Motorcycle Repair: tested, spark plug fouling, honda vt500c

Question
QUESTION: hello i have quite a challenge on my 83 Honda shadow VT500C .the front coil  plug 2 front cylinder right side fires constant and the other 3 plugs not so much or at all. all fuses are fine all 4 plugs are new all the plug wires are new and making good contact tested using ohmmeter the 2 coils test fine as well the 2 cdi boxes have been swapped back and forth with same result i just cleaned and shined up the pulse generator it is a lil worn i have checked everything i think i was told you cant test the cdi boxes and i don't think its in the timing i am stumped  almost i dont like to give up  the guys at the honda shop help a  little bit but not alot

ANSWER: Joshua, the coils are double-ended and fire both plugs at the same time. Did you test the spark plug cap/connectors or replace them? They should have about 5k ohms resistance values. Always test for spark with both spark plugs grounded to the engine. Otherwise, you have opened up the circuit and won't be able to get a decent test result.

Does the bike run at all? The coils should be around an ohm or so on the primary side and perhaps 12k ohms across the secondary plug wiring.

The pulse generator is a two-part assembly, one for the front and the other for the rear cylinder. Test the pulse generator wire pairs to see if they are both the same values. The pulse generators don't have anything to "wear" as they are triggered by a magnet off the flywheel/rotor. The wiring can decay, however, as well as the wiring connector.

http://www.cmsnl.com/honda-vt500c-shadow-500-1984-usa_model7640/partslist/E++08.

http://www.cycletronics.com/about.htm as a possible parts/repair source I found on a web search from the VT500 group

http://autos.ph.groups.yahoo.com/group/ascotvt500enthusiasts/

Make sure that everything else is okay... compression, valve adjust, carbs are clean and no damaged diaphragms which can cause spark plug fouling, which looks like no spark across the gaps, at times.

Bill Silver

---------- FOLLOW-UP ----------

QUESTION: yep i have tested everything possible i think it might be in the wiring harness or something   the bike runs but on one plug  caps tested wires new coils in good condition and tested fine swapped cdi/igniter boxes and get same outcome pulse generator wires both under gas tank and where it meets the cdi boxes are reading 450 ohms testing good so problem is between boxes and coils i think unless there is something im not looking at or for but im now mechanic just a tinkerer and parts changer. while running though on one plug i use a timing light and clip on each wire  then squeeze trigger and only one wire is lighting my light plugs are wet except the one thats firing thank you bunches for all your help

Answer
Test the pulse generators at the engine connection, first, before going further up the harness. The KILL switch and ignition switch both feed the circuit to power the ignition in series. Primary coil resistance values are 2.0 ohms at white/yellow and white/blue pairs. Secondary resistance is actually 20-27k ohms w/o plug caps and 29-40k with caps. Pulse generator ratings are 480 ohms plus/minus 10%. Air gap at the pulse generators is .012"-.035"

You should have battery voltage at the black/white wires on the coil leads.

I just found my VT500 shop manual so can give more accurate specs. Honda fours, back in that era had issues with spark units, mostly due to defective soldering joints. Repair ideas were found on the Honda V-4 sites.

When in doubt, look for bad grounds, too!

Good luck,

Bill Silver