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

JNaw

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Hey guys, new to the forum but have read through many many posts trying to figure this out and finally decided to just ask.

I got a 1979 xs650SE that I've modified lightly. New ignition coils, Uni pods, emgo tapered shorty exhaust, BS38 carbs original. Cafe style seat.

Went on a cruise today and I can't get the bike to go faster than 140km/h (around 6krpm) I am around 5'10 and 190lbs, not sure if that's just cause I'm a big sail or there's something else wrong. New to these bikes and understood it should hit 180?

Jetting for the carbs:
140 mains
32.5 pilots
3turns out adjustment screw
Passes slide drop test

Stock needle height (3rd clip position)
Plugs read a light tan, have yet to do a plug chop but bike does not hesitate or break up anywhere through the rev range, had the needle lowered but was weak in the midrange so brought it back to stock)
Charging at 12.9-13v at 2k rpm. 12.8 at idle)

Stock timing (yes it does go full advance @3k and above)
Unsure of compression numbers but bike has a strong compression feel with kickstart.
 
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You may be within the margin of error on each meter and it's an old bike. If your tach is accurate and you're 140 @6K, you'd be ~175 @ advertised redline of 7500.

Does it have a windshield on it?
 
You may be within the margin of error on each meter and it's an old bike. If your tach is accurate and you're 140 @6K, you'd be ~175 @ advertised redline of 7500.

Does it have a windshield on it?
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.
 
I've owned (3) 650's and on each one, 6K in 5th gear is 160kph. I'm 5'-10" and weigh 170. My '70 XS got there the quickest.
Okay, so I'm most likely just fighting old age with the bike then.

Motor should get a rebuild as Ive got no clue to the condition of the cam chain guide and general engine. She ticks/slaps alot at idle, rocker clearance are in spec so must either be something Internal or something rattling around outside the bike.

Just so many projects on the go lol.
 
2 questions...

Have you had it faster before??........... New problem ???

What is your sprocket ratio??............. Factory is 17 front 34 rear.
 
I grabbed the bike as lean running with missing mufflers and pod filters. So it's as good as I can get it with my prior knowledge of tuning and such.

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

Have you had it faster before??........... New problem ???

What is your sprocket ratio??............. Factory is 17 front 34 rear.
 
You have to keep in mind that these are only about 50HP bikes. Gear it too high, like much below a 32T on the rear, and it most likely won't even pull 6K in 5th, lol.
 
I grabbed the bike as lean running with missing mufflers and pod filters. So it's as good as I can get it with my prior knowledge of tuning and such.

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


Need to do a compression test. Set the tappets and do the timing.
 
Need to do a compression test. Set the tappets and do the timing.
Sorry I kept forgetting to add to the OP I have done that already, rockers in spec. Timing set to stock, carbs syncd and verified it gets into proper timing advance.

I think I'm just expecting too much as 5twins alluded to lol.
 
What have you done, if anything, to check all electrics, especially Grounds/earth fixtures
I've checked charging voltage, which is around 12.8-13.1v at idle. Rewired the fuse box and organized the mess inside the headlight assembly. (Verifying everything is electrically correct)

New start/engine kill switch assembly and New ignition key cylinder.

There isn't much grounds on the bike as is that I can see. Coils are brand new and wiring on the bike seems solid.
 
Sorry I kept forgetting to add to the OP I have done that already, rockers in spec. Timing set to stock, carbs syncd and verified it gets into proper timing advance.

I think I'm just expecting too much as 5twins alluded to lol.
Yeah, if you're looking for 180kpm you may be expecting to much. But I don't think 160 is unrealistic. Probably a combination of factors involved in getting that last 15 or 20kpm. Aside from the ones you mentioned, what tire pressures are you running. I know my best performance upgrade was loosing 40 lbs( 18 kg.) Me,not the bike. I
 
I've had my '78 up over 100 MPH quite a few times over the years, lol. Basically, I just do it occasionally as a "performance check". I never have pushed it to redline and it's absolute top speed, but I'm satisfied that it will break "the ton". On my '83, I can't do this, but I have buried the 85 MPH speedo quite a few times, lol.
 
Do you need faster than 140 ?
100 -- 110 long runs is a young mans game Neck and Arms ..
200 --300 km if up in years can soften the throttle hand

The charging looks low .If it is a New battery those numbers are on the low side.
If that is across the battery it can drop into coils and getting borderline low
Does it start OK ?

Charging at 12.9-13v at 2k rpm. 12.8 at idle)
 
Yes, I was going to mention that. But, full charging doesn't occur until like 3000 - 3200 RPMs, so that's where you should be checking it. Normally you should see low to mid 14s there. If it doesn't go much past 13 at those higher revs then you have charging issues.
 
@ 5twins

Most likely has charging issues. I was going to do a PMA upgrade over the winter as the bikes all original and as an insurance to prevent future problems. Im with you as it's a little low, though I don't know if it'll cause spark issues unless the coils expecting a 13.7+ voltage.

@Jan_P
Nah I don't need more then 140 all the time, just was curious on the performance of my "race bike" as I like to call all projects lol.
Yes the voltage test was done across the battery.

I'll do a compression test and check a few things over. Tire pressures are around 32psi. Last day of warm weather here in Calgary before we see snow so gonna do some riding today before she gets parked.
 
I've had my '78 up over 100 MPH quite a few times over the years, lol. Basically, I just do it occasionally as a "performance check". I never have pushed it to redline and it's absolute top speed, but I'm satisfied that it will break "the ton". On my '83, I can't do this, but I have buried the 85 MPH speedo quite a few times, lol.
I don’t think I’ve had mine going over 140 kph. Not saying it wouldn’t go faster, but with the hp available, it would take too long to get to that speed leaving me a target for too long, for the local police. On more modern bikes, a quick pull to higher speeds and quickly decelerating was much easier. I bought this bike to cruise around and not to be a “go fast” ride.. that said I’m quite happy with its performance.
 
I don’t think I’ve had mine going over 140 kph. Not saying it wouldn’t go faster, but with the hp available, it would take too long to get to that speed leaving me a target for too long, for the local police. On more modern bikes, a quick pull to higher speeds and quickly decelerating was much easier. I bought this bike to cruise around and not to be a “go fast” ride.. that said I’m quite happy with its performance.

Yeah that's what I got this for as well, a weekend or evening rider to get to the gym or wherever in the city or around.

I'm just trying to get it as tip top as I can. My 81 cx500 hits 175 easier then this which is why I'm kinda wondering what's going on.

I wonder if my jetting is on the rich side?
@5twins, if my jetting is slightly off would that be the reason? Stock on this bike was a 135 and thought a 140 should suffice with the extra flow that both the uni pods and emgo exhaust offer. Plugs don't scare me with how they look, they aren't black and it's not acting as if it's running lean.

I am currently dealing with a running lean issue at a 1/3 tank of gas, I have a plastic inline filter (filter element is plastic, sort of the same as the petcock internal filters in the 72 models that my bike has fitted on a 75 dual petcock manual tank)

I think it's due to the internal petcock filter full of crud as when I derusted the tank I couldn't get the crud off the bottom even with the nuts and washers inside. I won't know till I get it apart though.
 
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.
 
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