Carb issue or something else? 1979 XS 650

[jun8ior]

Hello,

Yesterday I was going for a gentle ride.
After a break, all of a sudden, applying the throttle caused it to cough and thump.
More gas, more thumping, too much and it would stall.
Eventually, it wouldn't start.

In the video, I'm using a tiny amount of throttle. No choke.
Recently, the right carb was pouring out a ton of gas, it came right but I'm thinking something crap might be partly blocking carb in some way.

No fuel filter and it has the original airbox.
Any ideas what's going on?

Cheers,
M

 

[member28833]

Mornin, played your video here at work. Sounds like you need to confirm that the timing isn't barely too far advanced. Do you have access to a timing light?
Just off idle like that it could be many things but advanced timing will try to "backfire" which sounds like a thump you are describing.

 

[jun8ior]

timing light

No timing light.
Why would that suddenly happen though. Its been running well for the last year with no issue like this.

Cheers

 

[member28833]

Runnin a year.. might be due for some tinkering. Like valve adjustment, cam chain tension, which plays into timing as well.

 

[hovel]

An inherent problem with a 2 into 1 exhaust system is it is harder to tell which cylinder it isn't running on. Pull the plugs, post a picture of them (the business end) and we can go from there. "Pouring out a ton of gas" could have fouled a plug.

 

[jun8ior]

An inherent problem with a 2 into 1 exhaust system is it is harder to tell which cylinder it isn't running on. Pull the plugs, post a picture of them (the business end) and we can go from there. "Pouring out a ton of gas" could have fouled a plug.

I took a look at the plugs, looked ok but I'll take a photo.

Cheers

 

[Jan_P]

Not Charging .. ??? -- It is not certain ( but almost ) but it is simple to check
Measure Voltage across battery and check that lights and other electrical is working.
Could also have lights on see if it changes with rev
And start machine turn all lights on see if it stalls
If it was the carburetor it would still run on one Cylinder.

 

[5twins]

The fact that fuel was pouring out of one carb seems to indicate some dirt had gotten in there and lodged the float needle open. It may have washed out but could now be plugging up some of the jets. If you can, drop the float bowl on the offending carb using a stubby screwdriver, carbs still mounted and full of gas. Observe the gas still in the bowl and look for any junk or dirt floating around. If you see any then you very well could have some plugged jets. The main and pilot jets are mounted in the float bowl on the 38's so if one has gotten plugged, you can remove and clean them with the carbs still in place. As long as the dirt hasn't made it's way by the jets and up into the carb body, this could fix it for you.

 

[Jan_P]

Been there done that
If we are talking a spring start -- it can happen the Float waive hangs BS 34 -s .and petrol flows out
I have never needed to open the carburetor .
Last time it happened last spring I gently tapped the outside of the Bowl with a Screwdriver handle
Closing the valve.
And it never happened for the remaining of the season.
I still think it would run on one cylinder.

 

[jun8ior]

my mechanic has had a look and said it doesnt look good. Looks like something mechanical in the top end has broken.

Thanks for all the feedback, I'll have to wait for a full diagnosis.

 

[Jan_P]

my mechanic has had a look and said it doesnt look good. Looks like something mechanical in the top end has broken.

Thanks for all the feedback, I'll have to wait for a full diagnosis.

Thinking out loud .. I am of a Suspicious nature towards Mechanics
He may be right but at this point in time I would test a bit more
If something have happened to Head end
I would suspect more mechanical rattle and more smoke out of the exhaust
But again he may be right

Perhaps take out a spark plug Right -- kicking the machine feeling the resistance
Then putting it back + taking out the Left spark plug kicking the machine feeling the resistance
Noticing any difference
I would check how the spark looks at the side that is out against the engine
If any difference

Then I Would try to start it with Plug in one side at a time It would run on one side I Believe
Be careful not Disconnecting the Unused spark plug .There can be precautions needed so not
Ignition system suffers .

Even if one can find the fault it is an advantage to have done these type of Procedures Finding on what side of the engine
.Making it simpler to find the exact location
more Easily

 

[AndersJ]

I’m not sure I would buy a diagnosis of “something mechanical”. That thumping could be it running on one cylinder for a bit (ask me how I know). I’d use this as an opportunity to get to know your bike a bit better.

 

[jun8ior]

well, that's from him seeing if its a simple fix. After looking at options, he is convinced something is broken in the top end.
Even if I had the garage for it, and the time for it, opening the engine to try and diagnose the issue could open me up to an even bigger bill.
I'm happy to do a fair bit of mechanical work, but I'm not opening the engine without guidance.


Cheers

 

[XSLeo]

Plenty of guidance on here. I have to go along with some of the others. Ceck the points and timing to elimanate them as issues then clean the carb, either by just dropping the bowls and cleaning them. Using a spray carb cleaning spray can with the tube and removing the mix screws to back flush the carb passages may help. If not, then pull the carbs for a good cleaning. www.amckayltd.com/carbguide.pdf will help with that. Shouldn't cost anything but tgime and a spray can of carb cleaner to find out. A mechanic will charge $75 + per hour to work on your bike and may take weeks. It's up to you though.
Leo

 

[nj1639]

I'd sooner a mechanic work on me wife than me bike and even that ain't happenin' .........
Jack Nicholson ripping the fountain up in "One Flew Over the Cuckoos Nest". He didn't get it done but "At least I tried."

These bikes do require a bit of tinkering from time to time. A manual or two, this forum, basic tools and a willingness to learn is all it takes to get the most of it done.......kinda goes along with the ride.

 

[Jim]

my mechanic has had a look and said it doesnt look good. Looks like something mechanical in the top end has broken.

Listening to your video and reading your description.... the last thing I'd suspect is a mechanical problem inside the engine. As others have said, you've got a carb or ignition problem. Given your stated experience, the first thing I'd swap out.... is your mechanic.