Weber Carb info, very detailed
Posted: Sat Nov 12, 2005 2:39 am
This was posted on the Yahoo Paso site by Mr. Goose:
Earlier in the year I advised the group that I was going to make the Weber work right, or else! Having purchased the bike new in the year 2000 (that's another story...) I wanted to get more miles on it before modifying the carburetor. Here's how I made my 1988 Paso 750 run right:
Having determined to my satisfaction that the bike was way over-carbed with the 44mm DCNF, I decided to put on a 40mm DCNF instead. The one I used is a "40 DCNF 102", which is nearly identical but not entirely. (Just like with other Weber carburetors, the castings have been changed a bit during the production life to suit changing needs and applications. All of these carbs are/were manufactured primarily for automobile use.) If you select a "40 DCNF 102" you will see the top cover is slightly different, and so are the idle jet screws. That's about it - - oh, the choke cable pulls in the opposite direction.
I decided to re-use the plastic float and the 3.00 float chamber needle valve from Ducati's 44DCNF because I figured there was some good reason for them to switch from the differently-shaped brass float, and for using the larger-than-normal 3.00 needle. Maybe it has something to do with leaning the bike over in turns. I fitted the 40 DCNF with 32mm venturis, 4.5 long nose auxiliary venturis, 135 main jets, re-used the F44 emulsion tube, and used 190 air correctors. I continued with the 55 idle jet, the F7/110 starting jet, and the .35 accelerator pump jet. These decisons were made after months of studying several Weber books, and represent my final jetting choices.
It should be noted that my U.S. version 1988 Paso came with 34mm venturis, not 36mm as stated in the manual. All other carb jets, etc., were the same as stated in the Ducati specs. I suffered, like everyone else, from the mid-throttle "hunting" when cruising, and the gasp and hesitation when opening the throttle further. These are normal symptoms of a momentary leanness caused by too much pressure drop from a too-large carburetor barrel. Otherwise my bike as stock ran OK ( I had installed a fuel pressure regulator when new, set at 2# pressure).
For the backwards choke mechanism, I cut off the cable mount end of the bracket and rewelded it onto the other end. Likewise I removed the 44 DCNF throttle "winder" on the end of the throttle shaft and re-used it on my new 40 DCNF. Everything now fit correctly on the new carb, so it was bolted into place and it started right up on the first try.
In actuality I first used 130 mains and 180 air correctors, but the jets listed above were soon substituted. After balancing both barrels per the factory instructions, I found that the bike starts very easily and needs little choke before settling down to a nice idle. I tested the running characteristics and acceleration in hot 90F August weather here in the south of the USA, and recently tested it again in 50F October weather. The bike runs perfectly smoothly, with no mid-throttle hesitation once it is warmed up. It accelerates like a rocket right up to the redline with great power. I tested this beginning in 2nd gear, cranking on full throttle from about 3000rpm, and again in the upper gears. A friend who followed me in a very fast car said there is a wisp of richness smoke in the higher RPMs, so the carb. might need slightly larger air correctors. However I am tired of removing all the bodywork, tank, and air box at the moment, so I'll leave it as it is. I think the air/fuel ratio is real close for best power.
Before I decided to change carbs I ran a "before & after" on an engine dyno computer program, and it looks like with the 40mm Weber and 32mm venturis, one loses 1 or 2 hp at 8000rpm, but you gain 1 lb.ft. of torque. Good enough for me, since I don't ride at 8000-9000rpm.
I realize changing to a new carburetor is not inexpensive (but you can buy them cheap on Ebay), however it does solve the Paso problem. It's possible you can rejet the 44DCNF very close to my 40DCNF and get nearly as good results, however you'll probably still have some hesitation since the carb throttle plates are still tooo large. If one were to try this on a 44, you might also try the trick of drilling the small hole in the throttle plates opposite the idle running holes, which has been described by other people, and is a recommended Weber modification.
Mr. Goose
Earlier in the year I advised the group that I was going to make the Weber work right, or else! Having purchased the bike new in the year 2000 (that's another story...) I wanted to get more miles on it before modifying the carburetor. Here's how I made my 1988 Paso 750 run right:
Having determined to my satisfaction that the bike was way over-carbed with the 44mm DCNF, I decided to put on a 40mm DCNF instead. The one I used is a "40 DCNF 102", which is nearly identical but not entirely. (Just like with other Weber carburetors, the castings have been changed a bit during the production life to suit changing needs and applications. All of these carbs are/were manufactured primarily for automobile use.) If you select a "40 DCNF 102" you will see the top cover is slightly different, and so are the idle jet screws. That's about it - - oh, the choke cable pulls in the opposite direction.
I decided to re-use the plastic float and the 3.00 float chamber needle valve from Ducati's 44DCNF because I figured there was some good reason for them to switch from the differently-shaped brass float, and for using the larger-than-normal 3.00 needle. Maybe it has something to do with leaning the bike over in turns. I fitted the 40 DCNF with 32mm venturis, 4.5 long nose auxiliary venturis, 135 main jets, re-used the F44 emulsion tube, and used 190 air correctors. I continued with the 55 idle jet, the F7/110 starting jet, and the .35 accelerator pump jet. These decisons were made after months of studying several Weber books, and represent my final jetting choices.
It should be noted that my U.S. version 1988 Paso came with 34mm venturis, not 36mm as stated in the manual. All other carb jets, etc., were the same as stated in the Ducati specs. I suffered, like everyone else, from the mid-throttle "hunting" when cruising, and the gasp and hesitation when opening the throttle further. These are normal symptoms of a momentary leanness caused by too much pressure drop from a too-large carburetor barrel. Otherwise my bike as stock ran OK ( I had installed a fuel pressure regulator when new, set at 2# pressure).
For the backwards choke mechanism, I cut off the cable mount end of the bracket and rewelded it onto the other end. Likewise I removed the 44 DCNF throttle "winder" on the end of the throttle shaft and re-used it on my new 40 DCNF. Everything now fit correctly on the new carb, so it was bolted into place and it started right up on the first try.
In actuality I first used 130 mains and 180 air correctors, but the jets listed above were soon substituted. After balancing both barrels per the factory instructions, I found that the bike starts very easily and needs little choke before settling down to a nice idle. I tested the running characteristics and acceleration in hot 90F August weather here in the south of the USA, and recently tested it again in 50F October weather. The bike runs perfectly smoothly, with no mid-throttle hesitation once it is warmed up. It accelerates like a rocket right up to the redline with great power. I tested this beginning in 2nd gear, cranking on full throttle from about 3000rpm, and again in the upper gears. A friend who followed me in a very fast car said there is a wisp of richness smoke in the higher RPMs, so the carb. might need slightly larger air correctors. However I am tired of removing all the bodywork, tank, and air box at the moment, so I'll leave it as it is. I think the air/fuel ratio is real close for best power.
Before I decided to change carbs I ran a "before & after" on an engine dyno computer program, and it looks like with the 40mm Weber and 32mm venturis, one loses 1 or 2 hp at 8000rpm, but you gain 1 lb.ft. of torque. Good enough for me, since I don't ride at 8000-9000rpm.
I realize changing to a new carburetor is not inexpensive (but you can buy them cheap on Ebay), however it does solve the Paso problem. It's possible you can rejet the 44DCNF very close to my 40DCNF and get nearly as good results, however you'll probably still have some hesitation since the carb throttle plates are still tooo large. If one were to try this on a 44, you might also try the trick of drilling the small hole in the throttle plates opposite the idle running holes, which has been described by other people, and is a recommended Weber modification.
Mr. Goose