This is a weird issue

I started having power issues with my 82 seca750. No guts, I barely made it home up a hill. I had just swapped the engine oil and filter exchanging the 10w30 for 20w50 because we're moving into warm weather. I thought maybe that was the issue but everyone I talk to says no dice.

It seems like a fuel thing so I cleaned the carbs perfectly and even swapped the tank for the one from 650 to rule out petcock, tank and bad gas issues.

She idles like a dream now, but still has low pickup on the throttle and backfires regularly. I parked and checked my exhaust, pipes 1&2 are blazing hot, but 3&4 are barely even warm.

It would explain the power problem and the backfire if those cylinders aren't firing, but I can't figure out why that would be and I'm not getting anything out of Haynes to explain it. I'm pretty sure it's not the ignition coils because those two cylinders are fired by separate coils.

Any suggestions?

 

sounds like ignition to me, im not an expert on xj's but do have alot of experience with alot of different bikes. i had an issue identical to this on a gs1100 not too long ago that ended up being a bad coil. the bike had 2 coils, 1 firing cyl 1 and 4, and 1 firing cyl 2 and 3. 1 of the coils was bad causing the problem. that would be my first guess. seems in my experience most i-4 bikes are setup this way. id check into coils before doing anything else.

 

Read your plugs. If they are stark white your lean, if they are black your rich, if they are light brown your good. If they are soaking wet you have no spark. Also check your spark:use a spark tester or if not stick a spark plug in the wire and rest it on a metal piece on the engine and see if you have a good spark. A coil can go partially bad and only fire on one of the two wires. I know you said you cleaned the carbs I dont doubt you did a perfect job but you may have loosened something that has now gone and stopped the float from closing and if your petcock isn't working right would just flood the cylinders with with gas. But check your plugs first and then your spark and we should be able to go from there. Good luck.

 

I have had issues fouling plugs when tuning - the long times at idle do them no good. Clean them well and you can even heat the ends with a propane torch then clean (I saw this on a thread I posted about 4 fouled plugs).

I did this last Saturday for exactly your symptoms and she is running like a top.

 

+1 on burning the gunk off the plugs to "unfoul" them. i usually just spray em down with carb cleaner and light em on fire to burn off the excess. works like a charm.

 

Sounds like your coils are connected wrong. If you have a bad coil, it's be 1and 4 or 2and 3, not 3&4. If for any reason you had them disconnected, you reconnected them wrong.