XS650 trouble

[pablohoney]

Hi there,
Here's what is going on, my 1979 xs650 starts right up and idles fine, but it does two bad things,
1: If I give it any choke at anytime it dies instantly, if I keep it revved with choke on, the plugs foul like mad.
2: If I let it just idle for a minute or two, then give it a little gas it dies. If I rev it periodically it is fine. It has stock exhaust and air filters, and pamco electronic ignition. Any ideas? Thanks

 

[Lunatic]

Hi there,
Here's what is going on, my 1979 xs650 starts right up and idles fine, but it does two bad things,
1: If I give it any choke at anytime it dies instantly, if I keep it revved with choke on, the plugs foul like mad.
2: If I let it just idle for a minute or two, then give it a little gas it dies. If I rev it periodically it is fine. It has stock exhaust and air filters, and pamco electronic ignition. Any ideas? Thanks

When was the last time is ran properly and what has changed?

Plugs will foul with the choke on when the bike is running properly, but it sounds like the bike is running really rich. Stock carbs or have they been rejetted?

 

[pablohoney]

It hasn't run for over a few minutes in a year and half. I just recently finished rebuilding the engine and everything. The compression was low. Stock carbs and jets.

Here is the kicker, when it had air leaks before, it did the same thing with the choke and it was really lean then.

 

[pablohoney]

I checked the plugs and they were rich, very rich, it also has a slight pinging, when I let off the throttle, none at idle. I am sure that the ignition isn't advanced.

 

[Lunatic]

You might want to post up something at xs650.com, you'll get specific xs650 advice and probably quicker responses.

Having said that, if your jetting is stock, have you tried playing with the mixture screws?

I would double check the pamco, make sure the timing is correct, when I did mine we found that we were way out and the bike idled fine but ran like shit.

I'm also assuming that the carbs are nice and clean?

 

[pablohoney]

I did that, thanks.
I did play with the mixture screws, didn't help.

I triple checked the timing, and it runs pretty good, makes a lot of noise though, that pinging is pretty damn bad when I let off the gas and let the engine brake while at speed though.

I cleaned the ever-loving crap out of the carbs.

 

[pablohoney]

I took the bike up to the top of my hill and rolled it down with it in gear,and the ignition off and I could still hear the pinging. Excessive valve lash?

 

[diesel450]

I was gonna say, I'd re-adjust the valves. I'll bet there not right.

 

[pablohoney]

I adjusted the valves almost all of them were too tight. Lots less pinging now, still a little bit every now and again. Is a little bit normal?

 

[teazer]

Pinging is the term used to describe a combustion related event. Your issues sound more like piston slap, valves hitting pistons or other mechanical mayhem like a loose cam chain (most likely). Those motors are not silent, but they should not be making unusual noises. I'd try to find someone with an XS and listen to it to have a basis for comparison.

Rich at idle is either leaky floats, wrong jets or a leaky choke/enrichment device. Strip and clean the carbs.

 

[djelliott]

Sounds similar to what's going on with my 79. In my case it turned out that the vacuum activated petcocks were shit. I'm ordering some of the mechanical ones from Mikes XS to replace them.

 

[pablohoney]

If the valves were hitting the piston wouldn't it always be making sound rather than just every few minutes, the cam chain is tight. I got the rich mixture taken care of, it was the float level.

I have mechanical petcocks, too.

Thanks

 

[pablohoney]

I did a compression test and the left side was about 80psi and the right was about 120psi. They both have new rings. If a valve was hitting the piston, would the noise get worse when the engine was warm, cause there is no noise when it is cold, but warm it is definitely there.

 

[diesel450]

Could have ring travel. where the rings either weren't set right or moved before they had a chance to seat. So the openings align.

 

[pablohoney]

would that make noise?

 

[diesel450]

I would just try to work systematically. You know you have bad compression on one cylinder so I'd fix that problem first. (maybe both cylinders, What is good cold compression for this motor?) Try to make everything you can control correct and go from there. You adjusted the valves and things got better so keep going!!

 

[crazypj]

re-check the valves and make sure you didn't set them on overlap.
Did you fit new tensioner and guide when you had motor 'down'?
That seems awful low compression, maybe broke a ring and it's rattling around?
BTW, my 77 has original vacuum taps, no problems with them at all ;D

 

[pablohoney]

re-check the valves and make sure you didn't set them on overlap.

What do you mean? Overlap?


New guide, yes.
new tensioner, no.
Tensioner seems to be in good condition though.

It does seem to be low compression, but why would a ring make more noise when the bike is warm, and only right after shutting off the throttle.

 

[teazer]

When you roll out of the throttle, the motor is still working hard and creates significant vacuum in the intake, so it could be something on that side. Or it could be that when the throttle is shut, there's less mixture to burn and less pressure on the top of the piston, so the rod stretches slightly.

If you stick it in gear with the front wheel against say a wall, and start to let the clutch out - carefully, does the noise get worse? If so, it's a big end with too much play in it.

If it's only there when you shut off, then it may be piston slap. That 80 psi reading is a good indication that something is seriously wrong and that the motor should come out to be fixed.

 

[crazypj]

What do you mean? Overlap?

The fact you have to ask means you probably did it wrong
Piston hits TDC twice
once with valves closed (compression)
once with valves slightly open (overlap)
I've seen hundreds of bikes 'done exactly by the book' where engine was at TDC but on wrong stroke.
When setting valves, watch intake valve open then close.
Then, as piston is coming up, look for TDC mark.
That way you know your on the correct stroke
Make sure you have spark plugs out