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Discussion Starter #1
Hi

I am currently rebuilding a Yamaha XT250 1982 model.

It is a 'barn find' and has not been run for about 15 years. I have managed to get it running but it is lumpy. The ignition system is fine but whilst it ticks over fairly evenly and will accelerate reasonably well, once it reaches about half throttle it refuses to rev any higher. It also spits and pops on the overrun with the throttle closed going downhill.

There is obviously a fuel problem, I was not sure whether it was running too rich or too weak so I stripped and rebuilt the carburetor, Mukini VM28SS which is the original as fitted by Yamaha.
Everything is clean, all jets are clear and all gaskets and O rings are good. I suspected that the fuel level was incorrect; I have checked the needle valve and it is working correctly so it is not permanently flooding.
I carried out the fuel level check by using a clear plastic tube attached to the overflow and unscrewing the drain plug. The manual calls this test 'float level adjustment', the measurement is taken from the bottom of the carb body to the height of the fuel in the tube.

This is where I have come unstuck.
The measurement I have is 40mm which is way too low which would account for the failure to accelerate above half revs and the misfire on the overrun as it is obviously running far too weak.
The specification in the 'manual' gives two measurements, 'fuel level' is given as 4mm and 'float height' is given as 23.5mm. Either way at 40mm mine is much too low.
Can anybody tell me which of these figures is the correct one? I would rather only adjust it the once, it is a pain getting the carb on and off so I plan to make the adjustment with the carb off the bike. I don't want to guess at which is the correct figure and have to do it all over again.

Hope somebody knows the answer
 

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I tune a lot of Mikuni's, but HSR's not VM's. Generally, the float height measurement is going to be the height of the float arm or float itself as measured from the arm, or the bottom of the float, to the surface where the bowl attaches. What you do is remove the carburetor, turn it upside down, and take off the bowl. You then measure from the bottom of the float, (which is now facing up,) to the gasket surface where the bowl mates to the carburetor body. On some models you measure from the float arm to the gasket surface. That would probably be your 23.5mm measurement, although it's different on other models.

As I said, that's how I do it for most HSR's, but maybe that will get you close on your old VM. Good luck!
 

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are you saying 40mm fuel level? turn the carb so it seats the float needle (just barely) upside down or on an angle, measure from the bottom of the float (as the they would sit on the bike) and set it to the 23.5mm. ALSO make sure you check the vent tube for clogs!
 

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get a pen or pencil that writes. and a short measuring stick like a 1/4" dowel around 5 or 6 inches long.
pull the carbs off.
turn them upside down and suport them with the bowls up.
take the bowls off.
pull the gaskets out, the measuring is done without gaskets and from the carb body itself.
now inside the lip of the carb body, where the bowls drop into and seats on the gaskets, put a little measuring stick upward toward the float hinge lever that holds the floats in the bowl.
put a little mark on the measuring stick.
take your rule, and measure the length to the mark. Does it match any of your carb specs?

if it does or is close, make any adjustments by bending the float needle lever.

if it doesn't, check to make sure your specs don't call for a reading from level, at the float bottoms, and if it does, you will need to make another mark farther up the measuring stick. by sighting across the bottoms of the floats, while the stick is standing on the carb bodys(carbs upside down with the little stick pointing to the sky, and the short mark lined up on the metal float holder frame, sight across the bottoms of the floats lining them both up to the same height or level, then make the second mark on the stick)
any way the specs tell you to test them, use that as a gauge but remember, some bikers change from the stock floats to aftermarkets because they find some rust on the originals, that means the measurment will not be proper unless the short method is used.

The adjustment you make, is only a change of about 2 or 3 millimeters if any. so don't be wrenching around on the float needle adjustment.

like on my xs11 somebody has put K&S aftermarket floats in there. They are alot smaller, have no rust and make the adjustment only possible with the short mesurement.

take your time, don't wrench around on the float adjustment tang.
 

