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Doug E.
Doug E., Kawasaki Master Technician
Category: Small Engine
Satisfied Customers: 2876
Experience:  Professional mechanic since the late 70's on Kawasaki, Arctic Cat, Jonsered, John Deere and others.
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I have a 1996 Yamaha VMAX 600xt 2 cycle snowmobile. The sled

Customer Question

I have a 1996 Yamaha VMAX 600xt 2 cycle snowmobile. The sled will start and idle fine, and once it gets above about 60+ mph, the engine seems to take off and run ok. But from a stop to the higher speeds, it struggles to speed up. I have a second vmax sled that once i hit the gas, it just takes off like crazy, which is what i think this sled should do. I have checked compression on both cylinders, 124 lbs which is good, i had the carbs cleaned by a 'supposedly' reputable guy and also had the carbs pressure checked at a dealer to make sure the needle/seats were ok and they said the carbs were good. when running the sled, there is a very strong smell of gas and the sled gets about 3 mpg on the trails whereas my other vmax is well over 6 mpg. the 6-7 mpg is about normal. so for some reason, it seems to me its being flooded. the sled will run on one cylinder(checked both of them) too(pulled a plug wire). plugs seem to look the same too, dark in color and a little wet. someone else told me to check the fuel pump as it could have dirt in it. i took it apart and seemed clean. diaphram wasn't in best of shape, but didn't think it coud be an issue. i've had multiple primary clutches on it as well as belts and that hasn't seemed to do anything to help.

i'm out of ideas... any suggestions?
Submitted: 3 years ago.
Category: Small Engine
Expert:  Doug E. replied 3 years ago.

Doug :

Hello,

Doug :

Pull the impulse line from the fuel pump to the engine block. Do you have fuel in that line? If so, your pump is leaking gas directly into the block and will need to be rebuilt or replaced.

Doug :

You shouldn't be smelling gas while running and your poor mileage is also an indication that something isn't right.

Doug :

If you have no gas in the impulse line, then the float levels may be set too high.

Doug :

The best way to check that is with a fuel level test. The problem is that you need special fittings to fit the bottom of the carbs so you can see the level of gas. I can give you some ideas on how to make the fittings. If the fuel pump checks out, that is what I would do next.

Doug :

I have to log out for the evening, but will be on in the morning.

Customer:

if i remember correctly when i pulled the pump out, there wasn't any fuel in the line between the pump and block. the small line that went from pump to oil injection there was fuel, but that made sense i guess as i figured that was needed to help the gas/oil mixture. i also pulled the reed cages out to look in the cylinders(dealer suggested might be a bad piston) but everything looked great in there. not that i could tell if there was extra fuel in it i guess though. how do you check the floats? or make adjustments if needed? thanks!

Doug :

The floats can be checked by taking the bowl off, turning the carb upside down and measuring the float arm height. It is adjusted by bending the tang in the center of the float arm. You can also check the fuel level in the bowl using a fuel level gauge. You can make these with a clear piece of hose, but you have to find a way to fit it to the bottom of the carb. Your carbs use two special drains, so it's not a simple test like on a Cat. I have the manual at work so I'll see if I can work up a simple solution for you. Just a 2mm change in the fuel level will make a big difference in gas mileage.

Customer:

maybe a quick check to see if its the floats would be to pull the carbs off my other vmax and put them on the bad one. that would tell me if its a carb issue quickly, wouldn't it? to adjust the float arm, do you need a special tool to measure? or if this is the problem, is it better off to have a dealer set the height? i know the drain plugs you're talking about, and that could be tricky to fabricate an adapter that would screw into the bowl and also slide a hose onto. or if i can measure the height, what is the proper height? kinda makes sense though given the horrible fuel mileage i'm getting. any chance you know what the average fuel mileage is for this engine? thanks.

Doug :

I'm going over some service bulletins on fuel issues for your sled. Give me about 10 minutes.

Doug :

OK, the first bulletin tells how to adjust the carbs and says the sled should get 8-10mpg when done. It does have diagrams, but you turn the carb upside down and measure from the body to the float arm. You will only need a small metric rule and a small flat blade screwdriver to tweek the tang on the float arm to set the level. Remember that this method only works if you have good float valves and floats. I prefer to use a fuel level test that shows the actual amount of gas in the bowl. That works best on older machines with worn parts.

Doug :

The second bulletin tells how to check the TPS. There is an error in the shop manual and this corrects that. Those are the only issues I could find for your sled. I will upload both for you in a few minutes.

Customer:

what is the TPS? sounds like i might be able to do this as long as there are a couple good diagrams to see what exactly to measure and what it should be. i would love to get 8-10 mpg... if that works, i'll be checking both sleds...

Doug :

The throttle position sensor sends data to the ignition from the carbs. If it's out of adjustment, the timing will not be correct which can cause poor performance, bad fuel mileage, etc.

Doug :

I'm having some problems uploading one of the files. Give me a few more minutes.

Customer:

no problems.

Customer:

i'm at work now and won't be able to look at the sled til at least tonight.

