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Need some brainstorming

  • Thread starter Thread starter kungpaodog
  • Start date Start date
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kungpaodog

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I've had the carbs on my '81 gs650gl all apart and tweaked and tuned and synced and adjusted and plug chopped and cleaned and leak checked and rebuilt and sealed and anything else that I can do to make them run as well as possible. No leaks, no worn parts, good compression, good air filter, good spark, good idle, good running, etc.

BUT after riding the bike hard, it's like a switch comes on and in an instant it runs a lot better. Usually it's at high rpms that this change happens, but it can come and go. It's not a gradual change, it is very quick. When it is not running well, it won't pull past 65 mph, but when it is running well, it runs up to 90mph no problem. I suspect it may be something electrical, but I really have no idea what is going on. Please ask me any questions you can think of, and please throw out any crazy ideas that I can apply towards making this bike work well all the time. I really don't want to give a local shops hundreds to tear the whole bike apart, and I know I can beat this in my garage. Coils? Wires? Exhaust? HELP!
 
So when you pound the snot out of it, consistent red line activity, bang-all of a sudden it runs well? Then if you just putt around town at low RPMs it eventually starts to run poor?
 
I wonder if Uncle Mike will chime in here about valve clearances... you might want to read this thread from a while back!
 
chiphead said:
So when you pound the snot out of it, consistent red line activity, bang-all of a sudden it runs well? Then if you just putt around town at low RPMs it eventually starts to run poor?
Click to expand...

Yes, pretty much. It is always a SUDDEN change when it does switch over, and it has changed over at lower rpms, but it seems the best way to get it running well is to pound the snot out of it. One time it popped over to good running when I downshifted to first and dropped the clutch hard enough to make the rear wheel chirp. Generally unpredictable, not like I know I can flog it at redline for 20 minutes and it will run well.
 
I'm thinking maybe electrical. Get anal on all the grounds. Also throw a fresh known good battery in it.
Check the mechanical advance unit on the igniter plate. Take it apart and clean it.
 
You could be dropping a cylinder once in a while. Bad connection at the plug or coil, or poor grounding. Check the wires that run to your coil and make sure your grounds are good. Checking your battery as above post says is a good idea as well.

At start up, check your exhaust pipes to see if they are warming up evenly.
 
mine does something similar and its the transition between pilot and main jet
 
mark said:
You could be dropping a cylinder once in a while. Bad connection at the plug or coil, or poor grounding. Check the wires that run to your coil and make sure your grounds are good. Checking your battery as above post says is a good idea as well.

At start up, check your exhaust pipes to see if they are warming up evenly.
Click to expand...

The pipes seem to warm evenly.
I don't mind trying a new battery, but it is pretty recent.
I also know that the mechanical advance is well lubed and working well.
What's the best way to check the coils? And if it is new coils that I need, where could I get new ones? Any good online sources?
By the way, all the parts on the bike are stock, and it only has 11,000 miles on it.

Thanks guys! Anyone else got ideas for what I can go over tonight?
 
Sounds like an intermittent electrical issue to me.
Something I've learned after restoring five non-running bikes: I originally thought that electrical components (igniter, coils, R/R, etc.) either work 100% or don't work at all. Now I know that is not true. All of the electrics can fail gradually and show up as intermittent problems until they finally fail totally.
I've had R/R (on two Suzukis), coil and electrical rotor (on a 1980 Honda CB750) that failed gradually. I spent tons of time messing with carbs when the problem was electrics all along (which I found out when the component finally died totally).

There are many posts on this forum detailing how each of the above components can be tested.
 
Last edited:
Thanks for the ideas to all that responded!
I had time to do a few tests with a multimeter tonight. The no load alternator test was fine, but the battery test was a little low.
The ohms off of the pickup sensor were right where they were supposed to be at 330 ohms.
The manual said that the voltage coming off of the battery was supposed to be between 14 and 15.5 volts, but I read it at between 13.2 and 13.5 volts.
Is this too low, or is it likely that this slightly low reading is just due to the contacts being a little dirty, or even a small discrepancy in my multimeter? Or is this unacceptably low? Does this mean the rectifier needs to be replaced, or that it's on it's way out? Or does it mean that the battery is shot?
(I know that's a lot of questions, thanks to anyone who answers them.)
I plan on testing the coils after work tomorrow, and maybe checking the timing while I'm playing with my electric testers. Anything else I should check while I'm at it?
Thanks again to all the help all the GSRers give me!!
 
What's the gap on your spark plugs. This may sound silly but if your plugs aren't gapped or your running rich you can "clean" your plugs while beating the snot out of your bike and it will fire all right until they foul again. Same applies to oil burning.
 
