Checking coils

I am rebuilding the carbs on a 750 Maxim for my neighbor. I figured I would check the coils because it seemed that I was losing cylinders erratically. Sometimes #1, sometimes 1 & 4, sometimes 2 & 3. I decided to pull the carbs and rebuild them to rule out any problems with them. They were not really as bad as I expected. I should receive the kits tomorrow and can re-assemble them.
First I did a check on the pickup coils and checked OK. Next I did a check on the primary side of the coils, well within the limits. Then I performed a check on the secondary (high tension) side of the coils. No resistance at all on either one :?: . According to the Haynes manual, it should show a resistance of 11 K ohms, +- 20%. I get absolutely nothing on any scale on my DVOM, which indicates an open. Strange that both coils could be bad at the same time. It seems highly unlikely. But, still no reading.
When buying new coils, what typically is the best replacement coil to use on this bike and who handles them at the best cost? I thought about buying a used set, but then mine are used so I am kind of reconsidering it.

Ed

 

Funny I happened acrossed this post. I too have this same problem. I had a rod go in my motor about 7 months ago. I just got the rebuilt motor on yesterday. Being so excited to ride again, I attempted to fire it up. Nothing. Motor turns over, but no start. I first checked the plugs, no spark(on any of them) I checked and rechecked the wiring, all good. Checked the pick up coils, had 702 ohms resistance(haynes says 700 ohms +/- 10%) Checked the coils(primary and secondary) Had well within tolorance resistance on the primary, but absolutly nothing on the secondarys.

I too find it really odd for both coils to go south at the same time. When it was running 7 months ago, it was firing good for about 2 hours of rideing. I was loseing number 4 cylinder and sometimes number 1. I knew one of the coils was growin "weak". Would sitting for six months make them go out?

I know this is far off base, but if the CDI was bad, and the coils where hooked up, would that cause a "false" reading on the secondary side of the coils? Is there a way to check the CDI? I took the cover off, the circuit board dont have any burnt spots. Also, when tring to start the bike, I did notice the back side of the CDI was warm to the touch. Am I looking at new coils and CDI?

 

There is supposedly no check for the CDI other than replacing it with a known good one. I didn't have the primaries hooked up when I checked my secondaries. It was running before I pulled the carbs to rebuild them but ran like crap and erratically like I described before. Rather than buy a set of used ones I want the opportunity to consider a new set first. A lot of it depends on the cost.

Ed

 

I figured that about the CDI. I have been tring to find a good place to buy new ones. The cheapest I have found is around $190. Still looking. I will post if I found a cheaper site. The reason for the high cost is that they sell everything seperatly. Meaning you have to buy the coils, then the HT wires, and the plug caps. If I find anything else, I will post.

 

Try unscrewing your spark plug wire caps, then check primary wire for 11k ohms?