air setup, but I have a long way to go before I start messing with the guts of my new critter)

--Jaeger

I did that using the standard paper filter. I used a 1 3/8 hole saw on both. It fits inside the little positioning bumps on the bottom of the airbox but still gives you lots of extra air intake. I'm still waiting on the the proper TBS needles to get here but with only the main jets at 132 and two shims on the stock needles and off road pipes it ROCKS. I have less than $200 in performance mods and it leaves most everything behind.

I just did mine today.... then I read your post...

I also used a 1 3/8" hole saw because I happened to have 2 snap in button plugs that size - in case things turned to crap...
I didn't have a lot of time afterwards but I managed a short test ride. The bike seems a little stronger, but I don't think I got 9 more hp out of this mod. A longer test ride is in order.... here's hopin'!!!

here's a theory my tri dealer mechanic told me: by drilling the bottom of the airbox, i created a vacuum because of the motion of the rear tire and the air flow under speed. so under speed the rear tire pulls air out. the snorkle was out.

i had 135s and the large hole for the airbox and paper filter and (grump's) tors. my plugs were sooty.

he set mine up now w/ 128s, 2 shims for each needle and 45 pilots w/ the unifilter. the plugs are chocolate brown.

it runs great and no bogging. before on windy days (nebraska gets windy) i did get the feeling w/ the drilled box that i was getting too much air and the bike bogged pretty badly.

my theory is if we can make an air scoop under there to block the vacuum created by the rear tire we will take advantage of the air flow under power.

i haven't gotten under there to look at the setup but i don't think there's enough room to use the snorkle under there. but some simple 3 sided plastic fastened under there might do it.

what do you guys feel about this? a dyno might not even measure his theory since a dyno run doesn't produce wind currents that wd. be present under speed.

cat

Well the dyno might be interesting to compare Grump's setup since he could do it with or without the plug in the bottom, direct, instand comparison, ya know? AND the rear wheel IS spinning so the vacuum theory would hold true.

That being said, wind blowing by the bike at 60 MPH is going to have a far greater effect on airflow than whatever breeze the tread on the rear tire has. If anything, the air going by the bottom of the bux would greate it...
THAT being said, there's so much stuff down there creating turbulance, I doubt there's smooth enough airflow under the box to create a good suction.

Looks like my airbox, except I removed the entire bottom of the UNI and cut the same size hole in the airbox. I also removed the inner snorkels and the inner wall that they attach to.

Cat,
I don't buy the rear tire vacuum theory. As somebody mentioned in another post on here, when you twist the throttle, the engine sucks hard enough to move the carbs forward.... I don't see the tire doing anything at any speed that will negate that amount of suction to any noticable degree....
But I do have a theory of my own...
Based on the amount of dirt and tar laying on top of the swing arm directly under the hole I drilled in the air box, I am going to install some sort of screen over the opening. I can see this hole allowing all kinds of unwanted debris into the filter. I did this mod with a stock filter that has maybe 1000 miles on it, and it already had a few small twigs and other bits of debris in it before I cut the holes.

I don't want to start finding chipmunk livers, tortilla chips, and other roadside goodies in my air filter....

I may lose a little of the new ooomph, but I'd feel better about it.

I doubt some screen would make much of a difference, and certainly less restrictive than squirrel spleens.

Tad,

Got to take a good run today and this modification does seem to make the bike pull better and stronger and even the throttle response is slightly improved. I don't think it's 9hp better, but it is better.

But, my right pipe started turning brown again..... I have had nagging adjustment problems with the right carb on almost every mod I've made since day one. I'm going to have to check the float level on it one of these days, that's about the only thing I haven't checked yet.

For reference, if anyone's interested, here's my current set up:
Stock pilot jets
130 mains
TBS needles - no shims
TBike pipes
Snorkel out of the airbox
pilot screws out 3.5 left, 4 right, (now 4.5 - lets see if the brown goes away..)

Mmmmmm, chipmunk livers....

Have you tried increasing your pilot jet by one step? Al

Al,

Why would I want to do that? The bike starts right up, pulls hard with no flat spots or stumbles, and there is no popping or backfiring on startup or on decel. I'm happy with it except my right pipe keeps turning a little gold from the head down about 4 or 5 inches. The 4.5 out adjustment on the pilot screw seems to have helped....

hi grump, your right about junk entering the airbox with this mod, what i've done is bought some air filter foam that is used on lawn mowers, cut it to size and put it between the filter and bottom of box this should keep things clean i hope.

Also i noticed you mentioned you've had probs with ya right carb, has this only been after you removed the snorkels from the box? because when i removed part of the devider that holds the inner snorkels in place (some time after i took the snorkels out),i noticed there was a trail of oil going straight up into the right carb, because the high suction of the carb sucks oil and fumes straight out of the engine breather tube that enters the airbox right under the right carb, the snorkel stops this happening, so my next job is to get a breather filter and bung the hole off.

You wont notice this untill you remove the divider that holds the two intake snorkels in place.

Phil,

Thanks for the info. My bike never had the inner snorkels in the airbox. Except for drilling this hole and removing the main snorkel (a long time ago) my airbox is still intact.
I'll have to check that breather line out... thanks again.