boging and half throttle

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#1
boging at half throttle

hi just had sons 2003 rm 85 bottom and top end redone.reed valve looks good . when my son rides the bike he says it has a bog at half throttle. so i am thinking it is the carb ? i have the air mixture screw out at least 3 turns. i looked inside the carb and the main jet is a 125 the other jet the slow jet there is no number on it.. should i get smaller jets and how can i tell the number of the slow jet

i am mixing the gas and oil at 32:1 with klotz

if i stand behind the bike it smells to rich

and in the garage when started it will burn your eyes out


thx again
 
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#2
the 125 for the main is stock. Your slow jet is a 50 stock, simple things like this you can get from your manual, like I did.
For some tips on jetting, you can read spankies jetting guide, here it is. :cool:

-A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband. A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.
Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting.
The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless.
Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best.
It's very important that you start with the pilot circuit. The reason is simple. The pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.
Before you start to rejet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting.
Before you start the jet testing, install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect your jetting all across the throttle range.
Warm the bike completely, and shut it off.
As already stated, start with the pilot circuit. Turn the airscrew all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idleing. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the airscrew for the best response.
Now is the time to determine if you have the correct pilot installed in your carb. The airscrew position determines this for you, making it very simple. If your airscrew is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet.
Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the airscrew for the fastest idle, it's time to tune the airscrew for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the airscrew slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.
The airscrew is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the airscrew to compensate for changing outdoor temps and humidity. An airscrew setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.
Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong.
Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.
Once you have a little bit of experience with jetting changes, and you start to learn the difference in feel between "rich" and "lean", you'll begin to learn, just from the sound of the exhaust and the feel of the power, not only if the bike is running rich or lean, but even which one of the carb circuits is the culprit.
The slide is also a tuning variable for jetting, but slides are very expensive, and few bikes need different slides, so we won't go into that here.
Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit. Two-strokes have an air screw that you screw in to make the jetting richer, and screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer.
 
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#3
thx when bike is at stand still and throttle is turned half way it seems like a rev limiter goes on. i looked at the reeds they seem not to be cracked anywhere.? does it seem that the bike is getting to much fuel.before it got rebuilt the bike ran fine.i just dont know where to go from here. new reeds ?new jets ?
 
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_JOE_

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#4
If it does it at half throttle it should be needle or main jet related. Did you have the carb apart during the rebuild? I would start by verifying that everything is spotless in the carb and the float height is correct.
 
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#5
_JOE_ said:
If it does it at half throttle it should be needle or main jet related. Did you have the carb apart during the rebuild? I would start by verifying that everything is spotless in the carb and the float height is correct.

i had it apart last night and both jets look ok but they could be wore out?

what would you guys recomend ? new jets (stock) or bigger or smaller than stock.
 

_JOE_

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#6
Start with what you had in there. If you really want to replace the jets get the size off of the old jets and go buy new ones. They usually don't wear out though.

I would look very closely at the floats. Check that they don't leak (liquid inside) and the needle/seat aren't damaged. Verify proper height.
 
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#7
today i took the entire carb apart and cleaned it very good. every jet every hose every piece of the carb i cleaned. adjusted the seat up and down and i still have this same problem? is there a chance that the engine being to its full potential that the carb needs rejetted either one size bigger or one size smaller or are the reeds shot and i cant tell there shot. if i turn the air mixture screw out 4 turns it still will burn your eyes out with the smell. thx for any ideas on this

also looked at the floats and they dont seem to be leaking fluid

needle also looks good

seat looks good also
 

_JOE_

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#8
Replace the float valve. Be sure to set the float to spec using the procedure in your manual. This assuming you verified the float itself is not leaking.

If you still have issues take the bike in for a leak down test.

Jets usually don't cause a random rich condition unless they fall out. They tend to get restricted.
 
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#9
i had the reeds checked today and they are fine. i went out and bought a couple of jets a 128 122 and 120. tried the 122 and it got a little better. put in the 120 and it runs fine. seems to have changed the power band acording to my son. bike runs good and it is snappy.strange why the smaller jet the 120 worked when it calls for a 125 stock.maybe me running synthetic klotz oil changed something not sure.
 
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#10
rmrider45 said:
i had the reeds checked today and they are fine. i went out and bought a couple of jets a 128 122 and 120. tried the 122 and it got a little better. put in the 120 and it runs fine. seems to have changed the power band acording to my son. bike runs good and it is snappy.strange why the smaller jet the 120 worked when it calls for a 125 stock.maybe me running synthetic klotz oil changed something not sure.

Altitude differences, or piston wear can effect jetting.
 
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#12
whenfoxforks-ruled said:
You did not read the jetting information? This winter you will be putting the 125 back in. How many turns out is your air screw?

2 turns out . i will be putting the 125 in come winter ?

that 125 jet we used winter and summer with no problems till this year . only change was going from cheap oil to klotz synthentic + new bottom and top end :ride: :ride: :ride: :ride:
 
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#13
2 turns out should mean your pilot jet is still in the ball park. Summer means you need to lean the jetting, winter needs richer jetting. Your engine could have been worn last year, and all the jetting in the world would not matter. Its not the oil. Reeds and silencer packing got changed hopefully also?
 
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#15
arghhhhhhhhhhhhhhhh back to Square 1 . yesterday son drive the bike about half a mile and he shut it off and it would not start. pulled the plug and it looked lite gray (too lean )changed the plug and somehow got it home . opened back up the carb and took out the 120 main and put in a 128 main. bike fires up but i need to hold the throttle wide open to start it .just cant think what could be going wrong. on start up seems not to be getting enough gas that why i need to hold it open ?