Author Topic: 1975 xs500b runs great for 20min then dies everytime???  (Read 3893 times)

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Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #15 on: August 24, 2018, 12:01:27 AM »
Mega bummer still same problem dies after 20 min.  Not sure where to go from here, actually seemed to have less power then before adjusting the timing. 

I have the the each points timed to the second mark from the left but this might be the end of the dwell time mark.  It appears T, mark, mark, FL, mark, mark.  I have it set to the second mark after the T before the FL  It was as far as I could get it, but I got both points exactly on the second mark for each cylinder.   

When I twist the throttle the advance moves to the two marks after the FL (T, mark, mark, FL, mark, ** mark) but know further dont know if thats correct.
« Last Edit: August 24, 2018, 12:14:41 AM by Cobra »

Offline guylr

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #16 on: August 24, 2018, 12:46:21 AM »
What's  your idle speed?  The advance won't settle all the way back to the F marks running but if you are seeing full advance at the two advance lines then it is in time.  If the governor function is stable and not changing when hot and going into a deep retard at idle then timing is not likely why the engine cuts off.

So, are the carbs in sync? 

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

Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #17 on: August 24, 2018, 02:46:11 AM »
The govoner advances from the second mark from the left, to between the two marks after the FR/FR marks.  It is only about a centimeter to movement hen when I left off the throttle it moves back to where it is set at the second mark after the T.  It never goes over the 2 marks after the F mark.  Shouldn't it advance (move) more then a centameter.?

I have been trying to keep it a 1200 but usually its at 1500 but Im almost certain the tach is off, at 1200 its not the same rhythm as the youtube videos which sound more like 1600 on my bike.

Offline guylr

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #18 on: August 24, 2018, 03:54:50 AM »
The timing will always be partially advanced at 1600. Even at 1200 it will probably not be on the F mark. As long as the timing advances to the two lines at 4000 you are getting enough advance. You are getting full advance at 3000+.  I suspect your carbs are out of sync which is why it won't idle smoothly at 1200. Before syncing the carbs you need to reset the valve clearances and adjust the timing chain tensioner.

Guy
« Last Edit: August 24, 2018, 04:01:54 AM by guylr »
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Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #19 on: August 24, 2018, 04:16:56 AM »
how do I do the timing chain tensioner (1975 xs500b) ? its just an acorn nut and a washer.  The black plug is even with the screw when I take the acorn nut off nothing happens even when running.  How do I compress the tensioner to let any slack out?  It feels tight as hell whne I have the valve cover off.

The manuals didn't have a very good description, and mostly online was about the newer style with a cap and lock nut which made more sense.
« Last Edit: August 24, 2018, 04:19:57 AM by Cobra »

Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #20 on: August 24, 2018, 12:49:46 PM »


This video makes it seem that the tensioners on 1975 and earlier bikes will always be at full tension since the lock nut will not hold the tensioner slacked.  This appears to only help when assembling the engine since you need to compress the tensioner fully for the lock nut to engage.  I can not compress the tensioner with the head on the engine.  So I can never slacken the chain on bikes of this year?

This means that the tensioner is always at its maximum tension on an assembled bike?  So just loosen the acorn nut (adjusting nut) and if no click is heard (FSM pg43) no big deal because it was already at max tension and I should just tighten the nut and move on to timing the valves?

Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #21 on: August 24, 2018, 10:39:43 PM »
So I loosened the cam tensioner bolt, ran the back wheel forward and backward a little in top gear, then tightened it.  I reset the valve clearances, checked the timing, synced the carbs w/ motion pro manometer, and still the issue persists.

 

Offline guylr

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #22 on: August 25, 2018, 03:28:13 AM »
When you say it cuts off after 20 minutes does that mean both cylinders quit or just one? Can you keep it running by revving it or does it just stop? How long before it will run  again?