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Fuel

The other guys are giving you the info for measuring the float height, upside down, which in turn determines the fuel height, which we assume is correct, but can not see.
But some Mikunis can reveal the fuel height while on the bike. That is what you are doing with the clear tubing. That is not an overflow hole, as it is at the bottom of the bowl. At least the hole to use is at the bottom of the bowl. With the plastic tube in place, you unscrew the screw next to it and can check the level. I am thinking, but not positive, the fuel level is the 4mm number you gave, from the top of the bowl. That looks like 1/4 of an inch from the top.

Just checking some info for my XS400. Says 3 plus or minus 1mm from the mixing chamber edge. Picture shows that as the edge where the bowl meets the body of the carb. The XS1100 does it the other way.

So if you know what your fuel level really is, you can adjust the little tang on the float accordingly. Much easier to do it with the carb off. If it has a rubber boot it slips / slides into, and it is a tough fit. Warm up the rubber with a heat gun, use a little vaseline and a crow bar.

I can change the main jets on my XS400 ( two carbs ) in 35 minutes.

Unkle Crusty*
 

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Some Mikuni carbs have the drain and overflow come out of the same opening in the bottom of the bowl. (They use a simple brass straw in the bowl chamber) The level can be checked with a clear tube while on the bike.

I just went through a particularly nasty carb on an XT225 and still am having no luck with getting the engine running. Today I'll replace the carb with a known running one and make sure that's actually where the problem is. I've already checked over everything else I can think of, so hopefully I'll have it figured out by the end of the day.
 

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Discussion Starter #8
Thanks for the comments and advice.
Just to clarify - the test using the clear plastic tube according to the manual is ''float level adjustment'' and yes, Uncle Crusty it is the float bowl drain, not the overflow as I mistakenly said.
The specifications at the end of the chapter give two measurements which are shown as ''fuel level'' at 4mm and ''float height'' which is supposed to be 23.5mm.
I have once again checked the float height by using the method described by Pillen140 and Yahmyxs, by turning the carb upside down with the float bowl removed and measuring the distance from the bottom of the carb body (where the float bowl attaches) to the bottom of the float.
I have adjusted that to exactly 23.5mm. I was assuming that with this float height set the fuel level, using the clear plastic tube would be 4mm but it was much greater than this, probably nearer to 15mm. I didn't actually measure this because it was so obviously greater than 4mm there didn't seem a lot of point.
Just to confirm I then bent the adjustment tang so that the needle valve would only close just before the floats hit the carb body just to see what the maximum fuel level could be. It was around the 12mm height.I could get nowhere near 4mm fuel height, the floats would be touching the carb body and the needle jet would be permanently open.
I have a second bike which i am cannibalizing for spares so I have stripped that carb as well to check the floats. They are identical in both carbs and appear to be original fitment, they are plastic so no rust problems and they are not leaking or damaged.
I have checked all the jets etc and they are the correct size and clean as a whistle. Everything appears to be ok so I put the carb back in having adjusted the float using the 23.5mm adjustment. It runs exactly as before - it starts first or second kick on choke, ticks over really well and accelerates smoothly up to about half revs then coughs and splutters if I open the throttle further. If I open the throttle fully above half revs it just 'dies' as if it is either being starved of fuel or alternatively getting too much. It then lurches back into life for a couple of seconds before dying again. It still backfires on the over-run going downhill with the throttle shut.
I'm now wondering whether the problem is not the carb at all but something else. An air leak would produce similar symptoms but the carb to manifold joint is good and tight and I have replaced the manifold to cylinder head gasket. This made no difference.
Any other suggestions anybody? I even toyed with the idea of an ignition problem but if that was the case I don't think it would tick over or pull so well in the lower rev range. The ignition is electronic CDI and is non-adjustable.
I have removed the plug and rested it on the cylinder head while kicking it over and there is a good healthy spark.
I'm beginning to run out of ideas now.
 

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i would double check the vent hose for clogs if you havent already. without a vent, it wont let gas in the bowl... or it will flow gas from somewhere it isnt supposed to.
 

