77 xs7502d mixture screws

[jjz28]

My xs750 idles up when warmed up. Well, I put new butterfly shaft seals in last year and It helped some but I didn't run it much after that. Yesterday I put it back together after pulling the carbs again and it idles up when warm. I started messing with the mixture screws and cant tell a difference, even with them screwed all the way in. The tips aren't broken off. Someone made a homemade airfilter by taking the old one apart and just putting 2 layers of that prefilter on it. My exhaust has a bad leak on the throttle side wich has 2 of the 3 carbs going to it. Bike has almost 14,000 miles on it. Any ideas?

[IanDMacDonald]

I'm kind of confused by this post. First of all, the MKII's used on your 2D are almost identical to the BS34's. Hanging idle is usually sign of a vacuum leak. The reason a vacuum leak causes a high Idle in our bikes is down to the way CV carbs work. When you twist the throttle, you open the butterfly and allow more air through the carb. This causes a Constant Vacuum above the diaphragm, which raises the slide, and needle, and allows more gas into the mixture. When you close the throttle, the vacuum is reduced and the slide falls. This drops the needle, reduces the gas and drops the engine speed. If you have a leak on the intake side, the carb can't tell that the butterfly has closed because it is still getting air .... so the slide stays high and gas continuse to flow. This will hold the idle as high as about 4000rpm. It won't go much higher because above that with no load, the gas would be controlled by the main jet which is not affected by the air leak in the same way. But, a vacuum leak is not my first thought. I always abide by KISS: keep it simple stupid. Have you checked your throttle play to make sure you're not kinked? Pull the throttle linkage from the carbs and see if it still runs warm. Have you checked the intake manifolds for cracks? Laatly, you mentioned that you turned mixture screws for idle correctness. Did u mean the idle screws? Also, have you bench synced those carbs? Is your choke working properly? And finally, have you registered @ www.yamaha-triples.org?

[jjz28]

I am on the triples site just don't like it like this one. The carbs are made quite a bit different. the mixture screws are in sideways. Boots are new, butterfly shaft seals are new, no kinks. Manifold nipples plugged off. Should be no leaks. After warmed up it will rise up like it does when you pull the choke in the beginning then it will hang there. If I turn the mixture screws all the way in, it makes now difference and the tips are not broken off. its just very strange to me.

[cpmaynard1980]

Im curious what carbs your talkin about. I have the 750 and my screws are on top not the side. Are your handle bars turned. Mine had an un controllable idle the previous owner told me. He had ordered a new cable from yamaha but it was just a little to short so depending on the bar movement ot would affect the rpm. Another thing i would say is make sure the carbs are synced and the linkage is free. Is there any chance you sucking air at the shaft seals? I would use starting fluid to help find a vacuum leak. Just my thoughts.

[IanDMacDonald]

I'm curious too, as the 750's from '76-on used only two carbs: MKI's and MKII'S . The MKII's were lighter with a few mods such as adjustable needles. The 850'sused Hitachi carbs. I dunno any Mikuni carbs with screws on the side.

[jjz28]

the screws aren't on the side. I said "sideways", They don't go up and down, they screw in sideways but are on top of the carbs. They are original carbs.

[IanDMacDonald]

Yeah, I gotcha. U have the MKI's. But, you still never answered my question: if your idle is what you're after to adjust, why are you turning the mixture screws? Either turn the idle screws between the carbs, or the main idle screw.

[jjz28]

I was just wanting to see if they made a difference and they don't. I can turn them all the way in and it runs the same. It definitely not just and idle adjustement though. Its like an air leak but I cant find one.

[IanDMacDonald]

Valves been checked? If u turn that screw all the way in (lean condition), bike will stall out.

[rpgoerlich]

Had one a few years back I rebuilt.
I would check valve clearances, correct what needs to be then sync the carbs.
I think then you'll find the pilots will make more of a difference.

[jjz28]

I really do need to check the valves but haven't. I just thought It was weird that I turn all 3 carb screws in and it make NO difference.

[IanDMacDonald]

Well, they say when u no-longer hear the valve chatter is when they're out of spec.

[jjz28]

Well, all 3 intake were too tight and one exhaust was too tight. The other 2 exhaust were in spec. I guess I need to order the tool now. Anyone have any thoughts on the best vavle shim tool (easiest to use)?

[IanDMacDonald]

The Motion Pro is what I use. I'd just pull the cams though.

[jjz28]

I am scared to pull the cams. Just don't want to get the timing messed up.