Carb or ignition problem?

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tobydmv
Posts: 58
Joined: Wed Apr 16, 2014 4:59 am

Carb or ignition problem?

Postby tobydmv » 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"

ranton_rambler
Posts: 135
Joined: Sun Jun 04, 2017 11:33 am
Location: Stafford UK

Re: Carb or ignition problem?

Postby ranton_rambler » Fri Jul 06, 2018 5:37 am

What else changed at the same time as the carburettor?
Why did you need a new carburettor?

Buster
Posts: 13
Joined: Fri Dec 22, 2017 12:56 am
Location: Reading, UK.

Re: Carb or ignition problem?

Postby Buster » Fri Jul 06, 2018 8:03 am

Throttle valve cutaway too big maybe?

tobydmv
Posts: 58
Joined: Wed Apr 16, 2014 4:59 am

Re: Carb or ignition problem?

Postby tobydmv » Fri Jul 06, 2018 12:12 pm

ranton_rambler wrote:What else changed at the same time as the carburettor?
Why did you need a new carburettor?
The intake manifold and rubber gasket from guzzino were introduced at the same time as the new mikuni.
The bike came with an old mikuni setup that was beyond repair and idk if it ever worked correctly. With the new carb im on proven config at least for a monza. I searched this forum and found a few threads about the sudco mikuni setup. But no data on the older mikuni.

tobydmv
Posts: 58
Joined: Wed Apr 16, 2014 4:59 am

Re: Carb or ignition problem?

Postby tobydmv » Sun Jul 22, 2018 2:56 am

Ignition problem solved by timing the alternator flywheel and using the correct DC ignition coil. Thanks all fir the help.


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