Throttle stumble just after idle

WV Sparky

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Hello folks, first post here but read a pile of posts which have helped solve many of my issues but I have one left that I can’t crack yet. 1978 xs650 bobber build that I bought about 80% complete 2 weeks ago. It has the Uni high flow filters on it, the untapered ones and a “straight pipe” true dual exhaust. Carb was just rebuilt 6 months ago with every gasket and jet replaced. That being said the bikes stalls to the point of unrideable immediately following a sharp crack of the wrist then picks up after probably 2-3 seconds of the carbs pulling air in. The diaphragms appear fine and passed the test I’ve seen guys do on them of plugging the hole and seeing if it plummets back to the carb or drops slow. So I replaced the gasket on the intakes, did the ole WD-40 test and that’s all cleared up now. Set the floats which were a tad off and adjusted the needle clip back to stock as it was at the lowest setting when I got the bike. I’ve cleaned the carbs meticulously twice now. Above 1/4 throttle the bike is pretty happy and revs out fine. Also the fuel air screw doesn’t make a difference at 3.5 turns out and does kill the bike when ran completely in. I’ll attach a pic of the new spark plugs and their condition after the most recent tuning episode. I appreciate any and all help guys thank you
 

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jpdevol

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Welcome fellow Mountaineer!

The CV carb isn't known for snappy throttle as you describe, but stalling from idle or reasonable throttle-up isn't right.

Pilot jet size should be 30ish? Pilot circuit passages could be blocked. Choke passages could be blocked. Choke plungers could be leaking. I like to spray contact cleaner through passages to verify they are open and flowing. Finally, are the carbs synchronized?
 

WV Sparky

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Welcome fellow Mountaineer!

The CV carb isn't known for snappy throttle as you describe, but stalling from idle or reasonable throttle-up isn't right.

Pilot jet size should be 30ish? Pilot circuit passages could be blocked. Choke passages could be blocked. Choke plungers could be leaking. I like to spray contact cleaner through passages to verify they are open and flowing. Finally, are the carbs synchronized?
Nice man what are the odds of another mountaineer anyway?! Current pilot jet is at 27.5 which from what I’ve seen is stock, however I just ordered a 30 & 32.5 I believe it was along with larger main jets. I sprayed carb cleaner through the pilot circuit and had it exiting everywhere it’s supposed to. The choke plungers and gaskets are brand new. And I did a bench synch on the carbs but nothing too fancy as of yet.
 

jpdevol

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Hoping the larger pilots do the trick.......post back with results please.

Nice bike!
 

teamWicked

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Often times , when you open up the intake and exhaust you need to go up 2-3 sizes on mains and one or two sizes on the pilots. My air mix responded similar to yours when I had stock pilots. Upping my pilots, now 2 up from stock, made a world of difference.
 

nj1639

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I recently upped my '78 BS38's to 32.5 pilots and 140 mains and like TW says above "a world of difference."
On an aside, mebbe relevant....
While working with my carbs I had done the slide drop down test and it had passed beautifully, slow, slow, slow descent. After changing out the pilots and mains and in the process of getting everything back in order I did the drop down test again for laughs. Slides dropped like a rock, much to my consternation and confusion. A sigh of relief came as I noticed the choke had been activated in my handling of the cabs.......wonder if that could be a test of choke function........
 
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WV Sparky

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I recently upped my '78 BS38's up to 32.5 pilots and 140 mains and like TW says above "a world of difference."
On an aside, mebbe relevant....
While working with my carbs I had done the slide drop down test and it had passed beautifully, slow, slow, slow descent. After changing out the pilots and mains and in the process of getting everything back in order I did the drop down test again for laughs. Slides dropped like a rock, much to my consternation and confusion. A sigh of relief came as I noticed the choke had been activated in my handling of the cabs.......wonder if that could be a test of choke function........
This is an interesting thing you’ve noticed which I will definitely be testing myself! I’ll report back
 

WV Sparky

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Often times , when you open up the intake and exhaust you need to go up 2-3 sizes on mains and one or two sizes on the pilots. My air mix responded similar to yours when I had stock pilots. Upping my pilots, now 2 up from stock, made a world of difference.
I’m hoping that is my issue but what makes me wonder is that my new plugs which have only had about 30 minutes idle time on them and tinker time with the fuel air screw are now that shade of black which makes me think maybe the pilots aren’t the issue at all? The bike also has the xs charge PMA i believe they’re called? I’m nervous about it being a timing issue as well because of the color of the plugs even though on paper the bike should be in a lean state?
 

teamWicked

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I’m hoping that is my issue but what makes me wonder is that my new plugs which have only had about 30 minutes idle time on them and tinker time with the fuel air screw are now that shade of black which makes me think maybe the pilots aren’t the issue at all? The bike also has the xs charge PMA i believe they’re called? I’m nervous about it being a timing issue as well because of the color of the plugs even though on paper the bike should be in a lean state?
I may be way the hell off here but my plugs were often sooty. I suspect that the mains are feeding what the pilots can't because you have to set your idle position (butterfly) far enough open to bring mains (metered by the needle) into the equation. My idle would sometimes take off to 2000-2200ish. With pilots upped 2 and they must be close now, the idle is reliable at where I set it. A caveat is my sync was off a bit too. Fortunately, mains and pilots are pretty easy to get at on BS38s. I don't know about BS34s yet.
 

teamWicked

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Keep in mind there have been reports of "carb kits" containing incorrect parts. Someone did a side by side comparison of a stock needle and a kit contained needle. Different length and obvious different taper.
 

