RD Float Height Measurement

hey forum!

so after a good clean up of my RD carbs im no setting them to standard float height.

the manual says that i should measure from the area where the gasket of the float chamber sits....
the picture in the manual shows the carb lying upside down (like in my pic) is this right the way i measure it with my sliding gauge?

manual says 23 MM plus 2,5 MM....i set it to 23 MM is this ok for a first start up or do you guys have any experiences on float heights on the first start up?

You should have a depth gauge at the other end of your vernier calipers. I find it easier to do it that way:





It should be +/- 2.5mm.....so 23mm would be in the "middle" which is where you should start.

hillsy said:

You should have a depth gauge at the other end of your vernier calipers. I find it easier to do it that way:

It should be +/- 2.5mm.....so 23mm would be in the "middle" which is where you should start.

Click to expand...

thanks hillsy!so the orientation of the carb (like in my pic is right?)

do you have any experiences on the adjustment of RD carbs....


i would start with premix...check if the oil pump is working (i will deaerate the pump before first run).
the adjustment of the idle screws sound complicated in my manual...

Your measurement technique is fine, as long as you are measuring from the gasket mating surface and not the top of the lip that locates the gasket. Using the depth gage part of your caliper is probably more foolproof. I like the carbs upside down for this, but there is the notion that the little spring in the needle gets compressed afflicting accuracy.
This style carb with the floats that slide independently on vertical shafts have an important issue to keep in mind, especially if there is a lot of miles in them.
First, the shafts must be absolutely flawless with no evidence of wear. In operation, the floats live at nearly the same place all the time, and vibration wears parts in that particular spot. The effort that the floats exert towards shutting off the fuel when the level is high enough is very tiny, and it takes incredibly little to hang the mechanism so that the carb weeps or even pours fuel through the vents as if the needle and seat were bad or stuck.
The shafts are very hard in your RD's bowls, and not likely to have a problem, but check carefully.
Second, and this is far more common, the brass arm that the floats push against wear where the little pin in the float contacts it. There can be a (sometimes very) perceptible little groove there that will try to lock that pin in place and cause the needle and seat to hang up slightly. I think a lot of needle and seat assemblies are blamed for leaking (in this style carb) when it is actually the mechanism. I learned this on a pair of 34mm TZ carbs that I just couldn't make stop having an overflow problem. The brass arms did have wear, and as an act of desperation, I smoothed out all the wear on the arms and the little tab that contacts the needle, and BAM. Instantly solved! Since then, that's one of the first things I look for if the floats stick, and it turns out to surprisingly often to be the problem. I sand out any trace of wear with #600 sandpaper and smooth that out with #1000. Harder than it sounds actually. I can't remember the last time I needed a new needle and seat; I do this regardless on a "new" bike, and now actually EXPECT the carbs to never overflow. Get some genuine Mikuni bowl gaskets and I can almost guarantee you will never see a drop of fuel.

I do not use pre-mix on new engines (unless they have no pump of course!). Assuming you have cleaned the carbs (emulsion tubes out, flawless jet of carb cleaner from all holes and passageways, etc.), and have all authentic Mikuni parts (if you have used an aftermarket kit, take your carbs back apart, throw away all the aftermarket parts, and put back the originals or get new), your RD should be fine. Make sure the float bowls are on the correct carb (one has a drilling in the bottom for the fuel enricher (choke)). Don't forget the little hose that connects the choke between the carbs. Turn the air screws out 1 1/2 turns. Pull the oil lines off the carbs. Bleed the oil pump, then hold the throttle wide open and turn the pump by hand until you see heavy ooze of oil on each stroke of the pump (the two lines alternate on pump strokes) from each carb fitting and no air. If you have just assembled the motor, there should be plenty of oil from putting it together. Put the oil lines back in the carbs, and start the motor. If you are concerned, you can reach down and pull the oil pump cable to hold the pump at max rich for a bit. Once it is warmed up and running ok, you can pull each carb oil fitting out briefly one at a time while the engine is running to check that oil is pumping ok. You won't hurt anything in the short time this check will take.

hey moeb...once again you are a big help!

somehow i still have some questions on your post though it is extremely helpful!

My carbs were professionally cleaned (ultrasonic) the technician told me that the shafts are ok but i want to check anyways...what do i look out for?is there a way to see any wear without any special tools (they are so narrow i dont see a way way)?

