Adjusting the idle screws?

Buzz · Full Member Joined: Jun 27, 2005 Posts: 37

I have a 1982 XS650 Heritage Special I've gone through just about everything including rebuilding the carbs. My problem is; when I stop at a stop light for more than 2 minutes is cuts out when I accelerate and levels out and runs fine. It's like the plugs are fouling from a rich idle. Now this brings me to the adjustment of the idle screws. I have heard everything for 3/4 turns to 3 turns...whats right?

Thanks Buzz

flavaz33 · Posted: July 2, 2005, 9:49 pm Post subject: Re: Adjusting the idle screws?

It was advised to me to tighten the screws all the way down, then back them out 3 full turns. From there, scrrew them down by 1/4 turns (which i believe is to the right when the carbs are onthe bike) until they seem tuned. My carbs are jetted, pod filters, and pipes open, so i cant give you a definite turn ratio becuase your set up may be different. Just keep turning them 1/4 turns until they are running smooth. Hope this helps.

Kevin

jdschooler · Posted: July 4, 2005, 10:56 pm Post subject: Re: Adjusting the idle screws?

Are we talking Idle Screws or Air Mixture Screws....Idle Screws are best set with a set of Mercury Sticks to Balance the Carbs which definitely needs to be done if you tore down the carbs....The directions flav is talking about sound more like air mixture screws....You also will want to take a look are the plugs and make sure they are burning good and white.

grizld1 · Posted: July 4, 2005, 11:57 pm Post subject: Re: Adjusting the idle screws?

Let's clarify a few things here. First: the component listed in Yamaha service literature as the "idle screw" is the throttle stop screw. It's on the left carb near the cable mount. The synchronizer screw is located between the carbs. That's the one used to set the balance. The fuel screw in the '80 and later carbs is described in another article in this forum. It meters the flow of fuel from the pilot jet.

Before attempting adjustment, make sure the carbs aren't sucking air through the boots or throttle shafts, the fuel tap vacuum line is sound, air filters are clean and tight, and valves and cam chain are adjusted. Start the bike and pull one plug lead so it's running on one cylinder. Set the idle screw for the lowest idle the bike will hold. Turn the mixture screw 1/4 turn at a time, in both directions, to find the direction you need to move in. Set the screw for the highest idle. As the bike begins to idle higher, adjust down with the idle screw, and again find the mixture setting that yields the highest idle. Do the same on the other side. If you haven't done this before it will take awhile to get the feel of it. Keep some fresh plugs handy, keep a heavy fan on your engine, and don't overheat it. If the bike doesn't respond to the mixture screws, the pilot jets may need to be cleaned.

If you're running modified exhausts or air filters you may need to change to larger pilot jets. The rule of thumb is to increase pilot jet size one step for every three steps of increase in main jet size. Good luck!

Buzz · Full Member Joined: Jun 27, 2005 Posts: 37

grizld1 wrote:

Let's clarify a few things here. First the component listed in Yamaha service literature as the "idle screw" is the throttle stop screw. It's on the left carb near the cable mount. The synchronizer screw is located between the carbs. That's the one used to set the balance. The fuel screw in the '80 and later carbs is described in another article in this forum. It meters the flow of fuel from the pilot jet.

Before attempting adjustment, make sure the carbs aren't sucking air through the boots or throttle shafts, the fuel tap vacuum line is sound, air filters are clean and tight, and valves and cam chain are adjusted. Start the bike and pull one plug lead so it's running on one cylinder. Set the idle screw for the lowest idle the bike will hold. Turn the mixture screw 1/4 turn at a time, in both directions, to find the direction you need to move in. Set the screw for the highest idle. As the bike begins to idle higher, adjust down with the idle screw, and again find the mixture setting that yields the highest idle. Do the same on the other side. If you haven't done this before it will take awhile to get the feel of it. Keep some fresh plugs handy, keep a heavy fan on your engine, and don't overheat it. If the bike doesn't respond to the mixture screws, the pilot jets may need to be cleaned.

If you're running modified exhausts or air filters you may need to change to larger pilot jets. The rule of thumb is to increase pilot jet size one step for every three steps of increase in main jet size. Good luck!

