fuel/air mixture help

[globalmaa]

so i followed this tech guide and i think i got it but i'm only at about 1/2 turn out and i'm still running rich. i have mikes xs pods and original exhaust with baffles out. my jets are two sizes up from stock. When i turned the mixture screw out 1.5 turns it idle really rough so that said I started at .75 turns out and i'm now at 1/2 turn out and it is creating black plugs even with BP6ES any idea???? the vin is 447-107453 so i take it it is a 75'( .75 turns out from seat) and i believe the carbs are 77'??

"Warm the engine to operating temperature, then raise the idle to around 1500 rpm. On
bikes with breaker point ignitions, pull off a sparkplug cap. Adjust the throttle stop to
hold lowest steady idle, then move the fuel screw 1/4
turn each way, seeking highest idle. When you find the direction of improvement, set the
screw 1/8 turn in that direction from your starting point and again move it 1/4 turn each
way. As idle rises, lower it with the throttle stop, as changes are easiest to detect at low
engine speeds. Set the screw at the inmost (leanest) position that yields highest idle
speed. (And yes, I know; some owners advocate finding the point where idle drops due
to leanness, then the point where it drops due to richness, and setting the screw at the
center point. I do not concur.) Repeat for the other cylinder."

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[5twins]

All jetting and mix screw settings should be based on the year of the carbs, not the year of the bike. Figure out for sure what year carbs you have and then you can start tuning.

 

[globalmaa]

so then it looks like these carbs are 76-79 why is it they run like crap when the mixture screw is turned out more then 1.5 turns?

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[globalmaa]

so 5twins i've read some post of yours and you are a guru with carbs. forgive me i know you've been over this a 1000 times with newbies.
here we go my bike keeps running rich. the PO had set the carbs up but might not be correct. I was told by a really good friend who is a machinest that he drilled out the mains to increase 2 sizes.( he does it all the time and never had a problem ) so if i turn the air/fuel mixture screw out to the 2 turn mark it runs rough, if i turn it out to the .75 mark it runs good. confusing yes since these are 76-79 carbs. I'm running mikes pod filters and stock exhaust with the baffles knock out. My plugs keep going black very quickly so i'm running really rich and gas mileage sucks. that said i would have thought that if i adjusted the mixture screw inward to a very lean setting the bike should run lean but this isn't the case it is rich. Is there a air valve that could be blocked?


WAIT A MINUTE could it be the jets are 2 big now since they have been drilled to increase 2 sizes up,and therefore allowing way to much fuel in and is over taking any air flow causing it to run way to rich all the time. If i switched up the exhaust to a straight pipe it should lean it out some ?
I believe the PO was running straight drag pipes before i bought it .

 

[5twins]

As I said, you really need to I.D. your carb set exactly. There was a '76-'77 set and a '78-'79 set. The 2 sets are vastly different as far as jetting specs go. Your rich condition could be caused by incorrect float settings, improper jetting, or worn parts. Float setting, jetting, and mix screw settings are different between the 2 carb sets so unless you know which set you have, you can't correctly tune it. Here's a chart I drew up listing the stock carb specs. As you can see, there's quite a difference between the '76-'77 and '78-'79 sets .....

 

[globalmaa]

thanks i'll pull them off and check . i've also been told that the jet needle could be moved in order to adjust the lean/rich mix so that said i'll pull the carbs off and adjust the needle jet.

 

[5twins]

Post some pics of your carbs and I can probably I.D. them for you. Make a note of the jetting and needle #s you find and I would also recommend replacing those drilled out jets. No telling what size they are now.

 

[grizld1]

A few more things: when you've identified your carb set, be sure that the correct pilot jet series, as well as correct size, is in place, be sure that float level is correct, and be sure that needles and needle jets are in good condition.

 

[globalmaa]

so they are 78' i believe the needle is E502 and since the bike is a 78' . so next question i was able to get the top of the carbs off without taking them off the bike . how do i adjust the jet needle to a leaner position right now it is in the second position closes to the point , now i know that isn't right and should be moved to the 3position for stock.

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[grizld1]

The '78 and '79 carbs (they're the same) were the only BS38's on the XS650 to vent the float chamber through the intake bell. Refer to the pics in the Carb Guide. The needle clip is just an E-clip; hold the needle in one hand, grip the clip with a pair of pliers, and pull the clip off the needle; push it back on with same. It's a good idea to do this while holding the needle and clip inside a box, in case the clip gets away from you.

 

[5twins]

Yes, those are '78-'79 slides at least. They retain the needle in the slide with a snap ring. I'd have to see pics of the rest of the carb to tell whether the body is correct.

Mikuni needle slots are counted from the top down so you are in slot #4, one step richer than stock. Combined with your drilled out jets, that's probably why you're running so rich.

To adjust, pull the e-clip off and move it to a different slot. Stock is the #3 slot but with your larger drilled out jets, you may need to go another step leaner to the #2 slot.

 

[globalmaa]

i moved it to stock and now it is about 1.5 turns out it was in #4 . i'll pull the plugs clean them out and go for a ride. i'll se what they look like but i think i found the problem. if they are still black i'll move to number 2. thanks guys this really helped.

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[globalmaa]

so i think another problem is my floats are to high. they are the brass color floats and with gasket removed they are at 27mm. hard to tell from the first pic but i drew a line on the gren tape and marked it 27. i do believe they should be 24mm +/- 1mm starting to understand carbs a lot more now.

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[5twins]

Yes, if set at 27mm, they are too high by quite a bit. That would translate into a low fuel level when the carbs are right side up and operating. Mix screws don't respond correctly (or at all) is the float level is really far off. Your mix screws should be dialing in somewhere between 2 - 2 1/2 turns, not 3/4 to 1 1/2.

Also, as I said earlier, change those drilled out jets. They're not flowing properly anymore and you'll probably never get it tuned right with them in there.

 

[gggGary]

Take 5T's advice, ALL of it not just the parts that you like or are ea$y to do...........

 

[globalmaa]

thanks i changed the jets 140 main 30 pilot set the floats to 24mm needle jet to #2 lean position . now my left side is about 2 full turns out and the plug is a little black and a tan color the right side is now about 3 turns out and is running real lean almost a off white color. so what are my options?
put right side needle jet back to stock #3 position? i really don't want to back it out more then 3 turns. I also know that both carbs should be almost exact but i'm not having much luck with getting the left and right to be the same. so should i change the right needle to the stock position( 3) and leave the left in the 2 position

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[globalmaa]

well i think it is done. i made a few more adjustments took it for a quick little scoot and both plugs are a nice tan/brown color and left side is out 3 turns and right is out about 3.25 to 3.5 turns. i know the reason why 3 turns cause the needle jet is at the #2 position . pretty sure if it was at the stock #3 i would be at 2 to 2.5 turns. i'm happy thanks to all for your help and to 5twins for having patience .

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[5twins]

Your larger pilot should compensate for that needle drop but it's possible you need to go up another size. Every one of these bikes tunes out a little differently so it's possible that yours needs the settings you have.