The second thing you described, the wear which you smoothened out woth sandpaper...do you mean the point where the side mounted screw meets the slider in the carb or the little tongue that touches the carb needle?
i ask you because at dissassembly i saw this ...


how do i bleed the oil pump?i guess the philipps screw with the orange seal is the bleeding screw.
i would put 2 stroke oil in the side tank and then manually repeatedly twisted the throttle with the motor not running till only bubble free oil would come out of the 2 hoses for the carb.


turning out the air screws 1 1/2....from where?i guess from the point where i feel a little resistance...?

how do you go about adjusting the idle?how much do you tune with the screws on the sides of the carb?

Ryan,

You don't need any special anything to do everything needed for your carbs.

Looks like I offered some info that does not apply to your carbs. Should have looked at your pics more thoughtfully. Looks like your carbs have both float attached to a single arm. AFAIK, all US RD came with separate floats that ride on guide pins mounted in the bowl. Here is a pic for what it's worth. The two shiny spots near the end of the two brass arms are where the pins on the floats push against the arm. The area on this one is pretty broad, but this came off one of my own bikes, which I run with the fuel shut off until the bowls are empty because it sometimes is a while till the next ride. Ride the bike a lot, and this area gets worn down to a notch eventually and can hang up. So most of what I said does not apply to your carbs.

You should be aware that while ultra sonic cleaning if great, it isn't any more foolproof than anything else. I would spray carb cleaner through everything while it is apart to know with certainty that they are clean. It is vital that the emulsion tubes are removed and cleaned and checked. The emulsion tubes are part that the needle goes into. It installs through the top of the body where the throttle slide goes. To remove it, remove the main jet and washer. The brass part you see underneath is the emulsion tube. Note that there is a little brass pin that you can see that keeps the emulsion tube from rotating. Be careful not to damage it when tapping the tube out, and make sure it is aligned when putting it back. Set the body upside down on a surface that will not damage the threads where the top cover screws on, and with a wood or aluminum drift, tap it out. They can virtually fall out, or be nearly impossible to drive out, but they are not pressed in, just a precise fit, and the bodies get varnished which can make them very hard to remove. If they are difficult, use lots of carb cleaner and very patiently wiggle and slide them back and forth until they slide all the way out. Once they are out, make sure they are clean. Most have a series of vertical holes that bubble air into the fuel as it is supplied by the main jet. But there are different variations.
Next, make sure the air jet is clean. Looking into the intake side of the body, you should see a small round opening at 6 o-clock. Spray carb cleaner in there while looking through the bore where the emulsion tube came out, you should be able to see a clean jet of carb cleaner shoot through. That is pretty much it for the main fuel system.
The pilot system is easy too. Remove the pilot jet. If you do not have an absolutely perfect fit screwdriver, make one. Take out the air screw. That's the one you adjust to set the low speed air supply, the one you turn out 1 1/2 turns with a screwdriver. You can see how it works by looking again into the intake side of the body. The pilot air supply is another small round opening at about 7:30 0-clock. You can see the tip of the air screw needle through this opening. When it is screwed in all the way, it closes off the opening. The farther you screw it out, the more air it lets in. So it is basically an adjustable air jet for the slow speed (pilot) system. Anyway, take out the screw and spring, and pilot jet, and spray carb cleaner through both openings. You should get a clean stream vertically through the main bore, and another angled toward the engine. They are very tiny holes, easily clogged, but must perfectly clean. Not much more to it than that. Hopefully, your technician has already had them apart and you will have a very easy job verifying they are clean.

Looks like we have a slight terminology issue. I believe what you are calling shafts I am calling slides. These would be the large cylinders which the throttle cables attach to which throttle the engine. The needles are slid through holes in the top of the slides and held in by a plate which rests on top of an "E" clip on the needle. The marks in your pic are no problem at all. Just wear from the slides banging on the idle stop screws every time the throttle is closed. Your parts look perfectly fine, and assuming they slide smoothly and easily up and dow, should not be touched.

