That elusive weak spark

[dmq136]

I have had my 1981 XS650/H for four seasons now and I think it is one of the best bikes ever make. As an engineering student, I marvel at the simplicity of the bike and am amazed at the way people can modify it (this site is a real showcase for talented people !) and I love the wealth of knowledge that is here.

Anyway, I have been searching for some time to figure out how to diagnose a weak/intermittent spark on the left side. This problem parked my bike all last season (not only am are I slow to figure it out, my time and funds limited – like most everyone).
Okay, what have I done so far:

1. Thought that it was the carbs, perhaps dirty, flooding, whatever. Cleaned, re-cleaned, and re-cleaned again, then installed inline gas filter. Also replaced air filters (no change, left plug kept fouling out after a mile or so of riding)
2. Replaced coil, lead wires, spark plug caps, and 2 sets of plugs (still no success)
3. Apparently at some time (long before me) the battery breather hose was not connected and the TCI, rectifier, and wire harness connectors were all in pretty bad shape (replaced all connectors, corroded wires, etc. tried to diagnose the TCI, but didn’t really know how) with still no change.
4. Replaced battery (as mine was going on four years…) – no help there either.

Now, where do I go from here? I gather that it is time to electrically check the TCI, rectifier, and pickups? Perhaps follow the wires from beginning to end (wiring diagrams here are great!). I am at a loss of how to do any of this or if I can do any of this…

Any other suggestions or insights?

 

[xsjohn]

Have an 80 myself.....did you check compression........I did have a coil that finally give out on the right on mine after 74 thou...but it didn't act like you described...ebay fixed that...?.......when the engine is cold (below 60 degrees or so I hit the e start to crank it over a couple of times and then 1 kick......and when I first got mine I cleaned the entire wiring harness connectors (all of them) kill switch too....used dielectric grease.........

And for cold starts the float bowl has a starter jet in it.......carb spray has to go through there..........would check compression 140-150 PSI throttle open to start with just to be sure it's not mechanical.........

xsjohn

 

[yamaman]

Seeing how you've replaced everything, I'd say it's carb problems.
The TCI fires a wasted spark, that is both plugs fire together every time.

So you can swap the plug leads over if you like, & if the problem shifts, you've got an electrical problem. If, as I suspect it will, the problem stays on the existing side. Then it is likely a carb problem. There is a carb guide on 650garage (there's links to it from the tech section here) that is very comprehensive, and explains where people who think they have cleaned the carbs often come undone. Hopefuly a final carb clean will solve your problem

 

[xsjohn]

Usually the BS34 goes super lean if not cleaned properly.....like Yamaha man infered another cleaning may solve it and doesn't cost .........make sure that the cleaner comes out the pilot circuit in those 3 tiny holes in the throat........also book calls for .0025 thou on the intakes.....that's too close and can close holding the valve slightly open...should be a simi snug 4...........xsjohn

 

[dmq136]

Thanks, this sounds good, I will check the compression in each cylinder (as I recall, I did this once before but the throttle wasn't open and the values were in the 90s). I can switch the plug leads and see what happens (just straight up change them?). If the weak spark remains on the left, I will revisit the carbs and follow the guide that yamaman suggests. If it goes right, I may have damaged the coil at some point (perhaps unplugging the cap lead during engine operation)...
BTY, the fouling is a sooty carbon deposit (does this come from lean or rich mixtures?)

 

[xsjohn]

Lean would be clean a plug...........have you checked the valve adjustment.....and compression check is easy...I hate taking carbs off myself.....I would check those 2 above mentioned things first.........10 minutes if you have a compression guage.....maybe autozone has them for rent if not........35 bucks to buy I think......

guess a float can fill with gas too.........

xsjohn

 

[dmq136]

xsjohn - I have the compression gauge that will bolt right in. Let me make sure I am clear on checking the compression; remove plug(s), insert compression gauge (or thread it in snug), turn engine over 3-5 times. Repeat for other cylinder. Is there a simple guideline for knowing the threshold for "good" compression 140-150 psi. Weaker indicates worn rings etc.?
As for the values is there a link in garage to do that? I haven't done this on a bike only a 73 volkwagon...

 

[xsjohn]

Yea you got it......some say to prop the slides open to to test compression...I have never done that on mine and it reads 150.........140 would be ok and probably not the problem..........I just hold the throttle wide open and run the e-start for a bit.......both plugs out.....should lay the plugs on the cylinder so they ground.......hell I never do that either and have had no problems with the TCI.........

now on these the left takes a lot longer to warm up than the right..and also the right runs 25 degrees hotter than the left......how far are you running this thing before checking the plugs..........usually 10 miles is what it takes to get the engine to operating temperature and for the plugs to clean off from a cold start and show correct operating color.................

xsjohn

 

[dmq136]

Whoa, I don't ride ten miles... I do notice that the left side does stay cooler than the right side. I guess that I will know more once I switch the plug leads as far as where else to look.

