Custom Airbox

After setting up a lot of other mods, I don't like the look of the airbox on my bike. However, I also don't like the hassle I hear comes with pods, and do like a nice heap of low end tuned torque.

My idea is to build a custom airbox that has the same runner dimensions and volumes, just configured a bit differently. Any reason this change would affect performance adversely?

Also, I'd like to switch the filter so that it fits OVER the snorkle, and use a conical "cold air" automotive filter. Again, any impact on tuning?

The end setup would be this:
filter-> snorkle -> volume that matches colume of stock airbox (plus filter holding portion) -> tubes that run into carbs with same dims as stock

My understanding is that the purpose of the airbox is to act as a Helmholtz resonator, so if I maintain the same resonance by preserving the volume, neck cross section, and neck length, I should be good. I think a lot of the internal complexity of the airbox is just to make it structurally sound, keep oil away from the intakes, etc, and doesn't affect resonance at all.

Is this sound thinking? If it is, I may have a spare airbox for sale when I get done...

 

hello. great question. I am planning on building a custom airbox for my xj550 project. I plan to use same rubber boots, similar inside volume box and foam style filter. also I will not be using a lid. I'm under understanding it's volume that dictates proper function of carbs. any more input would be great. also tinkering with idea of going constant velocity carbs. and foam pods.

 

Streetbrawler750 made some velocity stacked pods that are similar to what you described. Others have made pvc intakes with some success and many different styles. One guy made a Maltese Cross shaped airbox for his bike but didn't show a decent pic of it.
Making your own box shouldn't be too difficult or you could look on ebayfor an original. Some members on this site may have a box to sell, just make a WTB post in the for sale & trade forum.

 

I Have been down the road of air boxes and PODS on my XJ750 put pods straight on made extended velocity stacks put cowling around pods. Bike never ran quite right unfortunatly I cut my air box up. In the end I made up what I call an air intake manifold out of PVC plumbing pipe and just used one POD and now the bike actually runs better than with a standard air box however the bike did have 4 into 1 ex on it so extra air from the non standard air filter might have been needed. I am no expert but the biggest issue with thease side draft multipal carbs on these motors anyway, and PODS is that they don't allways (PODs) maintain equal air pressure across all 4 venture's. I think the air pressure or vacum at the venture affects the main jet slides operation so if theydo not open the same the bike will run crap. I apoligise to the carb experts on this site if I have this wrong.

Can PM you picks of manifold and tips on how I made it if you like( I cant seem to work out how to upload picks to this site yet)

 

Share! Please share! :wink:

 

mos def on the custom airbox pics! not to add any fuel to the fire but i too have been down the pod road. tried everything from velocity stacks, air restricters, and all aorts of jetting combinations. don't get me wrong, bike pulled ok and looked stellar, and not even havve to say how easy it became to work on them. but, i put on stock airbox w/out lid and reg. filter, also lil fatter pilot, and that thing pulls to 9.5k w/out hesitation. this was on my xj650 seca. just sayn. we got a great used parts barn here in nh called coopers. i have been snagn wishlist parts for my 550 custm build, and plan to maybe grab some constant velocity carbs from dif. model (research involvd, lol).

 

The original carbs on these XJ's are constant velocity carbs( have little dome shaped hats on top). Constant velocity carbs don't like air or exhaust mods and the YICS ports don't help either.
Kehein CR's are great, but costly. Hard to find a rack of different carbs that have the same spacing as the XJ.
A couple of guys have done the EFI conversion, but that can be difficult and costly as well.
I did a 2 round slide carb conversion for about 500 bucks, but that required fabbing some custom intake manifolds.

 

Sounds similar to my case. I still have my (undamaged) stock airbox, but I don't have the stock exhaust (got a 4-1, probably the same one as you).

I plan to run one of those automotive cold air intakes mounted on the back of the airbox I build, so it looks like the bike has one (gigantic) pod! I don't have any side covers but I managed to relocate ALL the electronics AND the battery to be under my tank (its a Virago tank, has a much larger tunnel), so it should be rather eye catching. If it doesn't work out well, I CAN switch back to 100% stock carb config.

