Carb or ignition problem?
Posted: Thu Jul 05, 2018 2:22 pm
1966ish Ducati NC Scrambler 250 bitsa. Bike will idle well but wont rev at all, here's the details:
Well I broke down and bought a Mikuni VM28 from Sudco to go with a new manifold from Guzzino. I also bought a Malossi rubber connector to bridge the intake and carb. The bike starts very easily, 1st or 2nd kick and idles well now. The carb originally came with a 30 pilot jet that required the air screw to be run all the way in. So thinking the idle circuit was too lean I tried some bigger pilot jets and settled on a 60 jet with the air screw about a half to full turn out now. Seems too big but the air screw and idle are agreeing with it. Problem starts when I open up the throttle to rev the bike the idle speed drops immediately and if I keep the throttle open it kills the motor. I kept thinking I have a vacuum leak requiring the larger pilot jet but I cant seem to find it by spraying starting fluid all over the intake manifold pieces. So i've come to beg for advice from you folks and try to figure out if this is a fuel or ignition problem. I'm running a very simplified wiring system based on the shop manual for a scrambler, IE straight to the coil with another lead from that post up to a kill switch. There is no rectifier or regulator in place on the bike yet it idles very well like this. I had a mechanic tell me recently I have to use a rectifier but i'm not sure I agree and I dont see a rectifier in the ignition path in the scrambler diagram.
I'm wondering if my problem isnt carb related, maybe the AAU might be stuck, stator coils arent providing enough juice to let the bike run at higher RPM's, or something else I havent though of like timing. I'm really just kind of lost at this point. The bike has 130 psi compression so I dont think there's a problem in the cylinder head. Let me know what you think.
Intake http://guzzino.com/duinma250350.html "29mm-b"
Well I broke down and bought a Mikuni VM28 from Sudco to go with a new manifold from Guzzino. I also bought a Malossi rubber connector to bridge the intake and carb. The bike starts very easily, 1st or 2nd kick and idles well now. The carb originally came with a 30 pilot jet that required the air screw to be run all the way in. So thinking the idle circuit was too lean I tried some bigger pilot jets and settled on a 60 jet with the air screw about a half to full turn out now. Seems too big but the air screw and idle are agreeing with it. Problem starts when I open up the throttle to rev the bike the idle speed drops immediately and if I keep the throttle open it kills the motor. I kept thinking I have a vacuum leak requiring the larger pilot jet but I cant seem to find it by spraying starting fluid all over the intake manifold pieces. So i've come to beg for advice from you folks and try to figure out if this is a fuel or ignition problem. I'm running a very simplified wiring system based on the shop manual for a scrambler, IE straight to the coil with another lead from that post up to a kill switch. There is no rectifier or regulator in place on the bike yet it idles very well like this. I had a mechanic tell me recently I have to use a rectifier but i'm not sure I agree and I dont see a rectifier in the ignition path in the scrambler diagram.
I'm wondering if my problem isnt carb related, maybe the AAU might be stuck, stator coils arent providing enough juice to let the bike run at higher RPM's, or something else I havent though of like timing. I'm really just kind of lost at this point. The bike has 130 psi compression so I dont think there's a problem in the cylinder head. Let me know what you think.
Intake http://guzzino.com/duinma250350.html "29mm-b"