Engine surging backflow into carb?

[jmasisi]

Guys,

My son's yfm350x has a metal to metal noise in the engine every 5-10 seconds. When this happens a kind of air-shock toward the carburetor side occurs. I can feel the pulsation each time when touching the boot.

If cold, engine will stall when it occurs. It is exactly what happen to another person´s, check this video:

http://www.youtube.com/watch?v=3vZYWuf0yNg

Any enlightenment will be very much appreciated. The interval between occurences is random which makes it weirder.

Thnxs

 

[WV98YFM350]

May check the adjustment of the forward reverse lever and your clutch cable adjustment, sounds like its trying to jump into gear but thats just a couple free solutions I would try ...

 

[overtheedge86]

Mine sounded similar when it had a bad rod bearing. Not trying to scare you or anything, check your oil for pieces of thrust washer.

 

[Gen1Pat]

That sounds more like a lean pilot jet to me. That sound you hear is actually a small backfire into the carb on decal because the pilot jet (or mixture screw) is too lean, and either needs a larger pilot jet, or the mixtures screw needs to be turned counter clockwise an 1/8 turn at a time to richen the idle circuit.

 

[jmasisi]

Guys thank you!!, now I have a clue where to start looking for.

WV98YFM350: I have put it on neutral, first gear and reverse with clutch engaged and still makes it, so there I'm puzzled.

Overtheedge86: Will do...., that's the worst case scenario. I also was wondering if it has to do with bad timing...

Gen1pat: Very interesting sugestion, will never imagine it because this motor is stock except for the K&N air filter and the Aftermarket spark arrestor. Is that enough for requiring a larger PJ?, still wonder why the metallic sound. On the other hand I understand this motors are made lean on purpose from factory and with those mods will be even leaner..., so it might make sense!

Thanks guys, I will report what I can find.

 

[lbjamm]

Check valve clearances too. Possibly hanging the intake open intermittently.

 

[WV98YFM350]

Yea man if you put a aftermarket pipe and filter on it will need rejetted...

 

[Wildgasgas]

I have the same "problem" (if it's really a problem). I know a guy on youtube who is quite good at mechanics, and he said to me:

It could be normal because when an engine slows down very slow the engine can fire the opposite direction and kick the exhaust gasses back into the carburetor. Unless it sounds very bad and like the quad is gonna fall apart id just let it go. But check to make sure the timing is correct too. there is a little plug right on the left side on the crankcase there is a mark with t and f the t stands for top dead center which means the piston is at the very top. there is the timing chain cover off above and there is a line on the gear and on the engine. when you turn the engine to top dead center they should line up if not the valve train timing is off.

Hope that information helps!

 

[jmasisi]

I will play with the mixture screw and larger PJ's to see if there is any improvement, if not will start checking valves and timing. Probably intake valve is not blocking well, it is temporarily stuck or timing is off....

 

[Gen1Pat]

IF it was a valve issue you'd experience low power, and even low compression. But, yes, check your valve clearances, check your mechanical timing, that way you cover the bases. You are going to want to take the carb apart and see what size pilot and main jets are in there. My money is still on lean jetting. That sounds exactly like a lean idle circuit. Also, before you take the carb apart, turn the mixture screw CLOCKWISE until it stops. Count the number of turns along the way (360* turns not 180*). It SHOULD be no more then 2.5 turns out from it's full "in" position. If it's less then 2.5, take it out to 2.5 turns. If it is more then 2.5 you definitely need to install a larger pilot jet.

 

[jmasisi]

Gen1pat: You were right, I turn out the mixture screw until it almost fell and it stopped doing it..., so I will order a larger PJ though I don't know what is the next size..

Thank you, I swear I thought this was a big problem!!

 

[Gen1Pat]

You're welcome. You will first need to take the carb apart and see what size jet is even in there before you go ordering another. Also, don't just order the next size up, get the next 3-5 sizes up. And lastly, run the engine wide open throttle in 6th, and turn off the kill switch AS YOU PULL IN THE CLUTCH. That will shut the engine down and you will be able to make sure that your main jet is correctly dialed, once you pull the spark plug to inspect it's color right after engine shut down. Just don't let off the throttle when you pull clutch and hit kill switch. No sense in taking the carb apart more time then necessary. Hell, no need in ordering jets more then once.