 

The Carbs we have on our Bikes are C-V Carbs.
"Constant Velocity"
The Intake Air entering the Carbs Intake Horn needs to be accelerated by traveling through a constricted space.
Velocity Stack.
That''s what the Airbox Rubber Boot does.

The rapid column of Air entering the Intake Horn is necessary for:
• Reducing Pressure at the Emulsion Tube exit orifice creating the condition that draws Fuel up into the Air Stream.

• Reducing Pressure at the Diaphragm Pistons Bottom Orifice creating the Vacuum that deflates the Diaphragm lifting the Needle Valve as the Piston rises.

 

i lay more confused than i thought. i know this is a tread lightly suject and certainly dont want to go down that road. i tried hard to get same power characteristics; using all sorts of combinations in velocity stacks, filter material, tubes and jets. i would say i took "pods" head on, and regardless- that stock airbox pulls harder and a hell of a lot easier. i tracked ploblem down to what many of you have said; the air was rushing by emultion tube too quickly as to promote proper burn of fuel. im certainly leaning towards custom airbox. thanx so much for shedding more light on subject and fixing my ignorant ways. . i also want to say that i have learned so much from reading your words and thank you

 

Yep. Like I said, planning to mimic their length and profile in my new airbox. Or I might just use the rubber units I have, to save time; if the new airbox is easy to remove, I'll be free to replace them later.

 

Eventually, the problem to overcome is a Lean Condition caused by Main Jet Supplied Fuel not being Siphoned into the Intake Air Volume in sufficient quantity do to the Intake Stream not being shaped and accelerated enough aas it passes over the Top of the Emulsion Tube to Siphon the Main Jet Fuel up the Tube.

The condition is further exacerbated because the Turbulent Intake Air fails to reduce pressure at the Diaphragm Piston Orifice reducing the extraction of the Needle Valve from the E-Tube.

Also, ... it is likely that the Flow of Intake Air --> across the Kidney-shaped Atmosphere Vent at the Top of the Intake Horn causes a REDUCTION of Atmosphere in the Void surrounding the Diaphragm Rubber, ... thereby starving the MAIN AIR Jet of sufficient AIR to vent down to the cavity supplying the needed AIR to bleed through the Holes drilled in the E-Tube to assist in the siphoning process.

 

jeez rick, your the man. i can feel your knowledge flexing from here. 8O sebwiers, you got the right idea; stock rubbers into custom airbox. the rubbers will look right, be easy to remove/mount, and they are already made. have you have a modified space that that box is going in? i purposely built space into my custm frame; having a feeling all along that i would be going airbox.

 

Yeah, I've modified it. All the electronics (battery included) will be entirely out of view (under the tank) and there's no side covers (or need for them). If I ran pods, the entire "under seat triangle" would be empty space. My saddle is the same height but much thinner, and I removed the rear fender (plan a replacement, but it doesn't intrude on that space).

LOTS of room to play with.

 

I will make a concerted effort to work out how the get the picks on. As I was just trying to repalce the air box on an almost standard bike I did not spend to much time making it pretty. However I recon if you wanted to, you could make a preety cool looking air intake sytem for a customise bike if you wanted this way.

 

Yea I would love to see those pic Paul

 

paul has not been here after sept 2014

 

I'm in the thought process of this custom air box. While attempting to remove the carbs for a cleaning (p.i.t.a.) I figured the stock air box can go in place of pods. Well after reading about issues you guys are having, I'll go the custom route. Well, I ended up breaking an intake bolt. Turns out mine are very corroded.
Anyhow, being a fabricator welder for 25 plus years. This is what I'm going to start with for shape and dimensions.
Cardboard & sillycon
Its cheaper than steel or aluminum for trial and error purposes. Plus a custom air box will allow for easier carb removal.
My idea for the electronics is to move them right up under the seat where the fuse box is. Relocate the battery more inboard & cross ways from its factory location. Then fab side cover plates out of aluminum.

