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low spark trouble - help!

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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I rescued a 1980 GS750E from a "friends" basement. It has been sitting there for about 15 years. I'm not getting enough spark - just a little arc across the tip of the plug. So far I have checked the coils, ohms pretty close to specs. replaced the battery. replaced the plugs. Bike will start only with brand new plugs and battery freshly charged. plugs foul after a short period.
Any hints as to my problem? Igniter is working, but could it "cause" a weak spark?
 
I may be wrong but I doubt it is the igniter since the bike will start and it is getting signal to the coils. Coils can test okay with a multitester but still not throw a strong spark. Sometimes what appears to be a weak spark is just a matter of opinion. Have you cleaned the carbs yet? After sitting for 15 years a good thorough cleaning is in order. That would be the next thing I would do.
 
low spark

low spark

Cleaning and setting up the carbs is ABSOLUTELY essential if the bike has sat for this long. Then, have a look at your points. Make sure they are clean, oiled (only the pivits, not the point surfaces) and properly gapped. If in doubt, replace them. With the bike idling, measure the voltage on the battery. It should be 12.5 to 13V. It should be about 14V at higher revs. You might have a U/S regulator and rectifier. If they are U/S then you can replace them, I bought a single regulator/rectifier unit and it works a treat. If all seems fine then you should think about renewing the coils. Again, I bought new ones, its false economy to rely on anything second hand. My new coils where 20 pounds each and a direct replacement. I hope this is of help and point you in the right direction.
 
Thanks, Yes I have had the carbs cleaned by a shop. the only setup I see in my maual is balancing, which I haven't done yet. No points on this bike, all eletronic. Volts across the battery are 12.8 at off, drops to 10.6 when starting, about 13 at 2K rpm.
So "we" think I need coils??
Karl
 
Its all electronic. GOOD. I converted mine to electronic ignition, its great. Anyway, its a hard call. It could be coils, it could be carbs. If you remove the plug and hold it to the engine block when cranking you will see the spark. I must admit, its hard to tell a weak spark. However, its your call. Do you trust the work the shop did on your carbs ? I personally would do it myself. Strip 'em and clean 'em, then balance 'em. Even a weak spark will burn a fuel mixture, so if your plugs are fouling it sound like you are running way too rich. My advice, concentrate on carbs first.

Best of luck
 
Before you strip down the carbs, check out your airbox. (or have you replaced it with K & N s). Try removing the airbox side panel and air filter, then run the engine, see if it leans out your mixture. If it does, carbs are definately your problem
 
thanks all.
Original air box, new foam & oil filter. I not sure that it is any different with or without air box. It's been out mostly so I can monitor the battery. I had a shop clean the carbs because when I took the float bowls off I found the needles so caked with crusty old fuel I could not get it apart. Also I just don't have the work space for the soaking etc. Maybe I should try pulling them off and spraying them from the inside. Pulled the no 1 spark plug and used a grounding jumper to be sure I had a gorund. All I see is a small arc. Maunal describes a "fat blue spark," clearly it is not that.
Can't play with it again till tommorrow. I'll let you all know.
Karl
 
Check the restance in the red wire from the fuse block to the ignition switch should read good if not replace with 12 guage wire then check the brown wire from the ignition switch to the ignitor box if it reads good take your coils off they are grounded through the core clean the mounting surfaces on the frame I used a flap sander on a drill and used new bolts when reinstalling them I hope this helps.
 
it's running

it's running

Ok, I got it running! Hopefully it will start again tommorrow.
Here's what I did. I pulled the carbs off, not expecting to find them dirty (and they seemed ok). But I sprayed cleaner through all the jets to be sure. I also reset the balance adjustment screws so all the throtle plates looked uniform. While I had the carbs off I found the bolt securing the ground cable from the battery to the motor was badly rusted, bad enough that the head would not hold the wrench. So even though I had a good ground according to my ohm meter, I added a second cable from the battery to the engine block, using a different bolt.
After re-assembly. It started. After a few minutes, shut it down and checked the plugs. They looked pretty good. I layed #4 on the side of the engine to check the quailty of the spark (#3 wire was still off) and it started and ran on two plugs. Spark looks much better - but still not "fat blue" as the manual describes, but this is good enough for me at this point. I put 30 miles on it today, and it still is re-starting. Yea!
Thanks to all for your help!
Karl
 
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