The ones you have to drill out the protective caps to get to. I adjusted the "fuel screw" to 2 1/4 turns and it seems to be working. i tried it out on the way to work this morning, I jumped on it from a stop and i had one tiny misfire at about 5 grand otherwise its running fine.

mo650chopper · Posted: July 11, 2005, 4:50 pm Post subject: Re: Adjusting the idle screws?

I went from 2 1/2 to 2 turns and now mine is finally idling good without loading up as well not having the "revving Up" problem i had by trying to compensate for the rich idle mixture by adjusting the idle speed screw.

Buzz · Full Member Joined: Jun 27, 2005 Posts: 37

Thanks for all the replies. I was reading all the ways to set the idle on my XS and while looking at my XS in the shop, I looked up on the shelf and there sits a gas ratio analyzer, the kind that sticks up the exhaust, Well it hits me, why am I messing with manometers and such? So I broke it out and commenced to set the idle. I let it warm up and checked both sides. I set set the mixture screws at 2 1/4 turns each. The left side was at 13.1:1 and the right side was at 14.2:1. I adjusted the synchronizer screw till I got an even reading on both sides that was 13.75:1. I took it out for a road test and it ran great. You can pick one up at a Motorcycle / car swapmeet or maybe on ebay. I paid $35 for mine

grizld1 · Posted: July 15, 2005, 4:47 pm Post subject: Re: Adjusting the idle screws?

Glad that worked for ya, Buzz; but bear in mind that no two carbs are identical, and less-than-perfect valves, etc. can affect exhaust gas readings. You're working under the assumption that both fuel screws should be at exactly the same setting and dialing in your synch. from there, and that assumption has a lot of room for error in it. The exhaust gas analyzer is a great tool, but I'd want to use it recursively with the manometer.

Redball · Full Member Joined: Jul 13, 2005 Posts: 11

I am just coming into this conversation and I am new owner of an '83 (build '82) bike. Are the mixture screws (the ones hidden under the caps) on these late model units fuel or air bleeds? Mine were set at 4.5 turns and then sealed back up. I changed them to 4-- I think maybe still out to far?

grizld1 · Posted: July 18, 2005, 7:42 am Post subject: Re: Adjusting the idle screws?

They're fuel screws--out to richen the mix, in to lean it. Yeah, 4.5 turns is pretty far; but why were they set that way? Does your bike have modified intake and/or exhaust equipment? If stock, are the air filters loose or deteriorated? Og the Neolithic Wrench (he owned about 80% of the surviving Yam 650's at one time or another) may have tried to compensate for bad air filters, varnish in the pilot circuit, or leaking boots by cranking the fuel screws out. If intake or exhaust are modified you may need to move up a step or two on the pilot jets. Use the procedure I gave earlier in the thread and find out where your scoot is happy. If it seems to want 4 to 4.5 turns, cleaning, tuning, and sealing may be called for.

Redball · Full Member Joined: Jul 13, 2005 Posts: 11

The air filters were totally shot, and everything "was" stock. I have installed MAC 2 into 1 exhaust and pod filters and the long reach plugs with 1k ohm boots. I cleaned the carbs good, boots probably should be replaced- they look cracked on the inside but I can't tell if they are cracked all the way thru (wishful thinking!). this is a low ((9k) miles motor with excellent compression, valves and chain adjusted, also one size up on the mains (135) and carbs synced. I have not taken the bike out on any real rides yet as it is a custom hard tailed project just getting finished-- I 'll shall ride it this week though!

grizld1 · Posted: July 18, 2005, 11:19 am Post subject: Re: Adjusting the idle screws?

A few things, Redball. In my experience with MAC systems and pod filters(12 years on 3 XS650's), you'd be well-advised to start your plug chops six steps up from stock on the mains and two steps up on the pilots. Otherwise you'll be buying a whole bunch of intermediate jets you won't use. If the boots are cracked at all you'll save yourself a lot of frustration by just replacing them as soon as possible. Also, pod filters remove support from the back of the airbox. I drill the carb bridge and run an L-bracket from it to a breather box bolt; keeps the carbs from floppin' around and wallowing out the boots. Be sure to get a free-breathing filter on that breather tube, too! Mike's XS is the best source I know of for boots and jets. The prices are the best I've found and delivery is reliable and lightnin' fast.

Go easy on the hammer on that ride, bro! She's gonna be lean as a winter coyote!

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