81 KZ750 LTD Backfire?

I just bought the bike this last weekend and took it for it's first true drive today. The guy I bought it from said it was gone over and carbs cleaned, etc recently, but there was a little rust in the tank. So today when I was riding, the gas light came on and about 3 more miles, it ran out. So I stopped an switched it to my reserve tank. I get to the gas station and put a full tank of 93 in and head off. No more than a 1/4 mile down the road I get a loud backfire. Now when I rev to 3k and above, right about at 2k on the way down I get a pop or occasional backfire. There is also a blue flame that pops out of the right exhaust at the same time. It is ALWAYS the right exhaust. Any ideas what this is? Did the rust settle towards the reserve and just cause a few problems? Drain tank, clean and refill? Thanks in advance!

Chase

Welcome to KZr Chase.

It looks like you're on the right track in draining and cleaning the tank and petcock thoroughly.
Pulling and re-cleaning the carbs is also a good idea.
Even though the PO stated the carbs were cleaned, doesn't mean they were done thoroughly/properly, if at all.

It might be that your ignition is firing intermittently on either the #3 or #4 cylinder - gas flows through into the exhaust without igniting in the cylinder, then when the ignition sends spark through it ignites the fuel in the exhaust = backfire with blue flame.

It could even be that one of your exhaust valves is partially stuck open, and when the fuel ignites in the cylinder it's going past that partially open valve and causing the unwanted 4th of July show from your right side muffler. :ohmy:

Where can I get the tank cleaning stuff? I have heard that phosphoric acid, I think it was that, works well, but where can I buy 2-3 gallons of something like that? Thanks.

del

I tried draining the tank today and put in some 87. The same thing ended up happening, the right exhaust kicks out a blue flame sporatically and I notice the "Puff" coming from both the right and left exhaust. The noise however only seems to come from the right exhaust. I did not notice any debris in the 93 I took out, but I think cleaning the tank is sitll something that needs to be done. Any opinions? Carbs seems to be ruled out because it is not consistent, but we are going to check plugs and see if anything is running rich/lean.

Sticky valve seems like it could be the issue, but what does everyone else feel?

I would suggest that before you start pulling the head to grind in a new valve you should do like R4 says and check the ignition thoroughly - all the wires from the electronic ignition to the ignitor and from there to the coils and from there to the plugs. You might want put in new plugs while you are at it.
I had an intermittent spark once when the battery breather came off and battery acid had dripped down and corroded a critical connection in the ignition circuit.

If you dont have a manual look here

www.mbsween.com/kz750l3/index.html

I will definitely do that. Now that you mention it, the guy did say the battery breather leaked on the exhaust causing a spot in the exhaust that doesn't have the same shine. Wonder if it touched a little more than he thought...

do you have a fuel filter on the bike, if you are seeing rust in there it has prob made its way to the carbs,, you may want to pull them and give them a good cleaning.
Also check the valves at this time to make sure everything is in spec.

This is in addition to 4Rs help

Check the compression. make sure the valves are seating well.

Blue flame tells me that raw gas could getting into the exhaust and igniting? If the flame was from an ignition inside the cylinder, I can't understand how the flame could get all the way down the pipe and through the baffles and still be visible.

When I got my 650 I had erractic running at about 4500 RPM and above; also on deceleration. Put in new plugs and all was well.