Really loud backfire and nasty smell...

My foray into motorcycle repair is not turning out exactly how I imagined when I bought this bike a couple weeks ago!

It's a 81 KZ750 four cylinder. It ran when I bought it, but had some minor problems, brakes, dead battery, old gas, etc.

So I changed the oil, cleaned the carbs, put in a new battery, fixed the brakes, changed the plugs, and tried starting it today.

It turned over, and sounded like it came close to starting, but then there would be an abnormally loud back fire, the sound of something metallic spinning inside a case and a pretty nasty smell. The force of the backfire even blew out some perforated metal that was placed in the end of the tail pipe. Now I'm worried I did something serious to the bike.

Any ideas?

Upon seeing the title, my first thought was to cut down on the Taco Bell. Sounds more serious than that however. There are much more knowledgeable people here than me, but to start have you checked the timing?

If not already done, would double-check sparkplug wire connections to assure being routed to the correct plugs.

There are two ignition coils located underneath the fuel tank.

Each coil has two spark plug wires coming out of it.

One coil is supposed to have its two plug wires attached to the OUTSIDE sparkplugs (plugs #1 and #4).

The other coil is supposed to have its two plug wires attached to the INSIDE sparkplugs (plugs #2 and #3).

Good Luck!

Did you check for compression and spark? That's always the best place to start. If you don't have a compression tester just pull out one of the plugs and cover the hole with your thumb while cranking the engine over. Won't give you an accurate reading but if there is not enough force to push your thumb back off then your compression is probably too low and may need a valve adjustment.

"The force of the backfire even blew out some perforated metal that was placed in the end of the tail pipe"

Sounds like ignition timing or improper spark plug wire routing to me too. Do you have a factory manual? If so, you may want to check the timing. BTW, can you provide a picture of the tailpipe? I don't know of any perforated metal that could blow out unless the muffler has been modified. Ed

chrispysaki wrote:

..........the sound of something metallic spinning inside a case and a pretty nasty smell.
Any ideas?

My starter often spins a little making a "metallic" sound for a second after the motor fires. I used to worry but it's been doing this for the 7 years since I bought it.

The nasty smell is weird though. Critters nesting in exhaust ?

I'm with the rest of the guys on checking the wiring to the coils.

Remove the CD sized cover on the right side of the crankshaft and see if the mechanical ignition advancer is stuck. It has two weights that change from advance to retard as the rpm changes.

All parts should move smoothly. Make sure a light film of oil in on the pivot points and springs where they are connected.

Thanks for the ideas. The bike has the 2.2 ohm Dyna ignition coils, and they don't specify which coil lead goes to which plug. They are going to the 1&4 and the 2&3, however.

I'll check the timing advancer tonight.

It's a Kerker tailpipe, and it just looks like the guy I bought it from took some metal from something else and stuffed it in the end of the pipe.

The bike has the 2.2 ohm Dyna ignition coils, and they don't specify which coil lead goes to which plug. They are going to the 1&4 and the 2&3, however

Perhaps the coil with leads now going to 1&4 should instead have its leads going to 2&3.

Signal wires from igniter should also be routed to the appropriate coil. So ignition timing for 1/4 governs whichever coil has leads to outside sparkplugs #1 and #4. And ignition timing for 2/3 governs the other coil having leads to inside sparkplugs #2 and #3.


Good Luck!

Ahh, I bet it's the signal wires. I didn't even think to check those.

So is it correct that it doesn't matter which coil lead goes to which plug, as long as it's either 1&4 or 2&3 and that they are attached to the proper signal lead?

chrispysaki wrote:

Ahh, I bet it's the signal wires. I didn't even think to check those.

So is it correct that it doesn't matter which coil lead goes to which plug, as long as it's either 1&4 or 2&3 and that they are attached to the proper signal lead?

Yes, that is correct. But just as easy to connect things the oem way as shown in the FSM and wiring diagrams, which would likely avoid some confusion in the future when next dealing with these ignition items.

Good Luck!

ya know every once in a while I backfire and others have said there was an incredible horrible smell....although I have never thought so. I have also noticed the dog walk out of the room when this has occurred (they do have good hearing). BUTT, I HAVE NOTICED THIS SAME PHENOMENON IN OTHERS AND DAMN SOMETHING HAD TO HAVE DIED IN THEIR EXHAUST.

Check the wires and connections.