Motoscrubs.com

I am still pretty new to the vintage motorcycle scene even though I now have two old ducs.

I purchased a 65 250 Monza a number of months ago. It sat in the garage for a few months and just got it started this weekend. The exhaust appears to be pretty rich or it has a penchant for oil. I have been going through various threads here and any documentation I could find, but am not 100% certain which of the screws is the idle speed screw and which is the idle mixture screw. I believe the mixture screw is on the left and the idle screw is on the right. Is there a simple way to tune or just follow the instructions here?

http://www.ducatimeccanica.com/dellorto_guide/dellorto_3_4.html

Also, the slide on the carb seems to get hung up - meaning you open the throttle and it doesn't want to return to it's off/closed position. Is that common? Would that be a tired spring or could there be corrosion inside the carb body. I did the unthinkable and added a drop or two of earl down the throttle cable and it did improve temporarily?

PS - There will likely be a sucking sound associated with my posts, but I promise to give back as my knowledge increases. I have an old Porsche and recently crossed the 1,000 post milestone. 250 and car below, Monza Jr in front of the car - engine out.

Hex screw with locknut is throttle stop, which sets idle speed.
The one upstream of that is an air screw, to adjust mixture.
If it were downstream, it'd be a mixture screw, which also sets the mixture, but already has some fuel mixed with it.
Slide not returning well might be due to a sticky cable. Remove from carb to see if slide then works OK. If slide is sticky in its bore, could be due to body or slide distortion. A fine single cut file might help.
If Dell'Orto tuning guide is hard to follow, find an Amal guide - same principle.

Thanks for the 'upstream' explanation, makes sense. I will test the cable, but because it seemed free after a I lubricated the carb slide I don't think that is the issue.

Those flange mount UBF carbs tend to warp the body because the two mounting bolts compress the insulator at the bolts. If you take the carb off and put a straightedge across the flange you can determine if it’s warped. The warped body can also lead to air leaks at the insulator which will make proper tuning impossible.

the rubber gasket/isolator gets hard with heat cycles and age. the carb flange doesn't 'warp'. morons that don't belong around mechanical things over-tighten the nuts and bend the flange (potentially warping the slide bore, to be fair).

the last UB I went through was abused and I first put the carb in a press and straightened the flange as much as I could. then chucked it in the lathe and turned it down the last few thousandths.

the rubber in the photo above is well cracked. that is the first thing I'd look at (replace).

SP3 wrote:the rubber in the photo above is well cracked. that is the first thing I'd look at (replace).

Thanks. I thought the gasket looked bad as well, so I purchased a new one, but am away for a few weeks so the install is deferred. I also bought an ultrasonic cleaning machine and plan to get the carb a good cleaning.

The trouble with these warped UBF bodies is that it isn’t just the flange surface that warps, the slide bore is affected as well. If you try to put a new slide in a warped body, the slide will not move up and down the bore freely because the slide bore is no longer round. I took a badly warped body and c-clamped it to a flat, thick steel plate, placing the flange against the plate and the c-clamp over the air inlet. The UBF carb is cast from zinc which has a pretty low melting point, so I left it in a hot oven for a while and tightened the c-clamp a few times during the heat. Figured I had nothing to lose since the carb body was already compromised. It actually worked! Straightened the flange and the slide bore was noticeably improved.

More good info, thanks.