Mach 1 Stock Carb Tuning

[blaat!]

Hi All,
I'm getting closer. I'm able to start the bike much more reliably and it is idling steady. I did a 10 mile (16km) ride last week which went okay... considering I tried to brake a couple times and upshifted instead shifter on the right is a bit of a mind-bender

Questions:
1. The idle improves as I tighten the silver pilot adjustment screw on the side. Fully in is best. Why would that be? What else should I change in order to get back to having adjustment in either direction? I have read different info if this screw is air or fuel mixture... anyone know definitively?

2.JPG

2. In an ideal setup would the screw on top still be pulling the slide up some amount to maintain an idle? If I let his screw drop the slide fully the bike will stall.

3.JPG

3. What are the "first idle system" and "second idle system" referring to in the SS Dell’orto Carburetor Technical Manual on MotoItalia?

1.JPG

https://motoitalia.parts/ss-dellorto-ca ... al-manual/

There is only one adjustment screw, no?



Here are the details of my carb and bike:
Dell'orto SS1 29D
Slide: 80
Main Jet: 120
Needle Jet: 265
Needle: M14 on 2nd notch (from the top)
Stock Mach 1 cam
Stock Silentium exhaust

Thanks all!
Jim

[Duccout]

Hi Jim,

Perhaps the first suggestion is to check that the float level is not too high. Second: is the pilot jet too big?

Colin

[blaat!]

Excellent suggestions Colin.

It isn’t overflowing and it seems to be getting enough fuel while running but I should definitely confirm that with the clear tube method.

The pilot jet is a 55 and stock is supposed to be a 50 so it is ever so slightly bigger than stock. Since they are hard to come by I have been living with it. Maybe that could be the reason? If tightening the adjustment screw is leaning it out then that would make sense.

[Bevel bob]

I have soldered up jets and drilled them to the size required. The slide adjustment on the top is crude and a bit unreliable so some resort to just blipping the throttle . You could put a couple of blobs of solder on the bottom of the brass slide to hold it up a tad. If the silencer is a modern replica it will likely cause poor performance. Mach 1 silencers were less restrictive than those "fit all" replacements.

[Dave354]

Hi, here is the extract from Mick Walkers book…. Hope it helps

Dave

[Dave354]

He also seems to suggest that idle is achieved using a combination of pilot air screw adjustment and throttle stop?

[blaat!]

Ok. I'm actually going to find a fitting to convert the bottom float bowl screw to a barbed fitting and use a clear tube and check the float level. Will upload a photo. check with the bike running? That is a lot of vibes!

I'm going to check the pilot jet with a pin gauge tomorrow. The size is in metric decimal right? So a 50 is 0.5mm?

Always great suggestions Bob. I'm a little wary of my ability to accurately drill a 0.5mm hole after solder. Drill bits come that small but not reamers. Will wait to see if the the pin gauge is spot on or undersized from "patina" and proceed accordingly.

The Silententium is authentic (even if a little worse for the wear) so should be good there.

Thanks for the reminder on Mick Walker's book, Dave354. I dug mine out and looked. Not that it matters for this idle issue, but he has a 60 slide on the Mach1 carb but I only see 80 or 100 spec'd for Mach 1s in the parts books.

M1-slide-A.JPG

M1-slide-C.JPG

Where does the 60 come from?

[blaat!]

After re-reading the technical manual, I realize that the carb has one of two idle system, not both. Lost in translation One was for production model motorcycles needing a consistent idle, and the other was only for race bikes. The Mach 1 carb has the replaceable (fuel) pilot jet screw, and the variable air screw on the side. Seems that the silver screw adjustment on the side is definitely an air screw which means that tightening reduces air and makes the idle mixture richer.

If my pilot jet is a 55, but it is supposed to be a 50, then it is getting (ever so slightly) more fuel at idle making a richer mix than stock. I don't think the pilot screw could be to blame since the idle gets stronger by richening it EVEN MORE with the air screw adjustment. It should be the other way around. There must be some other culprit.

I'll check the float level next.

[ducwiz]

Jet reamers are in fact available, for example: https://www.marolotest.com/gb/coffret-d-alesoirs-de-gicleurs.html
Soldering a jet bore and ream it to a smaller dia is imho not recommended. The tin allloy is very soft, the bore dia may not stay constant over time.

Hans

[blaat!]

Yeah there is zero chance I could even clean and get solder down into that 0.5mm hole anyway. Then the tip of the pilot jet appears to be crimped into the screw so it is unlikely to be true so couldn't put it on the lathe to drill & ream. I like the idea and I probably wouldn't put enough miles on to wear the solder away again, but I'd probably just ruin the pilot jet in the process due to poor craftsmanship.

My float level seems to be correct:

level.JPG

I need the adjustment screw all the way in, but the idle is strong and consistent. I might just move on to other parts of the tuning, which may ultimately affect the idle anyway.

It's breaking up a bit under load above 1/4 throttle and then it kicks back in above 1/2 throttle which seems to suggest jet needle adjustment, but before I get to that I want to confirm that the 80 cut slide is correct.

The parts manual says 80 slide for the Mach 1, but Mick Walker's book says 60 slide. Anyone know which is correct?