um, newbie here, mixture screw?

Bigibee

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Just inherited a beautiful, virgin '78 Special II. I ride vintage dirt bikes, so I am somewhat familiar with carbs, especially Mikunis. The bike is great, but it backfires coming down from speed. I cannot find the mixture screws - anywhere. I've searched The Garage high and low, and no one seems to have this problem. I've found what I think are "vacuum port" screws below the throttle rod, but these don't turn out much - one is outside the left carb and the other is inside the right one. These can't be mixture screws because they were securely flush when I tried to turn them. There are a couple of vertical "ports" in the front of the carb with metal plugs - which don't come out, at least not easily. WTF????

I can't believe Yamaha/Mikuni would market a bike without mixture screws, but there you are. The model number of my carbs are 3GI 00 B136 MIC KOGYO. They have chrome CV dashpots.

I may not be the brightest bulb in the bathroom, but I know what a mixture screw looks like. There ain't none on this bike...HELP would be appreciated - thanks!!
 
welcome bigebee.
first thing I would suggest is for you to buy a copy of a workshop manual whether Haynes, Clymer or Chilton . You won't be able to adjust or set anything without referring to it.
Of the three my preference is for the Chilton as it has a useful Trouble shooting guide in the back.

Sounds like you have the same model as mine . The frame tag and stamp on the headtube says made 1978 but as they were registered in the 1979 period they are referred to as 1979 models.

If you have the original BS38 Mikuni carbs, the air mixture screws are set within the caburettor body and the adjustment screws are recessed.

i would strongly recommend that you do not adjust anything at the moment until you've checked some other possibilities.

Backfiring on over run is frequently caused by a leaky exhaust . The exhaust gasses draw air in the leaking joints which ignites any unburnt fuel in the exhaust. You could use a exhaust jointing compound and remake the joints to the headers and check the whole system from engine to tailpipe .

Check your spark plugs for correct colour. They should be a light biscuit colour on the electrode and ceramic insulator. This could highlight an incorrect mixture.

Its the pilot circuit which is responsible for idle mixture both whilst idling and overun. ie the slide , pilot jet and air mixture screw. If you make any changes to the settings make sure that you mark the original positions so that you can return there if need be.

There is an excellent carb guide on the forum written by several expert owners you'll need to read through that to get an overview of how the carbs work and how to clean and tune them
http://www.xs650.com/forum/showthread.php?t=43

have a look through all the guides in the technical section too there's some tremendous help in there
sorry best image I could find of the air mixture screw
 

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Can you post a pic of the carbs? Wonder if the stock carbs were switched out for something else at some point?

The mix screws on the BS38's are pretty obvious and very easy to find.
 
It seems your '78 is not quite a virgin (that dirty slut, lol) because you appear to have BS34 carbs. They didn't happen until 1980. But ..... there was no Special II in 1978. You may be mistaken about the year of your bike.

Yes, you have mix screws but they're capped so you can't fool with them. You have to drill the metal caps out and extract them to gain access to the screws. You must take care when drilling because the screws are just below the caps. If you go too deep, you'll ruin the screwdriver slot on the screw top, making adjustment or removal very difficult. Note the small "vent" hole in the metal plug .....

6w594s8.jpg


Using a 1/8" drill bit, start drilling into the plug. Drill a little and check the hole, drill a little more and check again. Watch that little "vent" hole in the center. As soon as it begins to get larger, stop drilling, you're almost all the way through the plug. Get a small sheet metal screw and blunt the point on it .....

WwXCtog.jpg


Screw it into the plug until it bottoms and the plug starts turning. Yank it out with vice grips .....

imPf1SZ.jpg


Twisting as you pull it out helps.

The BS34s were pretty E.P.A. strangled. They usually run best with the mix screws set somewhere in the 3 to 3.5 turns out range. You may find the factory setting down near 2.
 
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It seems your '78 is not quite a virgin (that dirty slut, lol) because you appear to have BS34 carbs. They didn't happen until 1980. But ..... there was no Special II in 1978. You may be mistaken about the year of your bike.

I suspect that he is going by the manufacturing date on the headtube stamp and label as I did initially. My bike is a 79 special ll made in 78 .

How did you determine that he has BS34 carbs ? I didn't notice the model number but that would seem to suggest BS38s ?:confused:

A picture of the filter end of the venturi :wink2:would definitely clear thing up
 
The first picture are 38's. The mix screws marked in red.
The blue circle marks the choke plate(with removable plug) wrongly installed.
It should go on the outside of the right carb.

The second picture are 34's.
Mix screws (marked with green) with the cover plugs removed.

These are parts carbs so commentary as to looks or function, missing parts, or suitability for use isn't necessary but have at it if you must.
 

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He mentioned the vertical "port" at the front of the carb with a metal plug in it. Also, he said they have "chrome CV dashpots", which I assume to be the chrome steel "tin top" top covers.

WER, the 34 pic nicely illustrates what drilling too far will do the the tops of the mix screws.
 
He mentioned the vertical "port" at the front of the carb with a metal plug in it. Also, he said they have "chrome CV dashpots", which I assume to be the chrome steel "tin top" top covers.

