Narrowing down air leak 1978 BS38s

moebius

moebius

XS650 Member
Messages
17
Reaction score
1
Points
3
Location
Scotia, NY
Hey guys, pretty new to the forum and new to working on bikes but I was wondering if I could get some help narrowing down an air leak.

I have a 1978 xs650, started riding it this summer and I've had some backfiring popping issues for awhile now, only on the left side. I checked the points then adjusted the timing, after that I ended up pulling the carbs off and rebuilding each with gaskets, pilot jets, main jets (both stock sizes) and mixture screws. Also cleaned with carb cleaner and compressed air but the float bowls didn't look too bad. The air box was fucked so I decided to just put pod filters on it.

After that I realized the left carb holder rubber was starting to crumble so I figured that was the issue, ordered new carb holders from MikesXS, but only replaced the left one as the vacuum nipple on the right was running into the valve cover bolt and wouldn't fit.

Anyway long story short that didn't fix the leak, I can spray starter fluid right underneath the left carb holder and the idle goes up. So it seems like the carb holder is still the issue. I tightened the carb holder clamp as much as it can go without completely stripping the screw, made sure I pushed the carb in as far as it can go.

My next step is to get gaskets for the carb holders because they never had any, but I'm not really sure how to narrow this down. Some one said to cover the holder with grease to see if that fixes it temporarily to narrow down the leak.

Anyway I read this thread: http://www.xs650.com/threads/very-persistent-air-leak.3005/ and a couple others but I feel a little lost at the moment. Thanks in advance for any tips!!

Oh and here's the bike:
http://res.cloudinary.com/moebius/image/upload/v1503972001/IMG_3155.JPG.jpg
 
Yes, you need gaskets on the manifolds. The originals were quite thick, nearly 1/8". This is a simple gasket shape so you could easily cut your own from sheet gasket material. Just make sure it's pretty thick. I add a bit of sealer between the gasket and manifold, not so much for sealing purposes but so it comes off with the manifold any time I remove them. I don't put any sealer between the head and gasket.

If you change the Philips screws on the manifold clamps to Allens, you'll be able to better tighten them fully. Although shorter than the original length, an M5 x 16 works well .....

ZC8Ganx.jpg
 
Last edited:
...and for gasket goo, you might try Hylomar Blue.

It was invented by Rolls Royce for aero engines and is great stuff. I haven't ever found it in a store but it is readily available on-line. This stuff isn't cheap - but a $20 tube will last for a loooonnnggg time and its worth it IMO.

https://www.ebay.com/sch/i.html?_nkw=blue+hylomar

Hylomar_Blue.jpg


Another possible source of the leak may be the throttle shaft seals and the OEM Yamaha seals are still available on-line for $3-5.00 each. Here is the part number: 256-14997-00-00 - try www.boats.net or www.PartsFish.com

As I have said elsewhere (this morning, in fact) that Yamaha OEM part number is a key piece of information. If you have that - you stand an excellent chance of scoring the correct OEM part quite inexpensively by simply putting that part number into your Google search window (with no other words or descriptors).

Try it for yourself - simply cut and paste 256-14997-00-00 - into your Google search window and you'll get a whack-load of listings of that very part at prices ranging from $3.05 (from www.boats.net) to around $2.95 (from PartsFish). BTW - I have bought parts from both of these sellers and they are just fine to deal with.

www.boats.net has all of the parts blow-up views on their website - including the Yamaha part numbers and a listing of which other bike models used that exact part. Even some unique engine/transmission parts can be found using just the parts number. Also, even if www.boats.net lists the part as "Obsolete - discontinued" - try searching for it on Google anyway - and chances are you'll find it somewhere else.

I got a desirable single piece long clutch push-rod for my 1976 for the princely sum of $7.00 USD - and it was still wrapped in its Yamaha OEM parts baggie.

TIP for Fellow Canadians: be sure to specify shipping by US Postal Service or USPS (do NOT use a courier like UPS or FEDEX etc.). The USPS shipment will swap over to Canada Post at the border and shipments under $1-300 entering Canada via the postal system are seldom dinged by Canada Customs. I think that the same guideline applies for shipments from the UK - be sure to have the shipper use the Royal Mail. Have them marked "Vintage Motorcycle Parts" - these packages nearly always come through with no duty or other crappola fees added on.