Customer:

if you need to email me directly too, itsXXX@XXXXXX.XXX

Customer:

guess i can't put emails on here...

Doug :

There, that should work.

Doug :

The carb bulletin goes with what I was first instructing you to check. It also has an airbox mod and some jetting changes. I think that's going to help both of your sleds increase their mileage.

Doug :

I wouldn't mess with the TPS unless it's already been adjusted or you continue to have issues.

Doug :

If it was ever adjusted, it's probably wrong being the factory manual has the wrong specs.

Customer:

sounds good. i'll read this over during my lunch hr and if i have any questions before i tear into the sled, i'll ask. thanks.

Doug :

Your welcome.

Customer:

so i've isolated the problem to the carbs. i took my good running vmax and swapped the carbs around on the two sleds. now the good running sled doesn't run well, and my problem sled sounds really good when i pick up the back and rev it up. to fix carbs, what all should be done? i measured the floats on the bad carbs and they seemed to be set right. 24mm. what are all the rmain replaceable parts that are in carbs that potentially go bad? the jets(maybe the previous owner put bigger ones in?), needles/seats? what else? i thought i'd just get jets(since i have no clue what size are in there) and needles/seats and see what that does. is there anything else i should replace? or anything else in the carbs that could be causing the problem?

Doug :

The only part that wears out is the float valve. All other parts are fixed, so don't wear. The jets can be plugged, as can the passages. I would remove all of the jets, clean the carbs and check the jet sizes. The numbers are stamped on each jet. You can compare these to your other carbs or from any site that sells Yamaha parts. www.babbittsonline.com is one of them. They have a breakdown of the carbs with the sizes listed.

Doug :

The biggest problems with carbs are blockages from varnish (old gas turns to a varnish) and worn float valves.

Doug :

If the jets can't be cleaned, then replace them. Some people use a drill to clean them, but many jets have tapers and steps in them and are difficult to clean.

Customer:

how many jets are there in the carbs? theres the two main ones, but i assume there are a couple smaller ones? can those change in size? if the float valve is bad, would it not stop fuel from entering into the bowl? because when i pushed the float down, it would stop anything from coming in from the fuel supply line.

Customer:

i'm going to go pull the carbs to see the size of the jets.

Doug :

The float valves control the fuel coming into the carbs. they can not allow fuel, or allow too much fuel. The main jets are the center jet that sticks down the farthest. That controls the high speed mixture. They screw into the needle jets. Those and the needles in the slides control the midrange mixture. To the side of the main jet is the pilot jet. These control the low speed mixture.

Doug :

I have to log out for the evening. I'll be back on in the morning.

Customer:

i pulled my jets and they're the standard jets. 152.5. i'm thinking the float valves need replacing.... at the beginning of the season, i tore the carbs apart and went through them. and i thought they were clean. then it didn't run right, and i had another guy go through the carbs and clean them. if theres crap built up in them after we both went through the carbs, we suck... is there a tell tail sign when looking at the floats that they're not acting right and the valves might be bad? i'll pry be calling the dealer in the morning to see about getting some new float valves...

Doug :

In my very first post I mentioned doing a fuel level test. This is a little difficult on yours due to the drain set-up. You would need to have the factory test fitting or you could make a fitting. You would need a spare cap for the bottom of the float bowl. Drill and insert a fitting that you can attach a clear hose to. Run this alongside of the carb and supply fuel to the carb. The level in the hose will show you the exact amount of gas in the carb. You can see how this matches up to your other carbs, but you should have a level about 4mm below the top of the float valve.

Doug :

That shows what the test looks like. It's the only way to deal with difficult float issues because it shows the true fuel level.

Customer:

so i assume you do this with the carbs on the machine, but you have to have the airbox off to get access in there? on the bottom of the bowl, there is a big drain plug, and also a little set screw that once unscrewed will drain fuel out the bottom of the bowl that has a little nipple on it which i think i can just stick a little clear hose to and make it work pretty easily.

Customer:

so what i'll need to do it determine where the float valve level is in relation to the bowl and kinda have a good mark as to where to check the fuel level once the carbs are on the sled as i won't be able to see where the float valve is with the bowl on, right? and i'll have to kinda measure off the bowl.

Customer:

once i hook the hose up, does the sled need to run at all to fill back up again? technically, fuel will drain from the bowl into the clear tube, lowering the fuel level in the bowl. so does that need to be replaced to 'current' levels? or is that little loss of fuel not an issue?

Doug :

Yes, you can use the drain nipple with a small clear hose. I thought your year just drained out of the screw, so that will be easy. I do the test on the bench with a small fuel tank hooked up. That way you can supply a constant fuel flow. The carbs need to be level.

Customer:

so what are the steps then to do the test? you need a hose from a small gas can to the input on carbs? then just push fuel into the bowl til the floats stop the flow of fuel? then check your clear hose and see what the level is?

