79xs750 middle cylinder wont fire

S

sumdood

XS650 Member
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Location
wisconsin
1979 xs750
15,xxx miles
stock mechanicals

I am at a loss and made this account to maybe find someone who can help, as ad states, I cannot , for the life of me, to get my middle cylinder back to life... Ive had bike for 10 years and never had engine issues like this before, I do all regular maintenance and have basic knowledge of working on the engine; however, this beyond me.

I comp tested all cylinders and they are all in spec
I visually see strong blue spark on plug and coil tested correctly
I removed carbs, cleaned them and put them back on the bike dry, fired one cylinder at a time with starting fluid, 1 and 3 popped , 2 did not, i switched coils and plugs just for the heck of it, reattempt to make it pop with starting fluid, nothing..

so we have compression, gas, spark.. this is where my knowledge stops because I am assuming a cylinder wont ignite if there is an internal issue even though there is compression

Ill take any suggestions , ideas, etc, because if not, ill pull the engine and take it to somewhere expensive... ewww (pic was from morning of July 4th, was running fantastic as usual , I took it back out late morning, made it a mile down the road and that was that, limped it home

ty for your time

1753897213776.png
 
Jim said:
If it's getting good spark and has compression... but won't fire on starting fluid, I'd be looking at (that cylinder's) timing being way off.
Click to expand...
how would one do that.. is that an internal check? the 79 has a TCI, which haven't figure out too much yet to diagnose if it has an issue
 
Thinking out loud
If there is a restriction in the carb .Intake Neither starting fluid nor gas enters
Is it possible get the middle spark plug wet ,,? perhaps kick with full choke full throttle open
many times ignition off, ? looking the same as other plugs
 
Jim said:
If it's that restricted, compression wouldn't be normal. OP says all three compressions are normal.
Click to expand...
Ok Ok Yes agree maybe wrong here
But wet plugs would give info Thinking compression can be air only
Machine Would say something if compression and spark
Unless as you mentioned it is entirely off ignition timing.
And that be something to investigate
Dont know how it i set up on this machine
The strobe light was mentioned and using some manual instruction
 
There's not much good information out there on XS750 TCI set up that I can find. From the limited information I can find, it seems that it's not possible to time the 3 cylinders independently. Seems to me, you make one timing setting and all three cylinders are done. OP is going to have to provide more information here, I think.
 
My thinking goes like this...
  • if it has compression, which is all mechanical, not firing is not due to mechanical reasons
  • if it is getting fuel, either gas from the carb or ether from a can, it is not fuel-related
  • if you get a fat blue spark at the plug with the plug out, is it possible that the plug is not firing under compression? I ask because I've had that issue.
 
I'm thinking your ignitor shit the bed. On a three cylinder it's probably three ignitors in one housing. it's also possible one pick up has gone bad. Those prolly can be checked with an ohm meter.
The good news is that dead/abandoned XS750's are everywhere so parts shouldn't be too hard to find. A first step would be verifying +12 to coil, then use a test light in place of coil at the connector, it should flash when you crank. then check the wire from pick up to ignitor is good.
1753971009229.png
 
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I will do these checks , some I already did, I do have 12v to coil, I will use the test light and borrow a strobe for timing check, (although as stated above, 1 and 3 are firing correctly, i dont think it can be timing related on this bike , but will check anyways to rule it out for certain) .. ty as always for everyone's input , its super appreciated.
 
Last edited:
Probably sounds stupid Apologize if so
But new set of sparkplugs could be something to consider.
If old.
Some replace plug wires also .But can wait here.
Not a big cost and something that shall be replaced now and then.
Know of people putting in many hours work .To find out that there was a defect spark plug.
And at the same time check the caps contact surface ..status perhaps add some contact grease
The same at the coil end .
 
ill replace all sparks plugs , it cant hurt, ... I will do that valve check, i have not cracked any internal components yet but I am here because it seems all external checks are failing to give me any decent leads, ... last time i was really trying to figure it out I had help (gentleman much smarter than I)anways we had the thermo gun out on all the cylinders and the 2 cylinder was "hot" but no where near the temps of 1 and 3, so not sure if that was from heat spreading or intermittent firing. As far coils and plugs and caps etc... i did all the swapping around and they work as should on the good cylinders, like wise, the known working coils, wires and plugs do not work on 2 cylinder and they all produce a strong blue spark while connected to the 2nd cylinder, .. i have also put gas directly into the cylinder to get it to fire , and still will not... I know im starting to repeat but it seems that there is either an issue before the coil (even though spark looks good, perhaps not good enough ) or there is an internal issue where the motor has great compression but denies combustion. ... I will get back when I can with some new data and testing info other than just my own thoughts and theories..
 
Good spark is logically outside ..it can be different inside not saying it is so ,, but there is different physics
especially if over rich mixture there.
Are the plug no 2 wet by fuel at all, ? at any time ? Do they look the same all 3 ?
 
all plugs look the same, all plugs work on any other cylinder, all wires and coils work on any other cylinder , the middle spark looks just the same as 1 and 3, good color, bright and in the correct position .. middle cylinder will not fire with any plug wire coil combo and or starting fluid, or manually dropping gas in the cylinder .... i did remove valve cover , every single valve is out of clearance by being to tight, not sure if this is the issue, but im going to correct this first, unfortunately i just cant adjust on the fly, i have to find out which pad i need for each intake and exhaust valve, buy them and a tool ... once this is accomplished, ill reassemble and ATF.. I may be way of track but this will be addressed and we will continue the march, thank you again for all input and shared advice , knowledge
 
sumdood said:
all plugs look the same, all plugs work on any other cylinder, all wires and coils work on any other cylinder , the middle spark looks just the same as 1 and 3, good color, bright and in the correct position .. middle cylinder will not fire with any plug wire coil combo and or starting fluid, or manually dropping gas in the cylinder .... i did remove valve cover , every single valve is out of clearance by being to tight, not sure if this is the issue, but im going to correct this first, unfortunately i just cant adjust on the fly, i have to find out which pad i need for each intake and exhaust valve, buy them and a tool ... once this is accomplished, ill reassemble and ATF.. I may be way of track but this will be addressed and we will continue the march, thank you again for all input and shared advice , knowledge
Click to expand...
Maybe you found the problem. I think Jets-R-Us stocks your shim if you don't have them.
 

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