Stock carb problems (I think)

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Stock carb problems (I think)

Post  benjamin1031 on Wed May 21, 2014 5:25 am

Alright guys I humbly submit myself to the BRP experts. My pig began to have problems the other day manifesting itself as "cutting out" when under acceleration. Starts first kick, idles fine, jetted properly (165, 65). Ran fine from 4500' elevation all the way to 12,900'. After the problem began I thought it may be time for a good carb cleaning session. So, I cleaned the carb and jets with no luck. I adjusted the valves, again no luck. I replaced the petcock, spark plug and checked all electrical connections. No luck. There are only two symptoms to my problem 1. Under acceleration the motor cuts out, then takes off over and over again as long as I hold on the throttle. 2. When upright it idles fine at 1400 RPM when I lean it over on the kick stand it revs to 2400 RPM. The bike is uncorked, stock cam and white bros. exhaust. It has run great with this set up for over a year and now I'm lost. Thanks for your help in advance. Ben
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Re: Stock carb problems (I think)

Post  Mad Frosty on Wed May 21, 2014 11:28 pm

Float level ?
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Re: Stock carb problems (I think)

Post  benjamin1031 on Thu May 22, 2014 3:44 am

I watched this YouTube video and tried to do it like him.
http://m.youtube.com/watch?v=WOO8puMGiqM

I think the float setting part is around 2:30
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Re: Stock carb problems (I think)

Post  R056rx on Thu May 22, 2014 8:27 am

Does it do it no matter how hard you accelerate?
0-1/4 throttle is controlled by idle circuit and pilot jet
1/4-3/4 is controlled by needle jet
3/4 to wide open is controlled by main jet.

If it only stumbles at hard WOT acceleration then it may point to the main jet.

You don't mention the air filter. Have you changed it or cleaned it?

I'm with Frosty though, sounds like either float height, or possibly a worn float valve, especially with the increasing revs on side stand symptom. I'd replace the float valve anyway as it's difficult to see if it's worn or sticking. They're not expensive. Go OEM.  rider
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Re: Stock carb problems (I think)

Post  benjamin1031 on Thu May 22, 2014 3:23 pm

Air filter is good to go, I cleaned and oiled it just in case. Throttle position doesn't matter it does it when accelerating but also with holding the throttle steady at all positions while running down the road.
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Re: Stock carb problems (I think)

Post  R056rx on Thu May 22, 2014 4:04 pm

You know that the YouTube clip you've posted is for an XR600? Slightly different to ours. Although the procedure is the same, the float level for a BRP is 16mm, with some setting it at 18mm if it overflows on the side stand. 
Have a look at this for the xr650r stock carb procedure....  http://youtu.be/3z_IOhrSsWE
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Re: Stock carb problems (I think)

Post  Mauser on Thu May 22, 2014 5:51 pm

Sticking float jet.....

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Re: Stock carb problems (I think)

Post  benjamin1031 on Thu May 22, 2014 7:38 pm

I knew that was an xr600 carb but I thought they were the same carb so on my infinite ignorance I did the same specs he said. Ordered parts today and had the guy at the parts counter offer to buy my piggy. Haha. I could never stay mad at this bike. Smile
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Re: Stock carb problems (I think)

Post  BuRP on Mon May 26, 2014 7:54 am

It's fuel flow, or rather the restriction of. Drain the floatbowl with carb on (upright & cold!!!) bike & leave tap open >> fuel flows freely? If it is water then this will provide a, possibly short lived, cure, and if it is then flush the tank.
Of course the floatlevel must be set correct, which is not the 18mm as per manual but 19mm (to prevent pissing fuel when leant over on stand) - which is measured by blowing air through a fuelhose whilst rotating the carb toward upside-down (ie NOT fully upside down). You'll find the angle at which the float-needle will close, and at this angle you'll have to obtain the 19mm (between top-of-float & carb-without-gasket).

R056rx, your list of throttle openings & which part does what is correct (sortof) only when continuous flow is present, not when accelerating. Then also, be aware that even at idle the main does deliver fuel whilst at WO also the idle adds its bit, ie it all overlaps.... which is why jetting a carb is such a nice job as it requires 'feel' as well as experience. No hard rules like you list but more indicators, careful does it there  naughty
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Re: Stock carb problems (I think)

Post  AURORA on Mon May 26, 2014 8:49 pm

Float level or float needle valve.
Slight change the fuel screen on the petcock.

Watch the plunger on the float needle valve to proper adjustment.
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Woes are gone but...

Post  benjamin1031 on Fri Jun 06, 2014 9:15 pm

So my work schedule puts me out of town for two weeks at a time. I was about to start my 2 weeks on when I was getting frustrated with my piggy so I took it to the professionals. Prior to taking it in (and before the troubles) my bike set up was this; Uncorked, stock cam, Uni filter, white bros exhaust, and stock carb with 168 and a 65 jet. Living in Grand Junction Co our elevation is roughly 4600' (1402.08m for my metric brothers). I usually ride in about 80 degree (F) weather (26.67C). I also like riding in the san juan mountains which take me to 13000' (3962m) and 40F (4.44C). Prior to my problems my bike ran great and started on the first kick at these elevations. The professional mechanic however changed my jetting to 188 and 68. On the ride home it ran great and continues to do so but, the 188 isn't even on the jetting charts I look up?!? This brings me to my questions. 1. Is this going to hurt my engine? 2. Will my piggy run decent at 13000' (3962m)?? Or perhaps I am fretting over nothing. It does run like a scalded dog! Wheelies in 3rd gear with just a twist of the wrist!!


P.S. The problem did end up being the float adjustment and the needle. Needle was replaced and the float was adjusted. My instructions when dropping off the bike was to ensure that the motor runs 100% as it should. That's why my mechanic took it upon himself to rejet the bike.

P.S.S. You guys are an amazing resource and I can not thank you enough for the knowledge you give to any schmuck that asks. Thank You very much!!
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Re: Stock carb problems (I think)

Post  Mad Frosty on Fri Jun 06, 2014 9:20 pm

Now go and ride it  BRP smile 
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Re: Stock carb problems (I think)

Post  BuRP on Mon Jun 09, 2014 9:55 am

Ooh Benjamin,

so you're a schmuck hmm, an asking one on top of that?
You dinna tell us that mate, you'd have zero replies otherwise....
 Bump 


68 IJ sounds fine/correct, 188 MJ sounds way too large, especially at that altitude.
If you go further up / higher then this probably will show up as such, so you could pack a smaller MJ and swap it somewhere.
If you've got an opened sidepanel then you've got another alternative: tape it shut, jet the MJ accordingly at your 'home' altitude. The venture out to the higher regions where it will start running rich (ie too little air), and then take the tape off the openings allowing more air in. This is said to work. The Idle-circuit can be tuned by the screw a bit if needed.
However, any carbed engine can only work "perfect" at a given altitude. A bit more or less will work fine, but outside that window one has to take a little 'off' for granted.


Added Edit: a TM40 carb is said to be quite tolerant for large altitude changes, it was one of the reasons I got one. Jetted for 1400 HASL I've found it working well between 0m and 1800m - and expect it to be working well still at much higher altitudes too.
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