From the factory my ATV was jetted rich. Even though it would do 50mph and ocassionally 52. there seemed to be a flat spot where the engine had a hard time getting the last couple hundred rpms before hitting the limiter. I went to smaller jets and fixed the pull but seemed to loose some HP, rightly so - you only get HP from gas. After reading about TPI air jetting, it made sence that I was not getting enough air with the 150 main jet and the right jet was a 142 (less HP also).

So a trip the hardware store and a couple 3/4" threaded nylon fittings to fit in a drilled hole of my air box cured it all. I attached the fittings to the drilled hole in the air box that was able to accept a 1/2" garden hose (3/8" ID) and routed the hose around the engine and up to exit just under my speedometer. When the throttle was applied the hose was sucking hard enough to take the ash off a cigarette. So I knew that the air box was in fact starving for air before. Since the single hose was sucking for air so hard I installed a second one. Whalla, now the little 400 pulls strong the entire rpm range with no flat spots, a noticeable difference that can be felt (the bigger 150 main jets are back in).

In the winter when more gas is needed I need only to plug one or both openings to the garden hose with a bolt or a rubber plug, which will bring it back to a richer condition.

A very inexpensive way for a boost in HP and throttle response and to eliminate a flat spot. The engine is now breething pretty darn good.

Parts needed: The fittings are 3/4" threaded on one end and a male recepter for a hose on the other end, couple of thin nuts one on each side of the air box wall, and a small hose clamp. Total $3.00 x 2 = $6.00, You will also need about 3 feet of rubber garden hose for each fittings and maybe some plastic ties to hold the hose out of the way. The fittings were in the pluming dept and the nuts in the electrical dept. lol...

Change in altitude and need a richer mix just plug one hose or both hoses.

 

Sweet idea man! You should post some pics so I can steal your idea and patent it:-) JJ

 

lol - Davidson Industries is already taking care of that for me. I am in the process of hooking one hose to a funnel and facing the funnel forward on the machine (ala - 'ram air')...

 

Some pictures of what I did.

http://mysite.verizon.net/buck...ovie/750-p1050198.jpg

http://mysite.verizon.net/buck...ovie/750-p1050199.jpg

http://mysite.verizon.net/buck...ovie/750-p1050200.jpg

http://mysite.verizon.net/buck...ovie/750-p1050201.jpg

http://mysite.verizon.net/buck...ovie/750-p1050202.jpg

http://mysite.verizon.net/buck...movie/750-p1050203.jpg

 

I have some results of my mpg test. Went from 20 mpg to 24.3 mpg. About 22% increase. If the same is true with regard to HP, then that is a substantial boost in horsepower. The testing was done with average driving, some fast (WOT's), some slow, some cruising, some high-range 4x4 as well as some low-range 4x4. One results was a 70 mile ride and another on a 92 mile ride, to include dirt roads, trails, mud, water, rocks, gravel etc. All this for $6.00, I'd say it is well worth it.

Might add that my air filter is treated with 40 to 1, 2-stroke oil. It seems like when treated with Marvel Mystery Oil I get a little better zip on the throttle response - hard to be certain but just feels like it. I clean and treat my filter after every distance ride or at least every 100 miles.

 

Anyone tried this yet? It works great.

 

I want to thank Westslope for bumping a post, and I began to think that there is some good info with this post that will help people with respect to gaining some power and also understanding about jetting. Simply putting in a bigger jet is not going to give you more power, the engine will determine what it needs for a carb-jet. If you go to a bigger jet then the chance of it working are slim unless, you increase the available air (not just the rate as with a K&N, but also the amount available) and get an optimal air/fuel mix.

 

I'm going to do it this weekend. It certainly can't hurt .

anytime you can get more fresh air into the breather box it's a good thing. The nice thing is that you don't have to worry about this fix leaning your bike out. It just make it much more efficient.

 

This thread interests me alot. I have an H1 650 and am taking a trip to the Paiute Trail in Utah in September. My current jetting works great for my current altutude of 1000 ft (Winston-Salem NC). The Paiute Trail will mainly be ridden in 6000 to 8000 ft altitudes but will vary from 5000 ft to 11000 ft.

Looking at a chart I found it looks like my carb will need around 7% more air. Could I just put several hoses in the top of my filter box above the current air intake (before filter) and not rejet the carb? Any idea on how best to figure the size? Maybe 2 of the 1/2" hoses and when it runs rough unplug 1, then the other if needed. Another thought is one 1/2" hose and several 3/8" hoses.... Seems alot easier than jetting. I do realize that too much air to the point of lean is harmful. To little air just makes it run rough.

 

<div class="FTQUOTE"><begin quote>Originally posted by: roamerr

This thread interests me alot. I have an H1 650 and am taking a trip to the Paiute Trail in Utah in September. My current jetting works great for my current altutude of 1000 ft (Winston-Salem NC). The Paiute Trail will mainly be ridden in 6000 to 8000 ft altitudes but will vary from 5000 ft to 11000 ft.



Looking at a chart I found it looks like my carb will need around 7% more air. Could I just put several hoses in the top of my filter box above the current air intake (before filter) and not rejet the carb? Any idea on how best to figure the size? Maybe 2 of the 1/2" hoses and when it runs rough unplug 1, then the other if needed. Another thought is one 1/2" hose and several 3/8" hoses.... Seems alot easier than jetting. I do realize that too much air to the point of lean is harmful. To little air just makes it run rough.</end quote></div>

You can do the hoses without rejetting. As you need more fuel then you only need to plug one or more hoses with a bolt or something. A quick test to check your setting is that at full throttle, start to apply your choke if your machine increase speed or smooths out then you need to plug one or more hose etc. If while applying the choke if it stumbles etc then you need to get more air to the carb by opening up one or more hoses. It is like fine tuning or rejetting on the fly so to speak.