BS 38 Carb Question

kshansen

kshansen

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I'm working on the carbs for my 1978 XS650 project and have a question about the pilot adjuster screws.

Every carb I have worked on since back in the early 1960's some going back to the 1940's always had springs on the adjuster screws to hold them in place after you got the engine running right. Be they on a old Buick a chainsaw or a Harley Sportster.

Now this 1978 with the BS 38 carbs there is nothing to hold the adjustment. There is a thin oring around the screw but that does not seem to put any resistance on how the screw turns. This is the same if reusing the original screws or the new screw in the carb kits I have.

I can understand the 40 year old orings to be a bit hard and set in their ways but the new screws have new orings on them. Has anyone had this problem or know of a source for this tiny orings so I can try some from another source?
 
There should be a tension spring along with the o-ring on there. Take a close look in the mix screw hole in the carb body, maybe the springs stayed stuck in there. The spring is listed on the parts diagram (#40) but, unfortunately, is N.L.A. .....

http://www.boats.net/parts/search/Yamaha/Motorcycle/1978/XS650E/CARBURETOR XS650E/parts.html

The '70-'77 carbs also have a spring and it is still available, but I don't know if it's the same. The 1st 3 digits of the part number differ. I happen to have both, along with the BS34 spring. Let me do some comparing and I'll let you know if they might swap.
 
5twins said:
There should be a tension spring along with the o-ring on there. Take a close look in the mix screw hole in the carb body, maybe the springs stayed stuck in there. The spring is listed on the parts diagram (#40) but, unfortunately, is N.L.A. .....

http://www.boats.net/parts/search/Yamaha/Motorcycle/1978/XS650E/CARBURETOR XS650E/parts.html

The '70-'77 carbs also have a spring and it is still available, but I don't know if it's the same. The 1st 3 digits of the part number differ. I happen to have both, along with the BS34 spring. Let me do some comparing and I'll let you know if they might swap.
Click to expand...

Note in the kits I have there were springs that almost look like they would fit but they seems to be a touch too large OD.
I'm assuming the springs are to go down in the holes and be below the threaded part of the needle?
 
Yes, the springs must fit through the threaded part of the hole, down into the smooth bore below it. I did some comparing and while the BS34 and earlier BS38 springs look to be the same, they're bigger than the '78-'79 spring, both in length and diameter. They won't even go into the threaded part of the hole in the carb. This wasn't a common spring. Yamaha only lists it as being used on the '78-'79 BS38 650 carbs. If you're scrounging, a few 400-450 Suzukis used it as well .....

https://www.partzilla.com/product/suzuki/13268-44012?ref=78a94422c3745102d57645229754759e47afc6ac
 
Yamaha never listed or sold the '78-'79 mix screw o-ring separately. If you bought the mix screw, the o-ring came on it .....

S2lH1ef.png


Therefore, no one really knows what the original size was. Even MikesXS admits to this and won't guarantee the fit of the one they provide with their mix screw replacement. I tried a 1.5 x 3 and while a little fatter than the original I was replacing, it did work. But, it shaved some of the O.D. off the o-ring as I installed the screw. I now have the next smaller size, 1.5 x 2.5. While still a tight fit in the hole, they don't get shaved down like the previous larger size did. I think they fit and work nice. I may eventually take another shot at this and try a smaller diameter size, a 1.2 or 1.3.
 
I did some close looking at the two carbs and there is nothing down in the holes, I have a flashlight with a fiber optic rod on it that will fit down in there real good! Also took time to empty out my ultrasonic cleaner on the chance they fell out during the cleaning and nothing in there. So I can only assume they were lost before I got the bike around ten years or more ago.

Well I'm thinking I may be doing some looking for an alternative spring, maybe start with my collection of ball point pens!

Any chance of you re-posting that picture of the screw above with the spring on it to give me an idea of the free length?

The oring on it is sure bigger than the ones on the kit needles I have. These do not even cause the slightest bit of drag while turning the screws.

I'm thinking the spring is not too critical as long as it holds some tension on the needle enough to over come the vibration of the engine.

One more thing while picking your brain, what is a good starting point for turns out on the screws? I saw some where on the forum a chart that said 2 1/4 turns. Before removing these I checked and both were right close to 1 3/4. I will be running with stock air boxes if that says anything.

