do i need new coils?

The plugs on my 79kz400ltd have been fouling very badly. after a 5 minute ride, they are completely covered in soot. I was wondering if the coils were bad, and tested them today.

THe primary resistance is 4.8 Ohms. (the manual says this should be 4 ohms.)

The secondary resistance is 11.8K-12.2K Ohms. (manual says 13K)

Are these numbers close enough that my coil should be good

Do these results indicate a good or bad coil? both of these numbers are close to what the manual recommends, but are not quite on.

If i do need new coils, which ones doe i need? the options on Z1 are 3 or 5. I am assuming i would need to go with the 5? THanks in advance for any help

I am not an expert by any means, but what is your VOLTAGE at the coil? That seems to be a big problem with these older bikes and it is pretty easy to fix with the coil mod.

CSB

your readings appear fine, I believe you can have aprox. 10 to 15% diff. due to differences in multimeters and conditions. verify jetting and like previosly mentioned voltage, also verify timing and any air box restrictions

WiredGeorge's coil mod

This coil mod is a have-to on bikes as old as ours, just to make sure that the coils are really getting full voltage.

I did the coil mod on this last fall, and do not remember the exact number but the voltage to the coils was correct. my next step will be to check everything w/ the carbs, but i wanted to try the coil first as this seemed like it would be the easier thing to do first.

Fouled plugs are sometimes the result of low voltage at the coils. Your coil seems to be fine and if you did the coil repowering mod, you are probably doing ok there too!

OK what else can cause sooty plugs... when was the last time the valves were adjusted... soot means incomplete combustion. Poor compression due to valves hanging open can be a real source of poor combustion. If your valves have clearance and your compression is good... well look at carbs.

AIR FILTERS? Do you use pods? What type? Stock airbox? What type airfilter... a VERY dirty or over oiled air filter element can cause soot. Same with wrong spark plug type. Use NGK plugs as the heat range recommended in your owners manual will work right. If you buy Champion or Autolite plug from some application chart, could be the heat range doesn't cross exactly and the plug may be too cold.

Last, well, the carbs. What air jets are in your 79? The bigger air jet, located under the carb top and the little banana shaped cover is the pilot air jet and the wrong one can foul plugs in a heartbeat if it isn't the correct size. The other air jet could as well but not as likely because it has relatively less volume effect on the air/fuel mix as the slide will be open a lot farther with a much greater volume of air passing through the venturi. Do you have PLUGS in your pilot wells? The pilot jets pick up gas from the main jet well via a cross passageway on your bike. If the plugs are not in place; too rich. Float levels MUST be checked wet... that is, service fuel level method must be used... you must use the clear plastic tube method to check fuel levels. I think it is 4mm service fuel level for your bike (from memory) but check your manual. If the fuel level is too high, you can foul plugs. If your mixture screws are not adjusted correctly, you can foul plugs...

I checked the valve clearance last summer. I believe i was able to set them correctly, although i had a lot of trouble getting the feeler gauges in.

I have a stock airbox. I just put a new air cleaner element (the sponge type part) in last summer as well.

I was just using autolite spark plugs, and now that i think about it, i have always just put in what was in the bike when i bought it last spring. When i bought it, the plugs already fouled. Can NGK plugs be purchased at a local auto parts store (i.e. NAPA), or possibly at my local motorcycle dealer?

I do not know what size jet is in the carb? in fact i do not know much of anything about the carbs. Where are the mixture screws located? are these the brass screws on the sides right above the vacuum cap screw? Also, i check the float level on the left carb and the level was close to 4 mm. (i didn't check the other one because i discovered my problem somewhere else.)

So i guess my next step will be to get NGK spark plugs and then move on from there.

I think you are being slightly mislead about your carbs. There are no removable air jets in the carbs you have. They are pressed in, and they aren't located under a cover either. And the idle mixture screws should have a limiter cap on them, making it very difficult to turn them the to the point where the idle mixture gets rich enough to make the plugs sooty. But you are right, the idle mixture screws are located next to the vacuum caps. And if the limiter caps are gone, someone has removed them in the past.

Standard jet sizes for the 79 400LTD:

Pilot jet #35, primary main jet #70, secondary main jet #80.
Pilot screw 2 1/4 turn out from lightly seated.
Fuel level in the float bowls 2mm +/-1mm below the carb gasket surface.

I would also recommend that you replace the points and the condenser, and adjust the points gap and the ignition timing properly. New plug caps won't hurt either if you haven't replaced them already. The built in resistance in old caps will increase over the years, causing a weaker spark. NGK also have non resistor plug caps that fits your bike. And when, or if, you replace the caps, cut off 1/4 of an inch off the end of the plug wire to make sure the caps have a good and clean connection to the core inside the plug wire. And your bike should have NGK B7ES plugs and your local motorcycle dealer should have them.
And except for checking the fuel level in the float bowls and the idle mixture screw settings, don't break into your cabs before you know everything else is good and adjusted properly.

I put is ngk b7es plugs today. They are still fouling only after a few minutes. In general, the bike ran pretty poorly. I rode about 5 - 10 minutes and during that time the engine died 3 times. It would take a few minutes before i could restart it. When i got back home, i was trying to check the idle. Once started it would only idle for about 10-15 seconds before it would die. During this time the idle would go anywhere from 800-2000 rpm. Also, if i tried to give it gas, it would die immediately? any ideas? this seams to me that this might indicate problems with the carbs, but i don't really have any idea of what exactly it may be.