Some jetting questions for a 78 650

Chrispy

XS650 Enthusiast
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Good morning gentlemen

this is both my first post here and my first real motorcycle. I have scoured this website over the past couple weeks and it’s plethora of technical information has been invaluable. I have absorbed more information from you guys than I did in four years of college. I have now hit a wall though and feel I must ask some questions directly, some of them will be dumb but sometimes that’s just how it has to be
Backstory on the bike, bought it off a very frustrated man who won it in a bike week lottery. It was not running when he got it but He sent it to a local guy specializing in old bikes who got it working. He had put about 1000 miles on it when a float got stuck and was leaking gas out the air filter. At that point he remembered why he had sworn off carbureted bikes, and sold it to me for cheap. I knew nothing of bikes at the time but I got the carbs out, cleaned them as well as someone who has never seen a carb can, and put them back and it kicked up.
Over the past couple weeks I have trouble shot like a madman, mostly electrical, and learned a lot about the bike, so here are the stats I have for you.
This guy is pretty stock 77 or 78. It’s got points and what seems to be the original alternator. Alternator has fresh brushes now (they were literally 1mm nubs) and the voltmeter I installed shows solid charging in the 14 range over 3000 rpm. The owner before the guy I bought it from had put those Red Mike’s xs filters on that everyone hates, and my guy put the stock air boxes back on. Did either of them re-jet? God only knows. Also at some point a 2 into 1 exhaust system was put in. Did they re-jet? See above exclamation.
Needless to say I’m pretty sure it needs a rejet. The bike feels really flat between 3-5k rmp and it not very responsive and you really gotta open it up for hills to maintain speed. Again I’ve not ridden a lot of old bikes so I’m going on instinct but I’m pretty sure it’s supposed to pull better than that.
So with all that in mind here are the questions I haven’t been able to work out myself.
1. Any recommendations for rejet with stock air boxes and the pictured exhaust? I know I could test a litany of jet and needle combinations and if that’s what’s necessary I’ll do it. I just thought maybe it’s out there already.
2. This one is embarrassing but I have to ask. How in the name of sweet heaven are you guys getting to the mix screw while it’s on the bike. I’ve bench synced them as best I can but my god I’ve tried every combination of taped together tools to reach that thing and I can’t. Please help me.
3. Points and original alternator. Should I keep em or sweep em. How likely are they to strand me somewhere.
4. Is there anything I might have missed that will need attention. I’ve done valve clearances, timing, and solved some shorts but planning a rewire.
5.
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edit; repeated pics removed. ggg
How you guys doin today? I appreciate anyone who read all that. Any help is a huge.... help. Thats my sign off line I guess.
 
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Hi Chrispy, welcome to the forum. Looks like you have scored a decent bike there and nobody around here will argue with your choice of first bike. Appears mostly stock, good condition and all tinwork present and correct.

Fully agree this place is full of useful information and I'm sure somebody will be able to give a pointer to jet sizes and carb settings. Just to give a baseline, while you had the carbs out for cleaning, did you take a note of the jet sizes that were in there?
 
Hi Crispy. Welcome aboard! Nice looking scoot, like the understated paint.
Yes you need to ensure the jetting is correct for your set up. A two into one like that can hole a piston in 1 or 200 miles of highway riding.
First pic ID's 78-79 carbs so you are sure what year of carb set you have.
78-79 BS38 float bowl vents.jpg
Chart is factory jets for various years
CarbSpecs.jpg
Pipe, intake like yours will likely be up 2 sizes on the mains and at least one on the pilot.
you also need to confirm both slides are operating correctly. Check that vacuum ports on intake manifolds are capped.
Float levels correct. What filters are in airboxes?
 
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Thanks for the quick responses guys
(And the edit ggGary, sorry about those duplicates)

Carbs do appear to be from the 78-79 era. I am putting in an order for fresh jets now based on your sizing suggestion ggGary, +1 each direction just to be sure. Do I tell if it’s correct just by feel when she runs or are there hard tests?

vacuum ports are capped
Filters are some yellow foam in the air box. I assumed that’s stock but I’m not sure how to tell otherwise.
Definitely not keen to mess up pistons as I have not gotten to learning top end rebuilds yet so I’ll stay off her until I can rejet. Is mikes alright for jets or are there better places? I know many have mixed feelings about the current state of mikes
 
Yes, as mentioned you will need to determine what size jets are in there now. The jetting recommendations given are good. All I'll add is that you will most likely need to lean the needle setting a step if you install bigger mains. The '78-'79 carb set requires this with pretty much any main jet size increase.

