idle mixture screws

[81zuke]

Carbs were dipped cleaned new o-rings and stock jetting and stock airbox. Carb #1 is 2 turns out Carb #2 is 1half turn out Carb #3 is 2 and a half turns out and Carb #4 is 1turn out. So my question is if all carbs are equally rebuilt then why all the different settings on the mixture screws? These settings are the only way that I can get a nice tan color on all 4plugs.

[BassCliff]

Hi,

Manufacturing tolerances?

Are you reading the plugs at idle or using the "highest idle" method to set the idle mixture screws?


Thank you for your indulgence,

BassCliff

[81zuke]

Those settings are at idle.

[laverda1200]

my first question is what is the recomended factory base setting for the idle mixture screws? I would like to know as I just totally rebuilt a rack of carbs for an 81 GS 750. I have tons of experience with Dellortto carbs and Mikuni Rs flat slide carbs, not so much with CV carbs but the principles are the same, although these Cv carbs use a different form of idel circuit.

to read idle mixture correctly you need to know a couple of things -

first, float height has to be set exactly the same on all four carbs, and set correctly. Float height differences will change the mixture, as the pilot/idle circuit on these CV carbs is a bit odd, rather than drawing directly through the pilot jet, the fuel has to be sucked up from the float bowl through the main jet to get to the pilot jet through a little side passage. Float height is critical. The odd idle circuit arrrangement is why there are the rubber plugs on each of the idle circuit jet tower plugging the end.

second, the colour of the entire end of the plug is irrelevant, especially for the idle circuit - more on that in a minute.

third, the mixture screw is not an air bleed as on a lot of carbs, it is a fuel circuit - screwing the mixture screw in (clockwise) is decreasing the fuel, screwing the mixture screw out (counter clockwise) is increasing the fuel. Think of the idle mixture screw on these CV carbs as to fuel flow as the tap on your house wall for the water flow from the garden hose......

fourth, the amount of vacuum the inlet is pulling to draw the fuel up from the float bowl (set at correct height on all four of course) is dependent on valve clearance being set properly, and to a lesser extent, to ignition timing being set accurately.

fifth, so before going any further trying to set up the idle mixtue and carb sync, go back and set valve clearances perfectly, and check and adjust ignition timing as needed. Then, with float levels set equally and perfectly, valve clearances correct, ignition timing correct, go back to the idle mixture and carb syncing with a vacuum guage. This assumes no air leaks in the intake manifolds, by the way......

sixth, before trying to "read" the plugs" to get correct idle mixture, stick in a brand new set of plugs, and have a magnifying glass handy. Set all the mixture screws equally (if anyone knows what factory setting is, please speak up) Fire up the bike, and adjust the idle screws equally one after the other until you get the highest idle , let it idle for a couple of minutes, then stop it and pull the new plugs. What you are looking for on the plugs is not the tip colour, you are looking down into the tip recess to as close to the bottom as you can see, a light helps as well as the magnifying glass. You will see a "ring" around the base of the plug insulator, that is what accurately tells you what your actual mixture reading is.

seventh, make absolutely sure each of your choke plungers are actually sealing and shutting off the opening when they close, otherwise a leaking choke plunger could be ffecting your idle mixture and overall carb setup.

HTH

Paul

[Steve]

my first question is what is the recomended factory base setting for the idle mixture screws?
...
fifth, so before going any further trying to set up the idle mixtue and carb sync, go back and set valve clearances perfectly, and check and adjust ignition timing as needed. Then, with float levels set equally and perfectly, valve clearances correct, ignition timing correct, go back to the idle mixture and carb syncing with a vacuum guage. This assumes no air leaks in the intake manifolds, by the way

To answer your first question, the answer from the factory is "THEY ARE SEALED, DON'T TOUCH THEM."

Of course, we all know that they were set lean at the factory and presumably actually run against some kind of meter for optimum mixture, but the bottom line is that I have seen stock settings between 1 and 1 1/4 turns out from lightly seated.

When I am setting up a rack of carbs (from fresh rebuild or whatever), I start with three full turns. This gives a nice rich mixture for easy starting and won't likely foul the plugs (much) before you get a chance to fine-tune the mixture. I then tune them like you said in "sixth ...". I have found that most of the ones that I have done have been happiest between 2 1/4 and 2 1/2 turns out.

By the way, your point #5 is the basic starting point for ANYONE that gets a "new" bike. You should really not plan on riding the bike until all that is done, then check the tires and brakes, too.

.

[laverda1200]

thanks, Steve

As an enthusiastic and highly experienced amateur I typically build and tune Laverda triples and twins, the occasional Benelli Sei, Ducati bevels, etc.

Last week I was setting up a 1981 Ducati 900 SS desmo. The fellow that brought it to me says "I can't get the carbs set right no matter what I do....." Turns out his idea of setting a carb is twisting the idle screw and the idle mixture screws......

on inspection, valve clearances were out of wack, timing was out of wack as the old Bosch pickups were misaligned, the carb inlet manifolds were leaking, the carb internal passages on the giant 40 mm Dellortto pumpers were full of crap, the float on one carb was set at 14 mm (stock is 18) and hitting the roof of the float chamber without actually sealing the float valve, the other was at 25 mm and starving that cylinder.

when I delivered it back to him a few days ago, his first reaction was irritation at the amount of the bill, the second was "to hell with the bill, this thing has NEVER run like this before, ever...."

all the same principles are applicable to any motorcycle engine, an awful lot of carb problems are valve clearance issues or ignition timing or vacuum leaks or misadjusted float height or some combination of all!

Paul