As for the carbs, here is the guide that I followed (I swear, I cleaned the sin out these puppies). http://www.amckayltd.com/carbguide.pdf

Does the coil really waste a spark and fire for both cylinders simultaneously? I read that a good spark is blue at like 6mm, I cannot really tell any color at all...

 

[xsjohn]

Go run it a while...forget checking 2 mile plugs...if they are white in 2 miles just think what is going to happen at operational temps..........

I probably covered everying that it isn't.....but at least you have some more ideas to check.............float set a bit high on the left would cause problens too.........22MM on the plastic ones ........27.25MM in the earlier brass ones.......and the mix screw is generally set a bit less out than the right........both the same usually is not right............due to the hotter right..........

then again the left may be ok and the right is too lean.....which is always the case ..everything else being correct....only way to find that out is to run it 10-15 miles..........or more................hope I haven't confused you too much.........I can be like that..............

xsjohn

 

[yamaman]

What coil did you use for your replacement?

 

[dmq136]

xsjohn - I would go out and run it like you suggest; however, after that initial mile or so the left side is so fouled that the bike just dies under load... Anyway, I will update after checking the mentioned items.

yamaman - I replaced the stock coil with one from mikesxs (high output replacement ignition coil part# 17-6803).

 

[dmq136]

To follow-up on the compression test - both cylinders were around the 140-150. I didn't do multiple tests; however, next time I pull the plugs I will check again.

As far as the wires that connect to the coil. With the key on one (red with white strip is live) what does the other one do (orange)?

 

[xsjohn]

Sounding more like the left carb has a problem.........a couple of years ago one member had a left with a passageway under the diaphragm down in a hole that had lost a ball seal.......he went nuts for a long time .....super low temps during winter storage can do strange things.....2 weeks ago a customer found that the replacement bowl gasket didn't have the correct holes.......how about float setting or fuel in the float........I had to replace a right carb a few years ago because the pilot circuit was way to rich......never could determine what caused that.....wish you had another set of carbs to try....I suspose the rubber plugs are over the pilot jets in the bowl...what size main jets......here is a simple wiring diagram I saved........xsjohn

ebay carbs 110511180228

xsjohn

 

[dmq136]

xsjohn - Thanks for the diagram and the input!
I have three more electrical things I want to check out before I finally pull the carbs and disassemble them.

1. Clean the kill switch contacts
2. Check the orange wire at coil to see if TCI is good (i.e. operating coil)
3. Change current coil with old one (just in case I did kill the new one...)

Hopefully, these will ease my mind of the electronic ignition side of the problem.

These threads have inspired me for the above items:
http://xs650temp.proboards.com/index.cgi?board=ID&action=display&thread=10600
http://xs650temp.proboards.com/index.cgi?board=ID&action=display&thread=7878&page=3

When I do end up tearing into the carbs, where can I find a list of parts that are replaced when one rebuilds carbs? Is there a general kit that one can buy that has stock jets, gaskets, etc.?

(Sorry that most of my questions show how naive I am...)

 

[xsjohn]

Can't remember if you swapped the spark plug leads and see if the problem moves to the right................those carbs on ebay are the early BS34's....may even be the brass float ones.......80

xsjohn

 

[dmq136]

Yeah, I went to switch the spark plug leads to see is the problem would switch sides; however, there was no spark at all on either side.... So, now I need to find out if the power is going from the TCI to the coil.

I will test this by connecting my multimeter lead to the orange wire on the coil and the other one to a ground. With the key on, the multimeter should read some value, then when I press the start button, this value should oscillate. If not, and it remains the same the TCI could be shot. Sound about right? (I hope this is not the case) BTY- the red/white wire on the coil is live with the key on.

 

[xsjohn]

Wish I was better at electrics........maybe somone will dive in for that....although I was trained at Yamaha I am basicly a part swapper when it comes to electrics...........if one plug fired fine then maybe its not the TCI.......

xsjohn

 

[xjwmx]

When I do end up tearing into the carbs, where can I find a list of parts that are replaced when one rebuilds carbs? Is there a general kit that one can buy that has stock jets, gaskets, etc.?

(Sorry that most of my questions show how naive I am...)

Mine is an '81 as well. It's rare to need to replace anything in the carbs, just clean well with a couple cans of cleaner, especially the pilot jet, and 3 small holes near the throttle plate. Look at diaphragm carefully up to sun for holes. They're very rugged though actually. Remove float pin using sharp diag pliers as a lever under the head of the pin. Then just spray her all down. Do not adjust anything. If you're lucky your mixture screws are still capped off from the factory.

Adjust your valve clearances. Improper clearance will imitate impossible to track down carb problems. And bad clearance probably more common than carb problems

 

[xsjohn]

""""""""If you're lucky your mixture screws are still capped off from the factory."""""""

An then the 1 1/2 or so turn mix screws would be feening almost no fuel through the tiny 42.5 pilot jet .....and the log main needle would be feeding like next to no gas through the tiny 132.5 main jet........

xsjohn