Your mix screws are easy to get at. They're on the outer sides of the carbs. The screw you're having trouble with is most likely the sync screw on the linkage between the carbs. Easiest access is with the tank removed, but then you need to rig up some sort of auxiliary fuel tank to run the bike. I can usually access it from the side with a long screwdriver. It may help to raise the rear of the tank slightly.

But your tank is going to need to come off anyway so you can "fix" your cable routing. First thing I'd do is roll your handlebars up to a more normal upright position. Rolled way down like you have them is only adding to the cable routing problem. Your throttle cable should run straight up from the carbs, not kinked back like you have it. Then run it along the top and left side of the main frame backbone tube .....

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Next, your clutch cable needs to be tucked up under the tank. Tuck it in under the puck holder for the gas tank .....

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And finally, reroute the wire looms for your handlebar controls. There's very little room for them run down under the ignition switch like you have them. Instead, run them under the bars and behind and under the top triple tree to get them to the headlight bucket .....

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2. This one is embarrassing but I have to ask. How in the name of sweet heaven are you guys getting to the mix screw while it’s on the bike. I’ve bench synced them as best I can but my god I’ve tried every combination of taped together tools to reach that thing and I can’t. Please help me.
The "mix" screws are easily accessible, so I'll assume you mean the sync screw in the middle? Others may have a trick, but I've only been able to adjust it with the tank removed (running from a small remote tank).
3. Points and original alternator. Should I keep em or sweep em. How likely are they to strand me somewhere.
The points are 2 individual systems... one for each cylinder. One advantage to that is if one side fails, you can limp home on the other. On the other hand, most people like the "set and forget" advantage to an electronic iggy. Keep the stock alternator. There's fixes and mods to make it very reliable... for a lot less than replacing it. There's a guy on the forum that rewinds the rotor fairly cheap. I hear he does good work. :rolleyes:
Oh... and welcome to the forum. Nice lookin' bike. :D
 
RE the sync screw: I put a small section of fuel hose on it. Makes it easier to keep a long screwdriver captured in the slot.

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My next suggestion is for the main wiring harness, specifically the two large looms that run up into the headlight bucket. They tend to sag down and eventually pull out of the bucket so I like to make up what I call a "harness sling". Yamaha was kind enough to provide a couple small holes in the gusset plates supporting the steering neck that can be used for this. You simply slide a couple cable ties through these and connect them together around the loom on each side. You then snug each side up enough to pull the harness up under the gas tank puck holders .....

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The above are early pics. The sharp sides of the holes eventually cut my cable ties so now I put grommets in the holes, problem solved .....

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Chrispy, great score on the 78 Standard model ! Lucky You are.. :D
Do you like the stockish styling or do you intend to modify that further ?
Tuning choices may depend on your intended riding style.
That 4 into 1 exhaust tells that somebody was seeking more top end performance probably which is in my opinion not the most pleasurable charactistic of these twins. You say you have flat ness from the 3k to 5k range? Okay that is exactly the range you need to tune in for sure. To get there I'd recommend first getting a smooth idle and nice pull from 2k to 3k first. (pilots and sync) that transition up onto mains is a common stumbling point with the modified 78/79 BS38'S which are wonderful carburetors by the way.
One of the first things I'd do would be to check the timing with a timing light. Points ignition is still a great way to go! Don't panic and pull that apart yet!
Once timing and ignition is confirmed, get back to thinking about that jetting etc..
I agree on upping the jet sizes but to guess a guy would have to know your riding intentions and environment even.
Example, 2500' west coast mountain passes in cool air may just require 1 size up on mains, 2 up on pilots and mixture screw fiddling..
Would really like to see more pics as you start into this great XS you have to play with now. Its a great journey you've started on !
 
Hello Crispy! Welcome to the forum, you’ve got a nice bike to work with, with a little love you’ll have ‘er singing in no time. Regarding the carburetor balance screw, I use a long shank skinny screwdriver and come in at an oblique angle, it’s a little fiddly and you may have to work it from both sides of the bike because you’re not going straight into the screw head. But it can be done. By the way, have you bought a manual yet?
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Yes, that's how I adjust mine. Fiddly but you can usually get 'er done.

I'm also going to mention that your rear brake arm looks off. It shouldn't need to be adjusted that tight. Either your shoes are badly worn or more likely, the arm is mounted one spline off on the shaft. Correctly set up, you usually only see about an inch of threads coming out of the adjuster nut .....

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And if you have the back wheel off for this, probably best you service the rear brake. Details are here in post #3 .....

http://www.xs650.com/threads/how-difficult-is-it-to-replace-the-rear-drum-brakes.25632/
 
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