Can't get bike to go past 140km/h, 1979 xs650SE

JNaw

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Your jet sizes are pretty much spot on for your mods. I have similar mods and pretty much the same jetting. A couple minor things though - 3 turns out on the mix screws is quite a lot, especially with you being 2 up on the pilots. Did you set the mix screws for best running, for fastest, smoothest idle speed? Also, I'm only one up on my pilots, but that may be because I did some porting clean-up when I rebuilt it. Before the rebuild, I was 2 up on the pilots too. I had read that better flow can require smaller jets because it pulls more fuel through them. Well, it seems that happened to me with the pilots anyway. The mains were fine and stayed at 140. The other thing I'll mention is that it's rather odd you can run the needles in the stock #3 slot with the bigger mains. On this carb set, usually any increase in main jet size, even one, requires you lean the needles a step to cure upper midrange break-up under heavy throttle applications. Now, normally you probably don't ride the bike like this, but these CV carbs are rather forgiving and will mask over minor jetting glitches unless you push them hard.

So, here's the upper midrange to main transition test, to see if your larger mains are making the upper midrange too rich - in 2nd or 3rd gear, starting at around 3K RPMs, roll the throttle full open and run the bike up to 5 or 5.5K, through the upper midrange and onto the mains. Watch for any stumbling or break-up between, say, 4 and 5K. If you get some, you need to lean the needle a step. But now you said you had it like that but it made the midrange sort of "flat". Well, you may be able to lean the needle a half step using a washer. That could be enough to fix the stumbling but not have such an adverse effect on the rest of the midrange.

Let's talk about the mix screws some more. The factory set point for this carb set is 2.25 turns out but usually they like a bit more, like 2.5 or 2 5/8 turns out. But larger pilots often make them dial in best at less turns out, sometimes as low as 2. Luckily on this set, you can hear the effects your adjusting is having. It does help if you turn the idle speed down a little, to around 1000. You'll be able to hear the changes better. What you're trying to find is what we call the "sweet spot", a range of about 3/8 to 1/2 turn where the engine idles fastest and smoothest. I start with the screw at the factory spec then pick a direction, in or out, and start adjusting it 1/4 turn at a time. Usually within a turn or so from that factory setting, the idle speed will begin to falter and drop off, and the cylinder will begin running rougher. Note the turns at that point and start going in the other direction until the same thing happens. This should allow you to find the "sweet spot" range. I like to set the screw at the rich end of the range (screw more open), as rich as I can get it before the speed drops off and rougher running sets in. I think this helps alleviate popping on decel.
That's so we'll explained, thankyou.

Yes I haven't retuned the pilots since I've resynced the carbs, I'll check that but that's where it liked it beforehand. I definitely thought it was odd too, especially when I was around 2 turns out on a size smaller pilot jet (which is even weirder cause I thought a smaller would require more turns out) so maybe I should go back. Wonder if the jets are stamped wrong, will have to measure.

And I'll try doing that test, though I don't recall feeling any stumbling consistently around 3.5-5k rpm, it might do it occasionally so I'll try a washer and see how it feels. Preferably a thin as possible one yeah?
It does seem to flatten out above 5.5k, though looking at dyno charts it seems the motor flatlines around those rpms anyways.
 

bosco659

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No windshield, and possibly though cruising with the guys I'm around 5-10km/h within their speed and my speedo.

Could be just a bad calibration but it's not at redline. Definitely around the 6k mark.

I did just resync the carbs as the right side was out by around a turn and half. Bike idles smoother and power is a bit smoother. Wonder if that couldve been the culprit.
Curious, what’s the bike revving at 100 kph in 5th gear?
 

5twins

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You probably won't get the stumbling unless you work the carbs hard. Like I said, you have to push them to uncover the "glitches". These CV carbs work on engine demand. Just because you give it full throttle, that doesn't mean the slide will be lifted fully. It only gives what the motor can use. That's why it's easy to over-jet CV carbs. They'll still run halfway decent even if the jets are too big because those jets aren't flowing fully. You may have read jetting articles about using the throttle position to determine which jet or carb circuit needs tuning or changes. That really doesn't apply too much to these CV carbs. You narrow down what jets or circuits need changing more by what RPMs the issues occur at rather than how far open the throttle is.
 

