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Tuning for Secondary Airbox Removal and Airbox Mod?

13K views 32 replies 10 participants last post by  Flyguyeddy 
#1 ·
I'm working on a '99 Legend TT.
I'm pulling the carbs to clean them up and discovered the airbox (and filter) are trashed and the secondary airbox is in pretty bad shape, too.
I've never liked pods so I will be purchasing a new OEM airbox/filter assembly. However, I am considering opening the airbox up by basically just cutting off the very back end of both sides. I'd also be leaving the secondary airbox off. I didn't find any info on this specific mod. What are opinions on a jetting combo to go with?
Exhaust is stock.
 
#4 ·
I have a '00 Legend. I have cut an oval opening on the right side of the airbox and left the other side stock. I have STOCK pipes so my jetting is a little smaller. I am using 125's in all cylinders. I experimented with the opening after installing the 125's. I found the one 2" hole punched with a hole cutter was not enough, so I drilled a second one underneath it and joined the two holes with a sabre saw. The oval opening is about 2" X 4 1/2". If you have a free flowing exhaust, you might go a little bigger on the jets. I ran this setup on a 125 mile run last week and was really happy with it. As they say "results may vary". ...J.D.
 
#5 · (Edited)
I've done the same thing to two of my bikes. I've kept the rear box on since it had the seat lock and mounting point for the side cover.

The air box is going to have the back cut out, here is the cut line I laid out. I'm keeping the secondary box to retain the sidecover and I wanted the airbox opening to be as big as possible.





More to come tomorrow!
How it looks installed on the bike.

Had some time on the garage today. I installed the rebuilt and re-jetted carbs and the back-halfed airbox. I balanced them using the Morgan Carbtune tool which is awesome and easy to read.




As for jetting I'm at high elevation. I have 40 pilots and 135 mains on one bike with stock, drill out exhaust and the other bike has 40 and 132 with 3 into 2 header.

You being basically sea level I'd start with a 42 pilot and 145 main and take it from there. With the airbox cut open, installing and removing the carbs is pretty easy, disconct the throttle cable from the grip end.
 
#9 ·
The point of elevation is hardly ever mentioned in these discussions. It is a valid consideration though. I tune for 3000', the elevation at my shop, but when I ride it varies from around 2000-4000' most of the time. My bikes do not run well above 5000' I also think what the factory had in mind when they restricted the airbox opening so much was noise dampening, but restricting the airbox means restricting the jets....J.D.
 
#11 ·
Great info. 110s obviously won't work, then. You've saved me at least one carb rack pull.
I think I'll go on the conservative side (135/40) for starters since I'll be running the OEM filter element and my exhaust is unmodified.
I think I'll go ahead and order 140s, 145s, and 42s, just in case it needs more. If not, I should be able to use those in my TBS w/TORS when I start to monkey with the intake on that.
I'll report back on how it works out.
 
#12 ·
It would be interesting to take an old rear half of the airbox and some sheet plastic and make up something. All the used one I have had were already too hacked up the be useful. Anybody have a complete rear half they want to part with ? I'm thinking I could make a mock-up and run it through a laser printer. ...J.D.
 
#13 ·
Wanted to report back.
Just to summarize... 1999 Legend TT, new OEM air box with back ends trimmed off of both sides, deleted secondary inlet box, stock exhaust, "sea level".
Put in 130 mains and 40 pilots with 2 turns out on the screws.
Since I deleted the secondary box, I re-hung the left side cover using rubber cushioned loop hose clamps.
The bike now has great snap off idle and pulls hard with a most satisfying intake roar. I'm calling it good for now.
 

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#33 ·
Wanted to report back.
Just to summarize... 1999 Legend TT, new OEM air box with back ends trimmed off of both sides, deleted secondary inlet box, stock exhaust, "sea level".
Put in 130 mains and 40 pilots with 2 turns out on the screws.
Since I deleted the secondary box, I re-hung the left side cover using rubber cushioned loop hose clamps.
The bike now has great snap off idle and pulls hard with a most satisfying intake roar. I'm calling it good for now.
just like this.
 
#16 ·
Old thread but not dead ;)

Anyway, did the airbox mod this weekend, opening it completely like Rollinslow84.
I have some questions for the people who has done this mod:

1. Did you buy a separate crank case breather filter or kept the stock through the airbox? Seems like there is a risk that water will get in to the airbox easier when its open like this, right?

2. I left out the silicone that seals the two parts of the airbox together, seems like that is not needed now as the only restriction is the filter now, right? Or am I thinking of this wrong?

Cheers!
 
#18 ·
Looks pro :)

Maybe I'll try something similar but only if needed, you have pods so it is a little bit different.

Here are the plugs btw (I know they should be tested at a specific throttle position but with my jetted Mikunis they were black every time I looked), one taken with flash and one without, makes quite a difference in color. I would say they are closest to tan, maybe a bit lean but it's cold outside, around 5°C (40°F) so probably makes sense?

716501
716502
 
#20 ·
Maybe it's the photo, but they look pretty black to me...hm
There's hundreds of plug color charts on the internet. Here's a simple one that looks pretty good:

Still some around the edges but the electrode is not black anymore, maybe because it's the same plugs that previuosly looked exactly like the carbon fouled ones in the pic you uploaded? I replaced them with new ones now anyway so we will see next time I pull them :)

Nice bike collection btw! Trying to build something similar, have a 2002 HD Fatboy as my cruiser and would like to add a 60s/70s Triumph/BSA to rebuild!
 
#22 ·
It's an oil/air separator. They're used on air compressors. The vent line is connected to the inlet. Oil, and any moisture that condenses, is trapped in the bowl and the air exits through the small breather filter on the side. I drain the bowl every time I change the oil. Keeps things neat and tidy, and keeps nasty stuff out of the breather.

Some people just stick the filter on the end of the breather hose, but there'll always be a little oil vapor coming out. If you're OCD like me, that won't do...
 
#26 ·
Well if you have new useful info to add I don't think its worn out ;) Anyway, I appreciate the info you and the others wrote in the thread these years ago, without it I would not been able to modify my bike with so little pain. Also, really appreciate your answers WSC, it's great you take the time to help out less experienced riders/modders!
 
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