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jetting

12K views 54 replies 17 participants last post by  Lee 
#1 ·
I took off the air box lid and noticed that the air filter is fixed, so I figured I would run it without the lid, however its to lean and it wont run. Anybody know what stock jetting is? And what jets should I run without the use of the air box lid? Trying to get some cheap temporary hp....
 
#3 ·
Be very careful with this idea. The main reason it won't run is there is no pressure differential in the air box and the slides in the carb are not working correctly.

The need for bigger jets is not because you are getting so much air but because the velocity of air going thru the carbs is so much less that you need the bigger jets just to get you back the fuel you lost.

Taking the lid off and it going lean is not the added air but lack of fuel able to get in the stream of air going thru the carbs.

This is a big misconception that we have seen time and time again.

Take that lid off and you will lose power, trust me. Remeber those people who tell you different are trying to sell you a fixed air filter or some kind air intake.
 
#5 ·
I put the velocity air filter kit on this weekend. I tried the 12 one inch holes in the air box lid but that was a little much, I ended up covering up 9 of the holes leaving three open. Stock Jetting is 135 and 140. I tried a 150 and 155 and put 2 shims on both the needles and just wasnt enough, so I jumped to a 160 and 170 and put 3 shims on the needles and the thing really came alive! Still needs some fine tuning but there was a huge power gain by doing this. Im also running the dual exhaust from LAC. I dont have a air fuel mixture guage yet so Im not sure exactly where Im at. But I kicked alot of ass with it this weekend.. I beat alot of my buddies that I couldnt beat before I added the air filter and jetting.
 
#13 ·
The 102 was with a set of 44mm webers with an open style K&N! The stock cv style carbs will not run right without a lid!
 
#10 ·
I just picked up a hand full of jets and shims from my dealer, they had the shims in stock. Im not sure the thinkness of the the shims, they looked like they were around .031" thick. I would have also tried a 160 and 165 main but all they had was a 160 and 170.
 
#16 ·
Sorry to Mis-inform anyone but my calculations on drilling the jets were wrong. I drilled them using a 5/64 drill bit which does not equal 178's

This link explains the Dynojet system vs mikuni sizes.

Road Star Clinic - Mikuni/Dyno Jet Conversion Table

As explained in the article, DJ jets are coded in millimeters. 160 = 1.6mm.

5/64 (divide 5 by 64 to get english decimal ) converts to .078''. To convert to mm, divide by .03937. In this case, 1.985mm or somewhere between 185 - 187.5.

Hope this helps,

I just got back from Glamis and it ran great!
 
#12 ·
The engine runs on demand (suck) The more you open the slides, the more vacuum is made. (made by the downward motion of the piston when the exhaust valves open) The more air it sucks, the more fuel that it requires to maintain the proper balance. It has nothing to do with pressure differential. The only pressure problems is altitude and humidity.
 
#24 ·
Since I am wrong can some explain to me what opens the vacuum operated slides?

Seems like there is a butterfly in there operated by your gas pedal and then there is a slide that is operated by vacuum.

I also think the highest vacuum exist at idle and when you open the throttle the vacuum drops. This is the very reason for the vacuum style carbs, the slide won't open until the engine builds up enough vacuum to open the throttle no matter what the butterfly is doing.

Not sure where the Teryx carbs get their vacuum from.

On a Rhino there is a tube running from the vacuum diaphragm to the intake tube between the air box and carb, if you take the lid off a rhino you lose your vacuum in that tube due to pressure differential.

If you dyno a Rhino with the air box lid off the curve will go up and down and the reason is the vacuum slides are opening and closing.

Like I said, I don't know where the Teryx gets it vacuum because I have not looked into it yet. What I can tell you is if you take the lid off it won't run at all unless some changes are made and simple jetting won't be the answer.

Not sure where the 102hp engine came in, last time I checked a stock teryx got a little over 27 at the rear wheels or at least that is what I got on our dyno.

If we had a mechanical carb instead of a vacuum operated we could do what we wanted to on the intake side within reason.

I am new to this vehicle and engine so I have lots to learn so until I get to that point in our R&D I can't say much.

I won't be making anymore comments on this subject until I do it and find out for myself.

You guys do what you want, I was just trying to help.
 
#25 ·
I got the Velocity air filter kit and installed it put the lid on did not drill any hole and rejet one size over stock. Rode it hard for about 5 min and the rear cylinder exhaust pipe was cherry red . Any ideas on how much higher I need to go on the jets? All I have is the VDI and the filter kit and a exhaust tip.
 
#26 ·
on all cv stle carbs the vacume is created from air flow through the carb. to stop the fluter you need to flow the air correctly. its alot like an airplane wing. google" bernolli's principle " this may help you. basicly if the air in is turbulent the presure diferential will change inside the carb rapidly up and down causing the slide to move up and down.
 
