Idle good, 2000 rpm's and above really bad

[bherr]

I've spent several hours researching this problem on the forum and found the thread about installing magnets, (4.8mm) and I assume these are mounted on the back side of the pickup coil. if wrong please let me know!!! I've measured the resistance from black to orange and gray wires and they are around 756 ohms and 1500 +/- between them. I've measured another pickup that I know is good and they are almost exactly the same. I've also noticed that the bike will continue to provide spark at the plug even when the run/stop switch is off but the key is still on, not sure what that means. Sometimes the bike will run great and then back to misfiring above 2000 rpm's. Another shop tested the TCI and that didn't fix the problems. Any ideas would be appreciated.

Ben

this is on a 1980 yamaha 650 special II

 

[XSLeo]

Never assume anything. If you have read about the magnets you also read they should be glued to the rotor not the pickups. It gets glued with a very strong epoxy. I like JB Weld.
Before you try that, try running it with the reg/rec unplugged. If it runs the same the ignition is not the problem.
You say it gets spark with the engine stop switch off, that sounds like wiring trouble. Power is fed to the eninge stop switch, from the switch to the TCI box and coil. Switch on power flows, switch off no power flows. If the ignition is getting power while the engine stop switch is off you have a short from some other power source to your ignition.
I would get a meter and a wiring diagram and figure out where that problem is.
That may fix the poor running. While your working the electrics, start at one end of the bike, work to the other, cleaning, tightening and greaseing every connection in the wiring harness.
Part of your running issues may be carb related.
Have you read www.amckayltd.com/carbguide.pdf ?
Leo

 

[bherr]

XS,
Carbs have been ultrasonic cleaned and synced. I did remove the reg/rectifier and the problem is still there.
You stated the magnets get glued to the rotor. Do I assume there are more than 2 as I see several round disks on the rotor. didn't count them but more than 2.
I'll start checking the wiring issue as this bike has been sitting for some time.

thanks,
Ben

 

[Dennis]

only 1 magnet on the rotor
been over a year since i put a new magnet on and no troubles

 

[bherr]

Well after installing a new magnet I still have the same problem, missing above 2000 rpm's. To recap, I've ultrasonic cleaned the carbs, synced the carbs, changed out the coils with known good ones. The TCI was also swapped. the bike still misses with the reg/rectifier unhooked. I have to fix the float needle on the left carb as it sometimes does shut the gas off and I get gas running out thru the air filter. I've adjusted the air bleed jets from 1.5 out to 2.5 and no change. I'm still looking for suggestions on fixing the problem.

Ben

 

[jd750ace]

You changed the coils. What about the leads? Have you checked resistance values on them?

 

[bherr]

New plug wires and caps installed by owner. What resistance should I see? the plug wires at the coils are sealed so how do I measure resistance? thru the coils??

I noticed that the spark on the right side appears weaker than on the right. right side is blue and bigger, left side is more yellow and smaller and appears that it is split. whats up with that?

 

[weekendrider]

Hi Ben,
The TCI board has been known to crack at the solder joints.
There are a couple of threads about this and the problems that surfaced.
IIRC one of the reoccurring problems is break up at higher rpms.
Not saying this is for sure your problem but it might be worthing taking a look at.

 

[bherr]

weekendrider,
Yep, I suspect the TCI as I've checked just about everything I can check. I read somewhere on the forum that there's a guy that fixes them for around $100, so I have to do some searching for that post. 2 years ago another shop swapped the TCI box and he said the problem was still there. also looks like I've got to replace the float valve seats and needle because I can't get the one carb to shut off the gas. I've adjusted it several times with no luck.
thanks for the input!
Ben

 

[XSLeo]

On testing the coil, test from the spark plug wire to the spark plug wire. The Clymer book calls for 10500-15500 ohms as spec. You might more if the caps are old. Caps as they age increase in resistance. Without the caps it should be around 5000-8000 ohms. Also test from the plug wire to the body of the coil, where it bolts to the frame, should be infinity reading.
You can test the caps too. They should be around 5000 ohms. If no reading then bad. If low reading bad.
Test the primary side the same way wire to wire and wires to mount. 2.25-2.75 phms on the wires Infinity wire to mount.
Leo