carb tune ?

[mrhammer2u]

After installing new pipes I now have back firing when I crack the throttle. Also idle seems a bit shaky. I looked all over the net and cannot find exactly where the idle adj screws are. Is there a separate screw for each carb? Anyone have a clear pic pointing out where this/these screws are? I was going to start here before checking into jetting (ugh!)

[Prisoner6]

Hey, Rob -

Do you mean the idle speed adjustment screw? If so, there is only one, and it is located facing up between carbs 2 and 3 on the airbox side of the carbs.

Based on your signature, I'm assuming you have the early '78-'79 carbs.

[James England]

The crackle can be reduced by adjusting each of the four air screws on the carbs. Each carb has one. I always get my two bikes confused but I'm sure someone will post about whether to screw them in or out. I believe the mixture needs to be richened slightly. The popping is caused by the mixture being too weak.

The air screw on the left carb can be seen in this pic. It has a spring round it to stop it moving...

[cajun31]

did you install new exhaust gaskets?

If you did not then that could be a source of the backfiring. I had the same problem. Once I bought new gaskets it disappeared. If you used new ones with the new pipes then the problem may be elsewhere.

[IanDMacDonald]

Backfiring, or poppingbfrom lean-ness? You'll need to check your plug color and adjust accordingly. Clockwise is rich, CCW is lean. Don't chase your tail confusing the idle screws with the mixture screws (very easy to do rushing). After you get your plugs looking tan, sync your carbs (Morgan Carb Tune works great- thought that's what the thread was about).

[mrhammer2u]

Once I retightened my exhaust, the backfire went away. Thanks for showing me where the idle screw is. Now I know!

[motoman]

The crackle can be reduced by adjusting each of the four air screws on the carbs. Each carb has one. I always get my two bikes confused but I'm sure someone will post about whether to screw them in or out. I believe the mixture needs to be richened slightly. The popping is caused by the mixture being too weak.

The air screw on the left carb can be seen in this pic. It has a spring round it to stop it moving...

FWIW, just to be clear so as not to confuse anyone James, Those are FUEL mixture metering screws, no AIR. Between the head intake and carb, those adjustment crews are ALWAYS fuel related. Between the air intake and carb, those metering adjustment screws are ALWAYS air related. Examples of single carbed bikes, street or dirt, whether the carb be a Mikuni, Kiehn, Anal, Solex, Bing, etc., the mixture adjustment is BEHIND intake bell of carb and is an air mixture screw.

[mrhammer2u]

Prisoner6...I have a 79 special. So that I know,....the idle speed and air screws are different right? Not to confuse things more,...the jets are a whole other tuning adjustment....right?

I've called 2 different bike shops and when I mention what bike I have they get all flustered..."4 carbs....whoa!"....lol. Guess I have to learn to tune on my own!

[motoman]

Prisoner6...I have a 79 special. So that I know,....the idle speed and air screws are different right? Not to confuse things more,...the jets are a whole other tuning adjustment....right?

I've called 2 different bike shops and when I mention what bike I have they get all flustered..."4 carbs....whoa!"....lol. Guess I have to learn to tune on my own!

Not so much the excuse of four carbs, but not wanting to admit they haven't a clue how to work on ANY part of the bike better fits there answer.

[motoman]

BTW, they'd REALLY be at a loss if it where a Honda Vaulk 1500 with SIX carbs.

[Prisoner6]

Prisoner6...I have a 79 special. So that I know,....the idle speed and air screws are different right?

Hey, Rob -

To answer your question ... Yes, they are different, but there is one big caveat ...

First, as I said in my earlier post (see post #2), there is only one idle speed adjustment screw.

Now, look at the photo above. That screw with the red arrow pointing to it is the Idle Mixture Screw, or PILOT Screw.

As James said, each carb has it's own PILOT screw. Adjusting these screws controls the air/fuel mixture at idle/low speeds.

The way the PILOT screws control the mixture is (as motoman said) by controlling the amount of FUEL, not air, allowed into the mixture.

A good rule of thumb to remember is this: When the PILOT screws are on the front of the carbs (engine side, like our XS's), they control/meter the fuel.

When they are on the rear of the carbs (airbox side), they control/meter the air.

Clear as mud, right?

[James England]

Yes, sorry...fuel screws not air screws! I slipped into Triumph Bonneville mode on that one. I'm not sure re my XJR...I think they are fuel screws too, not air....

[TopCatGr58]

Well,

To be truly correct NOW since we've seen the sawed in half views of the carbs, the PILOT screws are actually FUEL/AIR screws since the air is being mixed into the fuel flow path after the PILOT JET, and BEFORE the pilot screw...so what you're metering is both fuel and air...albeit still mostly fuel...just ATOMIZED a bit, and it's then getting mixed/atomized with MORE air when it hits the carb intake throat going into the intake manifold.

But for a simplistic view, yes, it's still a fuel metering screw.

T.C.

[MPittma100]

A Waste

After installing new pipes I now have back firing when I crack the throttle. Also idle seems a bit shaky. I looked all over the net and cannot find exactly where the idle adj screws are. Is there a separate screw for each carb? Anyone have a clear pic pointing out where this/these screws are? I was going to start here before checking into jetting (ugh!)

Don't waste your time or money talking to ANY bike shop in the future about these bikes - they simply don't have a clue. The technical knowledge of the XS1100 is preserved right here on this forum.

If you have a problem or question about your bike, feel free to ask us here. We're here to help. And we normally do not charge that much either (LOL).

[Prisoner6]

Well,

To be truly correct NOW since we've seen the sawed in half views of the carbs, the PILOT screws are actually FUEL/AIR screws since the air is being mixed into the fuel flow path after the PILOT JET, and BEFORE the pilot screw...so what you're metering is both fuel and air...albeit still mostly fuel...just ATOMIZED a bit, and it's then getting mixed/atomized with MORE air when it hits the carb intake throat going into the intake manifold.

But for a simplistic view, yes, it's still a fuel metering screw.

T.C.

Yes, TC, but a simple explanation is what I was going for, considering that Rob (mrhammer2u) is a new member and is unfamiliar with the XS11.

Don't want to confuse him in any way ...