Moderated by: Greg Fletcher  

Joined: 02-20-2009
Location: San Rafael, California USA
Posts: 28
To all the Dellorto Experts, i have DHLA 40N installed on my tangerine orange JH, the carbs have been cleaned and checked with new seals and gaskets, etc but the mixture screw on #1 barrel has no effects. It make no difference on idle or at higher RPM whether it is screw in all the way in or out, all other mixture screw have an immediate effect. Any idea what may be going on and how to fix it? Thanks to all.

Jim Ketcham

Joined: 03-13-2005
Location: Salt Point, New York USA
Posts: 192
Assuming properly installed mixture screw the most common problems I have encountered:
1) Respective cylinder not firing - check spark and plug
2) Air leak at manifold - check O-rings at mount
3) Idle jet or holder wrong or missing
4) Air bypass screw way out

Dan (Florida)

Joined: 03-16-2005
Location: Ormond Beach, Florida USA
Posts: 97
If it's anything like the Webers, the throttle butterfly has to be completely closed in order to work .  If only slightly open the low speed jet adds to the mixture.  At idle the throttles are completely closed and working off of the idle jets. the idle screw should not touch the stop.  Something I have found is that above idle the idle screws continue to contribute fuel clear up to where the main jets kick in, so I guess that the idle screws need to be as lean as you can make them (in) to keep low speed mixture from being too rich.   A "Gunson colortune" can be a  useful tool  for setting mixtures.

Let us know your findings.


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