1979 XS 650 idles fine , falls flat just off idle, carb is clean.

Prisondawg

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Hello. I swear the carb is clean. Not only did I meticulously clean them, but I bought an ultra sonic cleaner.

I will say the exhaust has a strong fuel odor, so i am thinking the fuel isn't igniting in the cylinder, I think it might be electronic in nature.

Perhaps the coils are worn out and the spark is weak? I have replaced the spark plugs.

Timing seems to be good at idle. I checked with a timing light.

Could this be a valve clearance issue? I have not checked this yet.
 
Check for free operation of the advance mechanism (use your timing light). remove clean polish lube reinstall the advance shaft, it's standard maintenance work on this motor.
search advance shaft for several good how to's Note it's possible to reinstall with the points 180 off which will result in nothing but a random backfire instead of running.
Check that both intake vacuum caps are sealed.
badly out of synch carbs.
Plugged pilot jets, you have to visually confirm the orifice is clear, it's tiny and easily plugged (replugged).
 
Original Airbox?
You let it warm up well?
when you checked timing with light, did you rev it up to look at it advancing?
Its bogging down throttling up off idle when your sitting in neutral or taking off to ride?
How many turns out are the idle mixture screws at?

Sure your idling on both cylinders, did you pull plugs to inspect? Idling for a while during warm up, they can be rich but should not be wet (sign of no spark but has fuel)
 
Make sure you are using the correct type of pilot jet. There were two types used in the 650 carbs and while they will physically interchange, they flow differently .....

650 Pilot Jets.jpg


Pilots.jpg


Your '79 carb set was made to use the VM22/210 type pilot. The way parts get swapped around so much on these bikes, it's best you verify that the carbs have the correct float bowls too. There were two types of those as well, one for VM22/210 pilots and one for the BS30/96 pilots. The difference is in where the air is delivered to the jet. The VM22/210 type bowl adds the air on top, or after the jet. The jet flows straight fuel and is sized in the 20's. The BS30/96 type bowl feeds the air down to the bottom of the jet. The jet flows the resulting fuel/air mix and that's why they are sized so much larger, in the 40's .....

BS38 Bowl Types.jpg
 
Make sure you are using the correct type of pilot jet. There were two types used in the 650 carbs and while they will physically interchange, they flow differently .....

View attachment 263756

View attachment 263757

Your '79 carb set was made to use the VM22/210 type pilot. The way parts get swapped around so much on these bikes, it's best you verify that the carbs have the correct float bowls too. There were two types of those as well, one for VM22/210 pilots and one for the BS30/96 pilots. The difference is in where the air is delivered to the jet. The VM22/210 type bowl adds the air on top, or after the jet. The jet flows straight fuel and is sized in the 20's. The BS30/96 type bowl feeds the air down to the bottom of the jet. The jet flows the resulting fuel/air mix and that's why they are sized so much larger, in the 40's .....

View attachment 263758
It seems to have 27.5 pilots with the open end, non bleed type. Which I believe is the stock jets. I recently purchased some bleed type size 30's. This error has been happening with stock air boxes and stock exhaust. However I just picked up uni pods, and I was planning to try out the 30's with the uni pods, but have yet to make the switch. Could this potentially be caused by the coils or the regulator?
 

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Yes, those are the original pilots with no bleed holes. They weren't needed as the air is added after the jet, but ones with bleed holes will work just fine. My thinking is Yamaha contracted with Mikuni to have a bunch of pilots made without the bleed holes and probably got them cheaper because they were easier to make.

Your mix screws appear to be aftermarket as I see no knurling on the heads? Are the tips stepped like the originals were?


78-79MixScrew.jpg


If they're not, if they're just smoothly tapered to a point, maybe that's messing with the idle mix and causing your issue.
 
I suggest Prisondawg should tell us if the bike did previously run fine with the present setup or was it put together and never ran.
 
Yes, those are the original pilots with no bleed holes. They weren't needed as the air is added after the jet, but ones with bleed holes will work just fine. My thinking is Yamaha contracted with Mikuni to have a bunch of pilots made without the bleed holes and probably got them cheaper because they were easier to make.

Your mix screws appear to be aftermarket as I see no knurling on the heads? Are the tips stepped like the originals were?


View attachment 263810

If they're not, if they're just smoothly tapered to a point, maybe that's messing with the idle mix and causing your issue.
The mix screws are the same. For the sake of trying I put the stock screws back in and got the same results. I will attempt larger pilots later today and report back.
 

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I suggest Prisondawg should tell us if the bike did previously run fine with the present setup or was it put together and never ran.
I bought this bike as a non runner. Someone else's project. The carbs weren't even on the. Bike when I got it. So who knows what was happening to the PO
 
The factory setting spec for the mix screws on your carb set is 2.25 turns out. I've found that set likes and needs at least that, sometimes even a bit more, like 2 1/2 to 2 5/8 turns out. If you set them around 1.5 turns out like many other carbs spec, it won't be enough.
 
The factory setting spec for the mix screws on your carb set is 2.25 turns out. I've found that set likes and needs at least that, sometimes even a bit more, like 2 1/2 to 2 5/8 turns out. If you set them around 1.5 turns out like many other carbs spec, it won't be enough.

I had it set to 2 and 1/4 turns out as mentioned in the carb guide. Today I switched in the size 30 pilot, and it doing the same thing , to compensate for the improve fuel flow with the larger pilot I tried adjusting the fuel screw in a half a turn. Which didn't seem to have an effect. I will say that the plug was super carboned up from previous attempts, so I hit them with the wire brush and put them back in, and it had the same effect. I will say the connection between the stock exhaust must be loose, and it's shooting quite a bit of liquid out which I assume is unburnt fuel. I will also note that my choke lever is tough to operate. I still think it's an electrical issue causing weak spark.
 

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Check for free operation of the advance mechanism (use your timing light). remove clean polish lube reinstall the advance shaft, it's standard maintenance work on this motor.
search advance shaft for several good how to's Note it's possible to reinstall with the points 180 off which will result in nothing but a random backfire instead of running.
Check that both intake vacuum caps are sealed.
badly out of synch carbs.
Plugged pilot jets, you have to visually confirm the orifice is clear, it's tiny and easily plugged (replugged).
I personally think the advancer is the problem. The weights don't seem to snap back into position when pulled apart. I took it apart , cleaned it and greased the rod. Which did seem to help if I rev the engine slow, however the arms still aren't snapping back, I think I need new springs. Will any springs do or do they need to be a certain strength? Where can I find replacements?
 
The points cam affects how quickly the springs retract, so check that the cam is not in a position that is opening a point set while checking the advance springs.
The timing light is your friend it should show smooth and proper timing changes while throttling up and down. The most common symptom of weak springs or a sticking rod is a reluctance to return to idle speed when the throttle is released. (timing remains advanced)
Several here feel the best you can do if the springs ARE weak is take the old springs, clip off the ear on one end and bend up a new one from a coil, slightly shortening the spring. The MikesXS replacement springs are reported to be of the wrong thickness and change the advance curve "too much".
 
The cam has a couple bushings in each end for the advance rod to ride in. The outside bushing should have grooves cut into it's I.D. .....

GreaseGrooves.jpg


They are there to retain grease and provide a reservoir of lube for the advance rod. Clean any old hardened grease out and pack them with fresh stuff. The factory shop manual recommends a moly grease of some sort .....

GreaseGroovesFilled.jpg


The manuals make no mention of it but servicing (cleaning and lubing) the advance rod and it's bushings is a required maintenance task. Do it as shown above and it will last a long, long time, a good 5 years at least.
 
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