CV - Constant Controversy

The Question - The neighbor wandered over the other day and popped this question. Is the CV carb system worth the money?? I explained the theory behind it, but was wondering if anyone has had experience with it. He's ported for upper mid by R.E.D., Toomey T-5's, Boyesen reeds, Cool Head 19 cc's, stock carbs tapered bored out 1.5 mm. MSD ignition. He's wondering if it's worth the money to buy the CV system over like say some 28 PWK's or 31 Mik. flat slides. Tough call for me having never actually rode/seen a motor with it..... He's the next size larger available than I, I think he's pushing 280 lb............

The Response - I had the CV manifold system on my Banshee, and I loved it! When it comes to the technical aspects of motors and other things mechanical, I am "functionally illiterate". When I hang out here, I find out how little I know. I really like reading the posts on this board when you guys (Rick, Backcountry, trax310m, High output, YSF200 and more) analyze and discuss what makes motors go fast and why.

My experience with the CV system , THE WAY IT WAS SET UP on my Banshee was as follows;

When I got the CV manifold system installed, I was told that it would increase torque and give better low end to midrange response. This was not what I experienced. But the actual results were far different and better than I dreamed of! What I got was not what I had expected. I did not notice much difference in throttle response, in fact the bike was very peaky. The bike pulled incredibly hard from upper mid-range to WOT. The bike was awesome in the way the motor would rev. I loved it! It was a screamer.

Although I don't know details like squish, etc. I'll list what I do know about the setup.

Trinity CV manifold, Trinity Stage III porting, Toomey T5s ,35 PWK carb, Cool head with 17 cc domes compression 180 in one cylinder and 182 in the other, usually ran a 165-170 main, a 168 pilot and DGJ needle with the clip on position 5 (all the way down). This motor was a high revving screamer. I NEVER got beat racing at the hills by any 350, 370 or 420 Banshees(at this point, out of respect to all you 4Trax enthusiasts, I am going to leave those comparisons out of this message) the only Banshees that beat this motor were big bore bikes. On the dyno, all the power was at high revs, peaking out at 74 hp. That was my experience with the CV setup. I LOVED it for what it was in my bike, but it was a peaky motor that revved high and pulled HARD at high revs.

I am writing about it in the past tense because I am just about to take delivery of my new motor (next Tuesday in time for Easter week at Glamis!). The only (non-Honda) bikes that beat that motor was big bore Banshees, so I had to have one! Trinity is building my 500 cc alcohol motor, and Harry got the aluminum sleeves today, so he's going to do the porting and milling the cylinders before he sends the cylinders to get the Nikasil plating. The crank is done (10 mm stroke) and I think they have set Thursday to do what they are going to do to the cases (match port? Or something like that). Anyway, I'm really exited!

Back to the CV system, I am selling my whole top end as described at the beginning of this message, ported cylinders, pistons, etc., CV manifold, 35 PKW, Cool head and domes for $1000.00 or best offer. (Toomeys not included) The price of the individual components is about $1600 (not including the porting).

John

They are really made to enhance low end power. I haven't tried them, but from what I here, "they suck!!". I think that he would be better off buying the larger carbs. He may even want 34 or 35's if top end is the quest. Allot of my friends are going to the 35's. How does he feel about that MSD ignition? Sounds like a super fast bike.

Brandon

Hey Brandon, thanks for the reply. I've heard good and bad about the CV system too, some people say its great; the best---then others say it doesn't rev worth a darn...... I have heard about a lot of people going to 35 PWK's. A local shop here, JSD Racing, recommends running regular 35 PWK's, not the Air Striker PWK, on most of their "hotter" ported motors for bigger guys. So we're trying to figure what's the best right now.... the CV system is about 4 bills, but a set of PWK's slightly used run around 220-250 around here, so there's a price difference involved.

