That mix adjustment screw.

[weekendrider]

When Max changed his screw setting did he change the air/fuel ratio OR is he simply providing more fuel of the same ratio?
And was this change an optimal fuel flow or simply a way to make up for what I called fuzzy jets? Jets that have some obstruction but not enough to shut you down? Jet hole pics are tough to do and these are piss poor but are examples of a fuzzy jet vs. clean one.

 

[xjwmx]

too bad there isn't a fix for the Square gear sound on the clutch side .... that Wrrrr is loud on this bike !....

That's not a normal sound, like tick...tick...tick... is.... I would use the starter to turn the crank to see if it's wobbling on the clutch side. Then I would take the sump plate off and put the bike on its side and try to examine the front cam chain guide. Not sure how possible the latter is...

 

[5twins]

When Pete adjusted his mixture screw, he was changing the amount of fuel/air mix being delivered into the carb main bore, not the air to fuel ratio of that mix. The only way to change that ratio would be to change the jet size.

Let me clear up some misconceptions here. Yes, you will see this screw called by several different names. Some call it an "air" screw, even some of the manuals label it as such. It is not an air screw and does not flow straight air. It is not a "fuel" screw and does not flow straight fuel. It is a true "mixture" screw. It flows and can adjust the fuel/air mix already made up by the pilot jet, but only a small portion of it. There are 4 small holes in the top of the carb main bore back by the butterfly plate. These deliver the fuel/air mix from the pilot jet. Three are clustered together in a group about where the butterfly plate closes. The amount of flow through them cannot be adjusted. The 4th hole is off to the side (BS38s) or slightly ahead (BS34s) of the 3 holes. This is from the mix screw. Fuel/air mix from the pilot jet passes by the mix screw and out this hole. You can adjust the flow/amount that comes through. So, this gives you control over roughly 25% of the flow coming from the pilot jet. That's why the mix screw only works for fine tuning. It won't compensate for a jet size that's wrong, it doesn't have the range, but it can indicate that jet size is wrong.

Carb manufacturers usually like to see their mix screws operate in the 1 to 3 turns out range. This places the tapered tip of the screw properly within the metering orifice to do the job and also retains some tension on the spring to hold the setting and keep the screw from actually falling out. If you must set the screws above or below this, that usually indicates a jet change is needed. There are, of course, exceptions to this rule. Newer E.P.A. mandated carbs, like the BS34s, often require a bit more than 3 turns out and the early BS38s spec slightly less than 1 turn out.

And this brings us to those factory recommended specs or settings. They are simply a suggested starting point, a "ball park" setting. The bike should start and idle with them but they are most likely not the absolute best setting. You need to fine tune them and find that setting yourself. As previously mentioned, if you must stray too far from the spec for best running, like a full turn or 2, that could indicate a jet change is needed.

 

[MaxPete]

Good question WER - I'll check.

OK - so, I just wound the mixture screws back in and gently seated them, and here are the results:

• the RH screw was at 1-1/4 turns and,
• the LH carb was at 2-1/4 turns (different by a full turn);

...and that was after my ride. Noting that the popping only seemed to be a big issue on the RH side, I think it is clear that the RH carb was set way too lean and the LH carb may have been too rich - but at least the bike wasn't backfiring on the left side.

Soooo...I have brought both of the screws back out to 1-1/4 turns.

But, there's more....when I got home, I noticed that my choke (enrichener) lever (the entire assembly) had fallen off during my ride this morning - dammit. I had an old assembly with a cracked rubber choke button in my parts box and was just about to send it to a friend who had attended the big First Annual SE-MI / SW-ON Meet-Up and needed one....(sorry buddy..) but I have now glued the rubber button back together with Gorilla Glue and will be installing the assembly on Lucille in a couple of hours (AND I'll be using some blue Loctite on the tiny little screw that holds the damned thing on).

So, I'm closing in on the correct carb adjustments now - I think. I hate to admit it, but when I cleaned the carbs last year, I simply assumed that the carbs had the stock jets installed and that the settings on the mixture screws etc. were also pretty much correct - so I left them alone. The fact that I've put more than 3300 miles on the bike without touching the synchronization, mixture screws or jetting - seems proof that this wasn't a totally faulty strategy. It is certainly now clear that some of the settings were pretty far off and that is a real commendation for how robust and well designed the XS650 engine and carbs really are.

Anyhow, I'm still reading and still learning from the true masters!

 

[DanielBlack]

When I first bought my Stella, I went into the shed that evening to commune with the ol' gal. Reached down to turn the choke on and the lever fell off in my hand. It was a good introduction to owning and maintaining an old bike.

On Stella, the tapped hole for the lever screw was stripped out, but there was plenty of thread further in. A longer screw, some blue loctite, 3000 miles, and a whole lot of starts later...

 

[retiredgentleman]

Pete.......................whenever anyone buys an unknown bike, they must remove the main and pilot carb jets for 2 reasons. First to confirm they are clean. Secondly to read the tiny jet size number on the side of the jets. If you don't know what size jets your carb has, you're flying blind.

