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Dell'Orto PHBL26 Fuel screw fine tuning

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I have a 2013 250 TXT Pro with the Dell'Orto carburator.  First I was surprised to learn that the mixture screw is actually a fuel screw and not an Air Screw.

Now, concerning fine tuning of the fuel screw, what's the standard range of turns?  Jim Snell had a video for adjusting the screw but I can't find it anymore.  Any other guides / recommendations for doing that?

Thanks.

DC

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as it is indeed a fuel screw it is a rather coarse adjustment as compared to an airscrew, thus many folks kinda set them and forget them with the screw all the way out at the limit of its effective range at 3.5 turns. the pilot jet then becomes your main setting, with most at the 36-38 range for normal altitude. possibly a 40 even. the pilot jet size also effects the transition range off idle until yhe needle takes over so it requires a balance.

they are great carbs, yet not the most precise metering device ever. I love them for simple function and reliability.

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I was told on here the screw can be either a fuel or air adjuster depending on its location. If its on the bellmouth side of the tower its air, on the Reed block side its fuel (I think). It's location reverses the in/rich, out/weak adjustments. 

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Well, almost like that. As above, the PHBL26 utilises a fuel passageway and tapered screw to adjust pilot metering, located close to the reed block end. There is a similar looking carb from DellOrto called a PHBH26 which utilises an air passageway with screw adjustment, located close to the "bellmouth" end, usually a starting point for this one is with the screw fully in (clockwise), start with it being 1 turn out. I don't like this carb as much as the PHBL, it had a 2 stage pilot jet that used to always block up, and riding at different altitudes used to noticeably upset the carburation, more so than on the PHBL. Bye, Peter B.

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I am pretty sure all Dellortos are a fuel screw not an air screw, winding in leans it and winding out richens, the opposite of a Keihin or Mikuni. There are actually three Dellorto trials carbies- PHBH 26mm or 28mm[large body]- PHBL 26mm largest [small body] and the VHST [D or flatslide] and its the one that's difficult to keep in tune. The PHBL & PHBH are quite good carbies and require very little adjustment once set up correctly, most people changing the pilot jet to get the feel they like.

Cheers Greg

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Hi Drca, I have recently watched the Jim Snell video regarding the tuning of the dellorto carb. He states in his video that the factory setting for the fuel/air screw is 3 turns out for the factory setting. The video also shows you how to check if your floats are bad.

Try searching tuning dellorto carb.

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7 hours ago, sherpa325 said:

I am pretty sure all Dellortos are a fuel screw not an air screw, winding in leans it and winding out richens, the opposite of a Keihin or Mikuni. There are actually three Dellorto trials carbies- PHBH 26mm or 28mm[large body]- PHBL 26mm largest [small body] and the VHST [D or flatslide] and its the one that's difficult to keep in tune. The PHBL & PHBH are quite good carbies and require very little adjustment once set up correctly, most people changing the pilot jet to get the feel they like.

Cheers Greg

Dellorto's can be either air or petrol screw, but as said, the petrol screw are more reliable in daily use.

Edited by b40rt

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1 hour ago, slack said:

Hi Drca, I have recently watched the Jim Snell video regarding the tuning of the dellorto carb. He states in his video that the factory setting for the fuel/air screw is 3 turns out for the factory setting. The video also shows you how to check if your floats are bad.

Try searching tuning dellorto carb.

I thought I had the video it bookmarked, but apparently not.  I searched youtube for "JIm Snell Dell Orto" and the only video I could find was his video on the Dell Orto carb (very useful but nothing on the tuning of carb).

From what I remember seeing in the video I am still looking for, the procedure was as follow:

  • Warm up the engine
  • Turn the iddle down at the limit of stalling
  • Crack the throttle open fast
  • If the engine bogs, adjust the mixture screw
  • Repeat until happy with the result.

Now... I can't remember which way to adjust the mixture screw function of the result of cracking the throttle...

Thanks again for all the help.

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