Once you are happy everything is clean, put it all together. Set up is very easy. Screw the idle stop screws out so that the slides are all the way at the bottom of there bores, and are no longer touching the slides. Adjust the throttle cables at the top of the carbs. The goal is to get both slides to start moving at exactly the same time from a fully closed position. There are a lot of ways to do this, but I stick my fingers into both bores at the same time, start cracking the throttle at the grip, feel for which throttle moved first, and then keep adjusting until they both move at the same instant.
Next, while looking into the bores, screw in the idle stop screws until each slide move up a few millimeters. We will come back to this later, this is just a rough guess. Just to clarify, the idle stop screws are the big ones you can turn with your fingers on the left and right of the bike. The pilot air screws or slow speed screws need a flat blade screwdriver and are on the left of both carbs when sitting on the bike. Turn the pilot air screws out 1-1/2 turns from fully screwed in. Think about how these work. If you look at them, they have a cone shaped end. This is so they can close off the pilot air supply when screwed in all the way. So don't ever screw them in too tight - just until they stop. They are brass, and soft an fairly easy to damage. Get everything else done and start the engine. Warm it up, and you can set the idle speed. It is a combination of how far the throttles are open, and how rich the fuel mixture is, and you can adjust both easily. If the idle is high, lower it by backing out the idle stop screws. Obviously, if it won't idle by itself, screw them in until it does! Estimate that you are doing this equally by holding the palm of your hand close to the exhaust outlet and feeling the pressure pulses. Compare the right and left side exhaust pulses and keep the strength of the pulses equal, and get the rpm you want. Then take a screw driver and adjust BOTH the pilot air screws in (or out - whatever is needed) 1/8 turn at a time, then rev the motor once or twice and wait a few moments for things to stabilize until you get highest engine speed. Keep the turns the same for both carbs. Start over if you lose track. When you have the smoothest highest speed, bring it back down with the idle stop screws. That is it. You may have to repeat this, and you may have to tweak the pilot air screws a bit after you ride the bike around to get it perfect, but that is all there is to it. If the carbs are clean, there are no air leaks, and the ignition and compression are good, this will work perfectly as described.

The oil pump is easy to bleed but messy. Obviously make sure there is plenty of oil in the tank, and remove the bleed screw (yes, in your pic, the one with the red fiber washer on it. These can be really tight, so get a new #2 phillips ( or JIS) driver and don't let it slip. DO NOT use a hand impact driver (the kind you use with a hammer) on the oil pump! If oil does not immediately come out, it will eventually. Hold the throttle wide open, and turn the nylon thumb wheel (looks like a gear on the bottom of the pump, it only goes around one way) about a million times or until no more air bubbles come out of the hole. I think if you just let it sit by itself for 5 minutes with the bleed screw out you achieve the same thing. Put the bleed screw back in, and thats it. You can bleed the lines to the carbs too as I described in the previous post, but likely you will have already achieved that bleeding the pump itself.
Hope this helps.
Good Luck!

jpmobius said:

The oil pump is easy to bleed but messy. Obviously make sure there is plenty of oil in the tank, and remove the bleed screw (yes, in your pic, the one with the red fiber washer on it. These can be really tight, so get a new #2 phillips ( or JIS) driver and don't let it slip. DO NOT use a hand impact driver (the kind you use with a hammer) on the oil pump! If oil does not immediately come out, it will eventually. Hold the throttle wide open, and turn the nylon thumb wheel (looks like a gear on the bottom of the pump, it only goes around one way) about a million times or until no more air bubbles come out of the hole. I think if you just let it sit by itself for 5 minutes with the bleed screw out you achieve the same thing. Put the bleed screw back in, and thats it. You can bleed the lines to the carbs too as I described in the previous post, but likely you will have already achieved that bleeding the pump itself.
Hope this helps.
Good Luck!

Click to expand...

your a big help moeb!

since this is a post 1976 model of the RD there is no nylon thumb wheel...only the philipps bleeding screw.
what i thought is that i maybe could spin the plastic wheel inside of the engine case (which grasps the primary gear) while the yet "empty" oil pump is connected to the oil tank....but since i already mounted the engine case and put fresh motor oil in i dont want to remove the case.

so my idea would be to open up the philipps screw connect the oil tank,wait till the oil is sucked down,maybe use my vaccum pump brake bleeding tool to suck it down to the pump....but how do i get the oil up in the 2 carb oil hoses?

the motor is in neutral...i could spin the generator in order to spin the primary to "activate" the pump...but my fear is that the fresh pistons I installed (with 2 stroke oil in the cylinders) will lose all their oil while i do this procedure.....


another question:

whats your opinion....i know its a "religious" question

oil pump or no oil pump?
should i run the pump or use mixture?
there are different opinions on that.

id rather run the pump....since the motor gets the right measure of oil at any RPM....

thanks in advance!!

I always use a pump if I can.

