1981 Darmah - Carb Mods - FWIW
Posted: Sat Dec 01, 2012 6:17 pm
Just thought I'd share my experience with my Darmah carbs.
This bike is comparatively low mileage - 20,000 - and when I got it, it ran pretty well when warmed up and up to road speed. However, the idle was always ragged and it didn't like off-idle transition to road speed. It always felt lean and would buck, lurch, and occasionally spit back when doing lower speed riding in traffic.
I tried all the usual idle adjustments, searched for manifold air leaks, move the needle a notch or two higher, etc.
Nothing made much difference. I finally bought rebuild kits and went completely through the carbs. I didn't find anything particularly wrong other than old rubber parts, but thought it was best to start fresh.
Long story short, that didn't make any difference either.
So - I looked through my old Delorto manual at options. There are many slides, needles, and needle jets (atomizers) for the PHF's, and I saw that there were richer (lower cutaway) slides than the 503 slides that were in the bike.
As far as I can find, 503's were the stock slides.
I did eventually find some 402 slides and installed them. Presto - it cured about 95% of the low speed transition problem.
Second, I raised the needle one notch to the middle (3rd from top)notch and that cured it 99.5%.
Now it responds to the idle mixture screw and slide stop properly.
It starts on the first hit of the starter with the chokes on, idles fast and a bit rich till it gets the heads a little warm, (maybe a minute) and settles down to its normal lovely cammy idle. Motoring off from idle it immediately picks up both cylinders evenly and cleanly motors away with very little clutch slip or extra revs.
For the record, the carb specs are as follows:
Stock PHF 32 AD & AS
Slides 402
Needle K16 in 3rd notch
AB1-262 Needle jet (atomizer)
#122 Main jet
#60 Idle jet
Idle mix screw 2.75 turns out
Idle speed about 1200 - 1300 warm
(the 402 slides have a later but quicker accelerator pump ramp than the 503, but it seems fine. Now at steady road speed of 60 mph or so, the slide must be sitting just below the ramp as hitting the throttle gives a response like the secondaries on a 4 bbl carb kicking in - A really nice hit -great for passing. Just in case you wonder about what this does for mileage, it gets low to mid 50's mpg at steady 60 - 65 mph.)
The bike is now an absolute pleasure to ride without having to contantly baby it in slower traffic. I live way out past the suburbs and and can enjoy low traffic secondary roads most of the time, but I used to hate taking this bike to town.
I assume Ducati set these a bit lean for the then emerging emissions specs but it didn't do the carburation smoothness any favors.
Has anyone else gone down this road ?
This bike is comparatively low mileage - 20,000 - and when I got it, it ran pretty well when warmed up and up to road speed. However, the idle was always ragged and it didn't like off-idle transition to road speed. It always felt lean and would buck, lurch, and occasionally spit back when doing lower speed riding in traffic.
I tried all the usual idle adjustments, searched for manifold air leaks, move the needle a notch or two higher, etc.
Nothing made much difference. I finally bought rebuild kits and went completely through the carbs. I didn't find anything particularly wrong other than old rubber parts, but thought it was best to start fresh.
Long story short, that didn't make any difference either.
So - I looked through my old Delorto manual at options. There are many slides, needles, and needle jets (atomizers) for the PHF's, and I saw that there were richer (lower cutaway) slides than the 503 slides that were in the bike.
As far as I can find, 503's were the stock slides.
I did eventually find some 402 slides and installed them. Presto - it cured about 95% of the low speed transition problem.
Second, I raised the needle one notch to the middle (3rd from top)notch and that cured it 99.5%.
Now it responds to the idle mixture screw and slide stop properly.
It starts on the first hit of the starter with the chokes on, idles fast and a bit rich till it gets the heads a little warm, (maybe a minute) and settles down to its normal lovely cammy idle. Motoring off from idle it immediately picks up both cylinders evenly and cleanly motors away with very little clutch slip or extra revs.
For the record, the carb specs are as follows:
Stock PHF 32 AD & AS
Slides 402
Needle K16 in 3rd notch
AB1-262 Needle jet (atomizer)
#122 Main jet
#60 Idle jet
Idle mix screw 2.75 turns out
Idle speed about 1200 - 1300 warm
(the 402 slides have a later but quicker accelerator pump ramp than the 503, but it seems fine. Now at steady road speed of 60 mph or so, the slide must be sitting just below the ramp as hitting the throttle gives a response like the secondaries on a 4 bbl carb kicking in - A really nice hit -great for passing. Just in case you wonder about what this does for mileage, it gets low to mid 50's mpg at steady 60 - 65 mph.)
The bike is now an absolute pleasure to ride without having to contantly baby it in slower traffic. I live way out past the suburbs and and can enjoy low traffic secondary roads most of the time, but I used to hate taking this bike to town.
I assume Ducati set these a bit lean for the then emerging emissions specs but it didn't do the carburation smoothness any favors.
Has anyone else gone down this road ?