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boging and half throttle
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Posted by: rmrider45---------------------

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

Posted by: RM85rider123---------------------

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.

-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.

Posted by: rmrider45---------------------

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 ?

Posted by: _JOE_---------------------

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.

Posted by: rmrider45---------------------

Quote:

Originally Posted by _JOE_

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.

Posted by: _JOE_---------------------

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.

Posted by: rmrider45---------------------

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

Posted by: _JOE_---------------------

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.

Posted by: rmrider45---------------------

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.

Posted by: RM85rider123---------------------

Quote:

Originally Posted by rmrider45

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.

Posted by: whenfoxforks-ruled---------------------

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?

Posted by: rmrider45---------------------

Quote:

Originally Posted by whenfoxforks-ruled

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

Posted by: whenfoxforks-ruled---------------------

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?

Posted by: rmrider45---------------------

reeds look fine but packing could use changing

Posted by: rmrider45---------------------

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 ?

Posted by: whenfoxforks-ruled---------------------

A tired engine would require a starting problem you have described. But, a bad float valve will flood the engine with fuel and require the same starting technique.

Posted by: rmrider45---------------------

Quote:

Originally Posted by whenfoxforks-ruled

A tired engine would require a starting problem you have described. But, a bad float valve will flood the engine with fuel and require the same starting technique.

tired engine? it has to short rides since fresh rebuild

float looks good. how can i tell its bad?

even tho it looks good i think thats the problem. as stated by another user.

not sure how to tell if its bad

Posted by: whenfoxforks-ruled---------------------

Its bad if its not replaced yearly, or when you are having screwy jetting issues off bottom. On another thread it was pointed out that some after market sets are an issue also, go back with oem. I sure got to think its a mechanical problem, maybe not even the float valve can fix. Especially since you have a new starting technique required. It sounds one step from needing a quad to pull start it? You're issue is getting worse. Jetting is not like that, aside from plug fouling. A long period on a lean main will cause premature wear/seizure. You should be able to tell on the stand if the engine is erratic reving or loading up. Never ride an erratic, surging engine long. Look at the plug. A run of 150 feet and back should be enough for a plug chop, and look at the plug.

Posted by: _JOE_---------------------

The main jet has no effect on starting. Look to the pilot jet, that is once you verify the float does in fact float(at the correct height) and the valve is new. How old is the lower end?

Posted by: whenfoxforks-ruled---------------------

Joe, this started as a half throttle bog? The air mixture screw at 2 out says the pilot is fine. It would be hard to say a worn slide even? Something mechanically failing seems more like it?

Posted by: helio lucas---------------------

the needle controls 1\4 to 3\4 throttle. i´ve seen many worn (very worn) needles that cause odd jetting.

Posted by: rmrider45---------------------

Quote:

Originally Posted by _JOE_

The main jet has no effect on starting. Look to the pilot jet, that is once you verify the float does in fact float(at the correct height) and the valve is new. How old is the lower end?

lower end ? if you mean the float its original i would say

brand new hot rods crank and new wiseco piston with less than 3 hours on it

Posted by: rmrider45---------------------

i have a five foot drop by my house (hill) if i compression it down that hill it fires up.if i try to kick start the bike it wont staRT . seems not to be getting gas.once ran after compression it will kick start . but still that sputter here and there

Posted by: whenfoxforks-ruled---------------------

A top end not sealing up good will not pull a fuel mixture into the carb properly. I have read that its a fresh top end, but? I reread through the 3 and 4 turns out on the air screw, then you told me 2? Can you achieve a high idle at all, adjusting it in and out to find the spot? Can you bite the bullet and pull the top end and measure the ring gap, skirt clearance, check for abnormal wear, cylinder out of round? You will need a bore gauge to check the last part.

Posted by: rmrider45---------------------

Quote:

Originally Posted by whenfoxforks-ruled

A top end not sealing up good will not pull a fuel mixture into the carb properly. I have read that its a fresh top end, but? I reread through the 3 and 4 turns out on the air screw, then you told me 2? Can you achieve a high idle at all, adjusting it in and out to find the spot? Can you bite the bullet and pull the top end and measure the ring gap, skirt clearance, check for abnormal wear, cylinder out of round? You will need a bore gauge to check the last part.

at first the air/mixture screw was out 3 or 4 then i adjusted it back to 2 .true you need good compression to suck in fuel .the ring may have lost its tension along the way of me putting the smaller leaner jet in.but it bogged like had a miss from day one of new rebuild before ever messing with the jets etc.i will try to do a compression test before i rip the top off.

also last year i got that bike to idle but after a 5 hour day riding the bike if you tried to start it the next day it would not start and if you pulled the plug it was loaded with oil.so i adjusted it that it would not idle and the bike ran fine for weeks
strange thing is when you give the bike gas it seems it cant burn the fuel fast enough .like its cloged up.

Posted by: _JOE_---------------------

Quote:

Originally Posted by rmrider45

strange thing is when you give the bike gas it seems it cant burn the fuel fast enough.

Probly can't.

Posted by: whenfoxforks-ruled---------------------

If the bike is loading up, and clears out past half throttle, right crank seal or the carb is flooding the hell out of the engine. A leak down test would resolve the seal question, replacing the float valve the other. BUT, a top end not sealing up properly can do the same thing. A leak down test can show the top end not sealing properly also. A compression test is only a good reference if you test it when its broke in, and keep checking it to see a noticeable drop, top end worn out. It can be real deceiving if not used properly. You should be able to smell the difference between the engine burning klotz and whatever trans fluid you are using?

Posted by: rmrider45---------------------

Quote:

Originally Posted by whenfoxforks-ruled

If the bike is loading up, and clears out past half throttle, right crank seal or the carb is flooding the hell out of the engine. A leak down test would resolve the seal question, replacing the float valve the other. BUT, a top end not sealing up properly can do the same thing. A leak down test can show the top end not sealing properly also. A compression test is only a good reference if you test it when its broke in, and keep checking it to see a noticeable drop, top end worn out. It can be real deceiving if not used properly. You should be able to smell the difference between the engine burning klotz and whatever trans fluid you are using?

bump

Posted by: rmrider45---------------------

whenfoxforks-ruled was correct. i pulled the Cylinder off and took it to a good machine shop. the Cylinder was off 6 thousands and piston (with maybe 4 hours on it ) was out 4 thousands. machine shop said he would call la sleeve to see if he was able to rebore the sleeve that is already bored 2mm over (.080) my ? is if i buy a new sleeve for this jug what will be the bore size? will it be standard ?thx

Posted by: whenfoxforks-ruled---------------------

You could send it the way it is, with the piston to Forward Motion. He will fix it correctly. He could probably even coat that liner! Your in Pa? Give Pete Payne a call! LA sleeve, nooooooo.

Posted by: rmrider45---------------------

wouldnt it be cheaper to get a new sleeve?

Posted by: whenfoxforks-ruled---------------------

Call Pete and see. I have not had a liner since 81, then it was new, questionable, and I did not personally know Eric and Rich. I could have been enlightened much sooner!

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boging and half throttle

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