I wanted to open my airbox as much as I could without elliminating it, given the fact that the intake boot provides more ponies than a carb mount filter and all the best dyno numbers came from airbox ellimination altogether. i chose not to remove the airbox because I need the protection to keep puddles and kicked up tree limbs outa my filter, but I wanted as much gain as possible none the less. So with lid removal and snorkel removing and drain hole cap removal I'm as open as can be without doing any cutting on the box, while maintaining filter protection and also the ability to throw on the lid and stick two plugs in the front snorkel holes for a carwash trip.

Well it seems the problem is fuel/air mixture changes at speed. At high speed the airflow ramming through the snorkel holes in the front leans the mixture a bit, and when jetted for 80 mph airflow through the box the bike ends up being rich when trail riding. Drop down to the appropriate jet for full throttle in lower gears on 20-30 mph trail rides, and then she's lean on highspeed field runs. Airbox ellimination does away with these variables since the air ramming into the box is gone and you get the best and most consistant power this way, but so is filter protection lost.

Seems there is no middle ground, either lose the box and stick to dunes, or keep a closed box for trails - or at least maintain the snorkels but lose the lid, but problem is its pretty hard to fit that right side snorkel around the carb throttle wheel of a Mikuni flat slide. I do dunes and trails enough to make switching the airbox setup back and forth a pain, not sure where exactly I'm gonna go from here so suggestions and experiences are certainly welcome folks.

 

Wow, good question. I don't know your set-up so I will give you a general overview of some of the things I have seen.
I've seen this in cars before but not on any other vehicle. This is what I know about it. You can get into trouble making your own ram-air. Some turn out real good, and you pick up power. Then there is the other side of the coin, some actually loose power. Why? Well, you know what happens in your carb when you pass high speed air over an orifice? It creates a low pressure area which forces the fuel up and out the orifice. This can happen with <u>some</u> ram air systems. On a car it can get bad because the carb is 90 degrees to the air flow. Bad news to not run an aircleaner on a car running high speed. It will create a low pressure area right above the carb opening and the air and fuel will try to reverse flow. This is impossible because the pistons going down in the holes will create a bigger low pressure area; but it will cause big problems with air and fuel flow none the less.
As a general rule, the higher the pressure over the orfice (jet) the more fuel it will pull with it. So, at low speed, the carb will pull very little fuel, and at a higher air flow the carb will pull more fuel.
What I'm saying is that it should not lean out at high speed if it is jetted right on the bottom end. The only thing I can think of that will viloate that rule is eratic air flow, or an air leak behind the carb. In other words, a bad ram-air design or a loose carb flange of some type.
Hope this points you in the right direction?

 

I'm sorry Doc, I may have presented my issue a little unclear, you may have misunderstood it, but at the same time I do see your point about airflow, pressure, and fuel delivery. I ahve no fancy setup of any kind, just a naturally aspirated motor - K&N in airbox with the two snorkels removed and the lid off.

What I mean is that by removing the snorkels from the front of the airbox the open holes now catch air as I move in a forward motion at high speed, forcing air into the airbox rather than allowing engine vaccum to pull it in. The air being forced through the airbox is traveling from front to back at a high rate, while the motor is trying to pull air from back to front, and this accidental ramming of air into the airbox is causing the eratic flow you mention.

I think what I need to do is let the box breath naturally, let the vacuum pull air as needed, and dependant only on rpm rather than mph. This requires the ellimination of holes in the front of the box, leaving the question of how to allow the motor to breath nearly as well as losing the box alltogether, but without actually losing the box because that's not an option due to the above mentioned protection necesary on woods trails.

One option may be to install 90 degree elbows on the holes in the front - no more air being forced into them from forward travel, but still open for vacuum to pull air through......this I will try......

 

Hightower I agree with you. If the air is just available to the carb there should be no problem. But then again, one hole in the wrong place could cause eddies in the airflow that only someone with flowbench or wind tunnel can figgure out. Like I said, I've seen it on cars; could this be happening with your quad? I really doubt it, but then again, strange things happen?
Just giving you some ideas to think about. Maybe you can spot something that is out of wack? From what you have posted I think you will figure it out, good luck.

 

Hightower,

Take a look at my pictures link - see if you can try mounting the snorkel tubes outside of the frame rails as I did - I think this will solve your problems. When I first mounted them there, I thought it might be annoying, but I've never bumped them when riding, or getting on or off the ATV. Mr. HP said he saw a 2 HP improvement just by moving the tubes to the outside of the rails like that on a dyno. I figure the cost of 2 zip ties is worth 2 HP. Obviously your carburetor wont see the same improvement as the stocker, but this mounting location might work just as well for you.

 

What if you put a whole in the back of the air box so the air passes through instead of getting pushed into the carb? Or possibly close off front snorkel hole and put holes in back, lot less chance of mud or water getting forced into box that way.

 

What if you took some outerwear material and covered the holes with it. It would stop the turbulant air and would let air come in more naturally. It would also add some splash protection.

 

all good suggestions, thanks guys.
After working all last night on the clock and pondering, thinking outloud on the forums and such, I went home this morning and installed a PVC elbow into each of the snorkel grommets and popped them in. The results are good - the fuel and air mixture is now uneffected by bike speed, whether its 2nd gear trails or 85mph on the road or field runs she's very consistant.
To top it off, splashes getting through the once open holes are elliminated, and the elbows are just long enough to offer plenty of protection from blasting with a spray wand at the carwash.

To recap - with the holes open, it requires going up two main jets for full throttle 5th gear at ultra high speeds, with the compromise being too rich at low speed. Now with the elbows on the holes I'm jetted down those two sizes and the fuel and air mixture is very consistant regardless of speed, not to mention the added filter protection. I'm still up two main sizes from having the holes totally plugged though, so some HP gain is still there.

I opted not to cut holes in the box just because I may one day want to close the box again, if I get into some hardcore XC with deep creeks or something.....and wanted the carwash protection without a lot of hassles too.