XR650R Carburetion problem.


10 replies to this topic
  • Mudshark

Posted February 19, 2006 - 08:08 AM

#1

I have a new ’06, I’ve :

1. pulled the snorkel and plastic thing from the air box.
2. Put a 172 main jet.
3. 68s Pilot jet.
4. The stock needle is on 4th clip from the top.
5. New "uncorked" carb. manifold
6. FMF Q2 slip on with stock header.

The problem is that when (at 3.5 – 4.5k rpm) I try to roll on the gas nice and smooth,
It doesn't, it kind of jerks / snaps in. the same applies when I try to roll off the gas. It kind of
Snaps shut. So I end up with a bit of an “On / Off” switch feeling.
It may in fact be doing this at higher rpm, but it’s most noticeable at these speeds
In the tight stuff….. right when you DO NOT NEED THIS !!!!
Help!

  • madxr4

Posted February 19, 2006 - 08:36 AM

#2

I am very new to this and just did the same mods myself with the exception of the 68s. Also a 170 instead of 172. On the advice of a local rider/racer/tuner I left the 65s and adjusted the air:fuel 2.5 turns out. (it's the recessed screw on the right side of the carb just above the float bowl) You may need to adjust that but I'll leave the specifics to those who know.
Did you open up the carb to head manifold?
Are all the vacuum lines in the right places on the carb?

  • Mudshark

Posted February 19, 2006 - 08:50 AM

#3

Did you open up the carb to head manifold?
Are all the vacuum lines in the right places on the carb?


Yes.. sory I fogot to mention that - I've updated the post to include the
manifold.
The lines were never moved... so I assume they are all okay.
It is running a bit rich, plug reads tan in the center with a bit of black
around the outsides.... nothing terrible.

  • madxr4

Posted February 19, 2006 - 09:17 AM

#4

Maybe the 172 is a bit fat?
Check/fiddle with the mixture screw?
I was told to set it 2.5 out from lightly seated and make 1/4 turn adjustments.

Again, I'm very new to this and speaking from even less experience with this bike.
Good luck with it.

  • Outrider

Posted February 19, 2006 - 09:27 AM

#5

Hey, mine ran too rich in the midrange so I went to the 3rd clip on the needle. It doesn't take long to adjust and can make all the difference.

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  • snaggleXR650

Posted February 19, 2006 - 09:30 AM

#6

The stock carb isn't real popular. That's why you hear so much about the Mikuni TM-40 pumper carb or Edlebrock Quicksilver pumper carb. Furthermore the extreme off idle torque and compression braking really adds to the on/off feeling. You will learn that you can lug in a taller gear and use the clutch which will elimate you having to get out of the throttle all the way. This will help alot.

Jetting wise, you might run the needle in the 3rd or middle clip position or even 2nd and see if that cleans some of it up. There is an HRC needle and needle jet, but I stayed with the stock setup when I ran mine.

The pilot jet takes care of idle to about 1/8 throttle, the needle jet takes care of 1/16 to 1/4 throttle, the needle takes over up to about 3/4 throttle and then the main jet up to full throttle. Mark your grip at "0" and the throttle cable cylinder at 0, 1/4, 1/2, 3/4 and full throttle. Watch as you ride, and test each 1/4 throttle position for throttle response, acceleration (or lack thereof). This way you can isolate which carb circuit to work on.

Finally, you didn't mention your airfilter. The stock filter is ok, but the UNI filter is a bit better for getting air flowing. Furthermore, the stock back fire screen (behind the filter) is very restrictive. I pried mine open and removed all but one of the screens. Or you can buy a high flow screen from SRC or XR's Only. This may really help your problems too. Good luck.

  • Outrider

Posted February 19, 2006 - 09:36 AM

#7

Another problem with my stock carb is that it would keep a fast edle when I came to a stop. I would have to wack the throttle a couple of time to get it settled down. Also, I once tipped over and let the bike stall while laying on it's side. It took me almost 10 minutes to get it started. Now I have a Edelbrock....

  • Mudshark

Posted February 19, 2006 - 09:53 AM

#8

Another problem with my stock carb is that it would keep a fast edle when I came to a stop. I would have to wack the throttle a couple of time to get it settled down. Also, I once tipped over and let the bike stall while laying on it's side. It took me almost 10 minutes to get it started. Now I have a Edelbrock....


The fast idle is because the idle speed is set too high. Mine did that too till
I turned down the idle, now it's fine. I have a feeling this has something to do
with the Air cut-off valve messing with the mixture as the vacuum changes
at varying throttle positions. I'll block it off and see how it goes.

I tried the needle at 2 from top and now it's at 4th from the top, with no
real difference. the air screw is at about 1 and 7/8 turns out and throttle response
is great, it's just this darned on off thing.
Chugging along in a higher gear using the clutch a bit does seem to help...but
I don't want to chug :thumbsup:
I do think that it needs more air though...
I dropped the Main jet back to a 170.... no change...except the Air screw is
at about 2 1/2 turns out... maybe a bit more now, to get the throttle response
back to where it was.

  • HawkGT

Posted February 19, 2006 - 10:59 AM

#9

Carb jetting aside, I've found the XRR does demand skilled throttle control on the bottom end/lower midrange cause of all the instant torque. This seems especially noticable (on the street) when getting back on the gas after braking coming up to a corner.

Regarding the carb, don't overlook throttle cable adjustment either. It's tempting to set them too tight. There should be some movement of the throttle tube before the cable starts lifting the slide. Cables that are too tight can contribute to a "snatchy" feeling throttle response because you loose a bit of fine adjustment in the carb slide movement just as it opens/closes.

Try playing with the cable adjustment and see if that helps. You may need to make adjustments down at the carb if the throttle tube assembly adjuster is close to full tight/loose.

  • Supplicate

Posted February 19, 2006 - 11:13 AM

#10

The fast idle is because the idle speed is set too high.

When mine was new, it had the fast idle for the first month or so no matter how I adjusted it, it mellowed out after being broke in decently.

The stock carb just takes a little patience and a lot of brass to get to run right.

Getting an edlebrock installed and tuned next wed :thumbsup:

  • Mudshark

Posted February 19, 2006 - 01:39 PM

#11

okay with the airscrew out to 3 and bit turns it's smoothed off a bit.
Shutting off the throttle is totally smooth now, turning it back on is still
a bit jerky, but better...... isn’t 3 + turns out for the airscrew a bit much
though??? :thumbsup:
It is about 30degrees out today and misty.... may have something to do with it?





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