jetting help on son's XR

[Firegod23]

Can someone please help me with the jetting for my son's XR80. I have installed a K&N filter and opened the top of the air box with vents. I also opened the exhaust by removing the spark arrestor from the end cap. Where do I need to start for jetting. It seems to run fine at speed, but bogs pretty badly off idle when given a lot of throttle. If you open the throttle very slowly then it does not seem to bog.

Any help is appreciated.

 

[girlsbike]

jetting

I think you should start by upping the main jet atleast 5 sizes,then maybe lowering the clip on the needle 1 position to richen the midrange and check the plug after you ride it some.

 

[HiG4s]

UP THE MAIN 5 SIZES? Never make more than one size change at a time!!!!
Besides I don't believe they make 5 sizes up for the XR80/100 carb.

How old is the bike? If it is over 4 or 5 years old take out both jets and make sure they don't have any corrosion on them. My son's old 96 xr100 had both jets so corroded that they wouldn't flow right. I replaced them both with new stock size jets and the bike runs great now. Also does it backfire if you give it gas fast. That can be a too lean pilot or a leak in the exhaust near the cylinder.
Oh and our XR has the air box vented, a uni filter, and the arrester opened up some.

Also check and make sure the idle screw and air screw are in good shape. If the air screw was over tightened it will grove the end and no longer adjust properly.

 

[Firegod23]

Sorry I left the year out.
The boy's XR80 is a 2001 model. I have never noticed a backfire at anytime , but it will bog until the motor dies coming from idle, when given sudden WOT. To keep it running you must tread lightly with the throttle until the rpm's increase a little, then it seems to take throttle just fine.

In other words, once the motor has some momentum in it, it will take throttle fine, but from idle, it dies with quick throttle.

thanks for the info,
Bill

 

[snaggleXR4]

Hey,
Read your plug. If you plug is black, then first of all try to lean out the pilot screw on the carb. If that doesn't do the trick, try to go a step leaner on the pilot jet. This should crispen up the bottom end. If your plug is very light in color, then you are too lean, richen your pilot screw and/or go one step up on the pilot jet.

If you notice backfire popping noises when you let off the throttle from speed, that means you are lean on the bottom. Jetting is a trial and error type thing. I recommend getting a step or two leaner and richer for both the pilot and main jets, as well as a few new sparkplugs. Again, check the plug you have in there now before you make any more changes. If it is already to lean and you lean it more, that won't be good.

 

[mtngoat]

Sorry to disagree guys, but the way I see it is the bike is running lean off throttle. Most non-pumper constant velocity carbs (which rely on engine vacuum to suck in gas-air mixture) will hesitate if you crack the throttle, but unless it's excessive (and it sounds like it is), it's not an issue with riding thumpers because you can roll the throttle when riding.

In this case, the motor has been "leaned out" by the modifications (K&N, airbox, exhaust) and assuming relatively low altitude and moderate temp, I think the engine needs a little more gas from the pilot/slow jet circuit. I'm not familiar with the specific carb, but if it has a pilot jet, I'd up it one size and test the result, maybe go one more size later if still disatisfied.

 

[bulldog71169]

XR80

I have finished tinkering with my sons 2001 XR80 and its about as good as its gonna get. I installed a Big Guns Race pipe, 2 one inch holes in top of air box, and removed all screen material off the filter cage.

The best setup I could find was a 105 main jet, stock pilot, and I dropped the needle one notch.

Still have a slight stumble from closed throttle to wide open if not rolled on slightly... but it will catch itself quickly now and not die. But there is never any reason to be twisting like that on an XR80. This one pulls much harder in the mid to high rpms but still about the same on the bottom.

 

[Firegod23]

Thanks for the info guy's. It's off to the parts counter to get a few jets, and start testing.

Thanks again!

 

[DualSportr]

The 2001 XR80 needs the spark arrestor in order to run correctly for off-idle throttle response.

This model does not utilize a CV carb, just a regular round-slide kehin.

The new exhaust systems that come stock on the 2001 XR80 and XR100 are excellent, free-flowing systems and there's no reason to mess with them (unlike pipes from earlier models which were better used as door stops).

You can try to jet out the off-idle stumble by dropping down one size on the pilot. Or you can replace the spark arrestor in the muffler and regain the lost power.

You haven't increased flow on the bike by much, so I believe the stock jetting (which is relatively rich to begin with) will be more than adequate.

 

[girlsbike]

I was under the impression that most of honda's "play bikes" were jetted lean from the factory,i'm surprised to hear that they are actually rich.If thats the case then jettiing may be spot on for the kn filter.Any ways just check the plug after some time is put on the bike it'll tell.

 

[DualSportr]

Plug readings using pump fuel can be notoriously misleading.

Because of additives in current pump fuel, a very rich condition can leave deposits on the plug which create the look of a lean condition.

Plug reading is best done as a 'second opinion' once a bike is running well.