Cheers,

Pete
 
@5twins thank you that's super helpful, I'll be getting those Allens as well as making myself some gaskets tonight!

@MaxPete thanks I'll try and score some Hylomar Blue. Where exactly are the throttle shaft seals on the carbs? Did some searching but I couldn't quite tell. Something to do with the butterfly valves or the choke plungers?
 
The throttle shafts are long shafts that run across the carbs (i.e. across the fore & aft axis of bike) and have the main throttle plate mounted them.

Search in the Forum under rebuilding carbs and you should find some good photos and even a video.
 
Actually - if you look at 5Twins photo above, the throttle shaft is the thing in the middle of the picture with the nut and torsion coil spring on it - just to the right of the throttle cable in the photo.

Each end of that shaft has a seal which is pressed (gently) into the body of the carburetor to seal the rotating ends of the shaft and prevent air leaks. Those seals are prone to leaking (after 40+ years - aren't we all prone to a little leaking...:D) and they often need to be replaced.

Apparently, the OEM seals are WAY better than aftermarket seals for MikesXS - and Yamaha OEM seals are still readily available for like, $3.00 each (you need four seals to repair two carbs).
 
@MaxPete hahah, yeah okay i see that. Maybe I'll order some and replace just to make sure that's not an issue now or in the future. I remember when I did my rebuild I attempted to take that out but the little screws on the butterfly valve started to strip a bit so I left it alone, hopefully I can get those out now.
 
OK - so here is Pete's Harangue #2:

- for goodness sake get yourself a set of JIS (Japanese Industrial Standard) screwdrivers.

Everything that looks like a Philips screw on a Japanese motorcycle or car (or outboard motor or camera) is actually a JIS screw and using a Philips driver on it is not going to work.

You can get a very nice set of Vessel brand JIS screwdrivers on Amazon for around $40 - and never have a problem working on your Yamaha/Toyota/Honda again. They're a pretty shade of blue-green and are very nice tools indeed. While you're at it - order a grey/silver Vessel Impacta
impact driver - really excellent for those crummy JIS clutch screws that MamaYama used on our bikes.

...or you can stick with Philips and wreck every screw on the bike or car
 
Howdy!

That one looks like a really nice tool but the one I have is a #3 sized JIS with a silver coloured handle and it is much less expensive. It fits and deals with the clutch screws (even badly damaged ones) VERY effectively. There is one advertised on Amazon right now for around $14.50.

I do like the one you suggested though.....
 
Apparently, Vessel is the company that invented the hand impact probably like 70 or 80 years ago. Their standard offering is the model #2500. I bought one from my local hardware store when I was about 12, a good 50 years ago. Back then it cost me like $7 or $8, lol. I still have and use it regularly. They still make it but obviously, the price has gone up .....

https://www.vesseltools.com/handtools/screwdrivers/impact/250001-detail

But, being around for so long, it's easy to find nice used ones on eBay for a fraction of the cost. I picked up a few more as back-ups .....

https://www.ebay.com/sch/i.html?_od....TRS1&_nkw=vessel+2500+impact+driver&_sacat=0

Over the years, the only thing I've needed to do is replace the bits occasionally. They will wear and occasionally you'll break one.
 
Well, yes it would, but I think I'd keep shopping. That one is all the way over in England and shipping costs more than the tool itself, lol. You should be able to find a nice one from this country for $20 or less. One more thing I'll mention - these come with either a 1/2" or 3/8" drive head. I have both and feel the larger 1/2" drive version "impacts" better.
 
Good call I definitely dont want to pay more than the driver in shipping, I ordered JIS screwdrivers and throttle shaft seals that have yet to come.