WV Sparky

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Keep in mind there have been reports of "carb kits" containing incorrect parts. Someone did a side by side comparison of a stock needle and a kit contained needle. Different length and obvious different taper.
This exact thought came to mind the 2nd time I tore into the carbs. Luckily the previous owner kept all the jets/needles he removed so I was able to compare them. I’m almost certain this is the bikes first carb rebuild on account of the discolor and conditions of the old parts he saved. As for your above post this would make a lot of sense for the blacked out plugs. I can’t give an exact number for my idle as the bike has zero gauges at the moment. I will say though that my idle screw is probably 1.5-2 turns from being maxed out.
 

5twins

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Starting with the choke will blacken the plugs and may make you think your jetting is off. It takes a good run down the road of a few miles to burn them clean again.

If your short UNI pods have the capped ends then they don't flow much, if any, more than the stock airbox did. The 4" long UNIs with the open foam ends are best but can be difficult to fit on a hardtail (they hit the hardtail frame tube). But, Uni makes a version with an angled mounting flange. With those, you could turn them down or out to clear the frame rail.

Yes, when jetting for mods on these, you usually need to change mains AND pilots, often lean the needles a step too.
 

WV Sparky

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Starting with the choke will blacken the plugs and may make you think your jetting is off. It takes a good run down the road of a few miles to burn them clean again.

If your short UNI pods have the capped ends then they don't flow much, if any, more than the stock airbox did. The 4" long UNIs with the open foam ends are best but can be difficult to fit on a hardtail (they hit the hardtail frame tube). But, Uni makes a version with an angled mounting flange. With those, you could turn them down or out to clear the frame rail.

Yes, when jetting for mods on these, you usually need to change mains AND pilots, often lean the needles a step too.
I do have the capped Uni pods, you are correct. But I can’t even get the bike to come up my driveway because off the low rpm struggle it’s having. Even tried giving it full throttle to get it to pull out of my steep hill and it just spits and sputters almost stalling out till I grab the clutch. Needle clip is in the stock position and all jets are stock. I have the xs charge system, could something with it have been installed incorrect do you think I just need a size bigger pilot and main possibly
 
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5twins

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Well, with your mods, you will definitely need some carb re-jetting regardless what the current issue turns out to be, so I'd start there. Yes, one to two up on the pilots and at least two up on the mains, also the needles leaned a step to compensate for the larger mains. On some of the 650 carb sets, you can get away with increasing the mains without leaning the needles, but not on a '78-'79 set like yours. Any increase in main jet size will require the needle clip change. This is because of the large main jet this carb set came with, a #135 (largest ever put in any 650). This puts the upper midrange right on the verge of being too rich right from the factory, even though they reduced the needle jet size. Any increase in main jet size (even just one) pushes the upper midrange into the "too rich" territory, causing breakup and stumbling under heavy or full throttle in about the 4K to 5K range. All this is because the 3 circuits in your carbs overlap slightly. Changing one influences part of the one next to it .....

Carb_Circuits.jpg
 

WV Sparky

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Well, with your mods, you will definitely need some carb re-jetting regardless what the current issue turns out to be, so I'd start there. Yes, one to two up on the pilots and at least two up on the mains, also the needles leaned a step to compensate for the larger mains. On some of the 650 carb sets, you can get away with increasing the mains without leaning the needles, but not on a '78-'79 set like yours. Any increase in main jet size will require the needle clip change. This is because of the large main jet this carb set came with, a #135 (largest ever put in any 650). This puts the upper midrange right on the verge of being too rich right from the factory, even though they reduced the needle jet size. Any increase in main jet size (even just one) pushes the upper midrange into the "too rich" territory, causing breakup and stumbling under heavy or full throttle in about the 4K to 5K range. All this is because the 3 circuits in your carbs overlap slightly. Changing one influences part of the one next to it .....

View attachment 220038
This is a huge help. I appreciate you breaking it down Barney style for me, that’s exactly what I needed. This is my first full carb breakdown and this forum has been a wealth of knowledge. The new jets should be here later in the week and I’ll report back with my results. Just to confirm, I’ll need to move my needle clip up one slot from current position which would be the 2nd slot from the top correct?
 

5twins

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Correct, when talking needle clip slots, Mikuni numbers them from the top down. Stock position on your carb set was the middle or #3 slot. To lower and lean the needle setting, you would need to move the clip up one to the #2 slot .....

Needle Slots.jpg


NeedleSettings.jpg
 

WV Sparky

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Alright finally got my new jets in and installed tonight. Put in the 140 main, stepped the needle clips up to the second slot and tried both the 30 and 32.5 pilot jet which seems to have made a small improvement. First rev from idle is perfect, if I drop it to idle and rev again it bogs for a second then starts to rev. Same thing with both pilots and I tried everywhere throughout the fuel screws range to find a happy median. There were places where the second rev was a little less hesitant but still not like the first. I’m lost folks
 
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