Another Coil Question

[Toolmaker Tim]

My 79SF is having the typical bad coil behavior, weak spark and suddenly running rich on 2 cylinders. The pick up wires seem ok, so I don't think that is it. So here's my question; I have a set of 4H7 coils that check 2.5 ohms on the primary side (?) and 11.8-11.9 ohms on the secondary side(?). Are these 81 coils, are those good readings and what mods do I need to make to use these coils if they are good. I also have a set of NGK plug caps that read 5.0-5.26 ohms. IIRC I need 5 ohms total through the ballist resistor and the coils, so how does that play into things? I have the carbs done, but now these electrical germlins have made an appearance. So now I need to be schooled in the ignition side of things.

[DGXSER]

Total resistance through the coils including any ballast resistor should be roughly 3 ohms. The coils seem a little low, but depending on the accuracy of your meter that could be the difference. You should remove the ballast resistor if you use the 3 ohm coils. The 5 ohms on the caps is good.

[DAVINCI]

Sounds like you got one of them auto-ranging digital meters. They can be confusing if you're not careful.

[TopCatGr58]

Hey Tim,

Yes, the 4H7 coils are the 81 series, and IIRC, mine were also ~2.5 ohms on primary side. You may want to recheck the plug caps and secondary, and look for a little "k" on the readout which should indicate KilOhms, the caps should be either zero...no resistor, or 5 kohms, not just 5 ohms, and 11.# kohms for the secondaries WITHOUT the plug caps in place.

As stated, just run a jumper on the HARNESS side of the ballast resistor connector joining the two ends together, remove the ballast resistor from the circuit, the 2.5 ohms should be sufficient for the TCI, and this way the 81 coils will get their full 12 volts ALL the time=stronger spark especially at low rpms.

T.C.

[Toolmaker Tim]

Ok, more info. Went out to the mancave after work and did some more checking. The coils on my 79SF appear to be ok. The primary was 1.5 ohms and the cyl. 1&4 secondarys were 15.8 Kohm and the 2&3 cyl. were 16.7 Kohm. Now the fly in the soup. I checked the caps and discovered the caps were different. There were 2 caps that were an unidentified short plastic type, one was a long plastic type, and the last one was made by Nippon Tokai. That one measured 11 ohms, the 2 short stem ones were 7 ohms and the long stem one was 5 ohms. I have a set of NGK caps that all measure within .3 ohms of each other, so I will try those.The problem may have been the caps all along. When I tried to put the NGK Iridium plugs in it REALLY ran lousy. IIRC I need 0 ohm caps to run those.

Randy, yup I got one of those digital multimeters, but I don't think it is a infinite reading one. I have to set the range and if the reading is outside that range you get no reading or the meter blinks, telling you that you need to change the range. The one I have is a Craftsman that my daughter bought me for Christmas, but I can't attest to it's accuracy.

If I were to get the Dyna coils for my 79F, which ones do I need? I also noticed MikesXS has Pamco coils for sale now. Does anyone know anything about those?

Thanks for the help guys.

[SFerinTEXAS]

When I tried to put the NGK Iridium plugs in it REALLY ran lousy. IIRC I need 0 ohm caps to run those.

I think I also have seen that said before to which I say BUNK.

NGK Resistor Spark Plug Cap 14mm - 90 degree elbow - LB05F
NGK Resistor Spark Plug Cap 14mm - 120 degree elbow - YB05F
The 05 indicates 5k ohms . Stock coils Iridium plugs
Runs exceptional.
Ive never pulled those plugs out since I put them in at least 10k or 12K miles ago

[James England]

I use iridium plugs and removed the resistors from my plug caps, although the engine ran fine before I did that. I don't see the point in having more resistance than necessary... doubling up on the resistance isn't likely to stop the bike working but it's hardly going to give a better spark, is it?

[natemoen]

When I tried to put the NGK Iridium plugs in it REALLY ran lousy. IIRC I need 0 ohm caps to run those.

The thing about iridium plugs is that they stay cooler than the regular steel plugs. Because they stay cooler they do not self clean as well (or much at all) and foul up easy, so if you are not running well you should stick with the regular plugs till you get things sorted out and then switch to the iridiums.

[Toolmaker Tim]

Plugs

Those Iridium plugs came out right away Nate. It's easier to figure out one problem without adding another. When everything starts running right I'll try them again. It's not like they are going to spoil while on the shelf. First I'll try replacing the plug caps tonight after work.