 

It might be a unique case but I have had absolutely zero issues using pod filter on my bike after getting it jetted and tuned. I was also scared to get rid of the air box but didn’t have a choice due to the luck of space and I am glad I did. Bike performs as good as factory.

 

bike ,pod brand and jet sizes please . any other mods besides pods? Standard CV carbs?

 

And have you ridden a well-tuned stock XJ before jetting for pods?
Dyno results say that pods are not as good as stock, but can be close enough that most people won't notice without making a timed run (or using a dyno).

 

83 XJ750 Maxim, UNI pods, 43/134, stock collector with aftermarket baffles, no mafflers. Factory hitachi carbs.

 

My XJ was well tuned and 100% stock prior to me modifying anything. Considering g that my bike is 105lbs lighter than stock, it’s difficult to compare acceleration wise. But drivability is as good as factory. Starts great, plugs look as should, no hesitation at any RPM, no dead spots, pulls very smooth from idle to redline, steady RPM at 1050.

 

The air box was off on my 650 so guess what im diving into. Granted the patterns are for front, back, top & bottom. The sides will pretty much fall in place.
Its pretty close to stock box dimensions. Only difference will be the use of a spectre 3" maf adapter which will accept any 3" air filter. Final product will be in sheet aluminum & tig welded.
As far as air flow, I'm guessing more than stock but less than pods.

 

Its getting there

 

The jet sizes are going to vary per application and its not that difficult to nail down which sizes to run especially since jets are so cheap. its the overall tuning of the carb which is key, jet sizes + color tuning + needle/plunger spring adjustments + float bowl level adjustment if necessary. I found on my end the float adjustment weren't needed but that doesn't mean Joe Schmoe wont need it.

Pavelk313's carb setup with 43 130 is a good baseline for his open exhaust setup. What i am interested in is if stock and pods are raced side by side on a quarter mile, will the lighter frame make up any difference on the lowend that pods might cause.

 

I have 134 mains. 130s were a bit too lean. Also, as you mentioned, I didn’t just throw new jets in new jets and everything was perfect. Had to do some tuning, once that was done, bike performs great. I would love to race a stock 750 to see where I am at acceleration wise.

 

That would be cool to do.
It would be interesting to test with different variables accounted for. Maybe three runs per rider on each bike (6 runs total) as-is, and three runs with some ballast on your bike to bring the weight to stock. That would give a good indication of what effect your carb setup has, what effect your weight reduction has, and what effect the rider has on 1/4 mile time.

 

That would be ideal, but not very likely to ever happen unless I buy another XJ. Lol
Plus, not sure how I would possibly add a 100+ LBS ballast to my bike.
I been wanting to take my bike to the track but I don’t thing necessary gear to run faster than 14s and I am confident that it should be faster than that.

 

I'm pretty sure I can pack 100 pounds of lead into a tankbag.

 

Downloaded an app today and tried doing 1/4 mile run.
What you know about that 8.1@32 mph Hahahaha

 

Might need calibrated a little

 

You could let me ride it!

 

Back in the 70's People ditched their airboxes for a single filter unit called a Breadbox. It used rubber extension stacks between the carbs and the filter box and worked great. Wasn't that pretty, but it worked and only required one filter to change over time.
Personally, I would ditch the carbs and buy 4 round slide carbs. Block off the YICS channel with a 3/8" steel rod.

 

Why get rid of carbs & YICS when everything work perfectly fine?

 

He has a different opinion. He's not wrong, but neither are you.
Choices, tradeoffs, opinions.

 

Well the reason was CV carbs are a big enough PITA to clean and sync for a stock setup. If you change the exhaust or air(Pods), then you get to go through finding the right jets to use and every time you sync these wonderful CV Carbs, you have to go through the 2 step system of blocking the YICS channel for 1st sync, then unblock for final sync.

The YICS was only used on these bikes and it does a great job, but the carbs, air intake and exhaust all have to be set up just right. If that channel has to be blocked just to start the sync process, you have to wonder if it really does as much beneficial application as it was intended and if all the trouble is truly worth it.

