(BS series) Throttle Seals: Who's done 'em, who needs to?

[bluebikerblan]

Before too many more carb clean outs....one will begin wondering: "where oh where may all this junk be coming from?" Fuel lines, filters, petcocks, screens, even old gas cap gaskets??? Answers may be any and/or all of the above. Don't forget the petcocks, you won't believe how much crud accumulates there. Soon thereafter comes the "from stem to stern" approach of fuel system cleanout, de-rusting, tank re-lining, carburetor re-building, new o-rings, tubing and gaskets. It takes just one speck of anything from anywhere to plug a jet, passage or valve. Finding it is usally a pure PITA.

BTW G's if you can do all that in extra 20-30 minutes?......I wish you were my neighbor cause you would be getting a lifetime supply of free beer. Blue

 

[Sandy]

...vm34's. be done with it.

 

[BA_Dave]

I tried the 1/16 x 5/16 quad o-rings with a liberal application of high temp silicone grease late last year and i'm having to replace seals again already. Yes, my shafts are tight in the carb bodies.

I pulled the shaft from one carb today and measured both the shaft and the bore.

Shaft = 7.95 mm, Bore = 11.55 mm

This means the gap between shaft and bore is 1.8 mm. The 1/16" (1.5875 mm) x-section quad o-ring is too small to provide an effective seal. I think I've probably been sealing on the silicone grease. A 1.9 mm x-section o-ring yields 5% squeeze (small for any application), a 2.0 mm x-section o-ring yields 10% squeeze (still small but should be effective on high tolerance parts).

Can anyone post results of using either 1.9 x 7.8 or 2 x 8 o-rings in either Buna-n (nitrile) or Viton? Does the 1.9 mm cross section provide a sufficient seal over the long term? Does the 2.0 mm cross section result in binding of the throttle shaft?

I have OEM seals on order from boats.net but they're not scheduled to ship until 06/25. I'd rather do the job twice in two weeks than be out a week of riding.

 

[gggGary]

Go get some oem from the local Yamaha dealer. Heck I have a few in stock if you are desperate.

 

[BA_Dave]

Thanks gggGary,

I can wait for the seals from boats.net. I had limited results with the quad o-rings but I'll install new ones to hold me over unless someone has had better results with one of the other alternates. In the past I've always found recommendations from this board (and it's predecessors) to be accurate but I should have run the numbers to verify the o-ring cross sections before diving in.

BlueBikerBlan,

Unless someone else has reported better long term results, I'd recommend removing the 1/16 x 5/16 quad o-rings from your list of alternates.

 

[YL82]

This reply may be too late to matter, but there's always Bikebandit.com if you're willing to pay a little more.

http://www.bikebandit.com/search?q=256-14997-00

 

[YL82]

Put the "square section" o-rings in a set of 38s last week backed em with a thin section o-ring to "fill the space". Doped em with silicone lube too. OK, a little brag here.

Just ordered enough Yamaha 256-14997-00-00 to do 5 sets of carbs Boats.net has em for 2.75 each. Now to find the best deal on stainless steel socket head bolt sets WITH the 4 to bolt the carbs to the angle iron. Next a Caswells nickel plating kit (maybe)

Hey, gggGary. I am currently re-assembling my BS38's (specifically throttle shafts, valves, springs, seals, etc.) and was excited to come across this image of your sweet carbs. To demystify how the two throttle shafts are connected (with that gizmo in the middle with the springs), would you [or anyone] happen to have images of the backside of that connection (w/ fuel connecting line removed)? Not sure whether a top view of this connection would help me as well.

I'm sure it will become self-evident as I begin to assemble, but images sure do help! I've got all of the parts, just trying to figure out proper sequencing and I'm waiting on 5Twins-recommended McMaster Carr oval head screws for the shafts.

Thanks.

 

[gggGary]

throttle shafts are connected (with that gizmo in the middle with the springs) it will become self-evident as I begin to assemble.

Thanks.

Yeah all the little springs and stuff in the middle stay together. Just slide the plate on one shaft between the screw and the spring on the other shaft and you are good.





The square O-rings with a regular o-ring backer turns out to be uh not so good, it made the shaft too tight and caused slow closing.

 

[YL82]

Great! I really appreciate the images, gggGary. Very helpful.

