Author Topic: 1975 xs500b runs great for 20min then dies everytime???  (Read 87 times)

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Offline Cobra

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1975 xs500b runs great for 20min then dies everytime???
« on: August 12, 2018, 09:29:04 PM »
Ok, I am at a loss I got this bike 4months ago and have been working on it ever since.  Bike has 5k miles on it and was parked in 1984 with half full tank of gas.  Needless to say I rebuilt the entire fuel system.  Caswell epoxied the tank ultra soniced the carb 4 times, all new o- rings, new plugs in the pilot jet tube (or what ever that little plug is).  I have since disassembled and reassembled them 3-4 times checking every hole and passage and removed all jets.... THE CARBS ARE CLEAN.  The bike had both left and right exhaust valves stuck open but was able to remedy that with a length of rope down the spark plug hole and lots of gentle persuasion from the kick start.  Set the valve clearances. Rebuilt the front brake system.  Changed the oil (didn't mess with the oil filter yet). Got her running everything dialed in.  Idles great runs great rides great for 20min then dies everytime.  Even with gas cap open.  Whether sitting at idle or riding around as soon as it returns to idle (stop light or downhill).  Seems to happen once it gets hot.  No amount of idle screw adjustment will overcome it.  I can turn up the idle for a while but everytime it slowly drops to 1000 rpm then dies.  Wont start back up until its cooled down.   Some vapor is pulsing out of the vent tube to the valve cover at this temp (but I had used a few squirts of evaporust on the valves so this could be the water vaping off at temp and i did change the oil).  When hooking up a spark plug tester light up to the coils they both work at this high temp once engine has stalled. Stater is making 17-18VAC no load from the three white wires and about 13VAC with load.  Battery looks to be charging at 13.5VDC at idle and a max of 14.75 with throttle.  (Thought it could be a charging issue but checks out).  WTH is going on It gets gas has spark and still dies when warm!?    Both coils have 6.6 ohms resistance on primary and 21-22k on secondary while hot.  While cold they measure 7.5 and 8.2 ohms on primary and same 22k on secondary.  Everything I read says bad coils but it starts fine and still spark once hot.  Read Guy say that he can count on both hand the number of times its been a bad coil after 40yrs of work.  Do I just buy new coils and give it a try? Any recommendation where to get coils? 


Offline Hamburg

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #1 on: August 12, 2018, 10:39:14 PM »
Have you changed the condenser?
1973 TX 500A
1994 GPZ 500

Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #2 on: August 13, 2018, 12:57:16 AM »
No have not replaced the condenser.  Do you have a recommendation for either NOS or an after market product?  Does this sound like the symptoms to a failing condenser?  Its kinda away from the heat but maybe it heats up on its own.   
Any way to test it?

Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #3 on: August 13, 2018, 01:42:38 AM »
I see several on ebay for that are NOS pretty cheap.  Here is a pic from a xs650 forum and if the specs are same for our bikes the I am way out of spec on both coils but I tested with the spark plug caps on I was afraid to twist them.  Do they unscrew like the smaller bikes? 

Offline guylr

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #4 on: August 13, 2018, 03:23:38 AM »
The plug caps do "screw" onto the plug wires so do take them off to do the resistance check. The best test for coils is dynamically on a coil tester but almost no one has one these days. In lieu of a coil tester you can make a simple adjustable spark tester with some clear fuel line and a short piece of plug wire. Screw the short piece into the plug cap and put the open wire ends into the clear fuel line about 6-8mm apart. Start the bike and if the coil will jump a spark over the gap in the fuel line then it is strong enough. If it will do that for a minute or so then it is loading the coil enough to heat it up and make it fail. If it keeps on firing then the coil is fine.

Also check the red/white wire voltage at the coils while running. Wiggle the mainswitch key a little to see if the voltage drops. With the engine off you want to see something within a half volt of battery voltage at the coils.

Guy
Now Retired Yamaha guy. Cut me and I bleed "Cinquasia Red"

Offline Hamburg

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #5 on: August 13, 2018, 06:34:41 PM »
I had loads of running issues with mine, including poor hot starting, I fitted a NOS Yamaha condenser and itís now running really well. Also check the wiring to the condenser, mine had been hacked about.
1973 TX 500A
1994 GPZ 500

Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #6 on: August 13, 2018, 10:37:21 PM »
Guy, Thanks for all your knowledge on this site it has helped me learn an great deal about motorcycles.

I tested without the caps and both primaries at 4.4 ohms and secondaries at 11.04k and 11.16k.  Seems a little out of spec but not much.  The caps both have 10k of resistance, is that normal? not sure what could go wrong with them...  Love that idea for the coil test!  I did jump the gap between me and the bike when I was trying to use the spark tester light I was holding the light on plug cap and touched the handle bar as I hit the start button and got a really nice zap.  That was after it die from running for 20min and hot.  It would also light the light.  My thought it is would still jump the gap of the tester if it can jump to me. 

At the red/white wires I get 12.1vDC and the battery is at 12.38 both wires check out.  Wiggling did not disrupt this voltage. 

Hamberg: ill order one they are about $10 so why not.   Thanks for the suggestion!

Any chance my oil filter is clogged and the bike is overheating? Or maybe the points some how? Or something else that I should check next?

Offline guylr

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #7 on: August 13, 2018, 11:15:05 PM »
Your coils are right on spec. The caps are just in also but at the top of spec. I'd say all good. Voltage drop at the coils is also good but your battery is on the low side even though the charging numbers are right on.

A clogged oil filter would just bypass to the sump so no extra strain on the engine.

Did you happen to strobe the timing with a timing light. That's a good thing to do on these bikes with the centrifugal timing governors.

Maybe double check those valve clearances especially as you started out with some stuck ones. Set at the middle to loose side of spec and not on the tight side.

Guy
Now Retired Yamaha guy. Cut me and I bleed "Cinquasia Red"