Stumped (bogging/cut out under throttle)

[Jonathan Wall]

Alright, so my mechanic and I are stumped. Carb has been gone through, Brand new valvetrain, compression at 140, new CDI, new plug wires, spark and fuel checked and are good. Fuel flow is good. Receipt for new stator from the previous owner. Bike starts fine. 

Symptoms: engine idles fine, and revs fine through the range while NOT under load, under load bike will rev if slow on the gas pretty good, but when I go to open up the throttle significantly the bike hits a wall and cuts out  (stays running) almost like hitting a fuel cut in a car. 

What could be causing this? Something with the auto decompression? Stator (even though close to new)?

[JoeRC51]

Sounds like carburetion , what bike ? 

[dirtjumpordie]

My bike started doing that once the weather warmed up.  For me, all I had to do was put a leaner main jet in.  No longer cuts out at high RPM under high load.  My XR600R was running too rich, and that caused it to cut out and misfire with wide open throttle in higher gears. 

I've also heard of valves causing that problem.  

[baldy110]

Check the carb diaphragm for tears.

[JoeRC51]

19 minutes ago, baldy110 said:

Check the carb diaphragm for tears.

That's what I was thinking if it's an L.

[Chasx]

What is your main jet size and elevation?  I agree with the guys above that it is a carb problem.  If you don't have an adjustable needle, then I'd drop the main jet one or two sizes and see what happens.

[Jonathan Wall]

5 hours ago, JoeRC51 said:

Sounds like carburetion , what bike ? 

Hey guys sorry, this is an 86 xl600r.

[JoeRC51]

It's sound like an issue with your secondary carb.

[Jonathan Wall]

50 minutes ago, JoeRC51 said:

It's sound like an issue with your secondary carb.

So diaphragm was checked ... and carb was run through by my mechanic ... are there problems with the secondary carb that aren’t obvious, or issues unique to these bikes?

Edited June 16, 2018 by Jonathan Wall

[JoeRC51]

Sounds like a fueling problem with the secondary. Clogged main jet , no main jet , no fuel in float bowl . . .

[Jonathan Wall]

6 hours ago, JoeRC51 said:

Sounds like a fueling problem with the secondary. Clogged main jet , no main jet , no fuel in float bowl . . .

Alright, so my mechanic says this regarding jetting... "I have gone up in jets. Down in jets adjusted all the way one way to the other" 

Is there anything quirky on these bikes that can be missed if you dont know em in regards to the carb? I own an old Saab and there are all sorts of things like that ha. 

[JoeRC51]

Nope.  

[baldy110]

If your mechanic checked the diaphragm in the carb and told you it was fine get another mechanic the XL600 doesn't have a diaphragm carb

[dirtjumpordie]

Dude no offense your mechanic is a dumbass

[Jonathan Wall]

10 minutes ago, dirtjumpordie said:

Dude no offense your mechanic is a dumbass

He’s a really good guy actually, and a really good mechanic, he just hasn’t seen this kind of bike before. We’ll get to the bottom of it. 

[baldy110]

Set / make sure the carb jetting is set to OEM specs that will give you a baseline to start from.  The honda twin carb setup is great once dialed in, they are simple and easy to work on.  The left carb controls from idle to about 1/4 throttle then the right carb comes in.

[Jonathan Wall]

1 hour ago, baldy110 said:

Set / make sure the carb jetting is set to OEM specs that will give you a baseline to start from.  The honda twin carb setup is great once dialed in, they are simple and easy to work on.  The left carb controls from idle to about 1/4 throttle then the right carb comes in.

I have adjusted those screws quite a bit. Interesting that matches what others have said about the secondary carb, as I have just about 1/4-1/3 throttle before she bogs/cuts out. 

One other clue is she is VERY hard to start once at operating temp. Requires turning the fuel off and kicking for a good five minutes. Once she starts I turn the fuel back on. 

[Jonathan Wall]

10 minutes ago, Jonathan Wall said:

I have adjusted those screws quite a bit. Interesting that matches what others have said about the secondary carb, as I have just about 1/4-1/3 throttle before she bogs/cuts out. 

One other clue is she is VERY hard to start once at operating temp. Requires turning the fuel off and kicking for a good five minutes. Once she starts I turn the fuel back on. 

Another thought, Could the Auto decompression system be causing this and not the carb ? 

[JoeRC51]

No auto decompression on your bike.

[Jonathan Wall]

Interesting I guess I thought it because it’s kind of irregular as to when the kickstart “catches” telling me it’s at TDC , seems like a few kicks one time and then 10 or 12 the next until it catches.