Tuning my CV carbs

[Anonymous]

Alright, today I got in my brand new carb stix, and some new mixture screws. How much vaccum should my carb stix read, because I am pretty sure its graduated in cm instead of in. Also, where do I start out with the mixture screws on the CV carbs of my '83 GS750E, with pods?
Any help with tuning this bike would be greatly appriciated. :twisted:

[Anonymous]

I've taken to making an archive of Earl's procedural wisdom. Here's a reprint for you. The thread was "High idle after carb adjustment"

Michael

On CV carbs, pilot fuel and air are combined into one mixture screw. I set the #3 carb to baseline vacuum using throttle plate adjustment, then set the mixture screw on it for highest idle rpm. Then set the idle adjustment knob to return to 1200 rpm idle. Re adjust throttle plate on #3 to 10 inHg vacuum. Then set the fuel/air pilots (mixture screws) on 1,2 and 4 to the same number of turns as #3. When you have 1200 rpm and #3 is drawing 10 inHg vacuum, you have your baseline and then can set 1,2 and 4 to draw the same vacuum as #3. Once #3 is set to base, you must not touch the idle adjustment knob again. Only when all 4 cylinders draw the same vacuum and rpm remains at 1200 may you change the overall idle rpm if you wish.

Setting the mixture screws doesnt do you any good if you dont have the correct intake velocity (vacuum) for the screw setting. After you set up #3, that is why you cannot make any more changes with the idle adjustment knob. Its impossible to synch vacuums if youre constantly sending the reading all over the scale with the idle adjustment knob.

Earl

[Anonymous]

OK, I understand that, now what do I do about the measurment of my carb stix? I got the shop quality carb stix out of the Dennis Kirk book, but I think it is graduated in cmHg, not inHg. What the heck is Hg anyway? :twisted:

[Anonymous]

in Hg stands for Inches of Mercury, chemical symbol Hg. You can measure the difference in pressure between two environments by how far the lesser one will suck a tube of liquid. Think of it this way: if there were marks on your straw as you suck water from a fountain cup at McDonalds, it would measure the difference in pressure from the suction of your mouth to the environment around you.

People use liquid mercury because it is 11 times as dense as water. You could have a much safer tool (Hg is bad stuff) by using water but the water would have to get sucked up 11 times as high, so 10 in Hg would be 110 in H2O, or almost ten feet in the air. And since your carbs sit at about two and a half feet above the ground, good luck getting a ten foot vertical suction of water.

10 in Hg = 25.4 cm Hg

Michael

[Anonymous]

Ok, thank you very much Michael, I am going to try to get something done on the bike and let you know how it goes.
:twisted:
Matt