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Type of carb

Is it a diaphragm carb. If so, check the diaphragm for tiny holes. The piston needs to slide very easily up and down the cylinder. Check the brass coloured cylinder for high spots. When in place, if you lift it with your finger, it should slide down quickly. Use very fine wet and dry sand paper, or metal polish.
Somewhere on this site someone wrote about this.

Stopping abruptly, as in choking can be too little fuel. Bogging momentarily but then sputtering, can be too much fuel. Did the bike ever run properly with this carb? Has anything changed with the engine, such as reduced compression, or change of air filter and exhaust? Also check the ignition timing.

On the XS11 site, many of the guys mention that some replacement floats and needles do not work very well. It is sounding like the needle you have is the wrong length, if you can not get the correct level.

Unkle Crusty*
 

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I got that XT running this afternoon. Besides the clogged up carb, the fuel valve screen was partially obstructed. Runs like a charm now.
 

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Sweet

I got that XT running this afternoon. Besides the clogged up carb, the fuel valve screen was partially obstructed. Runs like a charm now.
Real sweet when they actually run properly.
My XS11 developed an electric problem. Will be nice when it is running again.
I have arranged an appointment for it for next week. The repair guy has been busy working on his boat.
Arms and shoulders are beyond the ache stage, and moved to burning. But the grinding and sanding is done. Never had this problem 50 years ago.

Unkle Crusty*
 

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Is that 4mm fuel level as measured from the bottom of the bowl, or from the top? I can tell you that 1/4" of gas in the bowl is not enough to keep a bike running at open throttle. On most bikes the bowl should be fairly full.
 

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Top it up

Is that 4mm fuel level as measured from the bottom of the bowl, or from the top? I can tell you that 1/4" of gas in the bowl is not enough to keep a bike running at open throttle. On most bikes the bowl should be fairly full.
From the top. See my post number six. I took a photo of the XS400 manual page on the carburetors. Has a drawing of the carb and plastic tube.

Unkle Crusty*
 

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Hmm.... This is a challenging one for sure. Sometimes when I get backed against a wall on a job, I go back to basics. This bike fires up fine, idles well, and accelerates ok until about half throttle? Ike (the OP) suspected a float level problem. In all my years of fixing bikes, I have never encountered a float level problem unless someone altered the tab or the floats filled with gas.

A carb controls 2 things. It meters fuel and it meters air. It creates a proper air/fuel mixture.

I would be checking the air delivery system as well as the main needle that slides up and down thru the main jet. If the needle height is incorrect, it will usually become apparent at higher rpms.
 

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Discussion Starter #16
OK - sorry I've been so long but I have tried a couple of things.
In reply to Pillen140, yes, I have checked the vent hose and it is clear
Uncle Crusty - yes it is a diaphragm carb; I have taken the diaphragm out and it appears intact. You ask if anything has changed or whether the bike has ever run with this carb - truth is that it is a barn find and I have no idea even how long ago since it last ran. I'm assuming it ran with this carb but I really don't know.
What I have done is to take the carb off the second spare bike and try that. I checked all the jets, needle etc and set the float height by using the plastic tube method and set it so that the fuel fills the float bowl up to 4mm from the carb body as per instructions and I am getting exactly the same results with this carb fitted as I got with the original except that with this higher fuel level I no longer get the backfire on the over-run which sort of confirms my suspicion it was running very weak.
I'm now assuming the fault may lie elsewhere, I would be amazed to try 2 different but identical carbs and have identical problems with both.
What I have also discovered is a problem with the air filter. When I first checked it a couple of weeks ago i just opened the cover and took a quick glance and it appeared to be fine and clean. On closer inspection it appears that somebody has cobbled this together from a piece of half inch thick foam. It matters now what it was made from because it did not fit the airbox anyway and might just as well not have been there. The reason it was so clean is that no air has been passing through it, the bike has been running without an air filter.
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Is it possible that a lack of air filter could have such a dramatic effect on the performance? I realise that an air filter should be fitted but would the lack of one cause the engine to cough and splutter and not rev above half throttle?
I'll be ordering a new filter tomorrow but due to my location it probably won't arrive for about 3 weeks. I think I'll give the whole thing a rest until it arrives.
.
Any views or observations gratefully received.
 

