Need help testing coil

[Take Warning15]

I have been testing the coils with the help of a lot of threads on here but am still a bit confused. It is possible that the colors of the wires on my bike are throwing me off. I suspect a bad coil but haven't confirmed it.

Can someone possibly walk me through troubleshooting the coil? I have brown and orange wires to the coil.

They are stock coils connected to points. I've replaced the condensors and points. Every test I do shows either infinite or 0 ohms, none show any other number of resistance.

 

[jd750ace]

What kind of tester are you using? If digital, make sure the battery in the meter is good.

 

[XSLeo]

On the stock points coils the brown and orange wires are the primary side, these should ohm out at around 3.9 ohms. On testing the secondary side you remove the plug wire from the coil by unscrewing the cap then unscrewing the wire. Now look down inside the coil where the plug wire goes. you will see a screw. This is where you touch one probe of your meter. The other meter probe touches the mount.
You should get a reading of 8,000 ohms.
Use the lowest ohm scale your meter has for the primary side. Digital meters have a 200 ohm scale as lowest.
Now with the wires on the coil unplugged from the harness, touch one probe to each wire.
Now switch to a high ohms scale, like 20k, touch one probe to a wire and one to the mount. This test is to find out if there is a short between the windings and the body of the coil.
Now using that high ohm scale touch one probe to the mount and one to the screw down inside where the plug wire goes.
You can also test the plug wire and cap by unscrewing the cap from the wire. I'm not sure just what the wire should test but plug wires have a certain ohms per foot. I just don't recall right now. I have a bad coil from a later bike, I cut the wires off of it Each wire has maybe .05 ohms It flickers back and fourth between .1 and .2 ohms.
So as long as you get continuity they should be good.
On the caps test from the screw the plug wire connects to the inside where it plugs on the plug. I think they are zero ohms on the points bike. So a very low ohm reading is good.
Hope this helps.
Leo

 

[5twins]

As far as I know, all the points bikes had resistor caps too. Yes, you can use zero ohm caps if you like but that's probably not what you'll find on there now unless someone changed them out. And speaking of the cap resistance, the ones Yamaha used were a rather oddball 9K ohms or something, not the standard 5K ohms like the NGK replacements we usually install.

 

[Take Warning15]

I have the 5k NGK on now, brand new with new wires.

 

[Take Warning15]

Primary reading on both coils is varying between 5 and 6 ohms. BEtween the motor mount (if that is what is right) and the screw inside the coil, I see about 7000

 

[Take Warning15]

wires unplugged from harness - 5.2 and from either wire to mount (ground)- infinite.

from screw inside the coil to the mount i also see infinite.

I'm not sure about the mount though. Do you just mean a motor mount for ground or coil mount? Sorry if that is an obvious one.

By the way, my wires with caps read just under 5k and are about a foot long.

 

[XSLeo]

Your primary may be a bit high and the secondary a bit low.
On your meter on the lowest scale touch the probes together. This will give you a reading of just the leads. Subtract this reading from the primary side readings.
I have several sets of leads some read very low, like .1 ohms. One sets reads 1.5 ohms.
If I got the 5-6 ohm reading using the 1.5 ohm leads the actual ohms would be, lets average the 5-6 and call it 5.5 ohms. Subtract the 1.5 from the 5.5 you get 4. Very close to the 3.9 speced.
On the higher scales you don't need to worry about the leads ohms.
Do you get infinity from the primary wires and the mount?
If so I might consider those coils ok. But if they are the stock coils from 75 then I would upgrade them.
Those stock coils are not very strong, they are rated at 10k volts at 4000 rpms. Under ideal conditions it take 6 to 8 k volts just to fire a plug. With a low battery the output is greatly reduced. An engine that hasn't gotten the best maintenance, needs a hotter spark to get the plug to fire the cylinder.
I might suggest getting a Harley style dual fire coil from one of the older points or early electronic ignition bikes.
Like item # 251304164581, it says it's for electronic ignitions but it has a 4 ohm primary so it will work for you. To use it just hook both points wires together and then hook them to the coil. This coil fires both plugs at the same time. Just as the 80 up TCI bikes and most aftermarket ignitions for all years.
I ran a similar set up before I got my Pamco.
Leo

 

[Take Warning15]

It doesn't matter that the spark plugs are firing twice as often with that coil? I'll go check my meter right now. I did touch them together but I didn't remember to subtract that number from my readings. I'll go check.