5twins

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Forgot to ask - have you checked the gearing? Stock in the U.S. was 17-34. In Europe, they went 17-33. Some of the Specials in Canada though got 17-36. If that's what you have, it might explain why it won't go faster. This bike does benefit from a gearing change, but only a slight one. Remember, you're only dealing with about 50HP here. The 17-33 gearing is ideal if you have an 18" rear wheel. On a 16" rear wheel, a 32 works very well. Go much smaller than those and the take-off power in the lower gears really takes a hit, besides maybe not being able to pull redline in 5th.
 

MaxPete

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New to these bikes and understood it should hit 180?

Hi JNaw:

Welcome, from Canada's sunny southern coast near Harrow, ON!

I'd say that 180 km/hr is simply not achievable with a stock XS650 - of any year. When these bikes were new, all of the magazine tests said that they would hit just over "the ton" - which is 100 mph or about 162 km/hr. The usual top speed listed was between 100 and 105 mph or just about 162-169 km/hr.

I have had a very well-sorted XS650 doing somewhere around 160 km/hr (the speedo - and me - were vibrating so much that I can't be certain) - and she really didn't have a lot left. Based on that, I'd guess that if you tucked right down onto the tank, you might get 160-165 indicated - depending on the gearing, but everything would need to be dialed-in nicely for that and in any event, how fast you are really going would be a matter for debate.

The reason for the debate is that motorcycle speedos are notoriously inaccurate - especially 40-50 year old units that likely have dust and dried instrument grease inside.

But, if I may be so bold, why would one ride an XS650 in that way? They're so much nicer doodling along at 50-60 mph on a back country road than hammering along on a 110 km/hr expressway trying to keep up with some distracted soccer mom in a Grand Carabarge or some cell-phone-yakking-real-estate-agent-wanker in a BMW 3-series which dingly-balls hanging off the mirror. If you want a really fast bike - you'd be best to look elsewhere, I'm afraid.

Anyhow - I'd expect more than 140 km/hr - but not a heck of a lot more.

Pete
 
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46th Georgia

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Hi JNaw:

Welcome, from Canada's sunny southern coast near Harrow, ON!

I'd say that 180 km/hr is simply not achievable with a stock XS650 - of any year. When these bikes were new, all of the magazine tests said that they would hit just over "the ton" - which is 100 mph or about 162 km/hr. The usual top speed listed was between 100 and 105 mph or just about 162-169 km/hr.

I have had a very well-sorted XS650 doing somewhere around 160 km/hr (the speedo - and me - were vibrating so much that I can't be certain) - and she really didn't have a lot left. Based on that, I'd guess that if you tucked right down onto the tank, you might get 160-165 indicated - depending on the gearing, but everything would need to be dialed-in nicely for that and in any event, how fast you are really going could be a matter for debate.

Besides, motorcycle speedos are notoriously inaccurate - especially 40-50 year old units that likely have dust and dried instrument grease inside.

But, if I may be so bold, why would one ride an XS650 in that way? They're so much nicer doodling along at 50-60 mph on a back country road than hammering along on a 110 km/hr expressway trying to keep up with some soccer mom in a Grand Carabarge or some cell-phone-yakking-real-estate-agent wanker in a BMW 3-series which dingly-balls hanging off the mirror. If you want a really fast bike - you'd be best to look elsewhere, I'm afraid.

Anyhow - I'd expect more than 140 km/hr - but not a heck of a lot more
Grizld1's will run in excess of 115mph, however that's with a 700BBK cam and extensive cylinder head work and many hours of tuning combined with decades of experience and an immense amount of knowledge. I believe if it's crazy speed you're after, buy a modern bike.
 

bosco659

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Hi JNaw:

Welcome, from Canada's sunny southern coast near Harrow, ON!

I'd say that 180 km/hr is simply not achievable with a stock XS650 - of any year. When these bikes were new, all of the magazine tests said that they would hit just over "the ton" - which is 100 mph or about 162 km/hr. The usual top speed listed was between 100 and 105 mph or just about 162-169 km/hr.

I have had a very well-sorted XS650 doing somewhere around 160 km/hr (the speedo - and me - were vibrating so much that I can't be certain) - and she really didn't have a lot left. Based on that, I'd guess that if you tucked right down onto the tank, you might get 160-165 indicated - depending on the gearing, but everything would need to be dialed-in nicely for that and in any event, how fast you are really going would be a matter for debate.

The reason for the debate is that motorcycle speedos are notoriously inaccurate - especially 40-50 year old units that likely have dust and dried instrument grease inside.