I talked him into the MSD, I've got one on my R and WILL NOT ever go back to stock. The MSD is far superior to any thing that stock can ever do. The timing curve is adjustable which is a big plus, real hot spark.... the works, I really recommend it. I've gotten nothing but top notch customer service from MSD. Sand sport magazine did a dyno test on a slightly mod'd Banshee, and they said after they tweaked and twiddled they gained 5 more HP..... that's considerable..... You can run lots of initial advance, but the curve tapers to a more friendly retard at higher revs where less ignition lead is needed. Too much advance at hi-rev can lead to the motor working against itself.... BAD!!! I say buy an MSD you won't be disappointed. So what's up with your ride lately anyways, I liked peekin' in and reading your posts to Trax.......

Backcountry

CV carbs on a Banshee... Another attempt at "jet it and forget it?" Selling it as a low RPM thing (read - rob HP). The Banshee was never sold as a "slow poke" quad, so why is everyone trying to make them that way? What's the world coming to and why do we need low end on our Banshees? Why can't people let the quads be what they are and stop pretending to make them into something they're not.

I was looking at a set of RZ350 barrels that were getting a bore job today - you know the Banshee barrels WITH the power valves. They sure look like they should have been OEM on the Banshee. But then again maybe Yamaha didn't include them because they didn't intend for the Banshee owner to want or need low RPM power. The RZ350 street bike will spend much of its time at stop lights and in traffic where the grunt might be helpful but the Banshee is to be ridden fast and hard without ever looking back, needing to go below 6000 RPM or asking its owner to explain why or why not.

There are those who read "stones" to predict the future. You know, they throw the sacred stones on a table in front of them and "look" into them. When reading them it's not the position they land in that matters - its how the interpreter feel about them landing in that position.

Rick

The latency of the reeds and the ram effect of the incoming charge will keep the reeds open much longer than we'd like to think. Two stroke engines will still have their reeds open when the piston is passing 90° ATDC or more. They don't close at TDC or a few degrees after like it seems they should. Because of this, the twin cylinder Banshee is indeed feeding from a single 34 mm (or was it 35 mm) carb using the CV system. While one piston is going down at 90° ATDC (reeds still open) the other piston is going up at 90° BTDC (its normal intake cycle) - making the carb feed both sides at the same time.

Rick

Interesting, that (the latency in the reeds and the ram air effect) kind of backs up a tech. paper I read a year or so ago, that was on a strobe test done on reeds under various engine speeds loads, so on. The testers were very surprised to see how much movement there was in the reeds, torsional movement, wave like oscillation within an open reed, stuff that they thought that they wouldn't find. Like most of us, we figure a reed acts like a door, either its swung open or swung close, nothing too wild within that movement, however this paper said that the was LOTS of extra movement.

So I guess with that, the CV system is NOT a worthy investment with this type motor. What would you recommend for carb size and type for this application??

Backcountry

There's some good SAE tech articles like that too. I have that here somewhere... It seems that the reeds twisted, bounced of their seats and did any number of things that would seem curious, at best.

What first threw me was the name CV - constant velocity. This lead me to think of constant velocity type carbs - 4 stroke thing. In this case they are using one carb to feed both cylinders and in doing so they are mellowing out the power by virtue of under carbureting the motor. Anytime you take away CFM's from the intake the power will be more mellow.

I'm from the school of every cylinder gets its own carb and pipe. Installing and jetting smaller individual carburetors to each cylinder will produce similar results, though in theory it should have more on top. Small carbs carburet better as well.

Since the Banshee is a zero deck motor, it makes it more difficult to tame the porting to match to the smaller carburetors. More difficult, but not impossible. It is possible to mill 1 to 1.5 mm's from the bottom of each cylinder to lower the power peak as well. Adjust the head so the piston - it will come up above the bore - still has a properly set squish. It will be fine running this way, in fact I set up a LT500 (also a zero deck motor) that way several years ago that worked fine. With a stock TRX barrel it is possible to mill more than 1 mm from the bottom without having the piston come above the barrel. It is not impossible to mill a couple of mm's there (watching the liner to crankshaft clearance) to make it perform like a totally mellow ride. Give it a good squish and compression and you have the making for a wonderful learners bike or a bike for the spouse who is intimidated by sudden power surges.

Rick

MacDizzy's Two Stroke Technology Exchange is the Two Stroke Engine Capital of the Internet.