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Help! RPM's go down, power loss

  • Thread starter Thread starter scaylabs
  • Start date Start date
S

scaylabs

Guest
Hello fellas,

I haven't posted in a while, been busy riding :)
Until the last couple of days, that is.

It first happened last week in a rainy 5 mile ride: almost complete power loss and RPM dropped like crazy, had to give it a bunch of gas to keep it from dying. I was only a few blocks away from home and I was able to get it home like that.

It started fine on Saturday and run OK around the block a few times, so I figured something had gotten stuck in the carbs but had gone through.

Today it run fine at first, then on the way back the same thing happened except intermittently, which was like riding a wild bronco all the way home.

I checked the petcock screen and there is no dirt there, so I can't think how anything might have gotten through to the carbs.
About 550 miles ago I rebuilt the carbs (yes, the proper way ;)), new boots, orings, adjusted valves, etc. Build thread here: http://www.thegsresources.com/_forum/showthread.php?t=172955

I posted in the electrical section because I'm suspecting the coils. Is it possible one is failing intermittently like that? It certainly feels like I'm running on two cylinders.
 
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Hi,

Have you checked all 4 plugs for spark? Let's hope it's not an igniter.


Thank you for your indulgence,

BassCliff
 
Old Bakelite spark plug caps can do that if they have started to break down and get wet.
 
Here's hoping for spark plug caps! I never changed them and I should have (please don't be the igniter!).

The problem is how do I test it when it's running fine one second, bad the next, then fine again, etc?
 
Hi,

When you pull a cap off of a spark plug, you should hear the engine start to run rough or labored. When your engine has one of its bad moments, pull a plug to see if it makes a difference. If it doesn't make a difference you can assume that plug is not sparking anyway. You can also test by putting another plug into that cap, laying it on the head to ground it, and see if there's a spark. Note: Be very careful handling spark plugs if the engine is running. In fact, I would recommend against it. :eek:

.........
Sparkplug.gif



Thank you for your indulgence,

BassCliff
 
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OK, I was able to test while the engine was running bad. Removing #2 or #3 had no effect. #1 or #4 and the engine would die.
So.. coil going bad, ignitor, or what?

Thanks in advance!
 
Nobody can tell you if it's the coil. You'll have to test it to find out. Check all the associated wiring too.

I once had one of the coils' power wires break off (off!) right where it enters the housing. Instant loss of half the engine. While working on that, I discovered a break in the ignition wire which would kill the engine when disturbed. I highly recommend taking the time to inspect all of the wiring and connectors. Brittle wires, damaged insulation, dirty connectors - these are all time bombs. Getting stranded once or twice should be enough motivation, if you think it's too much effort.

I'd tell you how to test the coils, but I don't remember exactly. Something about measuring resistance across the leads. Someone else here will remember, or more likely, it's on Cliff's site.
 
Basscliff's website has a .pdf of the service manual for your bike. The electrical section has a pretty decent explanation of how to test your coils and igniter, with diagrams, starting on page 7-1 of the manual, which is page 184 of the .pdf.
 
Thanks, fellas, I'll get to testing the coils later tonight.
The wiring should be good as I redid the whole thing 500 miles ago, including a coil relay mod.
I'll let you know how it goes.
 
The coils check out fine.
So... that leaves the Igniter, Signal generator and the wiring in between them all.
I'm betting on the green/yellow wire from the igniter to the right coil, as the terminal looked a little suspect and it's the original wire. I'm going to jump that with a new wire tomorrow and see what happens, if not I'll go forward with the testing.
 
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start your bike in the dark.......look for any evidence of spark grounding........ have needles and seats been replaced? cylinder flooding can occur with out static fuel overflow.
 
One option is to swap the coils and see if the problem stays with the same cylinders which means the coils are OK. If the problem moves with the coils, then the answer is obvious.

A handy test for leakage in insulation is to mist the suspected components with a water spray. If the engine starts to miss or cut out when one coil, cap or wire is soaked the that one has insulation problems. This can be an intermittent or continual problem but is worth while doing as part of one's periodic (yearly or what-ever) check list as misting can reveal a problem before it leads to walking. Not fool proof but worthy.

Looking for sparks in the dark can be helpful as can connecting a grounded jumper wire to the shank of a long screwdriver and rubbing the screwdriver blade along the caps, plug wires and coils.

Resistance tests may check out as OK but a cracked or spark perforated coil can still fail. A coil which tests outside the recommended resistance range may function properly so the tests are not conclusive.

This is also true of trigger and charge coils (Ignitor). You sound to be onto something with that wire but if not, try swapping the Ignitor leads to see if the spark moves to the other side's system. Heating with a hot air gun/hair dryer may be helpful as can cooling to identify a failing ignitor coil but heat with restraint.

Any chance you have access to an osciloscope?

If not, a comparison can be made between the two trigger coils/Ignitors, but using an analog voltmeter or the indicator bar of a digital. The two coils should produce a similar voltage at the same cranking speed.

scaylabs said:
OK, I was able to test while the engine was running bad. Removing #2 or #3 had no effect. #1 or #4 and the engine would die.
So.. coil going bad, ignitor, or what?

Thanks in advance!
Click to expand...
 
Sorry for the delay in answering. I finally had some time yesterday to try a few of the suggested things but, of course, the bike ran perfectly fine no matter what I did.
It didn't last long however, it almost left me stranded today. :mad:

Any chance you have access to an osciloscope?
Click to expand...

I actually have an old oscilloscope that I had forgotten about and I just ordered some probes so I can try to do some troubleshooting this weekend.
 
scaylabs said:
Sorry for the delay in answering. I finally had some time yesterday to try a few of the suggested things but, of course, the bike ran perfectly fine no matter what I did.
It didn't last long however, it almost left me stranded today. :mad:



I actually have an old oscilloscope that I had forgotten about and I just ordered some probes so I can try to do some troubleshooting this weekend.
Click to expand...
Could still be a coil...

Griffin had a coil that when it started to go bad would be fine till it got hot. Then it would crap out on him. Of course, once the bike cooled off enough to test it, it tested fine. It wasnt untill he limped it home on two cylinders, left it sputtering and yanked the tank off in the driveway to test it that he found it was bad...
 
I finally hooked up the 'scope, and of course, there was no way to reproduce the original problem. Heat, no heat, reving to 4k for a few minutes, nothing.
It was still running pretty rough though, especially when cold with backfiring like I've never heard before in my life (maybe if you put your ear right next to a VW bus). I could literally see the backfiring on the right exhaust and on the #4 carb.
Any ideas on these new symptoms?

I think the coils were replaced at some point (these are 33410-31311) and the spark plug wires are removable. There is a flimsy plastic clip holding them to the coils, but I don't know if I can take it off without breaking it.
Does anyone know where can I find the right wires (and clips probably)? or maybe I can buy something generic at Autozone?
 
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