Throttle stumble just after idle

jpdevol

XS650 Junkie
Top Contributor
XS650.com Supporter
Messages
726
Reaction score
1,463
Points
143
Location
26187
So, use the rotor nut to turn the motor. Usually need something to track the piston through the spark plug hole to verify its actual top of travel. The existing mark should be around 5:00 5:30 as in the pic TDC position. Make a T mark on the case. Now we figure out the size of the rotor circumference, translate that to degrees rotation and make an F mark and advance mark on the case. So we need rotor diameter (preferably in mm)
 

5twins

XS650 Guru
Top Contributor
Messages
23,283
Reaction score
19,357
Points
813
Yes, your aftermarket PMA has no timing marks. They were on the original stator it replaced. When you install a PMA, you're supposed to make new timing marks but many don't, I don't understand why. It's really silly because you need them to set/check timing and valves. That sticker you see in jp's pic is from HughsHandbuilt. He used to sell them but I'm not sure if he does any more.

Your '78-'79 carbs have an o-ring on the mix screws that seal them into their holes. If you did all that carb cleaner spraying with the mix screws still in place, you may have destroyed the o-rings. Then the screws wouldn't seal and suck air. That may be the cause for your big flat spot off-idle.

78-79MixScrew.jpg


Yamaha never sold the o-ring separately but through trial and error, we've figured out the size .....

78-79MixScrew3.JPG
 

jpdevol

XS650 Junkie
Top Contributor
XS650.com Supporter
Messages
726
Reaction score
1,463
Points
143
Location
26187
5Twins - would using the right side of the case's drain groove be close enough for an F mark?
 

jpdevol

XS650 Junkie
Top Contributor
XS650.com Supporter
Messages
726
Reaction score
1,463
Points
143
Location
26187

5twins

XS650 Guru
Top Contributor
Messages
23,283
Reaction score
19,357
Points
813
I see the MikesXS instructions provide no full advance timing mark. You need that if you still use the old mechanical advance unit. As the unit wears, it can start over-advancing and you can hole a piston. That's why you need to be able to check the timing at full advance from time to time.
 

teamWicked

Amatuer
Top Contributor
Messages
2,387
Reaction score
10,227
Points
513
Location
Sheboygan, WI
So will my TDC mark be in the same place for both cylinders on the housing then 180 degrees on the rotor roughly speaking? I’ve never messed with timing in anything before pardon my ignorance lol
Both cylinders will/should fire on that F mark while idling or just a touch before that towards TDC. Probable a good idea to look at Both when you hook up a timing light. Then, throttle up and watch the timing mark advance toward the "Advance" mark. I see in your picture that there is a scribed mark at 10:30. It doesn't hurt to highlight you mark with soapstone or some other high contrast marking.
 

jpdevol

XS650 Junkie
Top Contributor
XS650.com Supporter
Messages
726
Reaction score
1,463
Points
143
Location
26187
If he establishes TDC, we can then use degree wheel to establish F 15* and advance marks I think ~38*

Mike's instruction say 15* is .5" left of TDC, so 1.25" left of T = 37.5*
 
Last edited:

WV Sparky

XS650 Enthusiast
Messages
43
Reaction score
21
Points
8
Location
West Virginia
Yes, your aftermarket PMA has no timing marks. They were on the original stator it replaced. When you install a PMA, you're supposed to make new timing marks but many don't, I don't understand why. It's really silly because you need them to set/check timing and valves. That sticker you see in jp's pic is from HughsHandbuilt. He used to sell them but I'm not sure if he does any more.

Your '78-'79 carbs have an o-ring on the mix screws that seal them into their holes. If you did all that carb cleaner spraying with the mix screws still in place, you may have destroyed the o-rings. Then the screws wouldn't seal and suck air. That may be the cause for your big flat spot off-idle.

View attachment 221073

Yamaha never sold the o-ring separately but through trial and error, we've figured out the size .....

View attachment 221074
I just pulled both fuel screws and luckily the o ring appears to be in good shape, no degradation or deformation. Won’t be making that mistake again, thank you
 

WV Sparky

XS650 Enthusiast
Messages
43
Reaction score
21
Points
8
Location
West Virginia
If he establishes TDC, we can then use degree wheel to establish F 15* and advance marks I think ~38*

Mike's instruction say 15* is .5" left of TDC, so 1.25" left of T = 37.5*
This all will take me a little bit to figure out, I’ll be back once I find TDC. Much appreciated guys!
 

WV Sparky

XS650 Enthusiast
Messages
43
Reaction score
21
Points
8
Location
West Virginia
Left cylinder is currently TDC, so where should I make my mark? There’s a line on the rotor included in the picture but I think I need to remake my own?
 

Attachments

  • image.jpg
    image.jpg
    278 KB · Views: 12

jpdevol

XS650 Junkie
Top Contributor
XS650.com Supporter
Messages
726
Reaction score
1,463
Points
143
Location
26187
Unless photo is reversed, that doesn't look right

I re-read Mike's instructions; the rotor is keyed to the crankshaft, so its position is fixed. They show TDC just to right of drain groove in bottom of sidecase window. If marked as pic, there won't be room for advance mark and barely room for F mark
 
Last edited:

WV Sparky

XS650 Enthusiast
Messages
43
Reaction score
21
Points
8
Location
West Virginia
Unless photo is reversed, that doesn't look right
I was afraid of that. No that’s the back camera, that line is pointing towards the front wheel. Is it possible that it’s that far out? most things I’ve found haven’t been 100% on this bike. The needle clips were at their deepest when I got it with stock jetting all around…could it be possible that line means absolutely nothing? Just a machinery mark?
 

jpdevol

XS650 Junkie
Top Contributor
XS650.com Supporter
Messages
726
Reaction score
1,463
Points
143
Location
26187
It's possible to install without woodruff key. Unfortunately, you'll have to pull rotor to find out. Now you need an impact gun and a rotor puller! Sorry

I'm pretty sure that's the timing line on rotor
 
Last edited:

WV Sparky

XS650 Enthusiast
Messages
43
Reaction score
21
Points
8
Location
West Virginia
It's possible to install without woodruff key. Unfortunately, you'll have to pull rotor to find out. Now you need an impact gun and a rotor puller! Sorry
I’m not sure if it’s pertinent information but when I was turn the bike over with the alternator nut, the nut broke loose with the smallest amount of pressure I’m talking maybe 3 fingers worth of pressure. Surely that supposed to be rather tight? Could the alternator have spun because of this? I’m sorry for all the questions
 

jpdevol

XS650 Junkie
Top Contributor
XS650.com Supporter
Messages
726
Reaction score
1,463
Points
143
Location
26187
No problem. Nut should be tight, no hand wrench should loosen. Could have slipped or never installed right. Check PM
 

WV Sparky

XS650 Enthusiast
Messages
43
Reaction score
21
Points
8
Location
West Virginia
Alright folks MAJOR developments have happened. My timing wasn’t making it to full advance, so I ended up having to spin my red piece (pardon my intelligence or lack there of) to the right. The bike and I just went for our first ride together, while far from perfect the rpm’s spin out perfect mid to high so I think my timing may need a little tweaking yet. Question is, my mounting screws are right up against the red piece. So how do I fix that? Dremel the sides a bit? According to my light I still need to go maybe an 1/8” to the left on my housing. Thanks guys have a damn good weekend
 
Top