Too lean ?

I bought a used 650R. Pulled the carb and it's got 68s pilot and 170 main. I've read to use 175 main. I ride between 500-3800 ft. elev.

It starts and runs great.

The exhaust is Big Gun w/header. Airbox appears stock. I want to put in a Uni and pull snorkels. Is the 170 ok or should I get a 175 ?


That sound about right. Just make sure the intake has been changed or the restrictor has been cut out. If the bikes runs good I say your there. You can always check the plug to see if it's lean

The 170 is ok, but better to run the 175 below 1000' in my opinion. Make SURE the intake boot is not the stock part! It must be the 40mm inlet. Also be sure the snorkel and baffle plate are removed from the airbox. Check the needle for proper position. Should be 3rd (middle)for B53E needle, or 4th (down from top) position for the stock needle.

If all is well, then the bike will flat out RIP !!!

"Too fast, too quick" as they say . . haha. enjoy.

L.L. :)

I ride at about the same elevation and I'm running a 172/68s. It works great but like mentioned above make sure you have the bike "opened up" all the way or it won't make a difference what jets your running.

It's best to try out a few combo's first hand and see what you think works best with the seat of the pants meter. While companies try to produce consistant orifice sizes with their jets, they can and do sometimes vary, so a 170 may not really be a 170, but it should be relatively close. You may find enough varience to where a 168 outflows a 175, which is why you can't always depend on a specific jet number to be the golden ticket.

The only easy way to know for sure how your jets compare to each other is to flow a known amount of fluid (like water) through a series of main jets (165, 168, 172, 175, etc) at least a half dozen times for each jet and then calculate the average time, standard deviation, flow rate, etc.

Buy a larger 172 & 175 main jet and maybe even a smaller 168. Then try each one out to see which one feels the best while running WOT and stick with the one that feels best to you. Then check your spark plug to make sure you're not running too rich or too lean and pay attention for the classic signs of an incorrect mixture. Changing the main jet is quick to do and doesn't cost a lot of money and you might squeeze out some more power or gain the satisfaction in knowing that what you're running is best for your bike & riding environment.

Great stuff guys. Thanks for your time. The bike is "open" other than stock filter (Uni's on the way). It seems plug reading isn't an exact science so I just want to err on the side of caution and not run her too lean. I'm not necessarily looking for more top end, just wanna treat my pig right. If it was a little lean would it be blatently obvious on the plug ? Would an oil temp stick give me a better indication of a lean condition (i'll probably get one anyway) ? Or should I just throw in a 175 and call it good, assuming it runs as well as it does now ? I guess what I should be asking is what happens when your main's too big ?



If your main jet is too big, WOT performance will suffer and your spark plug will have plenty of carbon / soot all over it, but no real harm will come to your engine except for fouling spark plugs more often. If you're running too lean, then you'll get some surging, the engine will run hotter, the mixture ring on the spark plugs insulator will not be visable and you'll likely see a white insulator (hopefully not melted). If you're running too lean, then you'll also shiney black or bright aluminum pepper like flakes on the spark plug with the use of a 10X magnafying glass or with the naked eye if its bad enough. Those shiney pepper like flakes are actually pieces of aluminum being blasted off the piston's crown and your engine is headed for trouble sooner than later if this happens.

Ideally, you want the spark plug insulator to be light tanish or off white with the mixture ring deep inside the insulator so there's just a whisp of it showing, but you'll need a good illuminated magnifying glass of 10X to 12X too see this or you'll have to cut off the spark plug's shell for a better view. The deeper the mixture ring appears inside the shell on the insulator, the more of your air/fuel mixture is being burned which is what you want. This means the combustion process is buring nearly all the air/fuel mixture. The farther the mixture ring appears down the insulator toward the electrode, the richer your mixture is. The mixture ring is simply the byproduct of unburned fuel which is nothing more than carbon by the time you see it. Also note the spark plug only accurately reflects how your engine was running the second before it was shut off. For instance, if you wanted to accurately check out your main jet, you'd have to run WOT for a good 8 seconds or so preferably under load (like a slight uphill), then immediately pull in the clutch lever and press the kill switch to kill the engine (preferably near your pits) and then come to a stop so you can pull the plug to read it when its cool enough. If you motor your way back to the pits after your WOT run, then the spark plug will not show you what was going on at WOT, but rather what was going on at the last second before you shut down which was probably close to idling. You can do the same thing for part throttle openings, but then it gets tricky trying to figure out which mixture circuits are contributing too much or too little. If you install a brand new spark plug, make sure to get at least 20 minutes of good hard riding on it for enough deposits to form to more accurately read it.

The hardest I've ever ridden my 650R was last weekend, when I was racing some lame street bike on the freeway. It was the first time I had ever held my bike completely wide open for a total of about 15 secs or so. My speedo showed 102mph and I was ducked down (stock gearing). Interestingly, the engine didn't sound like it was screaming or about to explode, like my old suzuki DR350 sounded at top

speed (90mph). Instead, the Honda was buzzing, but no freaky sounds or anything. This is a kick ass bike. And yes, I did waste the crap out of that Yamaha RoadStar 1600 even at top speed ! hahaha. I guess they run about a 100 or slightly above, but apparently need several miles to get there because although he was still in sight, he was quite a distance behind me. hahaa. Pathetic! (This was a roll on race from about 60mph on the freeway) I'm sure he was thinking "how in the hell can a dirt bike be that fast? It's impossible I say! impossible!" hahaha. Ah,, but it IS that fast my friend hahaa. Damn that was fun. .


P.S. I run the 175jet, and find it interesting about the previously mentioned variations in the same jet sizes. . hmm. did not know that. In any case, I'm running the standard uncorking kit, with the 40mm tip which has been modified (remove the spark arrestor screen and baffle beer can end). This tends to improve upper mid-range and top-end with a very minor loss of low end, very minor. :)

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