Well, it certainly is a nuisance there is no manual means to run the pump. I have had them leak at that seal, so maybe that is why Yamaha deleted it. Regardless, it won't be practical to "prime" it by turning the engine by hand. It is pretty heavily gear reduced. It is a pain with the thumb wheel, but 10 times faster than if you turned the gear from behind. I suppose that it is possible, just ridiculously time consuming. I think if it were me, I would make up about a liter of pre-mix fuel, and use that to start the engine and let it run the pump until all the air is gone. A couple points to keep in mind: If you assembled your engine like most people, you used plenty of 2-stroke oil putting it together. That usually makes a pretty good surplus of oil inside the crankcase, and you could likely run it for long enough to purge the oil system without concern anyway, but it pays to be careful. The oil pump pretty much fills up by gravity, and the bleed screw being at the top lets nearly all the air out without turning it. You could check it again the next day to see if any air made it's way to the top. That leaves just the lines to the carbs that really need to be filled by the pump. With the engine idling, you can manually reach down and pull the throttle cable housing to the oil pump to make the pump run wide open without increasing the engine speed. The carb feed lines fill up very quickly when you do this. So make up a little batch of pre-mix and have peace of mind that you will have plenty of oil. If you are running clear lines, you will see when the oil is getting to the engine. If solid lines, just pull out the injector nozzle out of the carb to see it is working. Once you know all is well, fill the tank with straight gas. Just leave your liter of pre-mix in there. a little extra oil won't hurt anything except the plugs, and you will go through several sets of those anyway getting the engine tuned. Then once you are riding the bike, run it almost empty and when you fill up again, you will have very little oil in the gas tank. Remember, the pump works with a piston inside that delivers oil in little pulses, and that is what you need to see to know it is working. Check both lines. The pumps are super reliable, and the only likely problems are the pump not getting oil for some obvious reason, or the check valves sticking. When the pumps sit for a while, sometimes the check balls stick. The check balls are directly under the pressed in brass fittings the carb lines connect to. Don't take them apart unless you know the pump is full and no oil pumps out. It is very unlikely you will have this problem unless your pump has been inactive for a really long time. One last thing: Yamaha set their pumps rich to start, and leaned them out at the first service interval. This is done with little brass shims on top of the cable pulley, which increases or decreases the pump stroke. You'll have to look at the service manual for your bike to know for sure what shims are needed. Most of the bikes I see still have the "rich" shims still installed, so likely not an issue. If you are worried, you can always add a couple of shots of oil to your tank during break in for peace of mind.

hey moeb...so long story short...

i followed youre steps concerning the different jets and looking for their utmost cleanliness....and i´m effin pissed now...as you said ultrasonic is not everything...

i paid 100 EUROS for my cleaning...and the technician didnt seem to take apart the main jet part which needs to get tapped out.

i tapped out the part of the right cylinder (choke carb) and it was COVERED in "dust" of corrosion,greenish sand.i cleaned it thoroughly with carb cleaner and afterwards i tapped it back in, I realised that the threads where detroyed so i need to buy these parts new....

i´m pissed.

i´m based in europe,should i stick to mikuni parts or are there any aftermarket parts that are good too?
for example:

http://www.ebay.at/itm/Keyster-Vergaser-Dichtsatz-YAMAHA-RD250-RD-250-Bj-76-77-Reparatur-Satz-NEU-/350752733216?pt=DE_Motorradteile&hash=item51aa7df820

the picture shows the parts i need to replace....

I rarely ever buy any carb parts except factory. The exception is automotive. Bike parts go with Mikuni (or Keihin, etc) only. Especially jets. Yes, you can end up spending a fair bit (but 100 euro buys a lot of jets). Sorry the threads are screwed up, probably have to replace that emulsion tube. Definitely the jet. Don't give up on the tube, I have had several cases where previous "techs" have damaged the threads and I have massaged the threads back into useable condition. Keep in mind that the washer under the jet is what seals so as long as there is good grip with new jets you should be ok. Also, replacing a bunch of parts on these things is rare. The odd float, (I put them in a container of fuel and compare how they float to other identical floats if they are suspect - very rare for that to be the case), needle and seats (also quite rarely) and have seen needles wear on ultra high mileage motors. Most all the hardware is permanent. Get factory bowl gaskets. O-rings for the screw on caps over the slides, along with grommets for the cable adjusters where they attach to the caps. I really don't do anything fancy, and have had people shake their head at me, but my carbs really do function flawlessly. I take them completely apart and glass bead the bodies. Clean them after with hot water and detergent, then carb cleaner and compressed air, meticulously checking for perfect flow through the tiny passages. All the rest of the parts just carb cleaner and brass hand brush. Occasionally I will run a tiny wire through a passage if something is stuck, and usually it cleans right out after that. Occasionally I will soak parts in a gallon can of carb cleaner, but 99% of the time I use spray cans. Thats really all there is too it. No tricks or special anything at all. Just determination to be thorough. If there is any trick, you do need the exact tool for most everything as the brass is very soft and fragile. That means exact, and I mean exact screwdrivers for things like pilot jets. If something does not come apart, heat it up. Your first attempt to take something apart can easily be the only one you will get.

