72 xs2 bogs down and backfires out of carbs, out of ideas

RaceCar1983

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First post ever so here it goes, my 72 xs650 chopper with a freshly rebuilt top end and all new electronics struggles to idle when started and backfires out of both carbs, if it does manage to keep started long enough to rev, it bogs down when given throttle. I did a compression test and read around 110 which is low, but I need to confirm with a gauge I trust. So far I have redone timing/valves, replaced both points, and replaced advance springs that were barely snapping back with no change.

I also should mention I made sure the carbs are PRISTINE, checking all the jets and clearing them. Different jet sizes and mixture adjust make no difference so I can only assume my problem lies with my advance/timing/valves. Any ideas or order of rechecking my work I should follow? Running stock points with xs charge PMA if that helps. Thanks for any ideas.

Edit: bike ran fine 200 miles after rebuild until this happed FYI so the rebuild is unlikely to be the problem


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Those symptoms sound like your mixture is too lean.
What air filters/pods are you using?
- and are those exhausts straight through (no baffles)?

Check also for carb manifold leaks as Its been a long time suggested.
Just checked and no manifold leaks. I am using a shorty k&n type filter, witch despite bad things I have heard they have never had any problems (bike ran good before paint and rebuild) and have tested without them and with filtered velocity stack. I am using these carbs https://www.mikesxs.net/xs-performance-carb-xs650-unassembled.html if that helps. The pipes are not straight through they have baffles at the end (also had no problems with pipes before rebuild). I also tried to richen the mixture with jets but the symptoms don’t change.

Long story short everything I have on the bike ran fine until now, put about 200 miles on it before the paint and rebuild with no issues. Then it ran fine after the rebuild for about 200 miles until this problem formed.
 
Stock alternator or PMA?
110lbs is pretty low for a fresh top end. Did you hold the butterflies open? Kick or electric?
You said new points.... new condensers also? Although to be fair, ignitions are independent of each other. Both sides would have to have the same problem for the "backfires out both carbs."
Did you by any chance get some pics during the top? Curious how certain you are the cam's timed correctly. Off a tooth or two would explain the low compression...
 
Stock alternator or PMA?
110lbs is pretty low for a fresh top end. Did you hold the butterflies open? Kick or electric?
You said new points.... new condensers also? Although to be fair, ignitions are independent of each other. Both sides would have to have the same problem for the "backfires out both carbs."
Did you by any chance get some pics during the top? Curious how certain you are the cam's timed correctly. Off a tooth or two would explain the low compression...
PMA, and I was holding the throttle open while kicking, I don’t trust that old gauge though so I will borrow one and confirm this weekend. Same old condenser on it. I didn’t get any pictures but I did make sure the tooth was aligned right.

Also if the cam timing was off I would have had problems right after the rebuild correct? but it was fine for about 200 miles till now.
 
First thougt weak spark
Bad ground or not enough Voltage on upside --- Battery there ? Old ?
Measure and / or install Voltmeter
Ground is good and clean (tried a different ground location during testing as well), tried two brand new batteries that measured good no difference in symptoms.
 
hello
Ground is good and clean (tried a different ground location during testing as well), tried two brand new batteries that measured good no difference in symptoms.

OK Sir what I would do is to check if the charging part inclusive regulator can be disconnected and run on the battery only
as experiment On a stock system that would be possible. Please do the read up so nothing fries
if that changes anything.
 
hello


OK Sir what I would do is to check if the charging part inclusive regulator can be disconnected and run on the battery only
as experiment On a stock system that would be possible. Please do the read up so nothing fries
if that changes anything.
My system as far as I know can’t run without the regulator rectifier (if that is what you mean). I did try a replacement one with no change.
 
My system as far as I know can’t run without the regulator rectifier (if that is what you mean). I did try a replacement one with no change.

On a stock bike at least on my 80 there are more or less separate circuits
Charging / Ignition etc

I Would measure actual Charging Voltage across Battery when the bike runs bad
and at points when not running

Never in 40 years had Carburetor problems on both sides at the same time and not many on one side
Have had bad compression bike ran fine
 
New points? Chinese? If they are -

I had new Chinese points and after a short time, they sometimes failed to close. Sometimes they did, sometimes they didn't. I only discovered this by checking them with a multimeter and turning the engine. Sometimes the points opened but didn't fully close. Sometimes they did. In other words, they were new, looked perfect but we're in fact junk. In the end I converted to electronic ignition. You can't trust Chinese parts just because they are new.
 
New points? Chinese? If they are -

I had new Chinese points and after a short time, they sometimes failed to close. Sometimes they did, sometimes they didn't. I only discovered this by checking them with a multimeter and turning the engine. Sometimes the points opened but didn't fully close. Sometimes they did. In other words, they were new, looked perfect but we're in fact junk. In the end I converted to electronic ignition. You can't trust Chinese parts just because they are new.
Yes the new points are Chinese ones from Ebay. I will give them a test. Is there a write up of the readings I should look for and the procedure?
 
Yes the new points are Chinese ones from Ebay. I will give them a test. Is there a write up of the readings I should look for and the procedure?
No not to my knowledge. I discovered that the points sometimes failed to fully close by putting a multimeter across the the points, ignition off. Then turning the motor and watch the multimeter as the points opened then closed. Obviously, when the points are open it's open circuit on the meter. When they're closed the resistance is minimal. A test light bulb would tell you just the same.

I have also had dead on arrival Chinese condensers too. Worth a check for sparking across the points when the engine is running. Good condensers should mean no sparking on the points.
 
No not to my knowledge. I discovered that the points sometimes failed to fully close by putting a multimeter across the the points, ignition off. Then turning the motor and watch the multimeter as the points opened then closed. Obviously, when the points are open it's open circuit on the meter. When they're closed the resistance is minimal. A test light bulb would tell you just the same.

I have also had dead on arrival Chinese condensers too. Worth a check for sparking across the points when the engine is running. Good condensers should mean no sparking on the points.
I just checked the multimeter measurements on the points and checked for spark going across while running. No luck.

I couldn’t keep it running long enough to measure the battery voltage while running, but I will give it a go tomorrow when it’s warm here.
 
What Voltage on Upside to Points engine off.
And to Coils if possible measure engine off.
Could you elaborate what you mean please, what is “upside”? And do you mean to measure points to coils or just between the 2 coil wires themselves, and key off or on?

Sorry if I am being dense.
 
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