First Time Poster... Long time Reader (BS38 Woes)

[OfflineForever]

So I bought my first bike.... definitely a parts bike from what I can tell, the US (which is weird in Australia).
Rode around on it for about a week but was having trouble with hesitation, plug fouling etc. Found air leaks around the intake boots so I decided to buy new boots, gaskets and rebuild kits from geoff's XS.

Motor: 447-205XXX (1976 XS650C)
Carbs: BS38CV (dual throttle, linked at top of carbs… early 70s?)
Pipes: Straight pipes w/small baffle inserts

New intake boots and gaskets.
New spark plugs (BP7ES).
Checked cam chain tension, valves, points and timing (all in spec)
Float bowl set to 25mm.

Previous pilot jets: #45
Previous main jets: #130

Current pilot jets: #27.5
Current main jets: #135

So after rebuilding my carbs with geoff’s XS kits, they are still running poorly.
I replaced and cleaned everything I could and triple checked everything.
I believe I have concluded that my jetting is way off now, due to the disparity of different BS38 models etc, I didn't check my original jets before ordering either...
Even if I set my mix screw all the way in, it will blow black smoke when rev’ed.
I also noticed fuel comes up around the needle jet, which from my understanding, would create a rich mixture. I will purchase new and replace.

What I’m trying to understand is, if my pilot jets are smaller than before, why are my symptoms SO much richer?
Is it because the air leaks were creating this somewhat balanced mix haha?
I attempted a ride after rebuilding and it felt SO sluggish and bogged. It actually stalled too.
Do I return to the previous jetting or is there another combo I should really look at?

While I'm at it, can anyone confirm the needle jet I need for these carbs?
From what I can tell, a Z6 Long?

Any help is much appreciated!

 

[grizld1]

Kits often contain generic bits that don't fit our application and/or parts of questionable quality. It's a safer bet to buy individual, genuine Mikuni parts.

Excessive richness can be caused by high fuel level, which in turn can be caused by defective or dirty float valves, leaking floats, float level set too low, defective or damaged mixture screw and/or seat.

Carburetor identification and correct jetting for each OE carburetor fitted to the Yamaha 650 twin is given in the Carb Guide. You'll find the link in the Tech section.

Some carbie issues can test a guy's patience, so remember: If you start seeing red, put the tools down gently, step away from the bench or the bike, and take deep, cleansing breaths until the desire to throw things passes. Good luck to you!

 

[650Skull]

Think you will find those carbs are 74/75

If the bike has a pass switch on the left bar switch it will be an Aussie model

 

[OfflineForever]

If the bike has a pass switch on the left bar switch it will be an Aussie model

The frame number is different to the engine number, bike is definitely a 'bitsa' and has been slapped together by an older guy.
Neither the frame or engine had been registered in Australia before so it was an interesting process getting it done... a story for another time!

Excessive richness can be caused by high fuel level, which in turn can be caused by defective or dirty float valves, leaking floats, float level set too low, defective or damaged mixture screw and/or seat.

I've put in brand new float valves, floats have been leak tested, float level is set perfectly with guide, mix screw is fine.

I think it's a combination of wrong jetting and the worn o-ring on the needle jets that have caused the problems... but I'm interested to hear any other thoughts or ideas!

 

[650Skull]

is the jetting for the right carbs............have you identified them...............74/75 or 76/77...........????
carbs kits make money for the dealers and cost the buyers heartache and problems

 

[OfflineForever]

is the jetting for the right carbs............have you identified them...............74/75 or 76/77...........????

I used this spec sheet from https://thexscafedotcom.wordpress.com/2011/01/10/xs650-carbs-mikuni/

Judging by my original jets (#130 Main, #45 Pilot and Z-6 Long Needle), I would say they are pre '76.

