Maxim 750 spark questions....

[dorkpunch]

Kay, working on an '82 maxim 750 for a friend. It has some odd spark issues going on. For example, with the plugs out, crank for about 10 seconds and stop. 2 plugs will fire after about 3 seconds of sitting. its not the same two plugs either, it alternates randomly. ALSO, if you turn the key off, then turn it back on, wait a couple of seconds, 2 plugs will fire. NO CRANKING. i can turn the key off and on a bunch of times and every time, 2 plugs will fire. Note- the two plugs are on oposite sides, one coil does 1 and 3 and the other does 2 and 4, right? WHAT THE HECK IS GOING ON? oh, and it wont start either. but, if you crank it for about 10 seconds and stop, in about 3 seconds you get a HECKUVA back fire...

[TopCatGr58]

Hey DorkPunch,

The phantom sparking is NORMAL, something to do with the electrical system providing power to the coils ALL THE TIME, then when the power is turned off, the field collapses, and you get a spark.

You're were a bit wrong with your wires and plugs assignment, it's the same as the XS11, one coil fires 1-4, the other fires 2-3, so you may have them connected wrong, which is another reason you're getting a big backfire. Check, swap the wires, and you'll probably get it running!
T.C.

[dorkpunch]

thanks TC, but the plug wires are correct. They only reach one way, theres no way to mix them up. its done the backfire thing a LONG time (according to the owner...) I have had it running in the past. i rebuilt the carbs because of crappy running. after that it ran fine for about 10 min, then fouled all four plugs instantly and died. new plugs and it ran fine for about 10 min, then acted like 2 cylinders cut out for a while, then it died completely. after it cooled a bit it started but ran poorly (like on 2 cyls only) and managed to limp home. once it cooled completely, it started and ran fine, but i never rode it again. its sat for about a month, and now it wont even fire on primer (except for a BIG backfire, when then engine isnt even cranking over).

[TopCatGr58]

AHA,

When you rebuilt the carbs, did you use K&L kits? If so, you probably got ahold of the wrong sized pilot jets, K&L are notorious for that. Put the old ones back in.

As for the wiring of the plugs, I was going by the electrics diagram on the Yamaha Parts Web Site, they show the coils feeding 1-4 pair, and 2-3 pair, it's an inline 4 cylinder, and I can't imagine Yamaha changing how they use their pair of coils and their waste spark system. Perhaps it was wired wrong to begin with, and you've been only running on 2 cylinders?

With it crapping out when hot, could be the coils going, they could short out or such when HOT, have you tried to check the PRIMARY and Secondary/high tension wires after it's warmed up?

Fouling plugs is either too much fuel, not enough air, or not enough spark! How's the air filter? Good Luck, keep at it!
T.C.