Simple Carb Mixture Screw and Sync Adjustments

Simple Synchronization for LINKED Carbs (1976 through 1983) with DUAL Exhausts:
First, you need to understand the function of the idle stop screw and the sync screws. Screwing the idle stop screw in increases the air flow to both cylinders, which creates more vacuum, which sucks more gas in, which makes the engine run faster.
Screwing the SYNC screw in ONLY INCREASES the air flow to the right-side carb, so only the right cylinder gets an increased flow of gas and more power.
Now to syncing: Idle the bike at 1200 RPM. Place your hands a few inches from each exhaust. The exhaust pressure you feel should be equal. If the right exhaust feels weak, then screw the sync screw in -- this only increases the flow of gas to the right cylinder, remember? -- until the pressure is equal from both exhausts. Re-set the idle to 1200, check again, and do a final sync adjustment if necessary.
If the left side exhaust was the one that felt weak, you do the same thing, except you screw the sync screw OUT, decreasing the flow of the gas to the right-side cylinder until the exhaust pressure from both exhausts feels the same. Re-set the idle to 1200, check again, and do a final sync screw adjustment if necessary.
NOTES: This should take about 30 seconds once you've done it a couple of times. The XS650 is not a space ship, and this does just as good a job as a manometer for all practical purposes. Some people like to sync at a higher idle, such as 1400 or even 1600 RPM. That's fine by me, but there really shouldn't be much difference.

Simple Carb Air Mixture Screw Adjustment:
Start with the mix screw set to specs. Set idle to an RPM that is low but still steady -- about 1000 RPM if you have a smooth-running bike. Now go to the right-side carb and screw the mixture screw in in half-turn increments until the engine starts running slower. Now, turn the mix screw out in half turn increments. The engine should speed up again, and then maybe go faster, but eventually it will start running slower again. The correct mix screw setting is the place where the engine runs the fastest. When you think you are there, you can check and fine-tune by turning the screw in quarter-turn increments. Keep in mind that slightly rich (erring on the screw being more turned out) is better than slightly lean.
Repeat for the left side.
NOTES: This should take a bout 30 seconds once you've done it a couple of times. You are welcome to complicate the above procedure by pulling a plug wire (points only!) or by doing the dead-cylinder method, but the only reason other people suggest doing this is so that you can better hear the changes in engine RPM as you move the mix screws in and out. I can hear the RPM changes just fine with both cylinders firing.

FINAL NOTES: These are really simple procedures to do on a simple bike like the XS650. Don't-over-think this stuff or stress yourself out or be afraid of doing them. They ARE necessary, so be sure to spend the 60 seconds it takes to do them and then get on with your life.

The dead cylinder method seems to have a dedicated following but I never use it either. I can hear just fine what my mix screw adjustments are doing with both cylinders running. You usually find a "range" of maybe a half turn where the motor idles fastest. I set to the rich side of that range. That seems to help alleviate popping out the exhausts on decel better.

Great post!
Once set and running right, I like to turn me head a bit while cruising along and listen to the pulse and compare the left and right sides to each other.......yeah, still good!

Thanks for the validations. I was expecting to get some push-back from the manometer and the dead-cylinder crowds on this.

I was trying to keep things as simple as possible, but I'm not entirely happy with my Air Mix Screw adjustment procedure write-up. 5twins is right, there is usually about a half-turn range where the motor runs best. Here's another way of setting the mix screw: you find the engine's mix screw sweet spot as I described before. Now, starting at the sweet spot, you slowly turn the mix screw in until you hear the engine run poorer. Remember this spot. Now, slowly turn the mix screw out, back through the sweet spot, until once again you hear the engine run poorer. The correct mix screw setting is exactly half-way between the two spots where the engine just begins to start running poorer. Then back the mix screw out an additional 1/8th to 1/4 turner to make sure that you are slightly rich, and not slightly lean. Running lean harms the engine.

I write this because most descriptions of these procedures are way too unnecessarily involved and complicated. When I did my first sync, I dutifully made my own manometer, which entailed a lot of running around buying hose and such. Finding the correct hose barb fittings for non vacuum-barb manifolds is is a whole 'nother headache. Then, I filled the hose with water -- it is really hard to get the air bubbles out of the hose. Then I tried to use it, and I sucked most of the water into the engine, and had to start over. If you have dual exhausts, then this is all just completely unnecessary. My method does a completely adequate job. You can check the sync any time in a couple of seconds, and if it needs adjusting then a few more seconds and you are done.