 

[OfflineForever]

Upon more digging and investigation, it gets worse.
Previous owner or someone along the line has used two different sized needles!!!
Shorter needle is 4JN19-4
Longer needle is 4N8-4


What's worse is, I believed that based on the jetting I found originally, I had Z6 Long needle jets.
Upon measuring both of them, they are in fact 30.5mm (which according to the below post, is the short needle jet)
https://xs650temp.proboards.com/thread/15096/1973-carb-woes?page=4 Post #11

At this point, I'm not sure what to do. I have Z6 Long needle jets on the way already... will they work? I can't seem to find anywhere online that stocks replacement Z6 Short jets either.

Any input identifying these carbs accurately would be greatly appreciated!
Alternatively, suggestions on the needle jet and what I should do moving forward would take a load off my mind.

*EDIT*
I believe them to be 74/75 carbs as 650Skull had mentioned.
Z6 Short Needle Jets are apparently the same as Z8.
Now to find 4N8 Needles in Australia...

 

[gggGary]

Yeah sorry you get to have so much "fun"
what intakes did you use?

"the disparity of different BS38 models"
This may be key. It's possible you have "incompatible" or non matched left and right carbs.
Take a look in the intakes. compare intake floor heights. Making sure the left and right carb are the same vintage seems like job one.
What intakes did you use? The later intakes for linked carbs set the carbs at a different angle. The top linking plate is not "correct" for early two throttle cable carbs. A matched set of early carbs could/would work with that plate and later intakes but may need a different float setting.
If you post up more pics, intakes, sides, float bowls off, we may be able to airchair sleuth both carb body years.
Float bowls usually have a letter stamp on the bottom, they need to "match" the year of carb body.
Hate to say it but a different, matched set of carbs might be the best/easiest way forward. 78-79 BS38's and BS34 are usually considered to be the "best" carb models.

 

[MaxPete]

I will certainly defer to the carb experts above for the details - BUT - I do have a little story about my experience with carburetors and the value of that Carb Guide and the little adjustment chart in Post #6 above.

My 1976 XS650C (the fearsome Lucille ) came with the stock 1976 carbs and it ran well but after a while, one of the float needles started to leak (which is a PITA) and so I ordered the parts to fix both of them - but at almost the same time, a pair of 1978 carbs appeared for sale on the Forum - for a very low price. The advert stated that the carbs had been mounted on a 1975 bike (remember that fact) but that the bike ran poorly and the carbs were thus, for sale and so on an an impulse, I bought them.

When the '78 carbs arrived (before the float parts for my 1976 carbs had arrived), I went over them and found that every float bowl screw and all of the top cap screws were quite loose. I checked the float height (it was fine) and so I simply tightened everything up and stuck them on the bike. It actually started OK but ran very, very lean with lots of popping and backfiring etc. Hmmmm....I wonder why it does that.

Referring to the carb settings chart shown above in post #6, I found that the mixture screws were set at 3/4 turn - which is correct for a 1975 XS650B on which the carbs had been mounted by their previous owner. However, as noted earlier, these carbs were actually intended for a 1978 XS650E and a mixture screw setting of 3/4 turn is MUCH too lean when combined with the jetting and other characteristics of the '78E model carbs.

Soooo, I simply opened the mixture screws out to the correct setting for a 1978 carb of 2-1/4 turns and the bike started and ran like a top! I really must get around to doing the floats on my 1976 carbs some day....

The moral of the story is - use the identification chart in the Carb Guide to be certain of what year carbs you have and then adjust the carbs according to their year - which may NOT necessarily match the year of the bike. With so many XS650s running around and being so similar from year to year, there has been a great deal of parts switching and swapping (sounds like a good old fashioned Renfrew County Ontario car-key party....) but the carbs are one item that really do differ from one year to the next.

The folks who wrote that Carb Guide really know their stuff - and if you follow their instructions, your bike should run waaaaayyy better.

 

[grizld1]

Aw, come on now Pete, why should anybody bother to read that stuff? You know that 5twins and I made up most of that nonsense and only posted it to kill time and confuse people.