 

[Take Warning15]

primary would then be 4.7 ohms

 

[xjwmx]

If the coil measures bad the coil is bad, but if it measures good the coil isn't necessarily good. Of the two TCI coils I've used up both measured fine with a meter.

What happens is that the insulation in the secondary winding begins to develop a low resistance path, maybe to the grounded core. Which makes it occur to me that you might be able to get by with such a coil for awhile if the core wasn't grounded.

This is why if you're testing big electric motors you don't use an ohmmeter to check the windings, but rather a thing called a megger which is an ohmmeter that uses 1000v as the voltage it uses to test the resistance. It will measure breakdown in the insulation that a low voltage meter can't. A big motor can test fine on a regular meter but not run, or even blow up.

 

[Take Warning15]

I suppose at this point, I'll start from the beginning. The reason I suspect a bad coil is as follows: I just rebuilt the engine, replaced plugs wires condensors and points. I have a strong battery. However, the bike will not idle for more than a few moments before quitting. I noticed yesterday that the right cylinder is running okay. I took the left plug cap off and got no change in idle. However, when I replaced the cap, I could here the left side come alive but only for a minute. I put it back on and let it run for a few moments more. The left plug is fuel soaked. That led me to believe I am getting weak or intermittent spark on that side. Thoughts? Of course, the carbs are suspect, but I'm going to focus on spark now because it has been a persistent problem even with the last carb set I had on it and no matter how thoroughly I clean the carbs.

 

[5twins]

You seem to be making this much more difficult than it really is or needs to be. Simple coil testing procedures are given in the factory shop manual. No tests are performed between the coil wires and ground, only between the various wires on the coil .....

If you take into account the "±" values, yes your primary is a bit high but your secondary is within the spec. However, you tested that to ground, not as the manual instructs you to. You may want to re-test that one.

 

[5twins]

I would switch the coils from side to side and see if the problem follows.

 

[Take Warning15]

Okay, will do. I checked the manual first (haynes) and all it says is to have a yamaha technician check it if it is suspected bad.

I'm still getting 4.7 and 7000. Is that too far out or would you say I'm good? Again, I'll switch coils and see what happens, but don't have time right now. Thanks for the help.

 

[retiredgentleman]

Forget about trying to make those old stock coils produce a spark. They were only marginal 30 + years ago. Put them in the recycle bin.

Buy yourself a single, dual output coil, of about 4 to 5 ohms. As XSLeo mentioned, simply connect both sets of points together in parallel.

Assuming you have a good battery , and strong charging system, you will have a strong spark. Get your ignition working first before you chase carb problems.

 

[XSLeo]

I was thinking the secondary side grounded through the mounts not the orange side of the coil. He seems to get the proper reading at the mounts. From the primary wires to ground just checks for a short in the primary. If you get anything other than an infinity reading it shows a short.
On most anything I'm talking about I mean on the part. As on the coil the mount i'm talking about is the one on the coil.
Leo

 

[Take Warning15]

I think that the coils have been replaced by the previous owner. They do not look original. I'm pretty sure they are the stock replacements from Mike's. Are they no good as well?

 

[retiredgentleman]

I think that the coils have been replaced by the previous owner. They do not look original. I'm pretty sure they are the stock replacements from Mike's. Are they no good as well?

The stock type coils only produce about 13k volts. Most "modern" coils produce 30k volts or more.

Put your bike into a dark garage and have a look at the spark you are getting. It is most likely a weak yellow spark at best and maybe no spark at times. Here's a pic of a healthy spark from my bike.

Mikesxs sells a lot of sub standard parts, so that's my opinion on those stock replacement coils.

Another thing to consider. If you are losing voltage through the kill switch, fuse holders and ignition switch, you may only have 11 volts getting to the coil primaries. That means your secondary high voltage is reduced as well.

 

[5twins]

It's not that they're no good, they're just very weak, even when new. They only produce about 10 or 12K volts. Nice aftermarket coils usually give you 30K volts or more. If everything else is right with your bike, the stock coils will work fine. I ran mine with them for the the first few years I had it. The factory built near half a million of these bikes with them and they all ran. They used the same coils on several other models as well.

If you have any future plans for a Pamco, I would advise you not to waste any money on new stock coils but rather apply that towards the Pamco. I would get the basic kit for about $100 and buy a nice aftermarket coil to go with it. Or one of those used Honda coils guys are starting to use. For about $15, you can't beat them.