The same with the dead cylinder method to set the mix screw. If you don't have points then you have to fret over safe-guarding your ignition system, and the whole dead cylinder thing is just an unnecessarily complicated added step.

I also write this because I know a lot of people are afraid of doing these vital bits of maintenance, or maybe they are just lazy. I re-build carbs and send them out to members, and half the time the new owners think they don't have to do this crucial final bit of tuning because they run so well out of the box.

MORE NOTES:
Sync and mix screw setting only effect idling. Once you are above about 2000 rpm, they have less and less effect on the running of the bike, and by about 3000 rpm they have no effect at all. If you have performance problems above 3000 rpm, then it's something else.
If no matter what you do to the sync and the mix screws your bike just doesn't idle right, then you either have a bad carb, or you didn't clean it properly. I cleaned several carb sets before I really understood how all of the tiny passageways work and where they go to and from. One plugged passage is all it takes to screw up a carb. I have seen fuel varnish build-ups that were like epoxy. Ultrasonic cleaners and the 1-gallon-can type dunk-basket cleaners will NOT ALWAYS remove the worst varnish buildups from the tiniest passages. Sometimes it takes gentle poking with a very tiny wire, and you have to know where the passages go to and from, and then you have to spray carb cleaner through them to make sure that you have removed the obstruction. If the carb yoiu re-built doesn't idle smoothly, then you probably didn't clean it properly.
The other possibility is a bad carb. There are two common possibilities here. One is that the mix screw orifice -- the opening at the bottom of the mix screw hole -- is screwed up. The 76-77 carbs are particularly prone to this.
The other common possibility is that the throttle plate doesn't fit the throttle bore (or "throat") very well, or that there is excessive slop in the throttle shaft. Bad throat fit or shaft slop are seen in carbs that have LOTS of miles on them that were possibly run in dusty environments. The parts simply wear out over time.

Good write-up Dogbunny! What you're hitting on here is that grey transition zone from book-tech to gear-head, that unique knowledge that doesn't easily go into print, the feel/sound connection to cause/effect, usually only acquired in the shop after many combat tours.

I, too, have been experiencing the epoxy-like idle jet plugging. My bad habit of not draining the bowls prior to extended rideless periods (sometimes in only 1 month), results in at least 1 plugged pilot (idle) jet. Didn't have it this bad years ago, the fuel is really different nowadays. The old fuel (and I only run premium) takes on a dark amber foul-smelling appearance quickly. Even finding dark clumps of goo in the bowls. It's not stuff that gets past the filters, but the result of chemical ethanol/water interaction/decomposition. Yukkk!

I use fine wire drills, numbered 61-80, in a pin vise, to clear the jets. This new fuel, while decomposing, will lay down a constricting coating in the jet bore, and just a 0.001" thick coating will reduce a pilot jet's effective flow to the next couple lower sizes.

The pilots are numbered by their metric bore diameter, in hundredths of millimeters. Ex., a #42.5 jet is 0.425mm (0.0167"). That 0.001" coating reduces the bore to 0.0147", (0.374mm), which would now be a #37.5 jet. The bike will idle, but not well.

To clear this jet, I use a wire drill just smaller than the bore, in this case a #78 drill (about 0.016"). Using just finger power, plus a couple drops of carb cleaner, the wire drillbit can be swabbed about in the jet, fully clearing it, and not cut the brass.

Fine wire drillbit sets of #61-#80 can be found on eBay for just a few bucks.

TwoMany, thanks. I have two suggestions for you:
First, use an Ethanol Stabilizer. There are lots of brands, I am sure they all work fine, but the brand I like is called Ethanol Shield. It is found at lawn mower/outdoor equipment specialty stores, not at auto stores. It is an ethanol stabilizer and also a long-term fuel stabilizer. It takes a little time for the ethanol to start doing the destructive things that it does, so I don't use Ethanol Shield in my daily rider, that fuel gets replenished too fast, but I do use it in my vehicles that sit for any length of time.
Second, the best solvent for dried fuel varnish is MEK (Methyl Ethyl Ketone). Home depot and Lowe's have it. It is expensive and extremely nasty. Soak your clogged pilots in it. You will still probably have to poke the plugged ones.