I got the gaskets on, my 137.5 main jets came so I put those in (pilots are still stock) and I got allen heads for the manifold clamps and tightened them completely so the ends touch. The bike started up, I double checked the timing (not sure if that would have been affected but I did need to adjust it a tad). Then I sprayed some starter fluid in the same spot and the leak seemed to be fixed. The bike seemed to be running okay but I was having some weird issues. When revving the RPMs hang around 2 to 3 thousand and sometimes stay there for quite awhile. I did the pamco ignition upgrade last weekend so maybe I fucked up the advancer somewhow?? The idle would also sometime just jump way up and hang there, although maybe that's because the bike warmed up I'm not sure.

I guess I'm going to try greasing up the advance unit and maybe checking out the springs. How can you tell if the advance unit is hanging? If I look at it when I rev it does seem to be staying open rather than closing immediately.

I also clearly have a lot more work to do on my carbs. I read the carb guide last night again but I was wondering if there is an order in which you need to adjust the carbs.. Do you sync first and then adjust pilot screws after or the other way around? Seems like maybe sync first.
 
Re: testing your advance. With the bike off pull on one of the bob weights and see if it "snaps" back quickly and fully. Any hesitation or drag would indicate dry or dirty conditions.

While syncing and pilot fuel feed are both done to get optimum fuel delivery one is not dependent on the other.
My guess is a lively discussion could be had as to which is most important. Kind of a chicken or the egg question.
 
You may not be able to set the mix screws as easily to their best settings if the sync is off. One cylinder would be running faster than the other that way. Mix screws are easiest to set with the cylinder running very slow, even below the 1200 RPM idle speed spec. In fact, it often helps with setting them if you turn the idle down to 8 or 900. You can better hear the effects your adjustments are having like that.

I like to roughly set the mix screws, then sync, then re-visit the mix screw settings to see if they can be made better. If they can and it alters (raises usually) my idle speed, I'll check the sync again.

Yes, service the advance rod before messing with new springs. That could be all that's needed. It's very common for these bikes to need this service done. Most have never been lubed since they were assembled at the factory like 40 years ago. I guess we can't totally fault the owners because this required service is not mentioned in any of the manuals. Pull the rod right out of the head and pack the grooved bushing on each end of the cam with fresh grease. Some "before" and "after" shots .....

dhXcM3V.jpg


cc8H22E.jpg


The "before" shot is about 5 or 6 years since the previous greasing. Still some grease remaining but I took the opportunity to replenish the supply. So, although required, this isn't something you need to do very often. The factory recommends a moly grease of some sort.
 
5twins said:
Apparently, Vessel is the company that invented the hand impact probably like 70 or 80 years ago. Their standard offering is the model #2500. I bought one from my local hardware store when I was about 12, a good 50 years ago. Back then it cost me like $7 or $8, lol. I still have and use it regularly. They still make it but obviously, the price has gone up .....

https://www.vesseltools.com/handtools/screwdrivers/impact/250001-detail

But, being around for so long, it's easy to find nice used ones on eBay for a fraction of the cost. I picked up a few more as back-ups .....

https://www.ebay.com/sch/i.html?_od....TRS1&_nkw=vessel+2500+impact+driver&_sacat=0

Over the years, the only thing I've needed to do is replace the bits occasionally. They will wear and occasionally you'll break one.
Click to expand...

Hey - I already OWN one of those - I just hadn't known that it was made by Vessel. I'll have to dig it out, grease it up and start using it again!

Pete
 
If it is a real Vessel, that name will be stamped right in the side. There are many copies out there, and many even have the "2500" number on them too.
 

Similar threads

ForestHasAnXS650Too
New-Old XS650 Special journey - My initial problems
2
Replies
22
Views
510
jetmechmarty
jetmechmarty
O
Yeah I'm an Old Rookie Running without carb manifold gaskets for 2 years
Replies
9
Views
810
Kevin Werner
Kevin Werner
johne67
Exhaust Sucking In Air
Replies
13
Views
1K
Slippery
Slippery
fenbrancis
One Cylinder Just a Touch Too Rich
2 3
Replies
40
Views
2K
dps650rider
D
L
Popping Noise After Carb Rebuild at Idle
Replies
13
Views
2K
Wingedwheel
Wingedwheel
Back
Top