Motorcycle Repair: 85 honda 700 magna, purge valve, pulse generators

Question
my bike has just over 19,000 miles on it. at the end of summer about 1 mile from home started missing. pulled plugs, 1 looked wet. replaced all.about 1 mile home, missing again. pulled plugs they look wet. have clemer manuel, check fire good blue fire all 4 plugs. put in new again and backfire out both pipes. even drained all gas and filled back, same thing. need help with this one bike has never give any trouble and is serviced regular. dont want to take parts off if not needed. thank you for your time, larry

Answer
Larry, the backfire was probably just unburned fuel lighting off in the exhaust system, after fuel fouling the plugs.

It could be a failed air cut valve diaphragm..

http://www.cmsnl.com/honda-vf700c-magna-1985-usa_model7599/partslist/E++1801.htm...  #9
aftermarket companies make replacement kits for them, if Honda is NLA.

Check the pulse generators in the stator... at the connectors with an ohm meter. If both read the same, then they are probably okay.

http://www.cmsnl.com/honda-vf700c-magna-1985-usa_model7599/partslist/E++07.html  #8

More out of the ordinary issues can come from the purge valve in the canister system

http://www.cmsnl.com/honda-vf700c-magna-1985-usa_model7599/partslist/F++3301.htm...
http://www.cmsnl.com/honda-vf700c-magna-1985-usa_model7599/partslist/E++18.html  #31

Make sure nothing has gotten sucked into the snorkel for the air intake..
http://www.cmsnl.com/honda-vf700c-magna-1985-usa_model7599/partslist/F++1901.htm...  #15

Hopefully, it isn't the spark unit:
http://www.cmsnl.com/honda-vf700c-magna-1985-usa_model7599/partslist/F++1801.htm...  #31

Once you eliminate the other ignition components as being an issue, it comes down to that bad boy.

Check whatever the first fouled plug's cylinder is attached to. If the air cut valve is on that carb, check it first. Also check any vacuum lines attached to that cylinder's intake system to see if other control units are attached to it and have failed diaphragms.

Factory shop manuals are always the best bet for troubleshooting, as they will have specific values for items like pulse generator coils and the ignition control module.

Bill Silver