New Guy Carb Question

[scottbly]

So, I have my newly aquired 83 Venture running and stopping fairly well thanks to all the threads in the first gen library. Thanks to all. However, I am little confused about carb adjustments. I seem to be getting poor gas milage and my plugs are black and wet. I saw the thread about replacing the plastic spacers on the jets, but I also see lots of references to set screws at 2-3 turns. Does my bike have fuel/air set screws and if so where are they? The bike starts and runs well. I have not ran it enough to get an accurate MPG, mostly because I have the speedo disconnected due to the darn noise (I guess that is my next project).

[5bikes]

As I described in "Improving Gas Mileage". Only the pilot screws are adjustable, set them to about 2 turns out. Don't change any jets.

[mbrood]

http://www.bergall.org/temp/venture/1stcarbs3.jpg

 

Unmolested carbs have brass sleeves (plugs) inserted, blocking access to the idle mixture screw that is referenced. These can be drillout rather easily, just use a small drill bit and then progress larger and larger. Before you get to the carb body bore sze, the brass will spin on one of the drill bits and walk right out. In any case, this bike is over 20 years and without knowing it's maintenance history consider the screws are going to be a bit tough to screw in or out. My method is to put a wide screwdriver in there and note the ABSOLUTE rotational position on the blade... now try to carefully ease the screw IN 1/2 turn... if that doesn't work so easy, screw it a bit in then a bit out then a bit more in, etc. to clean the threads. Remember, these are FUEL jets, so turning them IN make it leaner.

 

You REALLY want to KNOW the position of the screwdriver rotary position to start (on EACH carb) and then slowly work the screw in to a GENTLE seat as it bottoms. Now write down EXACTLY how many turns it was from the bottom originally. You may want to diddle with these jets but it's pretty vital to know where they started from so you can (at least) get back to that position.

 

The ideal gauge to use to set these is an exhaust gas analyzer. Second would be a high resolution tachometer (no, the dash tach is worthless for this fine change) and least favored is to adjust each one for maximum rpm by ear. Adjustments on a 2 cylinder engine is easier, the change is more dramatic, on our big 4 cylinders it's tougher to notice a "fine" change.

 

A general rule of thumb:

1.The mixture is lean if:

the bike surges while holding a steady throttle at about 4000 rpm in second gear.

there is excessive back-firing on closed-throttle overrun.

(The main needle jet needs raised for the above)

the throttle is lightly 'blipped' and the idle speed 'hangs up' before dropping to the set idle speed (set the idle to 1000 rpm to start).

(turn the idle mixture screws OUT 1/2 turn or so)

2. The mixture is rich if:

the plugs are excessively soot-ed up or fouled.

(The main needle jet needs lowered for the above)

the throttle is lightly 'blipped' and the idle speed drops below the set idle speed before rising up to the set speed.

(turn pilot screws IN 1/2 turn or so)

 

You also want to pull the plugs and clean them well (or just replace them) so you have a good visual starting point of visually watching how they are performing. Reducing the thickness of the plastic washer under the carb slide main jet will also creat a leaner fuel/air mixture at midrange and above, but the idle mixture adjustment is mainly from idle to about 2,500 rpm.

 

And with over 20 years of operation, jets DO wear so no absolute number of "turns" is magic for all bikes.

 

Above all, you MUST start and end with the carbs syncronized.

[6m459]

Has this bike been sitting around for a while before you got it? Likely with gas in the tank and carb bowls?

 

I'd run some Seafoam through it for a while before I went messing around with carb adjustments.

 

Brian H.