But, if I may be so bold, why would one ride an XS650 in that way? They're so much nicer doodling along at 50-60 mph on a back country road than hammering along on a 110 km/hr expressway trying to keep up with some distracted soccer mom in a Grand Carabarge or some cell-phone-yakking-real-estate-agent-wanker in a BMW 3-series which dingly-balls hanging off the mirror. If you want a really fast bike - you'd be best to look elsewhere, I'm afraid.

Anyhow - I'd expect more than 140 km/hr - but not a heck of a lot more.

Pete
Agreed @MaxPete, that’s what I have my XS for - putting around secondary roads. Also fully concur wth @46th Georgia - wanna ho fast - get a modern bike (unless you want to do a special build line many in this group).
 

5twins

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For shimming the needle, a plain old M3 washer works fine .....

CarbShimWashers.jpg


On a Mikuni needle, the clip positions change the height by 1mm. These washers are roughly a half MM thick so adding one allows you to achieve half step positions, and sometimes that's enough. A whole bag of them, in stainless no less, cost me a whopping 73 cents, lol .....

CarbShimWashers2.jpg
 
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JNaw

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Sorry for the late replies guys, work was busy.

@5twins, you'd be right, I was reading basic jetting guides and with my current basic knowledge of how a engine should run. We just got snow so I won't be able to do any testing for a while now lol. I'll definitely try a washer as you've stated, got a bunch laying around in the shop will just have to pull the calipers out and measure some.

It is a Canadian 1979 special so it could be possible it has short gearing. I'll get a count soon as it's parked at my shop now. Definitely has a shorter gearing feel with the first few gears being Hella torquey, but my only basis of evaluation is with my cx500 lol.

@bosco659 the bike runs around 4500 @ 100km/h if my memory serves me right

@MaxPete I don't mean to be pushing the bike always at 140, just my main friend group all ride 600cc+ sport bikes from the modern Era and keep leaving me in the dust, maybe I need new friends haha. It's also a performance measurement for me, I don't intend to be a street killer haha, I try to be a speedlimit kind of guy.

I could get it to maybe hit 150ish on the speedo if I'm tucked and on the ever so slightish downhill. Could be possible every time I try and push it I'm on an incline too, bikes vibrating so much I'm not focusing If I'm on an incline or not though haha, more so just making sure I stay Alive.


I don't like the look of modern bikes, in my eyes a perfect bike would be one of these styled bikes with the performance of a newer one, though my buddies wife's Ducati scrambler 800 is definitely a joy to ride, just out of my budget lmao. (R6 swap maybe haha?)

Sort of debating doing a rebuild with a 750 big bore, though it's just an idea at the moment, engine definitely should get done as I have no idea the condition of it and it's got 45k km on the odometer with a non matching Vin motor, (same year Vin though)
 
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bosco659

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If you want to keep up with your buddies, buy a cheap, more modern bike. Suzuki Bandits are cheap, plentiful and fast.
 
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650Skull

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What is your sprocket ratio??............. Factory is 17 front 34 rear.

I'll count the teeth but afaik it's stock.

today
Sorry for the late replies guys, work was busy.

@5twins,
It is a Canadian 1979 special so it could be possible it has short gearing. I'll get a count soon as it's parked at my shop now. Definitely has a shorter gearing feel with the first few gears being Hella torquey, but my only basis of evaluation is with my cx500 lol.

More than one of my questions have been ignored or dismissed.

The fixation on carbs when carb type issues can often be electrical and vice versa, it is best to make sure your electrics are top notch and sorted. (that means a full clean up of all electrical connections, bullet, couplings and grounds, removed cleaned and reassembled), including switches, (bar), ignition switch and brake switches. Having to repair the fuse box and the age of the bike indicates a full loom refurbish is needed.

This should be done before any carb work.

I have a couple of other queries but i am reluctant to mention them till some of the issues i raised have been addressed
 

JNaw

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today


More than one of my questions have been ignored or dismissed.

The fixation on carbs when carb type issues can often be electrical and vice versa, it is best to make sure your electrics are top notch and sorted. (that means a full clean up of all electrical connections, bullet, couplings and grounds, removed cleaned and reassembled), including switches, (bar), ignition switch and brake switches. Having to repair the fuse box and the age of the bike indicates a full loom refurbish is needed.

This should be done before any carb work.

I have a couple of other queries but i am reluctant to mention them till some of the issues i raised have been addressed
I've gone through all the bullet connections and wiring while I had the bike apart doing repairs. Connections to the battery are solid and I've gone through all the connectors in the headlight assembly, as well as on the frame and cleaned and/or repaired.

