How CV Carbs Works and air box mods

[Hunterworks]

I went to my shop this morning and tore down a Rhino Carb and a Prowler carb plus did some studying online. I also talked with two more guys in the know, one at Mikuni and one at Thunder Products

While I have NOT got it all figured out I did learn more about how a CV works.

The original issue I was thinking about was taking the lid off or modifying it.


The diaphragm on the slide works off a difference in pressure on either side. The bottom has "atmospheric" pressure under it and the top side gets"vacuum" from the venturi effect in the carb. The pressure above the diaphragm drops as vacuum builds thru the carb causing the pressure above the diaphragm to be lower than atmospheric pressure under it which causes it to over come the spring holding the slide down and opens the slide. There is at least one hole in the slide to allow the vacuum to pull the air out from above the diaphragm thru the slide. People understand vacuum better than lower than atmospheric pressure better.

I have the privilege of have receiving hundreds of e-mails and phone calls from customers who have made these mods and get less than expected results and want to know why.

Oh yea, my lesson.

The Rhino, Prowler, Teryx carbs all have the bottom of the diaphragm vented to atmospheric pressure but not really atmospheric pressure, that is a book answer, all three of these vent into the airbox by two different methods. Both the Teryx and the Prowler have in the intake side of the carb a inlet at the top of the carb that goes directly into the underside of the diaphragm and the Rhino has a tube from the underside of the diaphragm to the rubber boot that connects the airbox to the carb down from the carb about 2 to 3 inches. From what I know all airboxes have a pressure differential between real atmospheric air and the air pressure inside the box, this difference is minute but it is there. So the slide hole size, diaphragm size and most definitely the spring pressure on the diaphragm has been carefully calibrated to the inside airbox "atmospheric pressure" and should NOT be played with without a very good reason and with the right instruments.

When you take the lid off or drill all these holes in the lid you now equalize the airbox "atmospheric" pressure with real atmospheric pressure and the diaphragm can no longer work correctly. NO amount of jetting in the world will correct this issue, the reason you then need the larger jet is to get back the fuel you lost by messing with the airbox. Imagine a drinking straw, your mouth provides the vacuum to pull liquid thru it, if you poke a hole in the side of the straw the liquid will either no longer come up the straw or will be much harder because you lost your vacuum thru the new hole in the side of the straw. In our case removing the air box lid or putting holes in it is the hole in the straw. Now the engine can't pull the fuel up thru the main jet and you have to put a bigger jet in to get it to come up. Liquid flows better thru a larger hole. Most people think they needed a bigger jet for all their new air, and that is not it at all.

Also taking the lid off or filling it full of holes makes the slide go up and down because the pressure can't equalize correctly. Oscillations occur and this is why our dyno graph with lid off looked like a couple of letter M's side by side, the slide was opening and closing.

The airflow, pressures and pressure differentials change as you run thru the RPM range and things react different. The big point is the engineers at Yamaha, Kawi, etc. are way smarter than I am and I just don't go screwing with their set up just cause some magazine article or some forum post says you get more HP from doing it.

I just didn't think if Todd from Hunterworks(cause I ain't no expert) says don't mess with your airbox cause I said so you would do it. We all need some explanation why and I love to learn stuff too , I did on this just in the last few days.

My real point is and I think you know is you just don't go slicing and dicing up a airbox. If you put a weaker spring in the diaphragm you can open the air box up some, if you bore your cylinder you can open it up some.

The intake opening size has been calibrated to the needs of the engine all the while giving just enough resistance to make the slide open correctly. You have to do something on both ends to make this work out, like the lighter spring and larger hole.

I don't have all bad news, you can modify the airbox but it just takes more time and good equipment to take the proper readings and you also have to change the air consumption of the engine to need it first. We will be doing this in good time with proper instrumentation and will post results of the work we will be doing. One device that would work on the air box is a TPI valve from Thunder Products and by using it you will be able to adjust only the amount of opening that is needed. These devices are also filtered, but again careful planning and testing is required.

Now if the diaphragm was really vented to the outside then it would not matter as much as to what you did to the box but it is not.

Last thing, for God's sake don't turn your airbox into swiss cheese!!!

Update 8/09/2008

I just reread my own tech article and realized I need to update it, while the article is correct we have done some research of our own and came up with a kit for the Teryx that simply works. Here is the way the scenario goes

Need more power then you need more fuel, got more fuel then you need more air, got more air due to opening the airbox up you lose vacuum, lost vacuum then you need weaker diaphragm springs to offset the loss of vacuum. The HW Teryx carb/air intake kit takes care of all that.

Bigger jets for the fuel
Two TPI valves for more air
Much weaker springs in the diaphragms to offset the loss of vacuum

The Dynojet Teryx carb kit is fine if you don't open the airbox any, it will add a bit of extra power, but if you open the airbox any the springs won't be the right spring rate. So what we did is find the right spring to match the opening size we added to the airbox and we also drilled an additional hole in the bottom of the slide unlike what Dynojet does. The end result is a kit that really performs. On our dyno we got the largest gain between 15 and 25 mph which was about 6hp with a peak gain of about 2hp. This run was done with a programmable dynatek CDI with our timing curve and the exhaust tip removed. Of course the before run had the timing curve and exhaust tip removed too so we would only be comparing what the kit did.

We have sold a good many kits with very positive feedback and very little tech support which means it just works right out of the box.

Shootin' Straight,
Todd