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DellOrto carb design question

blackduc98

Damn Yankees
Joined
Dec 19, 2005
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#1
Hi all,
This is a technical question regarding Dellorto PHBH26CS carburetor. Since it is only found on a trials bikes, I figure this would be the right forum to ask. It was used on 97-00 Montesa 315r, and I think maybe some GasGas machines too. Anyway, here is the question: what is the idea behind the 2-part design of the slow jet? I am referring to parts (17) and (34) on this parts diagram:
http://www.apexmotosports.com/montesa parts book/e-10.htm

(17) is called "slow air bleed" and it resembles a miniature version of a typical Keihin slow jet, so it is really the atomiser tube with a calibrated fuel passage. (34) is called the "slow jet" and it is a plain vanilla simple jet, i.e. just a simple calibrated hole. (17) and (34) are installed in tandem separated by maybe 10mm. So that's where I get puzzled. Since both will restrict fuel flow, then it seems like the smaller of the pair will govern mixture strength, which implies that (34) is superfluous. Yet if you look at the parts diagram, you will notice that (17) comes in sizes 42-50, while (34) comes in sizes 42-46, so it seems like they were meant to be tuned independent of each other?

Any ideas?
 

Patman

Pantless Wonder
Joined
Dec 26, 1999
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#2
I wondered the same thing when I had pulled mine apart a couple times. Sorry I have no idea.
 
Joined
Oct 21, 2002
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#3
hi, sorry im not an expert. but have spent some time with dellorto carbs on cars etc.. and alot of them use an emulsion tube to bleed air into the fuel prior to it being feed into the idle or transition circuits. i guess that on a trials bike (and like on a car) slow speed and part throttle performance is very important, so having a slow air bleed tube is another way to fine tune the slow speed performance of the bike. i would also guess that the jet (part 34) simply controls the amount of fuel as in any bike carb and then the amount of air mixed with this fuel is controlled by the bleed tube (part 17).
hope this helps.