 

[5twins]

Yes, your carbs are '75 at the newest, but could be as old as '72. To narrow the range down further, you will need to look in the air filter end at the floor of the main bore, as Gary mentioned. There were some big changes between the '72-'73 and '74-'75 carbs. The floor through the main bore sat up much higher in the '72-'73 carbs. That required a longer needle jet to stick through that thicker floor, and a shorter needle to fit and work properly in that higher sitting needle jet .....

The floor was dropped in the '74 and later BS38s. This then required a shorter needle jet and matching longer needle .....

It's possible you have one of each mated together and that's just not gonna work.

If the carb kits you bought were for the '76-'77 carbs then the included 27.5 pilot is the wrong type, a VM22/210 style. Your carb bowls are made to use the BS30/96 type pilot jets. So, you should probably put the 45's you found in there back in there. But, do inspect them to insure they are the correct BS30/96 type .....

Your needle jets are the short style. The longer type has that added length on the top portion of the jet where the o-ring fits .....

Unfortunately, I don't think you can get the "short" Z-6 needle jets anywhere.

 

[5twins]

You can also check the number stamped on the butterfly plates. '72-'73 carbs would have a 125. '74-'75 carbs would have a 120 .....

 

[MaxPete]

Aw, come on now Pete, why should anybody bother to read that stuff? You know that 5twins and I made up most of that nonsense and only posted it to kill time and confuse people.

 

[OfflineForever]

Ok so a lot to unpack in these comments, I really appreciate the advice!
I have a Clymer manual that I've poured over and I have also gone through the carb guide multiple times, definitely a wealth of knowledge.
Even with the manual, the forums still have the finer details!

I got home this morning and took these photos, looks like they're both 74/75 carbs *phew*
For whatever reason, the previous owner has just used two different sized needles.
Both butterfly plates have "120" stamped on them too.

As you can see, both floats and float bowls are the same...except the right side is missing an emulsion tube?

As are both the carb bodies.

 

[gggGary]

Great pics!
Whew is right.
The missing tube is correct for 75 carbs. That's the choke circuit feed, the 75 uses one choke and that (braided) crossover to feed the RH carb.
those look like the 70-75 style intakes so it might be best to lose the butterfly bracket, should let the carbs seat to the intakes better.
Diaphragm drop and float dip tests are a good idea, may be this may be that, eliminator.

 

[MaxPete]

OK - well, you may have yourself a ballgame there. Certainly, two carbs of different models would be....challenging....to adjust.

Now, over to the carb experts....

 

[OfflineForever]

So if I lose the butterfly bracket, the carbs are fine to just hang in those intakes? I know it sounds silly but I just want to be sure haha!
I noticed on some BS38 models, there is a right angle bracket to hold the bottom of them... is that something I need to worry about?

I've done a float dip test on both, they are both air tight!
Diaphragm drop test worked on the right side. Covered the top port and it slid down slowly.
Left side, even when I covered the top port, would slide straight down as if nothing was covered.
I've inspected for tears, which the previous owner promised there weren't any *haha god I should stop believing people*... there were two very tiny tears.
I patched them with gorilla glue as per some info I've read online as gorilla glue is polyurethane active and will not only flex but also withstand fuel etc.
After all that, the bad diaphragm still operates the same so...
Someone in a facebook group was nice enough to send me a 'new' diaphragm today, so hopefully I can try it on Monday when it arrives.

Anything else I should be looking at / for?

 

[gggGary]

The diaphragm chamber seal depends on the choke being seated. mebbie try the "bad diaphragm" on the other carb with the "good" diaphragm. have you opened, checked the rubber seal on the end of the choke plunger?

 

[5twins]

You should have the tin shrouds over the rubber intake manifolds. They add needed support. Running without them can cause manifold tears, especially if you're running pod filters and have no carb support at the rear from the airboxes. Also, the cable holders, as seen in your 1st pic (post #1) are on the wrong carbs. They shouldn't angle out like that, they should angle forward. The pointed end on top of the bracket will face to the rear .....

That angle bracket across the bottom of the later carbs can't be mounted on the early carbs. The mounting holes for it are cast right into the later bodies and your early set doesn't have them .....