I vote cracked coil casing. or fine rust in the gas tank hanging and plugging carb intermitant.

Ive had the same experience on two seperate bikes. Rust in my 1100e and cracked coil casing on my Kaw 550. They both acted the same yet unpredictable.
 
I just put in new plugs as part of plug chopping, and it hasn't run well when cold with brand new plugs ever. I have them gapped to about .027", within the reccommended .024" to .028" (I think, that's from memory).
The coil casing doesn't look cracked, but I haven't had a chance to pull them off and inspect or test them.
I haven't done any treatment to the gas tank, but it is possible that it needs some help. I can't remember if the discoloration in the bottom of my float bowls was always the same shade, or if it's gotten any worse since the last time I cleaned them. How hard is it to clean and do the anti-rust treatment on a tank?

Thanks again for the tips, all!
 
Sounds like electrical to me too.
Check all connections first. Check the plug caps. Could be the ignitor if all the connections are clean/tight.
Sounds like it's running off and on on 3 cylinders. By your description, I doubt you could be losing 2 cylinders but that's possible too. If 2 cylinders, I'd suspect a coil or sensor behind the ignition cover.
 
Thanks, Keith. My understanding is that the way the plug caps are connected to the coils, it is easier to replace the whole coil than to try and splice new wires into the old. Is this true?
 
If you have access to an inductive pick up timing light, when the bike is "running poorly" hook it up to each plug lead and watch the light to see if you can detect a variance in the pulsing of the timing light on each cylinder. Repeat when the bike is running well. This will help you isolate whether or not it is an ignition issue.
 
When I mentioned checking/cleaning the plug cap connections, I meant to unscrew the caps and inspect the end of the leads. If the copper has any corrosion/green color to it, you can usually just snip off a 1/2" and give the lead a "new" connection.
Did you inherit this problem or did it run well before and then it started doing this? I'm trying to figure out if there could be PO jetting changes or whatever that could cause a weak spark/no spark condition or a part has just started failing.
This bike is stock intake and exhaust? Stock jetting?
What colors are all four plugs after some general riding around town?
If you start it up and warm it up, what happens if you then disconnect each plug lead at a time? Turn bike off between each disconnect to avoid a shock. Re-start. Any effect? I'm trying to see if you're already running on only 3 cylinders until you go out and ride it harder. A weak firing cylinder will many times let the pipe/that cylinder get hot but it's still not allowing constant/good combustion.
What plugs are you running?
Are all four mixture screws set approx' the same?
I'm ASSUMING the carbs are clean and adjusted well, per your post. That would leave only the jetting suspect if this is carb related. Trying to narrow this down.
 
Here's another vote for plug caps. Had similar problems myself, and replacing the caps fixed it right up. It would drop a cylinder or two when warm, but not always when warm -- it was somewhat random. Drove me crazy for a while trying to figure it out. New NGK boots are cheap at z1enterprises.com and they look nicer, too.

There is a resistance test you can do (GS caps are "resistor" caps), but I first found out by handling them while the bike was running and getting a shock, so you can give that a try if you're brave. ;-)

Also check the voltage at the coils. This old wiring often leaves you with less than full voltage by the time it gets all the way to the coils. Probably wouldn't cause the problem you're describing, but something to check nonetheless. You want as close to full 12v as possible.
 
I'm 99% sure that it's stock jetting, maybe I'll pull them again and check, but it seems to come and go too sharply to be jetting.
Intake and exhaust are stock.
The plugs are Autolite.
The color of them is all a good light brown, but I suspect that cylinder 3 may be firing intermitently since a brand new plug seemed to show less color than the other 3, but it may be just tuned better than the others since it looked like the perfect plug color.
The mixture screws are all set at about 2 1/8 turns out (I'm in Colorado, so this makes sense to me to adjust for elevation, right?).
I think I inherited the problem because it was non-running when I got it, but it ran with some fresh gas (poorly) before I even cleaned the carbs.

I checked the coils last night, and they look to be fine visually. All the connectors are in great shape, there are no cracks on the bodies. When I pull the caps off and put in plugs and run the bike, all 4 plugs look to fire well. AAAGHHHH!

There's a set of used coils and a R/R on ebay right now that I could pick up for $25 and $30, respectively. I don't want to just throw parts at it, but would it be worth a few bucks to try new coils?
Thanks for the time and concern, guys!
 
I suggest you take the coils to a shop that has a coil tester and have them checked under load. The coil resistance test can be within or near spec but when there is the high voltage generated the windings can short due to bad insulation on the secondary windings. I had this happen and new OEM coils worked magic! Good luck, Dick V
 
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