Once you know your jets are clean and the carb/manifold has no air leaks, you need to do the dead cylinder adjustment of the mixture screws in order to find the elusive "sweet spot". Once the "sweet spot" is found, life becomes beautiful and the sun shines every day (except for when a total eclipse occurs )

 

[xjwmx]

(except for when a total eclipse occurs )

I have a prediction. There will be reports of a million people damaging their eyesight staring at a partial eclipse. That will be followed by the media blaming themselves for not emphasizing enough to not do that.

 

[MaxPete]

Pete.......................whenever anyone buys an unknown bike, they must remove the main and pilot carb jets for 2 reasons. First to confirm they are clean. Secondly to read the tiny jet size number on the side of the jets. If you don't know what size jets your carb has, you're flying blind.

Once you know your jets are clean and the carb/manifold has no air leaks, you need to do the dead cylinder adjustment of the mixture screws in order to find the elusive "sweet spot". Once the "sweet spot" is found, life becomes beautiful and the sun shines every day (except for when a total eclipse occurs )

Yes, of course you are correct RG - and I freely admit that my approach with Lucille's resurrection was not entirely systematic. I guess I had so many issues (mainly electrical) and was so busy at work during that period that I took some shortcuts. Happily, it did work out OK, and now I am circling back to clean up some issues - and so, I think that the sun WILL shine very soon.


...and in the matter of solar eclipses, yes. I am sure that there will be some injuries which is tragic. In the 1960s, I was a little kid living with my family in Richmond Hill, ON and in about 1964 (I think), there was a total solar eclipse. We were sent home from school early that day with very strict instructions to NOT look up - it was a big deal at the time. ,

 

[5twins]

You should find the "sweet spot" to be more of a small range of like 1/4 to 1/2 turn, as opposed to one exact "spot". Many like to set the screw in the middle of that range. I prefer to set it to the rich side, actually as rich as I can get it but still in the range, just before the idle begins to drop off and falter. I find this alleviates the popping on decel better/more.

Pete, you've toyed with your mix screw settings some, now it's time to really dial them in. You'll need to work the screws in and out to locate that "sweet spot" range, both it's beginning and end. Then you can set them somewhere in it. Like I said, I like to go to the rich side. I don't use the dead cylinder method, I don't feel it's necessary on these carbs. I'm easily able to hear the differences in running my one cylinder adjustment is having with both cylinders still running just kneeling next to that cylinder I'm adjusting.

 

[weekendrider]

Pete for your buddy that is without
http://m.ebay.com/itm/222615461569?...Dosub%26srcrot%3D0%26rvr_id%3D0&ul_noapp=true

 

[grizld1]

Re. pilot systems, when you find the mix screw in front of the slide tower, opening it will richen the mixture by admitting more engine vacuum to the pilot jet, pulling up more fuel into the air stream. When you find the screw behind the slide tower, as in VM carburetors, opening the screw leans the pilot circuit mix by adding air, and changes will have a stronger effect on off-idle behavior than in the vacuum control system. Effects of mixture screw changes will be most noticeable at the lowest idle speed the motor will hold. Another (not better) adjustment procedure is to equalize settings for highest idle, back off the throttle stop, equalize for highest idle again, and go for a ride to reach full warmup. The setting will be lean; you can find the final setting by richening the mix screws 1/8 turn at a time until there's no popping from the exhaust under engine braking.

 

[MaxPete]

Thank so much guys - this is great info!

Pete

 

[Mailman]

I never tried the dead cylinder method until just recently, for me personally , I found it made it easier to fine tune each cylinder to its optimal range. I also didn't put a whole lot of stock into manometers , UNTIL I actually used one on this bike and after using both of the above methods the improvement was profound.

 

[retiredgentleman]

Carbs are actually crude devices as compared to modern fuel injection, using computer control with feed back loops, etc.
I'm always amazed that carbs work as well as they do. I like the dead cylinder method for setting the mixture needle and a manometer is extremely accurate for the sync. You first have to find the right size pilot jet to get into the correct range, otherwise the mixture needle will be unable to do the fine trim.

 

[5twins]

Float levels need to be correct as well or the mix screw will do little or nothing. If I'm adjusting one of these and get little to no response by about 1 turn past the spec in either direction, I begin to suspect the carbs may have other issues like incorrect pilot jets or float levels out of the spec range.

 

[grizld1]

RG, I share your amazement. Carburetors have to perform a series of very complicated balancing acts with very simple vacuum and fuel control systems, and their every function is a compromise. I have no idea how they can work so well; it's a minor miracle.

 

[Bob Kelly III]

... and making a better one is not as easy as it sounds... I've tried my hand at making a better carburetor and the results were worse than the original ! as far as MPG goes....
however getting into Vaporized gasoline works much better, but then you have to mix air and vapor at the correct amount at all RPM and that is another can of worms ! LOL
Bob.........