Running rich. Main jet size?

[Captkersene]

Got a 1975 XS 650 with mikuni carbs that has been setting for 16 years. Rebuilt carbs but it is running WAY too rich. I live at 7000 ft and will be riding the bike from 5000 ft. to 100000 ft. when I go over mountain passes. Does anybody have a suggestion on what size main Jets I need. Seems to idle well and doesn't run too bad but bad stumble in mid range. I could experiment with different jets but just wondering if anybody has any ideas

 

[5twins]

Did you use an aftermarket carb rebuild kit? Many come with a generic Y-22 replacement needle and it's wrong for your carb set. It's way too short which makes it way too rich.

 

[Melnic]

Question, since idle mixture screw is affected by the Pilot jet (correct)? is there a difference between moving up the pilot jet and just moving the idle screw richer? In other words, does the pilot jet affect throttle (like accelerating) more than the idle mixture screw does?
On another bike, I read that if you go up on the pilot jet, you are likely to need to lean the idle mixture screw. Is this also correct?

 

[Ratranger]

You want the fuel screw to be somewhere between 1.5-2.5 turns out. Too big a pilot and you'll have have the screw too far in, too small and you have the opposite. The pilot jet will affect the fueling the most in low throttle, but does have an effect all the way to wide open. Same thing with the needle and main, they mainly affect higher throttle positions but do change low throttle as well.

Start out getting a good idle when warm, change the jet to jet the screw in the right range. Next do wide open to get a baseline for the main. Finally adjust the needle for part throttle. When adjusting the needle if you have to drop it the main might need to go larger to compensate for more needle staying in the emulsion tube. The idle might need the screw tweaked to adjust.

My DRZ with big bore stroker actually runs smaller jets than a stock bike with a FCR carb. It's a lot of back and forth to get things dialed just right. And remember higher temps=richer fuel mix with no change and colder temps= leaner with no changes.

 

[5twins]

The mix screw is for fine adjusting only. The idle mixture from the pilot jet is delivered into the main bore through 4 small holes. Three are clustered together at the top of the bore and come directly from the pilot jet. The fourth is off to the side and comes from the mix screw. That means you can adjust roughly 25% of the flow from the pilot jet.

Usually changing the pilot jet size will require resetting the mix screw. As a rule of thumb, going up a size will need the mix screw turned in (leaner) about ¼ turn. But, you will want to adjust the mix screw any time you change the pilot to find the best spot for it.

 

[Kevin Werner]

Some folks think that a few small intake and exhaust mods will take a 42hp XS650 magically to 57hp with the need for that much more fuel. We need to acknowledge that a few intake/exhaust mods will take a 42hp XS650 right up to the heights of 43.3265hp and the fuel delivery requirements change only a very little.

 

[Melnic]

Some folks think that a few small intake and exhaust mods will take a 42hp XS650 magically to 57hp with the need for that much more fuel. We need to acknowledge that a few intake/exhaust mods will take a 42hp XS650 right up to the heights of 43.3265hp and the fuel delivery requirements change only a very little.

It might be cause of things they see in the Fuel Injection world where bikes are mapped to hold things back or control emissions. Some bikes even stock can be tuned for 5% more power. With cats in bikes now, removing restrictions can mean cheap power. Honestly, I have been working to QUIET down my Indian Scout. It came with V&H Slash muffs which are almost straight thru. I compared the sound after putting the stock muffs back on and they were 12db louder! stock muffs are obnoxiously long so put quiet baffles in and still too loud for my taste at about 8db louder.

I'll comment in my 78 Special thread but I just ordered some pilot Jets for it.