Tuesday, October 6, 2015

More Tinkering

Tinkering continues with another small changes to the main jet. Changing the jets is really is pretty simple on the Ural. This is a picture of the carburetor bowl that you would see when lying on the ground looking up at the carburetor.. First, the gas is drained from the bowl and the fuel lines on the left carb since it is easy to reach. The drain is the little tube at the bottom of the bowl with an allen head screw acting as a valve on the other side. Between the two carbs, there is about a cup of gas which I drain into a clean bottle that is not used for anything else. After draining the bowl, the four allen head screws which attach the bowl to the carburetor body are removed. The stock screws were replaced with the stainless allen head screws shortly after picking up the Ural to make them easier to remove.

The main jet is circled in yellow and is very easy to remove with a screw driver. I installed a 135 main jet which is quite a change from the stock 122 and the 127.5 that I had been running with the stock air filter housing. The low speed or "pilot jet" seems to be working just fine so I'm leaving that alone for now.

The behavior that I'm trying to change is a loss of power when the engine speed tries to go above ~4500 rpm with the throttle wide open. As I understand it, there are four parts of the CV (constant velocity) carburetor that are used to adjust the air/fuel ratio. The idle air screw, the pilot jet, the needle and the main jet. Each of these are used under different conditions and there is overlap with each other. The main jet controls fuel delivery when the throttle is in the upper third of it's range. It's "neighbor" which influences fuel delivery at mid throttle is the needle controlled by the slide.

I'll see how it works tomorrow. Last spring when I made the plastic air box I just didn't have the time to tinker with the jetting to try and get it running well.

Wednesday Update - Success! Last night, I installed the 135 main jets and the rig runs great. No hesitation or flat spots evident on this mornings ride into town. Granted, I haven't tackled any long hills but it definitely runs better than it did last spring when I first tried the airbox. I never tried this large of a jet last time as others posts on Soviet Steeds were suggesting smaller jets. Though if you have a freer flowing air box it makes sense to install a larger jet to compensate.

7 comments:

Dar said...

Richard there seems to be a lot of tinkering with your Ural, is this common? i think I am glad I didn't get one at this point, I am nit a wrencher and it woukd be expensive and problematic for me me fix. Hope you get it sorted out!

redlegsrides said...

Dar, the specific tinkering Richard is doing is common to carbureted motorcycles in general, not just URAL. Its one thing I don't miss since I got an EFI rig. Though of course, the EFI rig brings along it's own set of needs/wants. :)

redlegsrides said...

RichardM, yep, what you described in terms of each jet's function is how I understood it as well. Never did mess with the idle jet. You're much more methodical than I was though.

RichardM said...

The tinkering is more because I like to tinker. In this case it's due to my making an alternate air filter. Most would not bother with such things. And I like the Ural (and my airhead Beemer) because I ca tinker. With all of the computer and electronic controls on the newer bikes you can farkle but not tinker.

I met another Ural owner on Monday who is thinking of installing a 3 cylinder turbo diesel. That's serious tinkering...

RichardM said...

I changed the richened the pilot jet last year to start better at low temperatures. At that point, the enricheners weren't even needed unless it was below 0°F. Kind of overkill IMHO.

Dar said...

For me tinkering usually involves a glue gun and crafty stuff :) I will leave the mechanical tinkering to you guys!

RichardM said...

Same idea. In this case a piece of plastic, some aluminum flanges, some screws, and you end up with something that hopefully is functional.