Bike "burps" after you turn it off...What is that about?

[pnedac]

So I picked up an 83 XS that I am redoing (previously bobbed a 77) into a cafe. It was evident that the PO was trying to make a cool bike with $0.00 and it showed. Black primer on EVERYTHING and the bike did not run. Next day I got it started and took it for a little spin. Smooth through all gears and though rough, seemed not to be in too bad of shape. A lot of backfire. PO took the baffles out of the OEM pipes and failed to rejet. After I rejetted it it was much smoother, but I notice that you can turn the bike off and just a moment later it seems to "burp" (for lack of a better term). As if it has just one more turn left in it.

I have tuned up the motor to factory specs, run a compression test (good), cleaned the carbs and set the floats, and it still does it.

Now I have the top end apart doing a full blown rebuild and upgrading to a SPARX alternator and a Boyer Brandsen ignition with some custom exhaust. I think after the rebuild the problem should be solved, but can anyone help me identify what that is? It didn't seem to be too big a deal, but I know that it isn'y normal.

Any help is appreciated.

Pnedac

 

[blackbetty]

Mine has always done this too, before and after rebuilding it (700 kit, cam, PMA, boyer). I believe it's a piston under compression stroke when you kill the bike. The crankcase pressure slowly relieves itself through the breather. I have a brake check valve on mine and when I had it positioned above the filter I was not able to hear the noise, obviously bc the foam was muffling it. When I rerouted it after I rebuilt it's like a creepy voicebox on my motor.

If I slowly turn the motor using the kickstarter I can make it ggrrrroooooaaaaannnnnnnn. I tell my buddies she doesn't like to be bothered when she's sleeping.

 

[XSLeo]

The burp is from the igntion. When you turn off the power to the ignition it often fires the plugs one last time. If the engine is in a position where one cylinder is on the intake stroke, that one last spark burns the fuel in the cylinder.
Points won't do that. My all stock 81 does, as well as the 75 with a Pamco. My 03 Harley does it too.
More noticable without the stock airboxes.
Leo

 

[xjwmx]

I've never had it happen on my stock '81. I don't think it's normal.

 

[KJ790]

This can sometimes be a sign that you are a little too rich on the pilot circuit. Left over fuel in the exhaust is being ignited just after you turn it off.

 

[blackbetty]

It's not a sound like a piston firing. It's a slow groan on mine. The sound of releasing pressure....

 

[3Deuces]

It's not a sound like a piston firing. It's a slow groan on mine. The sound of releasing pressure....

Mine does that too. Kind of an "FRRRRNT" sound shortly after I kill it. Sounds like something in the carb, but I can't track it down.

 

[pnedac]

Well Gents,
I am going to put this bad boy back together and we'll see what happens when it is all buttoned up. I get the overall feeling that right or wrong, it is not going ot kill the bike or cause any long term problems. Having the upgraded gear is always a plus in my book in terms of reliability so it is something that perhaps I will just have to learn to love. Everything else is fine so I will pay it no mind. The XS has personality! Thanks for all the replies.

 

[pnedac]

Oh yeah in addition, I would say that it could well have been/can still be, an issie with the carbs. I am going with different exhaust and deleting the airbox so again it is somehting that will be revisited in the near future. I am sure we'll get it all dialed in and that will be the test of whether or not it is really a problem or not.

Thanks Again!

 

[grepper]

Mine does this too. Started when I switched over to the Pamco Ignition. The Pamco fires one last time when the power is cut off. I asked Pete about it and he said it's normal.

 

[XSLeo]

On a totally stock bike the air cleaner and air box act as a muffler, so you may not hear it. I was running my 81 for a short time without air filters and could hear it. I now have the stock filters wrapped with air filter foam and don't hear it.
Leo

 

[ReycleBill]

My '83 started doing it along about the time I removed the stock air boxes and switched to pods. Or, I noticed it about then.