Regarding the transition zone from book-tech to gear-head, yes experience makes things a lot easier, but I don't want to scare the inexperienced away from sync and mix-screw adjusting. Keep things simple. It should be easy for the inexperienced to do it my way.

Something else that 5twins and nj1639 alluded to is popping on decel. If one side is popping, then you probably aren't synced, or the popping side needs a mix screw adjustment, or you need to fix the oil leak that is fouling the plug on the popping side. If both sides pop, then both mix screws need richening, or you need to put baffles in your straight pipes, or you need to take your stock pilot jets out and go up one size like the Carb Guide told you to do.

I find the dead cylinder method, for mixture setting, and the manometer sync method, both very simple and very easy to do, for me.

However, there's quite often several ways to accomplish the same task, and I concur that DogBunny has posted some good clear instructions.

No push-back from me. Regardless of how you did it, if your engine runs well, when you have finished tuning, then you did it the right way

Just as I admitted to not using the dead cylinder method (I find no need to), I will also admit I do use a manometer. I have one of the old original Carb Stix tools, the one with mercury in it. It's always worked well for me.

I don't agree that the best mix screw setting is exactly half way between the lean and rich roll-off points. I think just under the rich roll-off point works best. I find both points when adjusting the mix screw, then go back to the rich side, just past the sweet spot so the cylinder starts running rough. Then I close the screw back up just enough to get it smooth running again. Basically, I set it as rich as possible in the sweet spot.

With a manometer you can achieve pin-point accuracy FOR A GIVEN RPM, however I personally do not think pin-point accuracy is strictly necessary. The fact is, the instant you change your RPM, up or down, your manometer reading will change. Close is better than no synchronization at all, and again, I am just trying to give a simple alternative for those who are afraid of or can't be bothered with manometers.

5twins has way more experience with these bikes than me, so where he sets the mix screw is surely a good idea. I'm just trying to show how simple the actual procedure can be.

Regarding manometers, I got the idea not to use them from Farrell's Shade Tree Corner, which is linked to Michael Morse's 650Central.com site:
http://www.650central.com/farrell intro.htm
The specific article is here:
http://www.650central.com/fcarb_balance_technique.htm
He calls it balancing instead of synchronizing. He does not use a manometer, but other than that, his procedure is just crazily complicated.

DogBunny................... I agree with most things you are saying. However, I see a potential weak area to what you say.

You are experienced with bikes, and when you say, hold your hand behind the exhaust and just make adjustments to balance the exhaust; you have the ability to do that. Inexperienced lads most likely don't know what equal pressures feel like. There's a certain skill required to do what you do, but newbies may not have that skill.

If a newbie uses a manometer to get the levels equal, there is zero skill required, 100 % fool proof. I could teach a monkey to balance carbs that way.

Here's an analogy:
A newbie rider says his battery is always going flat on him. I tell him to use a VOM to measure the battery voltage to see if it reaches 14.1 volts at 3000 rpm or greater. He comes back and says he doesn't have a VOM. An experienced rider joins the conversation and says to simply watch the headlight...............he says if it brightens when you rev the engine, then the charging system is fine. The problem is that the voltage may only be reaching 13.5 volts, so the battery goes flat again in a few weeks/months. Watching a headlight brighten is a subjective measurement, so its not very precise. If the newbie buys and uses a VOM, he would see the defective charging system and do a proper repair, resulting in a battery that does not go flat again.

It comes down to some lads would get good results with your methods, but I think some would have trouble.

DogBunny said:

TwoMany, thanks. I have two suggestions for you:
First, use an Ethanol Stabilizer. There are lots of brands, I am sure they all work fine, but the brand I like is called Ethanol Shield. It is found at lawn mower/outdoor equipment specialty stores, not at auto stores. It is an ethanol stabilizer and also a long-term fuel stabilizer. It takes a little time for the ethanol to start doing the destructive things that it does, so I don't use Ethanol Shield in my daily rider, that fuel gets replenished too fast, but I do use it in my vehicles that sit for any length of time.
Second, the best solvent for dried fuel varnish is MEK (Methyl Ethyl Ketone). Home depot and Lowe's have it. It is expensive and extremely nasty. Soak your clogged pilots in it. You will still probably have to poke the plugged ones.