Thanks for your help!
Ken in NY
 
2.25 turns out is the factory spec for the mix screws on the '78-79 carb set. It's a bit more than earlier carb sets most likely because these were the 1st with E.P.A. type stepped point screws. I'll have to get a pic for you, I'll measure them too. Ball park though, they're about as long as the stepped portion of the screw they fit on, and slightly smaller in diameter than the threaded portion of the screw.
 
5twins said:
I tried a 1.5 x 3 and while a little fatter than the original I was replacing, it did work. But, it shaved some of the O.D. off the o-ring as I installed the screw. I now have the next smaller size, 1.5 x 2.5. While still a tight fit in the hole, they don't get shaved down like the previous larger size did. I think they fit and work nice. I may eventually take another shot at this and try a smaller diameter size, a 1.2 or 1.3.
Click to expand...
Do you have a place that sells orings in those sizes? I'm a bit confused by the numbers as they don't seem to be anything like the measurements I have taken. The bore the oring fits in measures 0.233 inches and the grove the oring sits in diameter is 0.153 inches
 
These are metric o-rings. The 1.5 is the thickness or cross section in MM, the 2.5 or 3 is the I.D. in MM. I usually get them from McMaster-Carr. Here's the 2.5mm I.D. size I'm trying now that I think fits quite well, better than the bigger 3mm I.D. size .....

https://www.mcmaster.com/#9262K626

The groove that the o-ring fits in is a little over 1mm wide. If you look at the pic above of the new factory screw, you can see the included o-ring doesn't fill the width of the groove. The 1.5mm thick rings I'm using do fill it. I don't have any factory new ones to compare to, just used originals, but even looking at them, the 1.5mm thick rings I'm using do look thicker. That's why I'm thinking an original must have been a bit smaller, with a 1.2mm or 1.3mm cross section or thickness. I think I'll try this 1.3 x 2.5 next .....

https://www.mcmaster.com/#1302n123/=1ce9fwd

Your .233" bore measurement converts to 5.9mm. Your .153" groove diameter converts to 3.9mm. I chose smaller I.D. o-rings so they would be stretched and be a snug fit in the groove. Since I was also aware that the 1.5mm thickness was bigger than an original, I was hoping the stretch fit would reduce that thickness some. I suppose it has but as I mentioned, my replacements are still a tight fit into the hole in the carb. They obviously aren't exact size matches for an original but they work.
 
Well, if they don't fit, let me know. I have plenty of the 1.5 x 2.5 and can send you some, lol. You may also be interested in some others they have. Even though this one is an American size, it's an absolute perfect fit for the cam chain adjuster acorn cover nut used on the '76 and newer type D and E adjuster assemblies .....

https://www.mcmaster.com/#9452k81/=1cebe4w

..... and here's one that works well for the valve covers .....

https://www.mcmaster.com/#9262k408/=1cebgrb
 
Well this has to be one for the books!

I decided this afternoon to run some more small parts through the cleaner. Last batch of little parts was the fuel hoses and vacuum lines from the petcocks to the carbs. As I was using the air gun to blow the solution off one of the gas lines one of those thin wire hose clamps dropped to the floor.

Bent over to pick it up and noticed something strange on the floor. A tiny little coil spring. First thought was it can't be! Walked over to the bench and picked up one of the needle valves and then the carbs. Holy Sh!t !! it fits. Well at least now I have a sample to match up, I think.

Next thing was as I always blow off parts in that area of the garage so as to not disturb thins on bench I decided to look a bit more. Best way to find little things I have found is to just sweep the floor and then look through pile of dirt. After sweeping that area got down on hands and knees and ran fingers through the small dust pile. No luck did not see anything. But just for kicks decided to run a magnet through it a bit to see what I might have missed.

Any guess what was on the magnet when I piked it up?

both are right now in the carbs and will stay there until the new orings show up and then only one is coming out at a time!
 
Well good for you !!! That is one pretty obscure and difficult part to find. Good news is that once assembled with a fresh o-ring, it rarely needs to come apart again. The o-ring will last a long, long time. In fact, the screw might even function OK without it. I think the addition of the o-ring was an E.P.A. thing to make the idle circuit metering more precise. None of the older 650 carbs have an o-ring on their mix screws, only the E.P.A. mandated '78-'79 set and the BS34s.
 

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