For what it's worth, if I need really super perfect cosmetic appearance, I glass bead with BRAND NEW very large premium quality glass beads and then walnut shells if they look too bright. Very counter intuitive, the large beads peen the surface leaving it shiny. Small beads leave a very dull finish. There is no issue with "deforming" anything and I have never had any trouble with clearing 100% of the media from the carbs.

Looks like you have this sorted. The whole key is complete disassembly, patience, and elbow grease.

Fortunately i got you and my german RD specialist as a perfect resource of 2 stroke wisdom!!

Effin up the threads of this part also showed that the PO installed 160 mains (which my german yamaha guy confirmed to be wrong,115 s should be installed).


Another question to you moeb:

Is there a way to 'measure' of the long slide needles that go in the hole of the vertically drilled brass part are straight?

I used highest diligence to protect them from getting bent around my build but i just want to be sure....
Could be hard cuz the needle itself gets thinner to the end....

The needles surprisingly rarely get bent. The also do not work in a fashion that, if they are straight enough to not look bent, and they easily "find" the opening in the needle jet/emulsion tube when you install the slides, they are surely just fine. Roll them on a piece of glass, and if they don't look like a bent nail, carry on. They function by allowing a variable cross section exposed as they draw in and out of the emulsion tube with the slide which is the variable air supply, so they are basically a variable jet for the main fuel system, which is ultimately limited by the main jet. So unless they cause a mechanical problem, or are worn (takes a zillion miles, and occurs at the fat area) don't worry about them. Do make sure they are the correct ones though. Don't jump to conclusions about the jets. USA 350's came stock with 2 very different main jet sizes depending on the airbox style. 105's and 140's (I think). One of my own bikes, absolutely dead stock except for an air cleaner runs 160's. I just finished tuning a customers bike, totally stock porting, but carefully machined heads, high rpm pipes, reeds, spacers, and stock casting 28mm carbs that is finally rich enough with 270's. Yes, that is big, no, that is not too rich, and yes, It is not a typo, 270's, and it is surprisingly fast considering no changes to port timing. But it does have a crank that will let it spin all day at 10k. I think most people really frustrate too much about some mystery problem they expect regarding carbs. 2 strokes are pretty crude beasties, and can sometimes be hard or even impossible to get to run really cleanly through the rev range, but getting the top and bottom rpm sorted is pretty easy. Aside from assembling the carbs wrong, i.e, fuel bowls swapped, slides swapped, jet left out, etc, 99% of problems are air leaks from worn non carb parts and restriction caused by carbs not being clean. Only the important few bits actually are very sensitive to perfection and precision. Go back and take a good look at the pic I posted earlier. The carbs on that bike - AFTER- restoring them, look like the came off a ship wreck. They literally were submerged in MUD when I got them. Once all the corrosion was gone, parts of them look like they were porous enough to have holes all the way through like a sponge! But the brass is fine, and though they were a bitch to get apart without damage, once cleaned they work perfectly (needles and seats too!), though cosmetically not so much. Clean them, assemble them carefully, and they will work great.

If you have modified your engine, start way big on the mains, and absolutely stock or less spark advance. Better to be cautious than building a new motor.

Couple of points to consider here.

The "correct" way to measure float height is to tilt the carb until the float tang just starts to touch the spring loaded pin. Usually at their sides is OK but never upside down. If the pin is pressed in by the weight of the float, the adjustment will be way too low when it's full of fuel the right way up.

The better way to set fuel level is to do that and not the float level. Fuel should be 3mm +/- 1mm below that gasket surface and can only be measured by connecting and external tell tale. You can probably make up a float bowl drain screw with a pipe in it to place at the side of the carb to see what the fuel level actually is.



Needles are rarely bent as JP mentioned but it might be worthwhile replacing those needle jets if you can find a pair. IIRC different markets used different jets but they are probably 175 series in either O-8 or P-2 size. And the threads on that main jet look pretty bad too.

That slide is fine. Don't do anything to it - apart from adjusting it.

Oil pumps all the time every time.

Do not be surprised if you need to replace the needle and seat. They often leak over time and most of the time it is wear on the needle and sometimes it's pitting in the seat. Sometimes you get lucky and they are fine. Floats rarely stick but it does happen and more so on certain Keihin carbs than Mikunis.