Need troubleshooting help - xs400 dies after warm-up

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Carter Thomas

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'82 xs400 Seca

Hoping someone can suggest something I've missed. Love this bike and it really runs great, when it runs. Not a great pic of my bike but all I could find today

Starts up on first bump of starter and runs strong...for a little while
Stop for a light or make a low speed turn, it dies and won't start up again (usually happens on deceleration and usually backfires)
Let it sit 30 mins - hour and it's all systems go again...for a little while
Repeat above steps


Replaced:
battery (new)
plugs (new)
pick-up coils (new)
plug wires (new)
ignition coils (used)
CDI (used)
Also bypassed kill switch on bar
 

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First of all, that is a good looking bike! We need more pics of that.

When you say for a while, does that mean it runs good just until it's warm, or it runs for a half hour, an hour?
And when it won't start are you still getting spark?

Sorry for the additional questions, I don't have a lot of motorcycle experience, so the extra info helps. I'm sure one of the more experienced guys on here will be able to point you in the right direction.
 
The key word is backfire. Yank plugs and if black they are fouled and something running rich. It fouls the plugs to not start back up without waiting.

If so then run a compression test, an old engine will do that.
 
Will run ok for anywhere from 10 minutes up to maybe 20. Don't think has never run more than 30. Doesn't always backfire but it does happen a lot. Sometimes will just shut off at idle. Mechanic has checked compression recently. I don't remember what readings were but he said it was ok. Bike has spent a lot of time at mechanic with extensive carb work. Pulled the plugs don't look too bad
 

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When the plugs foul from being black or wet the engine has to sit for a while for the excess fuel to dry up off plugs before it will start up again, it's the typical physics of a flooded engine. Too much gas not being burned.
 
amc49 said:
When the plugs foul from being black or wet the engine has to sit for a while for the excess fuel to dry up off plugs before it will start up again, it's the typical physics of a flooded engine. Too much gas not being burned.
Click to expand...

thanks for the info, that's what I thought you were saying but wasn't sure
I've never looked at the plugs right after it happens I'll pull them next time and see if they're wet
 
Rode the bike again yesterday until it stalled out. Pulled the plugs immediately and both were clean and dry.

What else could be going on here? Bike runs so good for short time and that's it
 
EXACT compression numbers, Don't tell me the mech said it was good as it may mean nothing. Valves set lately? Miileage on motor?

IF the engine is solid then a fuel or electrical issue. Consider where you get fuel as you have ethanol in it there. Water in fuel can easily do that.

Check grounds and not just at battery. Frame too.
 
I wonder if it is something heat related. I was having issues with my Motorhome generator, it would run fine for 20 minutes or so and then it would just stop charging, it would continue to run, but no power to the house. After much trouble shooting and checking of regulators and such, I won't go into all the details, it turns out that the generator as it warmed up would develop an internal short and kick itself off. Could there be something in your ignition system that would be causing it? Faulty coils maybe?
Just spitballing...
 
If your running pods and a non-stock exhaust the bike may be too lean. I would also check float height and you pilot mix screws settings.
 
Have an extra new plug in pocket and quickly unplug a plug in head and plug the new plug in and grounded to head and check for spark with key on. Immediately when it dies of course.

Same idea with fuel line to carb, have it easily removeable and quickly yank it at the the die to see if fuel runs out. Need to check carb bowls for fuel too.

I've seen excess fuel line run too horizontal for a bit to not fill the bowls up and worse with low fuel in tank, fill it up. The bowls slowly get fuel and the engine use passes that up to run out of gas. Does that bike have the vacuum fuel demand valve between petcock and carbs? That will definitely do it if bad. Chokes leaking can do it too.

Describe the die as accurately as possible..............like turning key off? You mention backfire. Does one cylinder run slightly longer than the other before dying? Big clue.
 
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In response to the PM, you never came back with compression numbers (per post #11) and the warmer the engine the better.

Need exact compression numbers to get a handle on how close to leaking.

That appears to be the one with direct acting tappets with shims, When the valves wear they can recede to then fall deeper into the valve pocket and then with any heating the valve stem grows long enough to then take out your clearance if any and then the cylinder loses compression after getting good and warm to die. Nobody ever sets the valves because of the shim needs. The last couple thou of clearance can be non-existent as you show say .002" which would run except that it may not be real, it turns into zero with engine running because the clearance at cam caps is close to the valve clearance itself and they can neutral out to nothing (no clearance) with running. Yet you show clearance at checking with engine stopped because the cams then get pushed around in the caps because of valve spring load to give you fake readings.
 
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Went to the manual and I would set valves at .006" intake and .008" exhaust, at the extremes of looseness to make sure the problem is not that.
 
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