All my efforts to light up my H.O... AAen cam, K&N, muffler mods, are being thwarted by that Constant Velocity carburator. It only opens as far as the engine vacuum allows. I want some PUNCH!! I hear the TMX Mikuni flatslide carbs have the top end, but bog off the line. What else is there?? Anybody??
Silly Later,,,,Fourlix

 

Try to get a better understanding of the CV carb before you condemn it!

Call Rick Ritter, and he can explain to you why you DON'T want a flatslide for what you are doing.

 

Cruisin' the Web on Mikuni sites and Thumper sites
I've found the TMX carbs are for 2-strokes. The TM with accelerator pumps in 33mm and 36mm are for 4-strokes. This is why the problems with bogging off the line with a TMX, it is a 2-stroke carb. Works great on the 400, but it is wrong for the 500 Scrambler and H.O. So we got the 36mm TM or the Harley Mikunis at 40mm (discontinued) and the HSR 42 and 45. Are the floats designed for
off road jostling? What are the racing 4-stroke
motocrossers using? Is there a way to make this Keihin CV carb more responsive?
Silly Later,,,,Fourlix

 

You make a good point, DieselPro!

A CV carburetor, wide open at high rpm, does all any carburetor can do: deliver an optimum air/fuel mixture to the engine.

At full throttle, the butterfly is wide open, the slide also, the needle fully withdrawn and the main jet totally uncovered. We approach the theoretically ideal carburetor at this point; minimum restriction, optimum mixture possible.

Then why buy carburetors of other designs? Some may offer quicker throttle response. A carburetor with an accelerator pump, for example, dumps raw gasoline immediately into the airstream when the throttle is opened, eliminating the delay or lag time of a CV carburetor as the vacuum-operated slide is lifted.

While operating characteristics differ among carburetor types, the CV carburetor design in itself will not limit power output.

Diogenes

 

On a dyno or top speed the CV carb may work as well as a mechanical carb. But Throttle response and acceleration are a different story. Case in point. Holley 4-barrel carbs, Double pumper with mechanical secondaries VS. single pumper with vacuum secondaries. HUGE DIFFERENCE!! IT is all a matter of asking your engine to rev (CV or Vacuum actuated) or TELLING IT TO REV!! (Flat slide Mikuni or mechanical secondaries) Look guys I'm looking for horsepower here. Extolling the virtues of CV carbs, spark arrestors, stock mufflers etc is not what this is about. I have found that the TMX carb is just not for 4-strokes,
yet is being sold to Scrambler owners, despite the bog off the line, due to no accelerator pump.
There is a TM 36mm with an accelerator pump, for 4-strokes, and I'm trying to find a 38 or 40.
I can go back to the round slide VM series,
which is a lot cheaper. But I might as well try to get the new high tech carb first. Hey, Electronic Fuel Injection, That's what we should be dealing with. Altitude compensating, self-adjusting, This is the third millenium and we're arguing about CARBURETORS?? My last Hot Rod (10 years ago) really took off after I canned the Holley 4-barrel and put on EFI. There's a lot of my miss-spent youth I'd love to live over again, screwing around with lame carburetors is just not at the top of the list....
Silly Later,,,,,Fourlix

 

The carbs used by the new thumper dirt bikes are the BST 40 Mikuni for the KTM 400 and the Suzuki DR650. The Keihin FCR 39mm is used on the
Suzuki DRZ400 and the Yamaha 426YZ and WR. These carbs are going to work best. Now it's a matter of what fits, and what's available at a decent price......
Silly Later,,,,Fourlix

 

When Polaris designs an agressive ignition system with digital maps, and so forth making the 500 a high revving beast, THEN a carb will be necessary.

THEN the magazines (who keep saying the HO has a FOAM wrap) can say that the bottom end performance is soft.

TAKE THE LID OFF OF YOUR AIRBOX LIKE GREG SAID! When you see that the restriction INTO the carb is more than the restriction OF the carb, then you'll know.Then you can find a way to get better flow into the box and still keep the air clean/dry.

Harley uses a 40mm CV on a 1450cc Vtwin!

The 44mm CV adds about 5-6HP (rear wheel)

Since the PVT compensates for power output, and does react somewhat slowly, you could put all of the throttle response you want into it, and get little/no significant gain.

 

You make some good points DieselPro. Yesterday was my first Big ride with the new cam.
Up till now has just been test runs. It's running rich, especially up at 6,000 ft. And the clutch isn't letting the engine rev up to its' power peak. The H.O. comes with the scrambler weights, but there are lighter high altitude weights for the Scrambler. I forget the numbers.
Your remark about throttle response being irrelevant with the PVT transmission is a good one. But there are top end gains to be made with a better carb. On the other hand, dialing in what I've got is a lot easier and cheaper than
putting in a new carb. And as all these guys that have put the 42mm TMX on their Scramblers,
that extra bit on top comes at a price, smooth power delivery from bottom to top is gone. There is a better carb out there, but at this point I
have some tuning to do, so I'll let it wait and maybe somebody else will figure it out for me and post it here.
I'll start a new post for this one but, I put my Arctic Cat 25x10x12 tires on the front of my
Sportsman (they were just lying around) and MAN
What a Difference!! They just barely fit, I put a washer under the lugbolts to give them some clearance from the tierod end. But what an improvement in Handling. I couldn't believe it made that much difference. 25x9x12 would fit on the H.O. no problem, but I'm keeping these 10's!!
They are great in the sand. The standard 8's seemed too narrow, to knife in. And when you shift your weight forward for high speed slides,
they would knife even more. Now I find myself riding a new and improved machine.
Silly Later,,,,Fourlix

 

I just bought a Mikuni TM 42mm Pumper carb. Should be here by Friday, I hope. I'll let you all know what happens. Rick Ritter says he is working on a new pipe for the Sportsman, based on a Scrambler pipe with a bigger headpipe and an automotive glass pak (quiet). It'll be out in about a month. Now pull off the airbox lid and see what happens. Scrambler high elevation clutch weights are coming too, this higher revving engine needs to engage at a higher rpm.
Just checked my plug after yesterday's big ride,
couldn't be more tan..