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Hi I am having some problems with my honda xr650l (1995). During acceleration I experience a sort of stuttering loss in power. Almost the same feeling as runnning out of gas in the main tank. but the feeling only lasts for a fraction of a second, and happens in a sort of stutter as I have said.. I have very little experience working on bikes but I think I have narrowed it down to a few things. Either carb needs to be cleaned, carb needs to be adjusted, or water in the gas tank. I have bought the bike only a few months ago and Have been riding it at an altitude of 5000 feet, high desert. At-least 3500 feet above its previous home so I belive it may be that the carb just needs an adjustment. So I think it is the first thing I should try.

I should reiterate I have no experience with adjusting the carb and very little experience working on bikes in general. I hope it can be done by adjusting the air fuel mixture screw, and the idle speed adjustment screw... But I am not sure where to start, How does the air fuel mixture screw work ? Which direction increases air in the mixture and which direction increases fuel? and where should the ratio be at high altitude? How will adjusting the idle speed help? I would like to know any other pertinent information I am missing, caveats etc. I am having trouble finding a good guide on the topic so far.

  • Duckatikiller, did you fix your carb problem with the mixture screw or did you have to buy the new 150 jet? I am having the exact problem and wanted to know. – user20091 Jul 25 '16 at 23:57
  • @justin, I adjusted the screw and had some good results. I never tried the 150 jet. I began to play around with the adjustments too much and wacked things out, had a mechanic clean/tune the carb and it was running fine for a while after that. – BryanK Jul 27 '16 at 20:48
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I'm not sure if the answer is still relevant, but I own the XR650L big red pig and may say that it's rather safe to mess with the carb of this bike even if you don't have specific skills. It's very forgiving unless you do something really wild. I used to ride about 500 km on my with mixture being extremely lean and was lucky enough to have my engine undamaged.

Second is that I confirm that this bike feels relatively sick at these altitudes due to the mixture being too rich for a given air pressure. You'll need to lean it a bit. But first you have to confirm running rich. The simpliest way to do that is getting out your spark plug and inspecting its color. Please carefully clean the engine and the cylinder head before removing spark plug to avoid even dust entering the cylinder.

Spark plug should be dark brown, you can easily find pictures with proper color over the internet. I'd bet that yours is completely black. By the way, replacing spark plug with iridium one may help a bit with engine stuttering, but doesn't solve an issue completely.

Put the spark plug back in (using a tiny bit of motor oil on the spark plug threads (not the electrode, just threads) will help reducing risk of stripping cylinder head threads) and get to the carb.

I would say that first thing you need to do is grinding off the mixture screw tab as described in Dave's Carb mod here: http://www.4strokes.com/tech/honda/xr650lcarbmods/ Please also note that you don't need all these mods because they are enrichening mixture a bit, which is something you want to avoid, you need only the mixture screw tab grinding.

Anyway, mixture screw only adjusts the mixture when engine idling or running at low throttle while your stuttering should happen with the throttle wide open. Mixture at the throttle wide open depends much on the carb needle and the main jet so you will need to go for these parts.

When having your carb apart of motorcycle, please check if your needle is adjustable. If previous owner used any sort of jetting kit, it's very likely to be so. Stock needle doesn't have adjustments at all. You need to lower adjustable needle a bit to lean the mixture.

If the needle is stock then the ultimate solution is replacing your main jet with a smaller one. Stock is #152 and you may need #150. Find somewhere a #150 main jet that fits your carb and replace yours with that one. That should fix your stuttering issues completely.

Beware of any dust or sand entering carb or engine. Always operate in a clear room with proper tools. Get yourself an XR650L service manual - it's very handy. Think twice before acting. Buy yourself #150 main jet in advance - it's rather cheap. And always change one thing at once - e.g. never change both main jet and the needle altitude at the same time.

This all sounds complicated, but it takes about 3 hours to change main jet at this bike for an unexperienced mechanic.

After changing carb settings go for 30-50 kilometers ride, get back and reinspect the spark plug color even if the bike feels much better. If you make your mixture too lean (gray, almost white spark plug), it will feel good, but you could overheat and melt the piston.

Oh, and there is a simple solution to your problem, which is putting on an aftermarket exhaust with improved air flow. This will also lean mixture a bit.

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Because the air pressure being comparatively low at high altitudes, it might cause the above mentioned problems. And working on the carburetor if you don't have any experience is a bad idea, because it could ultimately lead to engine overheating and such problems that could ruin you engine. Four Strokes have a better tolerance towards an engine running lean or rich but still highly unrecommended. Also clean the fuel line. A clogged fuel line could also cause issues. Contact a mechanic and he should be able to adjust the AFR for you in no time. Also clean/replace the air-filter.

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I ride my XL650L (2015) at 4000-6000 or so feet (high desert also). These bikes are very lean from the factory.

I have removed the intake snorkel and installed a Uni foam filter. Carb is jetted about perfect with 158 main, 52 pilot, 1.5 turns on the pilot screw

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The above answers pretty much cover the situation. But it may be helpful to note the venturi effect is a function of air volume rather than density such that the carb jet meters the same amount of fuel to a given volume of either dense or thin air. TomO

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Mikuni used to recommend changing main jets for every increment of 2,000 ft in elevation, so for an elevation of 5,000 ft. you could go with two or three size decrease. I would not adjust the needle height and as another poster said the air mixture screw only affects idle, so this is not likely to affect your problem. FYI-adjusting the needle may fix one problem and create another Although all the components of a carburetor are overlapping and have an influence on each other, it is designed to react to throttle position. That is, each circuit (some carbs have more circuits than others) is responsible for delivering fuel depending on which position your throttle is in. For example, throttle closed - your air mixture screw will have the biggest effect, wide open throttle (WOT)- your main jet has the biggest effect. In other words, changes to a carburetor are based on how your bike reacts to a specific throttle position. This gets very tricky and changes are usually so subtle that you need a dyno (except idle adjustments) to be able to measure effects acurately. Using spark plugs as an indicator is tough because you will cycle through many different throttle positions putting 30-50 km on your bike. Adjusting a carb is a lost art and in my experience most pros just focus on the main jet, because dialing in a carb takes many hours. Sorry for the long answer I would leave your needle in it's stock position replace your main jet down two sizes since that is recommended for altitude adjustment anyways.

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