The Patience build.

5twins

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Well, if you back the idle speed screw out all the way so it's not touching the cable arm, both butterfly plates should be closed completely and the bike should stall .....

IdleSpeedScrew.jpg


Are you sure you have enough slack or freeplay in your throttle cable? If it's adjusted too tight, that will hold the butterfly plates open. Even if you back the idle speed screw way off, the butterfly plates won't close all the way if the throttle cable is too tight and holding them open.
 

Wingedwheel

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Cable has enough slack to move it, that’s the first thing I checked. Replacing those parts didn’t seem to help at all so back to page one. At least I now know it is the carbs.
 

Wingedwheel

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Butterfly plates would not choke out the carbs even with the screw turned all the way out. I can still see a sliver of light with this screw all the way out due to the cable side linkage being set and not adjustable. Passages are all clean and everything is how it should be according to the Carb guide which I went over several times to make sure I wasn’t missing anything. I’m seriously considering dropping the set of carbs off my 72 on here to see what would happen.
 

Wingedwheel

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This is where I’m at. Before pulling the carbs back off no left cylinder, idled great. After taking carbs back off and replacing everything due to having available kits and issue jumped to no run right side. If I place my hand over the mouth of the carb it revs way up so doing this on the right carb causes a rev and it starts firing because I tried it with the cap pulled and it did nothing. Can pull the left cap and keep it running if I use my hand to choke the right carb mouth Air/fuel mixture screws have little to no affect. Appears to be a lack of fuel to the right but figuring out why is baffling me at the moment.
 

Wingedwheel

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I’ll also note that before I could give it throttle and it worked well, now giving it any throttle kills it. I can rev it with the choke lever so again, feeding it fuel.
 

5twins

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I don't think you "get" how the sync adjustment works. Both butterfly plates are adjustable with the screw on the linkage between the carbs. Turning the screw one way closes one plate and opens the other. Turning the screw the other way does the opposite, closing the other plate and opening the closed one. You play with the screw setting, turning it one way or the other, until both plates are closed. You do this with the throttle speed screw backed out as shown above. Then, after you get both plates closed, you turn the idle speed screw back in until both plates crack open a hair.
 

Wingedwheel

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I “get” exactly what you’re describing however mine does on appear to function that way. The right side butterfly is closed by the return spring and the left is adjusted with the screw accordingly. Unless I’m missing a part I see no other way this works?image.jpg
 

Wingedwheel

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I put the carbs off my ‘72 on and it’s purring like a kitten. Those are the same set as on this ‘76. PO of my ‘72 swapped out the originals for a single throttle cable set. So what is that telling me I’m doing wrong? I built that set with no issues.
 

5twins

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Your carb set looks assembled correctly except for that angle across the bottom. The down leg should be towards the inside, next to the bowls. But that shouldn't have any effect on the sync adjusting .....

Finished4.jpg


Go ahead and play with that sync screw. Turn it in the right direction and you'll see that it will close up that right plate. Keep turning it in that direction and in a turn or so, the left plate will begin to open. That sync screw controls the position of one plate in relation to the other one. Right now, your left plate is too closed and that's holding the right one open.
 

Wingedwheel

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Your carb set looks assembled correctly except for that angle across the bottom. The down leg should be towards the inside, next to the bowls. But that shouldn't have any effect on the sync adjusting .....

View attachment 216170

Go ahead and play with that sync screw. Turn it in the right direction and you'll see that it will close up that right plate. Keep turning it in that direction and in a turn or so, the left plate will begin to open. That sync screw controls the position of one plate in relation to the other one. Right now, your left plate is too closed and that's holding the right one open.
The way mine is working is I turn it to the right looking at the carbs from the front(tighten) left butterfly begins to open. That screw has absolutely no affect on the right side. I can open the right side with the idle screw and then sync the left in that way but the throttle return spring tension is what holds the right side closed with no other adjustment other than the idle screw. BTW, I flipped that angle, thanks I never even noticed that and I did play with the screw but it seriously does not move the right side.
 

Jim

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That screw has absolutely no affect on the right side. I can open the right side with the idle screw and then sync the left in that way but the throttle return spring tension is what holds the right side closed with no other adjustment other than the idle screw.
Leave the idle screw backed out so it's not touching the throttle arm.
Lookin at the carbs, screwing the sync screw in will open the left butterfly. Screwing it back out will close it. When it get's down to where it touches the body, lift the throttle lever slightly. Now as you continue to unscrew it, you'll see that the left will close down further and the right one will be higher.

...all as you look at the carbs obviously.
 

Wingedwheel

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Are the butterflies supposed to be sealed tight to the body because I’ve got a small sliver of light that I set both to and I even used a thin feeler gauge. I’m also going to say that I really think it’s more internal and since my other set is working just fine I’m going to focus on a few of the other things left to take care of. Guys I do appreciate all the help and I don’t have near the experience a lot of guys on here have but I do know when to stop chasing a Huckleberry.
 

Jim

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Are the butterflies supposed to be sealed tight to the body because I’ve got a small sliver of light
Yeah, pretty much. It's possible your butterflies aren't set correctly in the shaft. The best way to center 'em is to have the screws slightly loose, open 'em and let 'em snap shut a few times. That'll center 'em in the shaft, then tighten the screws down. After that, go back and re-sync 'em.
 

5twins

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I think Jim may be right, that one plate isn't centered in the bore and it's binding on the side of it.

The sync screw has to work as I've explained it. I put some arrows on your pic to illustrate .....

ButterflyPlateAdjust.jpg


As you tighten the sync adjust screw, that will force the "L" shaped bracket on the right butterfly shaft down, and rotate the shaft as shown (red arrows). This will close that right plate. At the same time, the "C" shaped bracket on the left shaft will get raised and it's butterfly shaft will be rotated as shown (blue arrows). This will open it's butterfly plate.
 

Wingedwheel

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Tried that, tried loosening the screws as Jim suggested and tightening that adjustment screw only raises the top bracket and does not push the other down. I’ve tried raising the throttle and adjusting the other side to check binding even phoned a friend lol. Conclusion, it’s where it’s supposed to be or at least very close and with it jumping sides I’m convinced there’s something else going on especially since I can pump the choke or use my hand to block off the throat and get it to work. I have another set of buildable carbs that I’ll work on but in the meantime the set that’s on there and working will be fine for now.
 

Jim

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Not familiar with the -38's.... that a screw missing?



Untitled.png
 

Wingedwheel

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Last bit of parts came in today from YamahaXS650.com and I replaced the rubber bits on the side cover and new latches. I ended up cutting the springs down because they wouldn’t allow me to turn the knob and I can’t see where using one that was as stiff as they sent was needed. Bike is running good and other than a valve adjustment and and my horn not working I’ve reached the end of the build. There will still be small things to tinker with but overall I’m very pleased. I will continue to mess with that set of carbs as there’s just not something right. Called my agent and insured it for registration. I have an Appt with the Secretary of State Monday morning for tags. Next week is break-in week. Still have a little oil smoke coming out of the exhausts so I want to get those rings to set. Thanks to all the following guys.
5twins, gggGary, Jim, Beags64, Mailman, NielsB. If I missed anyone let me know….image.jpg
 
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