a developing saga

[Crashedkiwi]

Hello, Looking for advice again please.

1975xs650B standard non molested, apart from electronic ignition fitted. Compressions good. I recently imported the bike ex US to NZ; the bike had been sitting for at least a year without running, but started pretty much first go but ran rough. Fuel tank is clean/ no visible rust or gunk coming through the lines. I stripped and cleaned the carbs, brought the carb kits from MikesXS along with new butterfly seals, replaced and refitted. Standard jetting for primary and main jets. Reassembled, still ran rough. stripped again to make sure I'd not done anything dumb, reassembled; same result. I then fitted the electronic ignition as above purchased from also from MikesXS. Bike started and ran exponentially better with this mod. But still missing on start up and sluggish off the mark. Stripped carbies again, changed to larger primary jet (49) and double double checked cleanliness of carbies. Remounted and I'm extremely confident that there are no air leaks from flanges or rubber mounts (these are new also).
Damn thing starts - not quite as well as first fitment of elec. ignition and misses and is difficult to move off idle from cold. Once warm, runs like a train; noticably better for changing the primary jets. Still does not like to start or idle, will idle away around 1000 & just stop. I'm now running out of ideas to fix this so appreciate any suggestions. Its the only flaw in an otherwise really enjoyable ride so your help appreciated.

 

[jetmechmarty]

The jets. Are they MikesXS kit jets, or genuine Mikuni jets? Size to size, they aren’t the same.

 

[Crashedkiwi]

The jets. Are they MikesXS kit jets, or genuine Mikuni jets? Size to size, they aren’t the same.

Hi there thanks for responding- they're Mikesxs kit jets. The only one I changed was the primary - as fitted 42.5; changed to 49. This has improved pick up but I've still got the hard to start and miss issue.

 

[jetmechmarty]

Are the carbs retrofitted to later model BS34s or the original BS38? 42.5 sounds like a BS34 primary off the top of my head. When I went up just one size, my bike became significantly more difficult to start. If those jets came from the kit, you need to replace with genuine Mikuni. I expect you’ll find that the predominant opinion here.

 

[Crashedkiwi]

Are the carbs retrofitted to later model BS34s or the original BS38? 42.5 sounds like a BS34 primary off the top of my head. When I went up just one size, my bike became significantly more difficult to start. If those jets came from the kit, you need to replace with genuine Mikuni. I expect you’ll find that the predominant opinion here.

Thanks - they are the original BS38s The 42.5's was what was fitted when I got the bike.
Could you explain what the difference is between the MikesXS kit and genuine Mikuni is please? I mean is a size 49 Kit jet from MikesXS a different size to a Mikuni 49 sized Jet for example?

 

[jetmechmarty]

Hopefully, someone will pick this up. I’m out for a few hours.

 

[Jan_P]

Well now i am out of my area don't know much about carbs
But may I suggest check the tech section for Carburetors
Since it is at idle I would suspect the mixture screw
Or idle setting but please check the tech section
And wait for those that know more about this.

 

[jpdevol]

If the carbs are '75 (447) carbs, in addition to the aftermarket kits being "different", you may have ended-up with a mismatch of parts. Some kits list a duplicated 4JN19 needle when an OEM 4N08 needle (~4mm longer) was used to match to a "short" Z6 needle jet. A US spec '75 should have come with a 45 pilot jet, but may have been changed to a 42.5 if at altitude, a 49 would be too much.

Can you replace the aftermarket parts with the originals and new o-rings on the needle jets and try it?

Also, can you link to the kit you have from Mike's?

 

[650Skull]

CrashedKiwi

Click on the first link. Takes you to "the carb guide", holy grail on carbs.

https://www.xs650.com/threads/carbs-carburetors.43/

Page 27 has a Jetting graph on the different years for BS38's and BS34's

 

[willis]

Quality genuine Mikuni jets are available at Niche Cycle. Not sure in NZ if you could source more local. Shipping could be a deal breaker for you more than likely.

 

[joebgd]

I feel like the recommended idle is a bit low. My 1980 idles better at ab 1400. Idk if my bike electrics is working at it's best I can get a bit lower but 1200 I think is low and 1000rpm you can def go up a bit.

 

[willis]

Have you checked the sync of the carbs? With this being an unlinked set of '75 BS38's is it possible they are not equally synced causing the stumble at idle? I know it's tougher to sync the older model carbs, but might be worth a check.

 

[gggGary]

Thanks - they are the original BS38s The 42.5's was what was fitted when I got the bike.
Could you explain what the difference is between the MikesXS kit and genuine Mikuni is please? I mean is a size 49 Kit jet from MikesXS a different size to a Mikuni 49 sized Jet for example?

In short aftermarket jets = stock jets? no one knows.
But a maven of early bikes carb work Chatahoochee Skunk works just detailed that exact situation; jets from a kit marked same size as stock, bike ran like crap, cleaned reinstalled genuine jets bingo ran like a top.
The take away? Carbs are very complex and exacting, adding unknown variables while trying to fix "poor running" is a good way to quickly go bald.

 

[Dom]

This will be interesting, I'm about to strip my 650b's BS38 carbs after finding a fair amount of silt in the bowls today.

 

[jpdevol]

This will be interesting, I'm about to strip my 650b's BS38 carbs after finding a fair amount of silt in the bowls today.

Your bike has been running ok, right? If you do strip them, please document what's in there (US or Non-US spec., as inquiring minds want to know and..) so you'll have a basis for any future tuning or jet changes

 

[Dom]

Your bike has been running ok, right? If you do strip them, please document what's in there (US or Non-US spec., as inquiring minds want to know and..) so you'll have a basis for any future tuning or jet changes

Sure will JP, it was recently (2-3 years ago) imported from Oklahoma so I'd imagine it is as it was when it left. One thing is for sure, all the brass jet screws have been well chewed by some well meaning 'fettlers'
It runs OK as it is really but the silt in the bowls an some of the maintenance leads me to think a bit of a clean up wouldn't go amiss.

 

[Dom]

I've stripped my carbs today so can confirm it is as far as I'm aware set to US specs going by the carb guide on here. The one thing I have noticed is that the mixture screw was a lot more than the book starter point of 3/4s of a turn out, more like 2 1/2 on the choke carb which might explain the popping?

 

[jpdevol]

Yep, and the fuel screw and needle jet should match below

 

[Dom]

Yes looks like it. The jet needles on both of my carbs have developed a slight worn ring, does that render them done for?

 

[jpdevol]

Yes looks like it. The jet needles on both of my carbs have developed a slight worn ring, does that render then done for?

The one in pic looks ok; probably better than alternative spec if ya can find one.