Carb "mixture/petrol screw" set-up

[Tufftrax]

Assistance or advice please......

I am trying to find information about setting my carbs up correctly and thought I'd ask fellow riders for their help.

The carbs seem to have just one screw for mixture (or petrol flow) and one for idle speed (throttle stop). The "mixture" screw seems to merely allow alteration of the flow of fuel and there does not seem to be an "air" screw at all which I find a little puzzling!

The bike was running a little lumpy so I screwed both "mixture/petrol" screws in all the way and then backed both off by 1& 1/2 turns and applied a similar approach to the throttle stop screws. Bike seems to run smoother now and both spark plugs appear to be the correct (sandy/light brown) colour so I think I may have it just right but wanted to check with other riders as I do not possess a vaccuum guage set and cannot check this way. Bike used to hesitate on slow running pick-up in higher gears but this seems to have been fixed too.

I assume that if the carbs were out significantly it would not start or run properly so maybe I am being over-cautious anyway!

Thanks in advance for any advice,

Keith

[MickeyMoto]

I think the 'mixture' screws only work at tickover and a little bit. When winding on the throttle (any other way to ride a Morini?) the main jet etc will be the deciding factor.

The mixture screws are set at idle and with a warm engine, turn one in or out until you get the highest revs then repeat for the other. Repeat. Set the throttle stop screws to get a reliable tick over at the corret engine speed. You will need to do the gauge thing as this is for the slides moving in sync. You also need a bit of slack in the throttle cables. You can roughly set the sync by holding your fingers in the carb inlets and feeling the slides lift when somebody opens the throttle, but it is a rough adjustment. Vacuum gauges are best as they can iron out the small differences that require fine adjustment of the cables.

Maybe I am wrong but that is what I do...... be interested to hear of other people's methods, maybe mine will run better....

Mike.

[72degrees]

I reckon a u-tube manometer with "dancing columns" is better than "needle on a dial" vacuum gauges. But the basic technique for setting the idle mixture is the same.

A Morini can start with the carbs quite a way out of synch and run surprisingly well, but the difference perfect balance makes is well worth the effort.

[Paul the Destroyer]

I'm in St Neots and have a pair of dial type balance guages.

You would be most welcome to pop over if you wanted see how they are balanced.

PM me and I'll send you my number, address etc.

[MickeyMoto]

For gauges I use Morgan Carbtune dancing column thingys..... I agree they are more accurate than a dial gauge, all that is needed is to align the clumns and it is easy to watch the dancing thingys...

Mike.

[EVguru]

The carbs seem to have just one screw for mixture (or petrol flow) and one for idle speed (throttle stop). The "mixture" screw seems to merely allow alteration of the flow of fuel and there does not seem to be an "air" screw at all which I find a little puzzling!

What you are trying to do is adjust the 'mixture', that is the ratio of fuel to air. Some carbs do this with a fixed jet and an adjustable air bleed, whilst other carbs do it by adjusting the fuel flow.

The Dellorto sets the idle mixture using a combination of a fixed jet and the mixture screw. Just above idle you have a 'progression' circuit that smooths transition onto the main jet/needle combination. The mixture here is controlled by the idle jet and slide cut-away. I usually find that with modern fuel our bikes run too rich through progression and you get a 'stumble' when pulling away (often worse the longer you've been idling). I've gone doen to 43 idle jets from 50's and the stumble is pretty much gone.

Before you 'vacuum' balance the carbs you should check the valve clearances.

If there is a balance pipe between the carbs I blank it off. It's intended to make the bike less sensative to carb balance, but it upsets the vacuum readings and idle speed. I get a smoother and more tractible bike without it and being fussy with the carb balancing.

[Tufftrax]

Thanks for the replies guys.

Hoping to go to NLM next week for MOT so I'll ask Alex to check it while I'm there. In the meantime I've dug ot an old copy of the Harglo manual and temporarily set the carbs up following their guidance (removing the plugs one at a time, using the mixture screw to obtain fastest idle then using throttle stop screw to set idle at around 1,000 rpm and repeating for other side - slowing the idle speed as low as possible and then moving the mixture screw gives a more pronounced fast idle and a small deviation either way has the bike stalling) and it seems to work OK. Checked the operation of the trottle slides visually and both appear to work in usison so maybe it's OK - I'll find out for sure next week!

Thanks again,

Keith