bike wont start, backfires out exhaust or carbs

ok, so my friend had my kz750b for a while and he couldnt figure out why i wasnt getting spark, turns out he was messing with the wireing on the points and the shaft it rides on, but now i figured out how to rewire it so im getting spark, but i cant for the life of me get it to start, all it does is backfire out the carbs, and sometimes the exhaust too depending on how i move the previously mentioned shaft. also it seems like when the pistons are coming up they are blowing the pressure out the carbs, i dont know if my timing is off, but i dont think he messed with it, if anyone has any ideas, please let me know, it would be greatly appreciated

Just a quick thought, all the time you were messing with starting it, you might have fouled out the plugs. So i would use some starting fluid to clean the plugs, or better yet go get some new ones......you might be pleasently suprised.

uummm wrote:

i figured out how to rewire it so im getting spark, but i cant for the life of me get it to start, all it does is backfire out the carbs, and sometimes the exhaust too depending on how i move the previously mentioned shaft.

It is very likely that your ignition is 180 degrees off. The 2&3 cylinders are firing when the 1&4 cylinders should be firing and vica versa. I've accidentally done this before with the same result. Try switching the wiring at the points or on the input side of the coils.

Also, It wouldn't hurt to perform a compression test just to make sure the mechanicals are ok.

check the plug wires, make sure they are not reversed

The same thing happened to me after putting the engine back in my KZ550 after a rebuild. Turns out that I had the coil input wired mixed up. After I switched the wires, she started right up.

I had this on my 550. I put the advance mechanism (Under the stator plate) together 180 deg. off. Switched it back and it fired right up.

uummm wrote:

ok, so my friend had my kz750b for a while and he couldnt figure out why i wasnt getting spark, turns out he was messing with the wireing on the points and the shaft it rides on, but now i figured out how to rewire it so im getting spark, but i cant for the life of me get it to start, all it does is backfire out the carbs, and sometimes the exhaust too depending on how i move the previously mentioned shaft. also it seems like when the pistons are coming up they are blowing the pressure out the carbs, i dont know if my timing is off, but i dont think he messed with it, if anyone has any ideas, please let me know, it would be greatly appreciated

Hello Ryan, and WELCOME to KZR forum! We're glad you're here!

After assuring correct ignition timing, if backfiring and rising pistons continue blowing pressure out the carbs, would check valve clearances (especially intake valves), because failure to completely close (seal) might be allowing compression and combustion to go back through the carb.

Good Luck!

I think the KZ750B is a twin. Hopefully, a twin owner will chime in. I don't the four cylinder advice will be all that helpful hehe I honestly don't know how the bike fires the two plugs on this twin. I have never seen one. It does sound like a wire swapping (green and black backwards) on a 4 cylinder... If you get the rotor on a Dyna S on backwards, you will get the SAME issue on a 4 cylinder even with the wires on correctly as the firing point is being picked up 180 degrees from where it should.

before you spend more time diagnosing, just flip the points cam around it goes on two ways the righ way and the wrong way . it is hard to tell apart by looking at it. when i got my 750 twin someone was playing with the points and coil, i kicked on it till i was blue in the face and all i got was back fires and such. flipped the advance mechanism and it started in one kick.
ok found a pic of mine.
1.take out the three screws holding the points plate on
2.remove points plate
3.take out the little bolt in the middle
4.take the cam mechanism(what the red arrow is pointing at) off rotate 180 degrees, reinstall everything
5.kick it over.
6. time it with timing light

uummm wrote:

ok, so my friend had my kz750b for a while and he couldnt figure out why i wasnt getting spark, turns out he was messing with the wireing on the points and the shaft it rides on, but now i figured out how to rewire it so im getting spark, but i cant for the life of me get it to start, all it does is backfire out the carbs, and sometimes the exhaust too depending on how i move the previously mentioned shaft. also it seems like when the pistons are coming up they are blowing the pressure out the carbs, i dont know if my timing is off, but i dont think he messed with it, if anyone has any ideas, please let me know, it would be greatly appreciated

From the view when looking at the points with engine running, the KZ750 twin's crankshaft turns counter-clockwise (opposite direction of wheel rotation as the bike moves forward). Some folks refer to this crankshaft rotation as running backwards, like the BSA's.

At the risk of looking foolish, I'll assume there's an F mark (perhaps inscribed on an advancer) that passes a fixed timing mark on the engine case, followed by a T mark which shortly thereafter also passes the fixed timing mark on the engine case, all happening while the crankshaft is rotating counter-clockwise.

For timing purposes, ignore the T mark which stands for Top dead center, and NOT for Timing.

As known, when the points are "closed" means the contacts are joined together with no gap existing beween the contacts.

"Opening of the points" is when the gap begins between the contacts. This opening causes the coil to shoot high intensity voltage through the plug wires.

The points are supposed to begin opening when the F mark aligns with the engine case mark. The fiber heel on the points rides against the rotor cam which serves to open the points. In other words, at the moment when the F mark aligns with the case mark, the points should begin opening which causes the coil to shoot voltage and fire the plugs.

While hand-turning the crankshaft counter-clockwise, it should be easy enough to see the points begin to open just as the F mark aligns with the engine case mark.
If the points fail to begin opening as the F mark so aligns, something is wrong (broken, worn out, assembled incorrectly, or mal-adjusted).

A strobe type timing light is imo the best way to fine tune the timing by seeing the F mark align at idle rpm and watching the F mark move "backward" as the timing advances with higher rpm.

Good Luck!

Edit -- use the larger "fake" nut (?? 17mm ??) on the crankshaft to hand-turn the crankshaft. Do NOT use the smaller bolt head of the bolt which threads into the end of the crankshaft, because it might strip the bolt threads or the threads inside the crankshaft.

I take it that both plugs fire at once? The points plate being mis-oriented has to be the problem then... there is only ONE wire from the points/condensor to the coil.