Carb or ignition problem?

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tobydmv
Posts: 60
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: 143
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: 16
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: 60
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: 60
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 ignition coil for a magneto/ac system. Thanks all forthe help.
Last edited by tobydmv on Mon Oct 22, 2018 6:10 pm, edited 1 time in total.

terra mostro
Posts: 3
Joined: Thu Oct 18, 2018 11:49 am

Re: Carb or ignition problem?

Postby terra mostro » Mon Oct 22, 2018 5:48 pm

Glad to hear you got it sorted - I have a very similar problem - 67 scrambler. Can you or someone tell me how to get the flywheel timing right? Does the reference mark line up with anything?

thanks,
Blaine
67 Scrambler
mach 1 bitsa

tobydmv
Posts: 60
Joined: Wed Apr 16, 2014 4:59 am

Re: Carb or ignition problem?

Postby tobydmv » Mon Oct 22, 2018 6:24 pm

terra mostro wrote:Glad to hear you got it sorted - I have a very similar problem - 67 scrambler. Can you or someone tell me how to get the flywheel timing right? Does the reference mark line up with anything?

thanks,
Blaine
67 Scrambler
mach 1 bitsa


Blaine, somewhere in the shop manual you will find the degree info from TDC you need. Line up the timing marks and then rotate the flywheel back how many degree the shop manual prescribes. I found it helpful to convert the flywheel OD to mm(OD divided by 360). I believe it was very close to 1mm per degree, but i've proly lost a few brain cells since then. So find the static mark on the flywheel then measure back 27mm or whatever you come up with and scribe/mark, set, and torque. I cant remember if the case has an alignment mark or not. Maybe someone else can clarify that part. I think my biggest problem was having a DC ignition coil and using the magneto. The scramblers dont need a battery or rectifier to run.

ranton_rambler
Posts: 143
Joined: Sun Jun 04, 2017 11:33 am
Location: Stafford UK

Re: Carb or ignition problem?

Postby ranton_rambler » Mon Oct 22, 2018 6:57 pm

I think the timing instructions can be downloaded from the Tech section of this website.

terra mostro
Posts: 3
Joined: Thu Oct 18, 2018 11:49 am

Re: Carb or ignition problem?

Postby terra mostro » Tue Oct 23, 2018 3:43 am

Hey guys,

Thanks so much for the quick response. I looked through the tech section and I see one booklet with magneto timing pictured with a mark at about 11 oclock form TDC - no actual measurement. Can it be that general or does it need a more precise setting. My Clymers manual has nothing, and the workshop manuals in the tech section seem to be dead links.


Am I missing where it is? in the manuals you guys have, are they the factory workshop manual, the haynes?? So far i find the actual work on the bike to be quite reasonable, but finding the information has been the biggest challenge. I sincerely appreciate your help and guidance.

tobydmv
Posts: 60
Joined: Wed Apr 16, 2014 4:59 am

Re: Carb or ignition problem?

Postby tobydmv » Tue Oct 23, 2018 1:32 pm

terra mostro wrote:Hey guys,

Thanks so much for the quick response. I looked through the tech section and I see one booklet with magneto timing pictured with a mark at about 11 oclock form TDC - no actual measurement. Can it be that general or does it need a more precise setting. My Clymers manual has nothing, and the workshop manuals in the tech section seem to be dead links.


Am I missing where it is? in the manuals you guys have, are they the factory workshop manual, the haynes?? So far i find the actual work on the bike to be quite reasonable, but finding the information has been the biggest challenge. I sincerely appreciate your help and guidance.


This is the manual I have https://www.ebay.com/i/252468838707?chn=ps.

FYI, the magneto timing picture is a general reference. Your setup needs to match the figure in the table in the manner of the picture. The table should have a figure for flywheel timing. For instance, mine was 28 degrees I think, cant remember for sure and I dont have the manual in front of me. So use the picture as a reference and the table for the actual figure. The thread below was very helpful to me in my research, as this is a very complicated subject. If you dont want to go this route, you could simply use a battery and a different ignition coil(http://guzzino.com/igco40612vo.html) and run the bike off the battery. The flywheel should still produce enough charge to keep the battery topped up. You would probably need to rewire the system and introduce a rectifier to charge the battery.

I'd recommend verifying the ohms of your ignition HT coil match or almost match the ohms of the ignition stator coil. Mine did not match and caused a very poor spark that was misdiagnosed as a carburettor problem. I spent a lot of time trying to fix the carb when it was ignition the whole time.
viewtopic.php?f=3&t=1255&p=8580&hilit=magneto+flywheel+timing#p8580


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