Fuse box was upgraded to a blade style fuse due to the stock vibrating out the glass style fuses. I've gone through and checked continuety to everything and made sure the wiring was good.

I know alot of the main power runs through the ignition switch and or engine kill switch, which has been outright replaced with brand new, and handlebar ground is verified working with little resistance.

New coils and wiring have been installed as well along with new plugs.

I did not find any issues other than the low charging. Gearing I haven't been able to confirm as I've been working 12 hour days, I apologise, I am not trying to dismiss your help. Everything counts.

As I said earlier. I'm planning on a Permanent magnet alternator swap which will get a new battery as well.

I assume if there was a weak spark, at those rpms it would end up being a blow out of the spark itself, no?

I've got a list to look over now thanks to everyone's help.
 

MaxPete

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If you want to keep up with your buddies, buy a cheap, more modern bike. Suzuki Bandits are cheap, plentiful and fast.

...or an SV650 - even cheaper and more plentiful (in Canada at least) and even simpler. With 75 HP on-tap and a top speed of 125 mph, the SV should have plenty of snot to land you in the back seat of one of the local Mountie-mobiles. I had a brand new SV650 - and it was superb - I just found that I like my XS650s more, so I sold it on to a buddy who needed a new ride.

The only thing about SV650s is that many of them have been track-bikes and while the SV is a very hardy bike, the former track bikes are usually pretty clapped out. If you do look at an SV - check for any fasteners that have been safety wired - that is a very reliable sign of a track bike.

I concur with the other guys though - either get a modern bike or a new set of riding buddies - because an XS650 is never going to able to keep up with modern sport-bikes - unless you spend a whackload of money on it.

Pete
 
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46th Georgia

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...or an SV650 - even cheaper and more plentiful (in Canada at least) and even simpler. With 75 HP on-tap and a top speed of 125 mph, the SV should have plenty of snot to land you in the back seat of one of the local Mountie-mobiles. I had a brand new SV650 - and it was superb - I just found that I like my XS650s more, so I sold it on to a buddy who needed a new ride.

The only thing about SV650s is that many of them have been track-bikes and while the SV is a very hardy bike, the former track bikes are usually pretty clapped out. If you do look at an SV - check for any fasteners that have been safety wired - that is a very reliable sign of a track bike.

I concur with the other guys though - either get a modern bike or a new set of riding buddies - because an XS650 is never going to able to keep up with modern sport-bikes - unless you spend a whackload of money on it.

Pete
So what your sayin' is an SV650 is kinda like buying a Subaru WRX?
 

JNaw

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Small update, got around to checking the gearing, 17 front 34 rear. Rear tire size is MT90-16 M/C 74H Shinko E-240.

Compression test @ 0°c (seems like a worn engine to me)
80psi left
90psi right

I've attached a photo of the spark plugs as well incase anyone is curious.

Seems to me it's just a tired motor looking at the compression results. I'll have to try a hot compression test when it warms back up again.
 

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5twins

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Your plugs seem to be burning quite nicely. If the motor was really worn, I think they'd be blacker and have oil on them. Yes, do a compression test when the motor is hot, also hold the throttle wide open while testing, you'll get better (higher) numbers.
 

JNaw

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Your plugs seem to be burning quite nicely. If the motor was really worn, I think they'd be blacker and have oil on them. Yes, do a compression test when the motor is hot, also hold the throttle wide open while testing, you'll get better (higher) numbers.
Yeah I forgot to hold the throttle open, was thinking about that on the way home lol. Probably expect around 100psi cold with throttle open.

Maybe I'll try a new battery before doing the alternator swap and see what that does. Is it common for the whole charging system to just die or is it just maintenance related like replacing brushes/diodes? Read alot that it's common for the charging system to die but it seems to be a blanket term.
 

5twins

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No, it's not common for the whole charging system to die. It's usually just one of the components. Top of the list are the brushes (wear item) and the rotor going bad. Then sometimes the regulator will crap out, but it's pretty reliable. But, you can replace it with a low cost automotive regulator that is very reliable. I quite like the original charging system. It's a 3 phase automotive type system, well proven and quite reliable. Yes, parts are failing on them now, but they're 40+ years old. I would never switch to a PMA (Pain-inMy-Ass or Piss-Money-Away, take your pick, lol). They cost hundreds of dollars and fail more often than the old original system it seems.
 
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