#27 ·
I looked that info over, have studied that before but a refresh was in order.

Like I said I will look into the Teryx carbs more but let me put something out there.

By the way I have a couple of guys I lean on for information and to run things by. One is Lon over at thunder products and the other is John Knepp who designs engines and fuel injection systems for the big three automobile manufacturers. He is a physicist too for what ever that is worth. I am not smart enough to know the book answer and when I come up with something to explain something I run it past one of them and see if it holds water.

I want to explain on thing on the Rhino and at this point I don't know why it would be different on the Teryx but might get proved wrong, it won't be the first time.

The engineers designed the air box the way did for a reason now if it has a "restricted opening" it has a reason and here is what I think.

When intake valve opens it creates a low pressure area and atmospheric pressure pushes air into the engine via the carb. Some call it a vacuum and that is not really what it is, a vacuum is what you clean your carpet with.

Anyway, the diaphragm is operated by the tube in the intake that connects the carb to the airbox. By having a restriction of the correct amount on the lid when the intake valve opens opens it causes a "vacuum" or lower than atmospheric pressure condition inside the airbox and the vacuum operated slide has to open because of this vacuum.

If you were to connect a vacuum cleaner to a pipe and put a vacuum gauge on the pipe and then restrict the end of the pipe the vacuum would increase,

In our example the engine is the vacuum cleaner, the pipe the airbox and the restriction is the factory opening. Ok so now if we come along and remove the restriction, "air box lid" there is less vacuum available to open up the slides.

Like I said this is how the rhino system works and I will look into this more on the Teryx.

If you guys only knew how many people called or e-mailed me and said "I put so and so's air intake on and so and so's exhaust on and now stock Rhino's are out running me, why?" it would blow your mind.

Now I am not saying this can't be done but you need to take a more scientific approach to it. This kind of reminds me of the guy who told me I must the only idiot that didn't think a machined sheave was a good mod for a rhino. I told him I didn't say it was not a good mod but I have not figured it out yet and with all the problems with the then present offerings it didn't look good. Now after selling several hundred of our our own machined sheaves with no issues of reliability I think it is a good mod. We did take another approach than the others.

Guys I am here to help, I ain't selling you a air box. We spent about $30,000 on our dyno and thus far we find more what does not work than does, these factory engineers ain't stupid.
 
#29 · (Edited)
More than likely, the reason a Rhino is so sensitive to airbox and connecting tube issues is due to the fact that it is a large displacement SINGLE cylinder. The intake resonance wave is very strong in a Rhino due to that huge piston going up and down. Air is moving both directions through the carb. The CV carb operates on pressure DIFFERENTIAL. When the CV carb diaphragm is connected to the airbox, the pressure differential is more consistent because the intake resonance wave is also changing the air pressure in the air box.
CV carbs are also tuned by the spring pressure over the diaphragm, passage sizes, etc.
That being said, it is possible to tune a CV carb without an airbox. However, in some cases, like the Rhino, it is more difficult than others.


Only if the engine can use it. You can also kill velocity and change the tuned length by modifying the air box.
Not saying it shouldn't be done, just that more is not always better. :D
 
#31 ·
The stock Teryx intake already has a velocity stack for each carb built into the plenum chamber. It is actually the rubber boot that goes into the chamber, it is molded with a velocity stack shape.
 
#32 ·
S2K1, don't know who you are but you are talking my language. You are spot on, on the Rhino being a big single and the reverses being so powerful. This is just one of the many reasons I will really like this V-Twin!!!

I just wish I was further along than I am. We have a TEryx at HMF getting an exhaust made right now and until it is ready and I have one I have been putting off doing anything to the intake.
 
#34 ·
I am Benchmark Performance, in San Diego area. I spent the last 11 years working for Honda, the last 6 in R&D.
I am also waiting for exhaust, in our case the full system that jimmmyzz has, and is the prototype from AC Racing Flow. We are supposed to have the 2nd system soon and will be helping AC with jetting.
I am currently working on CNC ported heads for the Teryx, which will be used by HRT Motorsports on their Teryx that they are building for CORR. More than likely we will be ditching the carbs and going straight to EFI. Can't wait to make that V-Twin rumble!
 
#35 ·
S2K1,

Where does the Teryx carbs get their vacuum for the diaphragm? I have not had time to dig into it myself yet.

I for one am not so quick to start screwing around with what the engineers have designed on these vehicles. I know alot of people have a good many ideas that they got from some magazine article or a mechanic buddy that simply don't work for a lot of reasons. I for one take a much slower approach and do it in stages.

Because of the Rhino's issues we were not able to do much and gain anything but I do believe there will be some intake mods along with others that will make air in and out better on this engine.
 
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