While alot of people are changing out their carbs in search of that extra HP I think back to my hot rod days and all money I used to throw at my car. Recently I read an article about a guy that got off on tweaking stock car parts and beating the guys with all these mods and I wondered how much faster my car could have been if spent more time tuning it. With my DS I suppose my philosophy is how fast can I make my stock one go.
Some confusing things have come up about the stock carburation lately.
First is that some people think the stock carb does'nt flow enough for the stock DS. In hotrodding most people over carbed. A Harley with twice the cubes only runs a 45mm carb.
Secondly, at first Knutz recommended using a carb mount K&N filter and gained 3hp on the dyno by doing this. Then MisterHP said to use the airbox and lid for dyno proven results. As Hightower pointed out, that goes against all common theory of less restriction. The Dyno results of the Lectron 46mm with open airbox show gains across the board which is kind of odd. This might support knutz's observation that the airbox robs 3hp or more. It's usually rare to see a bolt on change gain power across the board. Most changes, cams for instance, usually give up in one area to gain in another or at least only equal the stock part in one rpm range. Therefore it seems to make sense that the vacuum opperated slide on the mikuni isn't opperating correctly with the carb mount filter or the air path is not straightened soon enough. Hot rodders use short airstacks to straighten flow so I am going to try one plus a filter for obvious reasons.
Lastly people are cutting the carb spring which makes the slide more sensitive to vacuum and drilling the air bleeds in the slide which makes the vacuum bleed off quicker. This gives a faster responding slide. I noticed that my slide flutters in the upper rpms so I'm going to try these tricks and some others to try to correct that. One idea is to tap a hole in the end of the filter and mount the carb vent in there or restricting the vent. If I can make any noticeable gains I should be able to tell as my friend and I are running dead even now. Hopefully I should be able to tell if the stock carb has any extra to give and I will post my results. Misterhp said he thought his stock carb was dialed in to within .5hp but was in the box with the lid on so maybe I can get the extra HP from less restriction.

 

having the 2 ds's is nice because you can run them and see what changes, although this is now gone the 720 is so much more than the stage 2 that it not close enough to compare. but anyways when i first switch to carb mount i didn't see any three horse power difference. as a matter of a fact it might have been slightly slower than the airbox quad. i think if i had to pick one way or the other i'd go with the mr. horsepower way. bottom line is most bolt on mods are very slight on the gains, and as you know the gearing for riding conditions and tires have made the biggest difference. well until i went 720 that was hugh.

 

Good Comment s Mystreid. I agree with your take on using mostly stock parts. My experience comes from cars too. Good books to get are "How To Build Horsepower" Volumes 1 and 2 by David Vizard. Vizard has been building fast car engines for lots of years, and tried all types of gizmos on dynos, and knows the horsepower boosters from the junk. He also writes for Popular Hotrodding from time to time.
Anyway, my take is that if you can move more air in and out, you will make more power. As you can see by my profile, my DS is mostly stock, but it's really fast. Anyway, check out these books, they are worth a look.

 

My Two cents:

I work on computers all day fixing them etc. and when a software company, perhaps even the largest software company in the world tells you how their product is SUPPOSED to work it's just theory - until you prove that it does or does not do what it should. On a computer, it's pretty easy to test theory - does the software do the operation correctly? With engines, there is only one true way to answer the question does the machine produce more power. This would be before & after modification dyno results done on the same day, with as like conditions as possible with the same dyno etc. (Seat of the pants is great, but can be deceptive when you look at the same changes on a dyno.) I recall when Knutz mentioned the 3hp difference he saw on the dyno with the carb. mount filter vs. some un-named filter in the air box he saw a 3hp increase. I don't think he was lying, I'm sure he did in fact see that. However I know Knutz at the time (and I'm betting still doesn't) have his own dyno. The difference I saw between the two testers is that Eric has made something like 60 - 70 dyno runs, trying out all sorts of small changes in tuning to see exactly what each change does. This to me looks like the most thorough testing I've ever heard of or seen done on a DS650. If money was no object and Knutz had the time a couple of years ago I'm sure he would have jumped at the chance to run 60 - 70 dyno runs to try out every trick he could think of. (If I recall correctly, I think his 3hp gains were on a TM45, not the stock carb. - someone may have to correct me on this.)

This extensive testing is what seriously impresses me and leads me to believe the person who performed it really did some home work to come up with probably the best solution possible. I've also seen comments he's made that he doesn't personally recomend something since he hasn't tested it. So even with 60 - 70 dyno runs there are still changes & configurations that have yet to be tested, so there could be even more potential horsepower out there. I've also spoken with people who have purchased some of his knowledge and they seem to be elated with the results.

I'm certainly not trying to take anything away from what was previosly said, I'm more or less trying to get the point across that I've seen only one person go the extra mile with testing which has really impressed me. IF we get a couple more poeple even doing a little bit of factual testing then I think the entire DS650 community benefits from it; especially when the results of that are posted here in a public internet forum.

 

[img]i/expressions/face-icon-small-cool.gif[/img] well said !

 

I've not played much with the stock carb, and the carb mount vs. airbox with it is something I can't really offer much on, and I think the recommended use of airbox with this carb is mostly due to its vacuum slide operation as theorized.
I do know that I'm told Mr.HP tuned Lectron users are running no lid for increased flow, which goes along with decades of race proven practices.
About the Harley cubes, this 45mm Mikuni was designed for the 1340cc evolution motor, and only one cylinder is intaking at a time. That's a 670cc cylinder sucking through the 45mm carb.
Have fun experimenting, that's what its all about, and let us know your results!

 

Thanks for the replies, I don't mean to take away or dispute what Mrhorsepower has found. I just find it odd that running an airbox is the most efficient way to get air in the engine. I really have no idea of the different configurations he tried but I recall him saying he did'nt try the carb slide mods which really seems to be the culprit. Would further restrictions like downsizing the airbox snorkles make it run faster? Maybe he did, my memory never seems to be 100% anymore and I read this after graveyard sometimes. The testing I will do is pretty simple and mostly free. I think I'll take a day and try some little things at the dunes. My friends quad runs very strong and he has hardly touched his bike. He runs S.T.U. competition cut paddles and we're going to swap. We've always wanted to find what difference if any that would make.
Sandman, I've said for a long time that gearing and tires are the most important things to a stock DS. I could'nt agree more.

 

OK, I'm sorry for repeating myself here, but since I don't have a dyno, the second best test for me was to find a hill I couldn't make it up with the air box lid on and try it with the lid off. That's what I did. Last weekend, at St. Anthony I tried to go up the hills to the right of Chokecherry, where the sand is still crested and very steep. I came up short several times. After I took the lid off, it became easy. This is as scientific as I can get without a dyno or timed runs on a track.

 

Ok, MisterHP does NOT need the airbox, he wants the air TUBE.... In fact, I was the fist person to get his 46 setup, He originaly tested it with OUT the boot and a clamp on. He lost liek 3-4 hp like that and could not get it tuned. He told me this and I agreed to run the airboot. Glade I did as it works well! I wanted the airbox with no lids or snorkles to stop the splash, I am a nasty muddy woods rider. He said his bikes run with only the boot, the airbox is gone...

 

I also like using the airboot with my aftermarket carb for the reasons Mudstud mentioned, as well as running my crankcase vent to the grommet on the left side that the stock carb vent tube used to connect at. no more oil mess on a mini filter attached to the end of the crankcase vent, positive crankcase ventilation, burnt emmisions, etc.