Oh, I know it. But I can't find anyone who has quantified the difference between pumper and CVK on a big bore - other than to say it's rad. That's why I follow what you are doing - at least you're putting numbers to the tests you are performing and providing something more than just a subjective judgement.

I'm not hating on pumper lovers, but I ride strictly road / gravel and my CVK is totally dialed. I don't want to blow $300 and weeks of tuning just for better throttle response, but would spring for one if it genuinely added REAL power. A bike length or two in a 1/4 mile drag matters huge to racers, but I would consider it marginal for my mundane purposes. Now if it picked me up 2 or 3 horsepower, I'd be interested.

Anyway I'm not trying to call you down or anything with what I said above - I know just how hard it is to quantify improvements without a dyno. It's just that differences along the line of 1/4 second are pretty much well within experimental error, given the tools you (and I) have at our disposal. Based on all I've seen so far I'm sticking with the CVK, but would be STOKED to see numbers that prompted me to think a pumper is worthwhile for me.

Cheers!

 

While I am confident that the timing is done very accurately, the 1/4 second between the freer breathing CVK and the 36 is nothing. Meaning it may be possible to nearly equal the 36's performance with the CVK if you can let it breath better. I've added a small K&N breather with a 1" inlet neck to an extra air box boot and will soon be seeing if that works as well as the misplaced boot did the other day. If it does then it'd be easy to add some hose and mount a small K&N filter higher up or even just run the hose into the air box above the air box outlet.



There is the thought that inlet at the top of the CVK could have been seeing higher atmospheric pressure when I was riding with the poorly installed boot and boosting the slide up quicker, but I don't think slide rise time was the factor in the increase in performance. Remember too that half of the boot's clamping area was hanging into the air stream and obstructing it, yet performance was increased.

My shift light is due to arrive any day now. Once that's paired with the stopwatch, testing will be easier and even more accurate.

 

Broussard, you won't see any increases in hp like that with a pumper. Again, it's that small kick of response that you get for the most part, and only in situations where you're cracking the throttle open in situations where you want/need a stronger response. Where I notice it the most is in a situation like climbing a rough, rocky hill where you have to navigate by going left and right. That's usually a serious momentum killer on the KLX. The pumper really kicks the power back up to stay in the power band when whacking on the throttle. To be honest, I'm thinking the pumper is much more beneficial for off road than pavement. I would probably have been just as happy with a 34mm Mikuni pumper "IF" the 34 were just like the 36 in specific design...and Mikuni actually made one off the shelf.

 

The modified air box tube/CVK combo felt very good just now in a short test ride, with a nice wheelie on the 1-2 speed shift and what seemed like an attempted wheelie on the 2-3 shift.. Timed testing will probably have to wait until the morning when the roads are nearly empty. After that I can also test with an unmodified boot/CVK combo to the compare that with the 36mm's previous time.

No increase in power with a 2mm larger carb TNC? You're trying so hard to be negative you're not making sense. Even BB's wife posted once about a HP increase (probably on their 351) when trying the 36mm..

 

According to the Sudco catalog they both have identical dimensions. No reason not to be the Guinea pig, eh Mark...

 

A cheap a-- guinuea pig!

With some jetting research I would think a used 250 two stroke carb could be done up for maybe a hundred. Some research on the round slide vs flat slide for an older four stroke, like an SR/XT/TT, to see how close they are in general. Look around to see if similar stuff was done with the old XR250s or the like and see what it takes.

I know the pump can help, but again four strokes ran without them before and still do. The kits for the Nortons/Triumphs/Yamaha XSs are almost all round slide VMs, it appears the cheapest of the cheap, KLR riders, are also doing round slides, and my first SR had a round slide, so there's no reason why the flat slide should not work. Especially with a smooth throttle hand - trials rider...

 

Yeah, but Mark...motorcycles also had wooden wheels at one...doesn't mean we should go back...LOL!

But seriously, it would be neat to see it done and hear how it works. My first KLX250 was one of those ancient KL250 motors stuffed into a KX125 chassis by Kawasaki back in '79. Several of us raced those seriously in enduros back then out of the shop we worked at. The handling and weight was extremely impressive...power not so much. They came with a 32mm Mikuni...no pumper of course. I installed what was called an "EI" flat slide carb in a 34mm size as I had a pipe and a Powroll cam...no accelerator pump of course...accelerator pump carbs were extremely rare in those days. It ran OK, and even the stock Mikuni had the tendency to hiccup in those herky-jerky off road situations where you were on and off the throttle a lot. Because 4 of us were racing these bikes at the same time, we got the stock carb to operate and jet-out pretty darned well in these engines that were mostly stock all the way to a 318cc Powroll kitted setup. Still, without an accelerator pump this is one reason a CVK carburetes so smoothly and cleanly...just doesn't hit it with quick response like a pumper. It's why most all carbureted performance dirt bikes ended up with pumper carbs until FI came along.

Here's the famous EI Blue Magnum carb...or infamous.

Laverda Racing ? View topic - What's an Ei Caburetor

 

I am so darn curious I'm looking to try just to see.

I remember the EI, remember I think the name was Lake injector,, as mentioned in that forum link? It had no float bowl and supposedly mounted any direction. I remember one magazine was doing a tricked out Van Tech (or the like) framed SL125 and had it mounted sideways to simplify the jetting changes. There was the Posa too. Makes me wonder about the diaphragm carbs used on snow mobiles...





For what it's worth, a VM38 on a KLR650

 

"For 24 karat gold add $75" with the basic unit starting cost being $72.

 

I put the 36mm with AP back on today after exhausting all options with the CVK. I didn't do any further riding with the AP disabled so I can't add to that initial evaluation, but based on the testing without the AP I did do - if I had to bet I'd say the bigger carb's mechanical slide control vs the CVK's vacuum slide is where most of the throttle response difference between the 2 carbs comes from.

I didn't try any WOTs from 2 mph etc with the AP disabled, but during the riding I did do I didn't notice any loss of throttle response just because of no AP shot.

Now that I've run out of simple things to fiddle around with, I may disable the AP again someday soon just to test it out some more under different conditions..