.
ah thanks .:wink2: didn't notice either of those points.:doh:
 
Thanks all. Exactly the kind of info I was looking for. I was told this was a '78 but now I have no idea. Are these 34s?

Is this an '80?

Great site...:banghead:
 

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the date the frame was manufactured is stamped into the headtube .have a look.:wink2:
There should also be a plastic tag fixed there but it may have been lost in time.
Tell us the first few characters of your frame number and someone will be able to tell you all about it.
 
Yes, those are BS34 carbs, used on the '80 and newer models. Yours may be even newer than 1980. It has the oil sight glass on the right crankcase cover. That wasn't implemented until '81. I think you have an '81 Special II. The 36 spoke wheels are another "tell".
 
Thanks all. Exactly the kind of info I was looking for. I was told this was a '78 but now I have no idea. Are these 34s?

Is this an '80?

Great site...:banghead:

Hi Bigibee,
The photos show an XS650 Special II.
And yes, they came with BS34 carbs.
Compare your frame ID stamp to the IDs in Skull's sticky to identify your bike's year.
 
Got the title just this afternoon...'81, engine no. 4N9016438...knew I was a little early writing but I wanted to get it running right this weekend. Takes forever to get through the legal stuff. The I.D. on the Steering head says 3/81...duh!..

Besides electronic ignition, does the '81 H have any other improvements? And what carbs are the best for this bike? I don't want to start modifying too soon - just rather have fun for now. Anything especially noteworthy on this year?

You guys are the best!!:thumbsup:
 
You have what I consider some of the best carbs. They were the last and most modern set used on the 650. Yes, they are heavily E.P.A. mandated, but you can work around that, starting with drilling the mix screw plugs out. The slide needles are non-adjustable and fixed but you can swap adjustable needles in if you like. Some aren't crazy about the Special styling and that fat, squat 16" rear wheel, but you can easily swap in an earlier 18" wheel if you want to. The 16" rides nice in a straight line but slows down the steering and the ability to throw the bike into corners a little bit.

If you want to make one change right away that will really make a difference, put some normal rise (or even lower rise) bars on there. It will transform the look and feel of the bike.
 
You have what I consider some of the best carbs. They were the last and most modern set used on the 650. Yes, they are heavily E.P.A. mandated, but you can work around that, starting with drilling the mix screw plugs out. The slide needles are non-adjustable and fixed but you can swap adjustable needles in if you like. Some aren't crazy about the Special styling and that fat, squat 16" rear wheel, but you can easily swap in an earlier 18" wheel if you want to. The 16" rides nice in a straight line but slows down the steering and the ability to throw the bike into corners a little bit.

If you want to make one change right away that will really make a difference, put some normal rise (or even lower rise) bars on there. It will transform the look and feel of the bike.

I have to agree that those roto-tiller handlebars need to be gone asap, although a friend of mine has a 1980 XS650 and even after riding my bike (with standard type bars)he prefers the roto-tiller bars.:shrug:

However, I can't buy into the theory that fat rear tires slow down steering ability. Take a look at the real sporty crotch rockets being sold at dealers, as well as pure race bikes. All those bikes designed to handle lots of curves, and not a narrow rear tire to be seen
anywhere. If narrow rear tires enhanced handling, I'm guessing the pure race bikes would be using them to gain an edge over competition.

Regardless, I like a fat meaty rear tire to handle the brute torque these "special" engines put out. Perhaps the "standard" engines don't put as much power to the ground?? Anyway, that's my story and I'm sticking to it:D
 
You have what I consider some of the best carbs. They were the last and most modern set used on the 650. Yes, they are heavily E.P.A. mandated, but you can work around that, starting with drilling the mix screw plugs out. The slide needles are non-adjustable and fixed but you can swap adjustable needles in if you like. Some aren't crazy about the Special styling and that fat, squat 16" rear wheel, but you can easily swap in an earlier 18" wheel if you want to. The 16" rides nice in a straight line but slows down the steering and the ability to throw the bike into corners a little bit.

If you want to make one change right away that will really make a difference, put some normal rise (or even lower rise) bars on there. It will transform the look and feel of the bike.

Thanks 5T. Already took out the mixture plugs and set the screws about 4 turns out - she still pops a little, but throttle response is much better. So thanks for that - I would have really screwed something up if you hadn't given me the tap screw trick. What are your best sizing for slide needles? - I'll have to go through the carbs eventually, but I can wait....I also need to find the "sync" screw - I think it's the one in the middle, at least that one I can see.

I love the "roto-tiller" description - I thought the bars were very weird, but after riding they kind of work - after all you're leaning and not steering - but I'm sure I'll change them out soon - any suggestions on which brand/style has best looks/feel? I don't want to sit too far forward

Thanks again for helping and not poking fun at me...easy to do....:bike:
 
I bought a standard seat and two pairs of low bars one of which was the Triumph T100 type....but...I've decided to stick with the roto-tiller bars and special seat .

Its a very relaxed style of riding and if you have a dodgy back like I have you'll welcome the laid back position and the extra lumber support you get from the special stepped seat. I know it looks un-cool but it sure is comfortable.:thumbsup:

Plus your head is further away from anything you might run into which is a big plus point:D
 
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