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Well jetted stock vs a pumper. How big a difference?


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I mentioned in my jetting question that I've got the FMF and Jet kit dialed in now and it just wants to flat GO! I've tasted the Kool-Aid and I LIKE it! I have zero complaints about the way she pulls. Especially over stock setup.

I'm wondering what I would notice if I went to a pumper. Is there that much of a jump from a well sorted and jetted stock carb going to the Mikuni? What's the seat of the pants difference?

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IMO the big difference is not in total horsepower; it's responsiveness. A properly jetted CV carb will probably make about the same power on a dynomometer as a pumper carb. Plus it will tolerate altitude and atmospheric pressure changes better than a pumper. BUT, based on my experience with several DR350s with both types of carbs, a CV carb will never provide the same instant throttle response as one with an accelerator pump.

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carbs wear out, when my carb was pretty new I couldn't understand it, judging off peoples posts I had about the same throttle response as a pumper, no problems at all doing roll on power wheelies in 1st, even 2nd with just the slightest incline

I had it dialed in pretty much 100% for performance, at te expense of a bit of fuel economy since I had to run it rich at idle

But then time went on, and I could never get it that good again.

When I start making decent money again i'll be getting a pumper for sure

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Throttle response is certainly a head liner for the cable controlled slide accelerator pump carb.... but it is not the only improvement over the vacuum slide carb on a big bore thumper. Thumpers don't thump with a CV carb... they herk and jerk or huck and buck.๐Ÿคฃ Just try running your BST'd DR650 down to 40 miles per hour in 5th gear. The exact same thing happens in any gear at thumping speeds. The engine runs so much smoother down low with a cable controlled slide carb it's amazing. The only thing CV carbs are good for is emissions. The mfgs can lean the engine out without the bike suffering huge flat spots since the slide really doesn't do what your hand does... and certainly not as fast. ๐Ÿ‘

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But wouldn't the CV be better for going from below sea level to 12,000'??

I keep seeing that posted in a lot of carb discussions but it doesn't make any sense to me. All the vacuum does is lift the slide... it has nothing to do with mixture control... it can't change the AFR leaner as the altitude goes higher or richer as the altitude goes lower. It's simply lifting a slide (air control) which has a fixed position needle (fuel control) just like on a cable operated slide. The volume of the AF required for X engine speed is no different between the two carbs. A stock restricted lean BST would certainly do better than a FCR-MX or TM40 at altitude.... but a DJ kitted BST would be in the same boat as the pumpers except for the AP circuit. So this bodes well for the adjustable AP for the guys that like to experience temporary nose bleeds every now and then. ๐Ÿ‘

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I keep seeing that posted in a lot of carb discussions but it doesn't make any sense to me. All the vacuum does is lift the slide... it has nothing to do with mixture control... it can't change the AFR leaner as the altitude goes higher or richer as the altitude goes lower. It's simply lifting a slide (air control) which has a fixed position needle (fuel control) just like on a cable operated slide. The volume of the AF required for X engine speed is no different between the two carbs. A stock restricted lean BST would certainly do better than a FCR-MX or TM40 at altitude.... but a DJ kitted BST would be in the same boat as the pumpers except for the AP circuit. So this bodes well for the adjustable AP for the guys that like to experience temporary nose bleeds every now and then. ๐Ÿ‘

I have a little experience with this riding 2-strokes at Deals Gap and theose bike REALLY let you feel the effects.. There's probably a 20% HP loss going from the valley to the peak of the Skyway ride (from ~1000 to 5600 ft or so) and the pilot circuit plays a pretty small role. Unfortunately the 2 hardest-to-get-to jets (Needle/Needlejet and main) would have to be tweaked aif you wanted to have any real effect. You could go for a happy medium I guess but then your bike would suck a little all the time. I jet for low altitude and just trudge through the higher stuff. You're not usually up there for long anyway and if you plan to be.. well.. then plan ahead.

Or get FI and be done with it. Now I wish there was a bolt on kit for THAT!

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I keep seeing that posted in a lot of carb discussions but it doesn't make any sense to me. All the vacuum does is lift the slide... it has nothing to do with mixture control... it can't change the AFR

I'm with you. I don't quite buy the whole idea that CV carbs 'self adjust' for altitude.

The theory is that the thinner air creates less lift at the slide. Of course even if that is the case it would only have a self correcting effect in the middle throttle positions not when running on the main jet or the pilot or the straight part of the needle.

A better strategy for running in a wide altitude range is to jet at the lean edge of the good running range. Then as mixture richens with altitude it won't go so rich as to create running problems. I've run mine (TM40) from sea level to over 9500 feet with no need to adjust anything.

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I'm with you. I don't quite buy the whole idea that CV carbs 'self adjust' for altitude.

The theory is that the thinner air creates less lift at the slide. Of course even if that is the case it would only have a self correcting effect in the middle throttle positions not when running on the main jet or the pilot or the straight part of the needle.

Yeah, but what would be self correcting about letting off the throttle? That's all that would be happening. The rider is still going to open the throttle to the point that he/she is satisfied with the power the engine is putting out. So if the CV "self corrects" (by backing of the slide) then so will the rider by opening the throttle to lift the slide back up so the engine produces the omph required.

A better strategy for running in a wide altitude range is to jet at the lean edge of the good running range. Then as mixture richens with altitude it won't go so rich as to create running problems. I've run mine (TM40) from sea level to over 9500 feet with no need to adjust anything.

Amen! ๐Ÿ‘

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Yeah, but what would be self correcting about letting off the throttle? That's all that would be happening. The rider is still going to open the throttle to the point that he/she is satisfied with the power the engine is putting out. So if the CV "self corrects" (by backing of the slide) then so will the rider by opening the throttle to lift the slide back up so the engine produces the omph required.

Ahhh.....

The 'CV carb / altitude' myth can now be declared debunked ๐Ÿ‘

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Oh please Jeff. Do tell!

No, no, no! I have my hands plenty full without going down that road. ๐Ÿ‘

I don't yet see much potential for a kit. There's just too much fiddling and fabrication to imagine an 'out of the box' kit that could be installed by the typical tinkerer.

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A better strategy for running in a wide altitude range is to jet at the lean edge of the good running range.

After months of reading here and at AdvRider I did the Dynajet kit (155 main, their needle, drilled the slide, etc.) and I did the usual recommended air box mods. After finally drinking the kool aid I am yet to be sure I feel the good effects...๐Ÿคฃ

I now have a stumble at idle to work out, but throttle response does not seem any different. The bike does seem to be running cooler.

Anyway, I've read this thread with some interest and would like to know more about what it means to "jet at the lean edge of running good."

As you have time to 'splain it to a clueless "carb n00b."

Thanks. ๐Ÿ‘

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