Mikey's XS 81 Special II

[MikeyDeepXS]

I had a XJ550 center stand that was from a parts bike lying around, decided to cut the tang off of it and weld it onto the 650’s center stand.

It didn’t end up exactly perfect, but it’s a huge improvement over what I had to use before.

I started by cutting the tangs off, then installed the remaining part of the original stand back onto the bike, rolling the rear wheel up onto a scrap 2x8 to aid in getting it lifted, then tack welded the new tang, set the bike back onto the ground/kickstand, and checked for clearance between the exhaust and new tang.

Everything worked out fine on clearances; so I gave it a send with the mig.

Finally I stripped the paint and cleaned up the grime with a wire brush and parts cleaner, painted, and reinstalled.

Works pretty nicely, although the foot plate isn’t exactly perfectly angled, its way better than the old FUBAR

 

[MikeyDeepXS]

So I’m thinking after my last ride that I have a fuel issue at wide open throttle. There was a bog that kinda seemed like it could be from a rich condition(although my first instinct told me it might be a lean condition) kinda made a wet gargley sound as i recall

The idle and midrange perform fairly well; so I would probably try adjusting the needle position, but I’m reading that the 81 special didnt have an adjustment for needle clip posiition… so I didnt even take that apart to look. I’m assuming its a mains jet issue


So I pulled the float bowls off today to check on the main jet size and they seem to be 137.5(possibly why idle seemed a bit nicer and the bike started a bit easier at 2 turns on the idle screw??)

One is heavily corroded, I know… I ran it anyway after carb cleaning.

So I’m trying now some “132.5” jets
Really one eBay 132.5 I had laying around and one other jet that was probably closer to a 122 that I drilled out to match the 132.5

The thing is, the difference in the jet orifice size is so minscule that I might have widened the second one closer to the 137.5. Idk it’s really tough to tell without some extreme precision measurements.


I do have some factory 130’s from xs750 mikuni carbs, and I might try those next if the new drilled out jet and “132.5” don’t improve things.

Reset the idle mixture screws to exactly three turns out from lightly seated.

Regarding carb syncing… can someone with more experience tell me, does carb syncing mainly effect Idle and low RPM, or can it drastically effect wide open throttle as well?

I have vacuum gauges; but havent done carb syncing with them.

Also, when it comes to carb syncing with the vacuum gauges, is it best practice to adjust the butterflies relative to one another first to balance the vacuum and leave the idle mixture screws alone, or is there also a beneficial way to adjusting the idle mixture screw with the vacuum gauges for feedback….

Or could I be better off trying to just using my ears to tune everything?


Btw: stock airbox, stock exhaust. Air filters are OLD but were cleaned and oiled(oil type) recently

 

[gggGary]

https://www.jetsrus.com/a_jets_by_carburetor_type/jets_mikuni_BS_series.htm
strongly recommend genuine Mikuni jets.
drilling jets is not a good plan.

 

[MikeyDeepXS]

https://www.jetsrus.com/a_jets_by_carburetor_type/jets_mikuni_BS_series.htm
strongly recommend genuine Mikuni jets.
drilling jets is not a good plan.

Do you think I could run the 130’s?

Unfortunately; I’m not really in a place to throw money at problems to solve them right now

 

[gggGary]

With stock exhaust they'll run fine on 132.5 I would NOT go smaller. 135 might be OK
Really; you'll endlessly chase your tail with anything other than the genuine jets.
Pic of the bike?

 

[gggGary]

I looked back in this thread, trying to tune to a random muffler from "some other bike" can add to the fun.
Trust me, my tail is covered with bite scars.

 

[MikeyDeepXS]

Those 137.5’s don’t look like genuine Mikunis to me. Not the same double square markings as the 130’s anyhow….

Well gggGary, I love to FA2FO, so I’m gonna have to run these drilled out jets just a little bit and see what it’s like

 

[MikeyDeepXS]

Are my mufflers not stock?!? Go figure

 

[MikeyDeepXS]

Pretty quiet exhaust, not sure why PO would have made them MORE silenced, but then again, who knows

 

[gggGary]

Are my mufflers not stock?!? Go figure

Not stock to an XS650 but look like stock mufflers from "some other bike". And that can be a problem if they are from a small displacement bike, they may be unable to flow enough. Yeah, voice of HARD learned experience. The carbs aren't up on the mains till WFO, 4-5K RPM

 

[MikeyDeepXS]

Not stock to an XS650 but look like stock mufflers from "some other bike". And that can be a problem if they are from a small displacement bike, they may be unable to flow enough. Yeah, voice of HARD learned experience. The carbs aren't up on the mains till WFO, 4-5K RPM

Thank you so much gggGary for pointing that out for me, I had no clue that the mufflers weren’t stock and I’m not sure if I’d have ever figured that out on my own.

I’m going to investigate further now.

Thanks again!

 

[MikeyDeepXS]

Lu Hao Silencers LTD.

Not much to go on besides this stamp

I did take the bike for a spin with the drilled out jets. It seemed better at wide open, not boggy like it had been before, but the bike didn’t seem to idle as evenly on both cylinders, which seems like it would be unrelated to the mains.

I did re-adjust the idle mixture screws afterall…

I’m going to do carb syncing and dead cylinder idle mixture adjustments next.

I kinda wonder, would it be better to do the dead cylinder adjustments first, then sync the butterflies, or butterflies first then dead cylinder idle adjustments

 

[MikeyDeepXS]

I’ve been having a fuel(I believe)issue since removing the float bowls and swapping main jets. It’s like the right side cylinder is either flooded or not getting fuel. I just know it’s running strong on the left cylinder, but having weak sounds and occasional popping on the right. Engine died immediately when pulling the wire to the left cylinder.

It looks like I’m going to have to pull the carbs off again afterall. I’m thinking I have a float issue.

I’ll update once I get some bench testing done to determine if the bowl is getting too much fuel or not enough. If I see no clear problems there, I might go back to the 137’s that were previously installed to see if that might clear up things too.

I did have an issue after swapping the mains where the right side bowl wasn’t filling with fuel, like the float was stuck closed. I was checking via the bowl drain screws, nada from the right; so in an effort to free things up without taking everything apart, I opened the gas cap, capped it with a rubber plug and blew some lightly compressed air into the fuel tank, which worked. A second later fuel was coming out the drain on the right.

Considering the amount of varnish that was previously in the tank, I’m wondering if some got missed and worked its way down into that carb and is making things sticky.

I’ll update once I know more

 

[Jim]

It’s like the right side cylinder is either flooded or not getting fuel. I just know it’s running strong on the left cylinder, but having weak sounds and occasional popping on the right. Engine died immediately when pulling the wire to the left cylinder.

If I see no clear problems there, I might go back to the 137’s that were previously installed to see if that might clear up things too.

If the popping and not running you describe is at idle or low speed, the main has nothing to do with it. It's in the pilot jet/circuit and or idle mix circuit.
That or there's no fuel getting into that carb.

If it was too much fuel, you'd likely be getting belches of black smoke out the pipe when it pops. And you'd likely see fuel coming out the overflow.