When I blocked mine off for my custom intake and round slide carb setup, I didn't miss it one bit. The system was easy peasy to set up and sync and it ran better and faster.

I don't get upset if people want to keep their bike original or mod it. Blocking that channel off permanently as I stated isn't exactly completely blocked. There's a tool for blocking the channel for the sync process that has rubber grommets on the shaft that completely seal the chambers for each piston, but those grommets have to be replaced after some time. if they were used all the time, the grommets would be completely destroyed in quick fashion.
What I did was to unscrew the bolt on one side of the channel, measure the hole's diameter(10mm) and put a 10mm diameter steel rod through to the other bolt and mark the length of the rod needed, I cut the rod to the desired length(I believe it was 18") put the rod in and screwed the bolt back into place. The rod doesn't completely block the channel for each cylinder, but it's enough to simplify the sync process. Over time, deposits will form around the rod in each cylinder area of exposure and permanently seal each cylinder for even better ease of sync and performance.

Gotta ask yourself, If it was such a great idea, Why was it only used on these bikes and only for so long? To my knowledge it was only used in the 80's on the XJ series. These engines were made great, just difficult to keep them running great(carbs), questionable YICS channel system and a terrible starting design(starter clutch that requires an entire engine tear down to fix or replace). Other than that, these are wonderful bikes.

 

+1 on jet sizes. I just finished a pvc tube using 2 90°'s and 2 T's. Cut to match distances between intake horns, hole saw on the opposite side and sanded straight and glued the 6 pieces cut off to give me about 4" . I put the extra stock fz8 muffler i had on it, jetted with 116s and 43s i ran for about 4 minutes and finally holding my choke on all the way doesn't make my engine rev high, I'm bench synced ready to do running synch valves in check compression was 148-151 all the way across. Have bpr7es so took out the resistor in the caps it's just been too cold to start this I'm afraid mid 30s will not give me a good tune, is that correct?

 

I know this thread died in Jan, but it can still be informative....and related to an exhaust thread.

Factory air boxes are serving 2 main purposes... Restrictor! and Filter. Mostly focusing on the first
That's why they vary in size and shape. It is simply when the manufacturer is designing the engine, they strive to achieve whats known as VE "volumetric Efficiency". I somewhat explained this in the exhaust thread already, but any ways.... different countries have different (smog) pollution and fuel economy restrictions, so they make air boxes to filter and restrict the (volume) of air allowed to enter the carbs.

Un restricted air flow allows a larger cyclone effect altering the ratio of air entering the CV cavity and overall carb venturi (Mouth). Unrestricted air flows such as pods allow the larger cyclone effect causing, more air, more fuel equaling rich conditions, and by effect altering the VE percentage. The airbox restriction, and exhaust diameter, exhaust length along with engine design and tuning dictate the end VE %. More air in requires more fuel = a larger fuel jet, larger exhaust need...

All countries and manufacturers strive for a VE of 100% which equates to a perfectly tuned and net zero smog rating. Thus a perfectly responsive, low consumption, and clean (no pollution) running engine. This is strived for, but does not exist....yet!

Average engine VE is approx. 70%. some newer engines can and have achieved a modern day VE of 85% and some diesel reaching a 90% with the introduction of fuel injection, catalytic converters, and VVT (variable valve timing), egr valves and turbos. I don't use the phrase lightly, as VE is what has been driving engine designers for decades now in their designs.

Any engine designed from the mid to late 70's to today was designed built and tuned to this vision of power to VE ratio with the two constantly conflicting the other.
So when you have a "stock" engine and you begin removing/replacing parts that were designed to improve the runability and work in unison, expect to see big changes..

Example: most notably the pod filter swap out usually not ending well for most.
mostly due to not having any understanding of the system they removed the part from. Other factors usually not considered are.. altitude, ambient temp, fuel octane to gear ratios to name a few.

So your airbox is to restrict, and clean the incoming air....nothing more altering the allowable volume has a chain reaction of required alterations all the way thorough the engine to the tip of the exhaust. there's a lot more factors and details, but you get the jest...