Guy

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

Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #23 on: August 25, 2018, 10:00:56 AM »
it becomes harder and harder to keep running no matter what I do when stationary. It has to be a gas problem. Everything electrical checks out.  So its either flooding (the oil doesnt smell like gas) or it cutting off the gas....I never replaced the needle valve assembly (the one the float connects to).  Both seemed to clean up fine but maybe they are becoming stuck once the bike gets warm (hard rubber tip maybe).  One for sure I have seen some sticky issues with but with cold gas there is never a problem.  But maybe the needle becomes stuck with heat at about 20 min.  Or a float problem but the bike died the same before I adjusted the floats (many months ago) and still dies which only leaves the damn needle valve assembly...there is literally nothing left....
« Last Edit: August 25, 2018, 10:11:44 AM by Cobra »

Offline guylr

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #24 on: August 25, 2018, 02:14:56 PM »
That is what it's pointing at. How's the oil level? My TDM850 recently had a petcock that wouldn't shut off so that I had 5 qts of oil in a 4 qts sump. The bike would run great for 5 miles then start dropping a cylinder at idle due to the over rich mixture.

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

Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #25 on: September 26, 2018, 08:18:46 PM »
I bought new needle valve assemblies installed them no change in performance.  I have the tank off so I can sync the carbs, which I did successfully (used a manometer).  I have 2 long fuel lines leading from left and right petcocks to both carbs  I notice when I first open the petcocks fuel rushes down the lines through the filters (i can see as the fuel flows through the filters as the bowls fill up) then the lines fill up all the way back to the tank (fuel line is completely full of fuel). 

As I start and run the bike the level of fuel empties out of the lines eventually look empty (but I can still see a small amount running through the filter while running but its not full like when I first fill the lines).  I was told as long as the fuel is seen flowing that this is not a big deal but it sure sucks seeing it and having this issue.  I opened the filler cap to make sure it is not vacuum.  This did nothing to change the issue.  If I disconnect the lines from the filters they flow like a fire hose.   

Offline dailydose1

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #26 on: September 27, 2018, 01:20:30 AM »
Have you tried running with the gas cap open? I remember I had a vent problem on my cb that cause it starve fuel at one point

Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #27 on: October 25, 2018, 06:15:02 PM »
Ok so I put new points in and timed them perfectly with a strobe.  Prior to this the cam chain was adjusted, valves adjusted, the carbs are clean and synced.  The fuel lines are full and the gas cap is open (to rule out vacuum in tank).  All new jets in carb and new needle valve, new pilot plugs and orings, bike has 115psi compression in left cylinder and 120 in right (tested while warm and throttle open, seem good).  There is no gas or level change or smell (gas) in the oil.  I did strobe the bike (just hooked up the strobe to see if the spark is missing) while it was about to die (after 20min of idle) just to see if there were any misses happening.  I didn't see any misses as the idle gets lower and lower, then just stops.  I didn't see any misses in the strobe then it just dies at about 800rpm.  Even after the bike wont start, the strobe still flashes as it turns over (tryng to restart) so it seems its getting spark.

What the heck am I missing?  Im ready to just buy a new set of coils and test them what is everyone doing for aftermarket coil replacement.   #Desperate    :-\

Is there any way an air leak in the case could cause this (there are no leaks in the carbs or boots)?  Pretty sure this is only a 2 stroke thing but idk. 

Offline guylr

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #28 on: October 26, 2018, 01:35:26 AM »
Your compression is low. Standard compression on 1973 to 1977 models is 140psi. 150psi on 1978 models. For readings below 128psi the manual recommends adding a little oil through the spark plug holes and retesting. If the compression rises then it's likely worn or stuck rings not sealing. Offhand it sounds like a tired engine if your compression gauge is correct.

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

Offline Cobra

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Re: 1975 xs500b runs great for 20min then dies everytime???
« Reply #29 on: October 26, 2018, 07:45:50 AM »
it only has 5k on the engine the bike is in really good shape.  It had stuck valves when I first got it from sitting since 1984 (is what I was told).  Could be the rings are stuck too.   wouldnt the bike smoke if the rings were burnt?  I burnt the rings on a 1971 cl100 and it smokes and burns a lot of oil.  Is there any way to unstick rings or am I looking at a rebuild here?