1978 Yamaha xs750 special help!

[xs750special]

Hey guys, so I recently picked up a 1978 yamaha xs750 special with 15xxx miles on it. When I got the bike it would start and idle fine around 1,100-1,500 rpms, and I could only rev it up about 15% to get the rpms to slowly raise, but when I gave it more juice it would bog out and die. I should also mention that the previous owner put uni pod filters on and i do not have the stock air box. So I took off the carbs and checked them, the previous owner said he rebuilt them and he did as he said. There were new jets in it, everything was super clean. The main jet size was 140, Pilot Jet was 45 and the air jet was 180. So it looked like the previous owner upped the jets to compensate for the filter pods. We put them on and started the engine, i adjusted the idle so it wouldn't run so rough but i still can't get it to rev up without it bogging out and dying. Any help would be great! thank you!

[jwhughes3]

Sounds too rich, not enough air...maybe those pilot jets are not quite as clean as you think - try checking the tiny holes again.


John

[Radioguylogs]

Plugs might tell something about the fuel mixture.

Spark advance is a centrifugally operated mechanical type inside the LH crank cover. It's easy to make sure it is not rusty/sticky.

[IanDMacDonald]

Well, I believe stock Main Jet is 130, so that means u jumped four jet sizes for just pods? Nuh-uh, not good. Jump back down to stock jets and get a bench mark. It'll probs run lean, but u have adjustable clips that u can adjust to compensate for that lean-ness. Worst-case is one jet size jump. I have K&N pods on mine and only adjusted clip setting, and it runs great.

[xs750special]

Do you know the stock size of the pilot and air jet by any chance? I'm going to take the carbs apart today again and double check everything and clean them again, but the are all new jets that are in there as well.

[motoman]

Do you know the stock size of the pilot and air jet by any chance? I'm going to take the carbs apart today again and double check everything and clean them again, but the are all new jets that are in there as well.

Genuine Mikuni?

[IanDMacDonald]

Yup:
Mainjet: 130
Pilot jet :42.5
Needle jet: Y0
Jet Needle: 5Hx6

[IanDMacDonald]

Btw, you do know we have a forum for the XS750's/850's right? Come-on over to Yamaha-triples.org if u did not know. I was actually a member here before that site.

[xs750special]

okay, will be putting in 130 mains and 42.5 pilots as soon as they come in, and i tried registering on yamaha triples and emailed them but I never got any account information after I filled out the info. Thanks for the help by the way. I will let you guys know how it goes once I put them in.

[IanDMacDonald]

Also, make sure those pods are not blocking the intake bells.

[xs750special]

could the floats not being set right cause the stalling problem as well?

[IanDMacDonald]

absolutely! Only got three carbs to worry about. Sync & u have ur baseline.

[xs750special]

Thanks again! the info you gave me was super helpful! i can't thank you enough, going to set the floats and put the new jets in tomorrow, if i get them in the mail. I ran the bike a little and pulled the spark plugs after and they were dark and wet so i believe I'm running pretty rich.

[xs750special]

so i put stock jets in today and still no luck I'm beyond confused and close to giving up for the season ;( any body else have any ideas?

[IanDMacDonald]

Yup, a few ideas: make sure air gaps on ignition are set correctly, make sure carbs are synced and air/fuel mixture is set, make sure intake bells are not blocked.

Run the carbs w/o the pods, see what happens.
Run the bike at different choke settings.
Condition of plugs?

Report back.