Take Warning15
XS650 Addict
I've been searching for the numbers, and ik now it is here but I cannot find them. What should my compression be on a stock 75 650b? I've got about 90 on the left and 110 on the right.
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Stock compression specs
- Thread starter Take Warning15
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- stock compression
Is this a bike that's been running or a barn find? If those are barn fresh numbers; put an hour or two on it, see how much oil she's using, retest.
If it's warm been used recently; more than that... #1 is even numbers left to right. Most gauges will show about 150 on a good engine. 130 is acceptable 100 is marginal at best. I don't worry about holding the carbs open to test, others will mention that. Squirt a bit of oil in the plug hole, retest, higher readings says the rings/cylinder are bad if they don't change much you have valve issues. Put some compressed air in the plug hole at TDC, listen to where it's coming out, there's your problem.
If it's warm been used recently; more than that... #1 is even numbers left to right. Most gauges will show about 150 on a good engine. 130 is acceptable 100 is marginal at best. I don't worry about holding the carbs open to test, others will mention that. Squirt a bit of oil in the plug hole, retest, higher readings says the rings/cylinder are bad if they don't change much you have valve issues. Put some compressed air in the plug hole at TDC, listen to where it's coming out, there's your problem.
Take Warning15
XS650 Addict
Thanks Gary, no this is a bike that I have been riding but bought recently. It was warm when tested, not hot. I tested it because my left cylinder seems to be firing inconsistently. That is, when I'm riding I can feel a boost of power when it kicks in. I have consistent spark on both sides, but it appears I have low compression. Not much significant change when I poured oil in the cyl.
Compression isn't great but your symptoms could also indicate a plugged idle circuit on one side Or an excessively rich mixture (leaky float valve), poorly synced carbs, or a weak coil, condenser etc. A sparkplug in a cylinder may be fouling even if it sparks good in free air.
Take Warning15
XS650 Addict
How would I know if I have a leaking float valve? Would I be able to see scratches or scoring on the needle? Or, would I just see fuel leaking out of the bowl?
Valves set, cam chain adjusted?
Pull the plugs after it's idling on one cyclinder for a bit what do you see? A throttle shaft or intake air leak is also a possibility.
Pull the plugs after it's idling on one cyclinder for a bit what do you see? A throttle shaft or intake air leak is also a possibility.
Most of the repair manuals list 145 lbs minimum. This spec is probably for a good running bike that gets rode regularly. A bike that has sat for long can read much lower.
The book calls for you to hold the throttle wide open. This can effect the reading by a fair margin.
You say you just got the bike but have been riding it. How many miles have you put on it since you got it? Sometimes the compression readings can come up on a bike that has been sitting as the rings reseaet. I might do the carb checks as gggGary mentioned.
Ride the bike a few hundred miles and recheck the compression. It may come up.
Leo
The book calls for you to hold the throttle wide open. This can effect the reading by a fair margin.
You say you just got the bike but have been riding it. How many miles have you put on it since you got it? Sometimes the compression readings can come up on a bike that has been sitting as the rings reseaet. I might do the carb checks as gggGary mentioned.
Ride the bike a few hundred miles and recheck the compression. It may come up.
Leo
Take Warning15
XS650 Addict
I've probably put 200 miles on it. However, the other day I forgot to turn the fuel petcocks off and when I went out to them fuel was spilling out of the left carb (the one that isnt running). Does this indicate a bad float valve?
The plugs indicate it is running too rich, no matter how I adjust the air screw. Stock pilots, stock airboxes, aftermarket exhaust with mufflers.
The plugs indicate it is running too rich, no matter how I adjust the air screw. Stock pilots, stock airboxes, aftermarket exhaust with mufflers.
"fuel was spilling out of the left carb Does this indicate a bad float valve?"
Sure does, tear down and rebuild is on your hot list now.
Do it right full rebuild (think throttle shaft seals)
Sure does, tear down and rebuild is on your hot list now.
Do it right full rebuild (think throttle shaft seals)
Take Warning15
XS650 Addict
Oh great... I'm terrible with springs. Okay, anyway, just to confirm, that would also solve my running rich problem that I've been chasing, right?
No guarantees, but it will be better with a good float valve.
The leaking fuel can be from just a dirty float valve, not bad just dirty. It can also be a poorly adjusted float, a leaky float. And yes a leaky float valve can run rich. The float valve is a metering device, not a shut off, that's why it has petcocks. If the float is adjusted to low it lets to much fuel into the float bowl. This too high a level lets to much fuel flow into the carb. If set to high the fuel level will be low and this creates a lean condition. The floats being of by a mm either way can have a strong effect on whether it's lean or rich.
Have you read the carb guide, www.amckayltd.com/carbguide.pdf ? It's the carb bible.
Leo
Have you read the carb guide, www.amckayltd.com/carbguide.pdf ? It's the carb bible.
Leo
Take Warning15
XS650 Addict
So, I'm about to start tearing this thing apart while it is winter. However, I think I just need a good kick in the ass about it. Is there anything I should do to be sure that it needs to come apart? Is there any other reasons why I would have poor compression?
Compression leaks are from several places. Leaks past the rings, through the valves, through the headgasket.
I might suggest doing a leakdown test before you tear it down. This test tells you just where leaks are.
This will give you a better idea just where to inspect closer.
But before I tore it down I would do the carbs and put at least 500 miles on it. As I said the compression often comes up after a few hundred miles.
Leo
I might suggest doing a leakdown test before you tear it down. This test tells you just where leaks are.
This will give you a better idea just where to inspect closer.
But before I tore it down I would do the carbs and put at least 500 miles on it. As I said the compression often comes up after a few hundred miles.
Leo
Take Warning15
XS650 Addict
Yeah, I've been riding it, trying to get some miles on it but it keeps fouling the plug in the left cylinder. I can't really tell if it is oil, too rich, or a combination of both.
OK here's a good kick in the ass....................did you feel it? No, there's nothing you can do that will all of a sudden, jump out, and tell you that its re-build time.
The reason you have poor compression is simple......................the engine internals are worn out and need to be machined/re-build. Do you want an engine that runs well. or one that ones like crap?
90 psi on the left and plug turns black....................sounds like the head gasket is passing oil from the camchain tunnel into the cylinder, and likely the same for the right.
Now is the time (its winter) to bite the bullet and do a top end re-build. You will want to read the Yamaha Service Manual (www.biker.net). Only proper measurements of the cylinders and pistons will reveal if you need to buy new pistons, cylinder rebore, etc.
Take Warning15
XS650 Addict
Thank you sir, I felt it! I'm gonna take her for one last spin and then get going on it. Now is the time, after all.
Take Warning15
XS650 Addict
I do have another quick question... What kind of vent is it exactly that is at the top of the engine in the back. The one with the hoses coming off of it.
Clarko
Yamaholic
PCV (ie: positive crankcase vent)
Sounds like another potential 'gas in oil' thread...