Questions: That screw on top in your first image that screws down onto the throttle shaft end plate (?). How tight does that need to be? Does it need to be set to some specific setting (x number of turns) or just snugged up?

Is there a name for this screw or piece? Yamaha carb images don't show these parts very well (understatement) in the Carburetor section.

 

[wherearewe]

Ah Im gonna chime in with a question here. Just redone my seals. Reset the butterflys according to instruction. All is good. Question re bench synching. gggGary - I read in a post you made somewhere that when setting the plates, they should fully close over the three bypass holes. But - using a strip of paper there should be a smooth drag. Setting them like that, I can just see a touch of the foremost hole in each carb. If i set them to close fully over them all, then they bite down on the paper. Is the key relative position of the two plates - ie that they match - more than the actual position in the throttle body? Sorry if its a dumb question!

 

[gggGary]

The screw and springs between the throttle shafts is the synchronizing assembly

There are about as many methods of bench synching as there are XS650 owners.
My method is to do the synch before the diaphragms and slides go back in. adjust the idle stop screw til that carb has a crescent of light under the throttle plate. Look towards a light through the bores. Match the light sliver size on the other carb by adjusting the center screw. Close the idle screw till one sliver almost disappears, rematch the two sides with the center screw. Repeat if needed. Two small slivers of light can be compared to extreme accuracy by anyone. I am usually bench set within 1/4 turn of the synch screw once the engine is running with gauges on it.
Yes relative position of the plates is what we are after, the opening size is idle speed.
I don't know about the idle mix holes, there is nothing you can do about their location?

 

[wherearewe]

Thats what i figured. Thanks mate!

 

[YL82]

Thanks, gggGary !!

 

[gggGary]

No, no, no it's not tight! it's the synch screw, start about where the one in the pic is set, then bench synch them.

 

[wherearewe]

gggGary - I used your light method last night. Worked well. I think i was able to get them pretty damn close, probably closer than with the paper method. Lets hope thats the case when i chuck em on the bike! Thanks

 

[YL82]

gggGary,

I must have had way too many Newcastle Brown Ale's on the 4th to ask the dumb question about tightening the sync screw. I GET IT NOW!!

Let me follow-up one dumb question with another, but hey I'm a first-timer on motorcycle carb work.

I bought a nice Dywer U-Tube Manometer to sync my carbs with and I know how and where to hook up the tubing.

I assume that this sync screw is what I adjust to sync the carbs, correct ??

Sounds like I need to follow the steps for bench sync followed up by u-tube manometer sync'ing when the bike is running. Is that correct?

Thanks for your patience and help!

 

[gggGary]

No need to apologize for tipping a few on a holiday. Sounds like you have it all scoped out now, tell us how it goes for you!

 

[YL82]

Will do, thanks gggGary !!

PS: Those NBA's go down so smoothlhy that they can sneak up on you!

 

[YL82]

I got some nice stainless replacement screws for the butterfly plates from McMaster Carr. These are slightly longer than the originals but that allows you to bugger the threads up on the portion that sticks through using needle nose vice grips so the screws won't back out. Yamaha peened the original screw ends but I don't know how, maybe they have a special tool. It's very difficult for the home mechanic to do.

UPS delivered my 100 Oval Head Screws (M3x.5 thread x 8 mm) and while I didn't have time to put them into service, I took some images to compare these SST screws vs. Mikesxs product (#48-5022) when screwed into a throttle valve through the shaft.

The McMaster Carr screw is the one further recessed into the hole and is the longer screw as evidenced on the backside (how much thread each is showing). That extra thread will sure make it easier to mangle the threads with needle-nose Vise Grips!

5Twins: Do you recommend the MotionPro T-Handle #1 Phillips (ground down or use as is?) or otherwise what tool do you recommend to secure these throttle screws?

A JIS Type Screwdriver or Bit for these McMaster Carr PHILLIPS heads is not needed is it?

 

[5twins]

No, you don't need the J.I.S. bit, the Motion Pro #1 T-handle is what I use. After careful measuring, I chose this length specifically so the threads would be easier to "mangle". Technically, screws that stick out too much could hinder air flow through the bore but I doubt it's going to effect us street riders. Racers would grind the screws down and some even grind away most of the shaft.