Doug :

When bench testing: Connect a gravity feed fuel source. Connect a clear line to the drain and run it up alongside of the carb body. Turn on the fuel and open the drain screw. When the gas stops rising in the hose, measure from the top of the float bowl to the level of fuel in the hose. To change the level, remove the float bowl and bend the tang on the center of the float arm. If bending that doesn't change the level or if the level varies each time you check it, then the float valves or floats themselves are faulty.

Customer:

so gravity is enough to fill the bowl up to the point where the floats should cut the fuel off? so really a small 16 oz bottle with a nipple on it is good where i can stick that to the fuel input on the carbs(using hose) then just flip the bottle over and hold upside down. so i'm assuming i want to see the fuel level in the clear hose below the top of the bowl, right? anything above is pry a flooding issue.

Customer:

how can a float go bad too? i can see the valve go bad, but what goes bad on the float and is there something i can look for on the float?

Customer:

how many times should i do the test to verify what i'm seeing is consistant too?

Doug :

Yes, gravity feed is all that is needed. Your sled's pump only produces minimal pressure.

Doug :

I'd do the test twice, just to verify the results, and repeat the test each time you have to adjust or replace anything.

Doug :

Brass floats can leak as can the hollow plastic type. You should hear gas in them if you shake them. The solid plastic type don't go bad very often, but may still absorb a slight amount of fuel making them a bit heavier than when new.

Doug :

Most Mikuni VM carbs run a float level 3-4mm below the top of the float bowl. If it goes above that or keeps going and doesn't stop, then you will have a flooding issue or an over-rich condition where you burn too much fuel.

Customer:

that 3-4 mm level is measured from the bottom of the float to where the float bowl meets the carb?

Doug :

Top of the float bowl on the exterior of the carb body to the line of gas in the clear hose.

Customer:

ok yeah i'm with ya now.

Customer:

i was thinking the other way with the bowl off and setting at the 24.3mm like the diagram said to do that you sent me.

Customer:

but doing the fuel test is a more accurate test

Customer:

so the floats 'go bad' if/when they aborb fuel in them making them heavier and thus making it harder to stop the flow of fuel into the bowl.

Doug :

That's correct. The measurement way works fine when all of the parts are new. The fuel level method works even on old worn out stuff. Yes, if the float leaks or becomes heavier, it sinks further and the gas level goes up. Even a 1 to 2mm change will alter your mileage and performance.

Doug :

Yamaha doesn't publish the fuel level readings for your carb, so I am using standard Mikuni levels.

Customer:

sounds good. i'll get the stuff to do the tests tonight to verify what i'm working with. when i drained the two bowls last night to pull the jets out, i did notice one bowl seemed to have more fuel in it than the other as it took longer to drain using those drain plugs. so i'm hoping that is my issue. i ordered new valves today but they won't be in til mid next week.

Doug :

Get back to me when you can. I'm betting that should get you fixed up.

Customer:

will do. thanks for the help

Doug :

ok

Customer:

sorry i haven't got back as i haven't had time to play with the carbs. well, i can't catch a break. before i put the new float valves in i decided to do the test. i did each twice and both are right where i think they should be. just a few mm below the top of the bowl. so i put the new valves in for fun, did the test again, and same result. so i put the carbs back on the sled and fired it up. no change. its still just flooding out somehow. i took the carbs back off and there is a lot more fuel in the carbs than there is on the other set of carbs that is running good. even the boost bottle is very wet with fuel where the good running sled you don't see the wetness.

Customer:

just by the way it smokes when running too it seems like theres just too much fuel there. any brilliant ideas?

Doug :

Test the level in the good carbs and set the other carbs to the same level. Also, verify the jetting is the same and use the instructions in the Yamaha bulletin to set them to the new specs. If the sled runs good with the other set of carbs on it, then that is where the problem is. Either something isn't adjusted correctly or there is a blockage in the air passage that is causing it to run too rich. Make sure the needle jets (main jet holder) are removed and inspected. They can build up varnish behind them and that will cause the gas not to atomize in the bore.

Customer:

i believe the pick up tube was the culprit. i took the carbs apart again completely and the top of the pick up tube has a hole on the side of it where the mixture can come in, then goes up by the needle. its a very thin little space that is wrapped around the main part of the pick up tube. this was partially plugged by a piece of dirt and had bad varnishing on it. after i cleaned it up and put the carbs back together, it seems to run ok now, although i haven't had a chance to take it out and test it. pry can't now too as the snow has melted... hopefully that is the problem though. if not, i'll bug you again. thanks for the help. sorry i took so long to get back. for some reason, the wife thinks the snowmobiles are last on the priority list of things to do... and if that was in fact the problem, its great to know i paid someone $100 to clean the carbs and tune it up for the season...

Doug E., Kawasaki Master Technician
Category: Small Engine
Satisfied Customers: 2876
Experience: Professional mechanic since the late 70's on Kawasaki, Arctic Cat, Jonsered, John Deere and others.
Doug E. and 3 other Small Engine Specialists are ready to help you
Expert:  Doug E. replied 3 years ago.
That's why I end up doing everything myself. I hate working on cars, but I find that I do a better job then the places I take it to. I always take pride in my work and can never understand why others don't.

Happy riding!

Doug

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