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Air filter

These diaphragm carbs can be a bit mystical, as they require air suction to lift the the piston via the diaphragm. I read many cautionary tales about what type of air filter should be used.
My 1983 XS400 has two Mikuni diaphragm carbs. I tried them without filters and it did not run very well. However at the same time the main jets were too large. Whatever goes on the back of the carb should not mess up the air flow at the carb entrance point. Paper pod filters are not good, according to many. Yet my bike ran okay with them. Foam pod filters are okay, and probably a good choice for a dirt bike. I have two foam pod filters on my 400. Trying to read plugs was confusing with these type of carbs. I followed a Mikuni tuning guide by doing full throttle tests. My bike was spluttering, too rich. After getting the jetting near correct, the bike runs great.

Long story to say: Try a foam pod filter, and check all the other things, like compression, valve clearance and timing. Check the quality of the spark. Some guys use a timing light for this. Also check for any obstructions in the exhaust pipe. Sometimes bikes that have sat for a while, will have some surprises. Like chewed electric wyres.

While reading about how paper pod filters will not work, I noted that many XS1100 ( 4 cylinder, 4 similar carbs ) run fine with them. After doing battle with the stock filter and box I know why they use them.

Unkle Crusty*
 

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Discussion Starter #19
@ Unkle Crusty - sorry I may have misled you. It is not a diaphragm carb, it is a standard carb but it has an accelerator pump which operates by a pushrod attached to the throttle lever shaft which in turn pushes on a diaphragm to squirt extra fuel in when the throttle is opened. I have dismantled it and checked it, there are no holes in the diaphragm.
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@ Yahmyxs - have checked the fuel flow from the tank and it's fine, the petcock is working and the fuel line is clear. If I remove the fuel line from the carb and turn the petcock on I get a good flow of fuel.
Is it possible to get such extreme symptoms purely from a dirty air filter or alternatively from having no air filter? Having checked mine it has been cobbled together from a piece of foam of unknown source, it may even be an offcut of upholstery foam for all I know. It was not sealed in the air box and no air was passing through it, air was just passing it by and straight into the carb intake; it may as well not have been there at all.
Could it possibly make that much difference?
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I have a new air filter on order but it won't be arriving for a few days. I'm not doing anything else to it until I have it fitted; until i know everything is as it should be I can't really assess what is wrong.
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@ OneEyedJack - I'm at the point where I'm not entirely convinced the problem lies within the carb itself. I have taken an identical carb from a spare scrapper and checked it over, cleaned the jets etc and set the fuel level to exactly 4mm from the bottom of the carb body, as per the specifications. This has resolved the backfire on the over-run but it still coughs and splutters above half revs. Until I get the new air filter installed I'm really only guessing at what the problem might be, perhaps the coil is breaking down at high revs?
Maybe the ignition timing is off - shouldn't be because it is electronic CDI but worth checking anyway. The bike has been stood for a number of years, perhaps the exhaust pipe is blocked or restricted? I am loathe to remove it at this stage because I know I will end up shearing off the exhaust manifold studs which will need removing with an easy-out. They've been there for 33 years, no chance they will come out easily. I'll check everything else first and leave that till last - I will have to remove the engine to fix that one.
 

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I had the same issue as you described Ike2248.
I finally solved mine dropping the jet main jet from 165 to 160 and lowering the fuel level in the float bowl to 8mm from the carburetor base without the gasket. (This is also just below the upper set of holes in the emulsion tube that the main jet threads into) I also made sure the holes in the emulsion tube were all clean out. This has been a problem hating this bike for many years and reading all of this helped so I figured I would pass on what worked for me
 
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