XJ750 White Smoke from Engine

Hey everyone,

I purchased an ‘83 XJ750 last week that was not running. After replacing the battery, changing the oil, and rebuilding the carbs (thank you to everyone who has posted on these subjects, this forum has been invaluable to this point), I got it back up and running.

However, while going for a test ride around the neighborhood, the motor started running rough and died on me. White smoke started emitting from the front end of the bike. I couldn’t find the direct source of the smoke but assume it was coming from the motor.

I am very new to working on motorcycles, so not sure where to even start with troubleshooting.

Any help would be appreciated.

 

If it wasn't coming out the tailpipes, are you sure it wasn't an electrical short and melting wires? The smell should give that away, though.

Oil too full? Can happen from carbs leaking, filling the crankcase with gas, but typically that smoke will be out the pipes in back.

Do things still seem normal electrically? Headlight comes on, neutral light on, cranks with the start button? Did you check if you still have spark?

Do you see any major oil leaks from the valve cover, oil filter area or underneath?

 

I should’ve mentioned that it wouldn’t start immediately after it died on me. But, after a few minutes of pushing it down the street, it fired right up.

The wires on the front end of the bike were warm to the touch. All 4 exhaust pipes were hot. Everything was in order electrically - headlight was on, neutral light was on, cranked over even on the failed starts right after it died in me.

No noticeable leaks. Unless it was leaking and burned off before it could pool on the ground.

I will take another look at the wires on the front end of the bike today to see if anything looks melted, but it was a substantial amount of smoke, seems too much for a burnt wire.

 

Oil on exhaust

 

I guess I was reading that as the smoke started, then it ran rough and died. But if it was just burning off oil leaks on the exhaust, they would be completely unrelated. Carb issue, running out of gas, restricted petcock...who knows. Then you notice the smoke because the bike isn't moving. This seems likely.

 

While I inspecting the gas tank when I was working on the carbs, I saw the longer tube (yellow in color with a screen on it, not sure what it’s called) of the petcock was unattached in the gas tank. I also found a spring? Not sure if the petcock has a spring in it. I took the tube out and fixed it back on the petcock. Put in a gallon of fresh gas and rode it for 5-10 minutes before it died. That’s when I noticed the smoke. Might be worth noting that I took it on a test ride a few hours later and it died on me and smoked again.

Could this be a carb sync issue? While rebuilding carbs I made sure floats were within the 17.5mm +\- .5 mm spec.

 

The yellow tube, I think you are referring to the inlet tube with a screen that goes inside the tank? If that came loose, there is a good chance you got sediment in your petcock or carbs that blocked fuel. Your symptom of not restarting at first, but restarting later could be due to restricted flow that slowly refilled the carbs after running out of fuel. Likewise with your description of the second test ride. With restricted fuel flow, everything might feel normal for a few minutes, then fuel is finally used up in the bowls and it starts to sputter.

The petcock has a spring in it, but that is part of a vacuum valve that shuts off fuel unless the motor is running. Do you still have that style of petcock with a vacuum line attached to one of the carburetor boots? Sometimes the petcocks get swapped out for a simple on/off style. And if you still have a vacuum style, the spring can't come out without disassembly and major fuel leaks.

Also with the original petcock, the inlet screen is retained when the petcock is screwed onto the tank, even if it is a bit loose to start. Some aftermarket petcocks with a pressed in screen might not fit the same.

Carb syncing and setting float levels are two unrelated things. Neither one would really have symptoms of running normally, then running rough and dying. Petcock issues and debris in the fuel seem more likely from your description.

 

Roast, you’re the man!! I will take the petcock out and give it a thorough cleaning and see if I can identify where that spring came from and update with results. I have a vacuum line attached to the petcock, yes. But no fuel leaks since I got it running. So sounds like the vacuum valve is working correctly?

The previous owner said that gas was leaking all over the place on the left side of the tank and carbs when they filled the tank up more than halfway. So I figured it would be a stuck float and issue with the petcock. Turns out it was both. So that makes sense.

 

If you have everything back apart again, it would be a good time to add an inline filter between the tank and carbs. They never had these originally, but it is a common and recommended upgrade.

The simple test with the petcock before tearing it apart is just disconnect the fuel line to the carbs and turn it to prime (PRI). If it is anything less than a steady flow then you can proceed with petcock cleanout.

You can also test your vacuum valve with this fuel line disconnected....fuel flows on Prime, no flow at ON or RES.

Also a test you can do is to open the bowl drain screw on each carb separately and make sure you have good flow and make sure the inlet screen at each float needle is not blocked. This can be a little messy (use a drip tray or tube) as you need to let the bowl fully drain before you know how much gas the float needle is passing. But this is a good test to do before going through the hassle of pulling the carbs off again.

 

Quick update:

Tried all the tests listed above and everything looked good, albeit, to my untrained eye. I got an inline fuel filter like you suggested and filled up the tank after installing it. Rode for like 30 minutes around the neighborhood with no issues. So hopefully I can chalk it up to the inline filter, running for the first time in a while, or being a newbie rider. Whatever it is, I’ll take it. As long as it doesn’t come back lol

Still planning on rebuilding or replacing the petcock because I have no idea where that spring came from. But everything else looks good, apart from a very slight leak where the fuel line connects to the petcock, but thinking a hose clamp will do the trick there.