Regarding the transition zone from book-tech to gear-head, yes experience makes things a lot easier, but I don't want to scare the inexperienced away from sync and mix-screw adjusting. Keep things simple. It should be easy for the inexperienced to do it my way.

Something else that 5twins and nj1639 alluded to is popping on decel. If one side is popping, then you probably aren't synced, or the popping side needs a mix screw adjustment, or you need to fix the oil leak that is fouling the plug on the popping side. If both sides pop, then both mix screws need richening, or you need to put baffles in your straight pipes, or you need to take your stock pilot jets out and go up one size like the Carb Guide told you to do.

Click to expand...

Alternative: You can find on the internet, all the gas stations in your area that offer Ethanol-free gas. That's all I run in my '77 which I bought new. Ethanol sucks! The government is forcing Ethanol on us. Protest it's use every chance you get!

oops! duplication

DogBunny said:

TwoMany, thanks. I have two suggestions for you:
First, use an Ethanol Stabilizer. There are lots of brands, I am sure they all work fine, but the brand I like is called Ethanol Shield. It is found at lawn mower/outdoor equipment specialty stores, not at auto stores. It is an ethanol stabilizer and also a long-term fuel stabilizer. It takes a little time for the ethanol to start doing the destructive things that it does, so I don't use Ethanol Shield in my daily rider, that fuel gets replenished too fast, but I do use it in my vehicles that sit for any length of time.
Second, the best solvent for dried fuel varnish is MEK (Methyl Ethyl Ketone). Home depot and Lowe's have it. It is expensive and extremely nasty. Soak your clogged pilots in it. You will still probably have to poke the plugged ones.

Regarding the transition zone from book-tech to gear-head, yes experience makes things a lot easier, but I don't want to scare the inexperienced away from sync and mix-screw adjusting. Keep things simple. It should be easy for the inexperienced to do it my way.

Something else that 5twins and nj1639 alluded to is popping on decel. If one side is popping, then you probably aren't synced, or the popping side needs a mix screw adjustment, or you need to fix the oil leak that is fouling the plug on the popping side. If both sides pop, then both mix screws need richening, or you need to put baffles in your straight pipes, or you need to take your stock pilot jets out and go up one size like the Carb Guide told you to do.

Click to expand...

did not see reply so sent it again

XS650DL, there are no ethanol-free gas pumps within 200 miles of TwoMany or me.

retiredgentleman said:

There's a certain skill required to do what you do, but newbies may not have that skill.
It comes down to some lads would get good results with your methods, but I think some would have trouble.

Click to expand...

Please actually try it for yourself, and then tell me if you think it requires experience.
It is very easy to detect an out-of-sync carb set by feeling the exhaust pressure with your hands. No special skill is required. Once you can no longer feel a pressure difference between the exhaust coming out of the two pipes, the sync, for all practical purposes, is good. Try it, compare your results to your manometer, and report back if you still think a typical novice probably can't do it.

Excellent write up, except I have a two into one exhaust lol. Still the write up itself is very helpful and easy to follow so great advice!

Subscribed!!

Just to be the devil's advocate, I'm surprised no one else has brought this point up.

From 70 to 75 the twin exhausts were completely separate. From 76 to 83 the twin exhausts had a crossover pipe.

So with 76 to 83 you are trying to sense a small difference in exhaust pressure, from 2 exhausts that have a pipe running between them. The crossover pipe would be causing some averaging of the 2 exhaust pressures, but you're trying to detect if there is a difference. It appears that you are trying to balance 2 pipes that already have a balance pipe between them.

Ah, good point RG, though in my applications I'm running separate duals.

Thank you DogBunny, just confirmed what I was doing already.
And also thanks to 5twins for the information on the mix screws,I'll be changing to your settings.

None of these settings are "etched in stone". Use what makes your bike run the best. I've found not only on my 650 but on pretty much any other bike I've owned that setting mix screws to the rich side of the "sweet spot" works best. I have freer flowing pipes on all these so the popping on decel can be a problem. Honestly, I think there is some popping present even with a stock exhaust. It's just that they're so quiet, you don't notice it that much.