Yamaha QT 50 oddity

Anyone have any hot leads on this kind of a problem?

I just bought a QT 50 from a kid ouside of town... the damn thing hasn't been register in 15 years... but runs great... err used to run great.

As of about a week ago, it started acting weird at full throttle. Though its kind of internittant, it always happens on level ground, or going down hill... I suspect its RPM related, as it never gets above 20 MPH going uphill... at about 24 MPH the engine changes note, to a quieter one, a lower pitched one, and ther eis a sudden drop in speed. This lasts from 2 -10 seconds... then picks up again, and sometimes will be fine for a few blocks, sometimes not, and start over again...

any ideas? or other information I could provide?

Ive checked the spark plug wire... it seemed loose on day... the air filter is clean... and its full of gas... I did notice that it seemed to run slightly rich today, when I came toa stop sign right after on of these incidents, and the exhaust caught up to me, and it smelled rich. It never did before... not even when choked.

Re: Yamaha QT 50 oddity

I just bought a QT50 from a kid in down, and the damn thing hasn't been registered in 15 years either! Wow! Except mine doesn't run... dammit! it needs a choke cable, battery's dead, what are ya gonna do for $20. What's the law on registering if you don't know where it came from???

Re: Yamaha QT 50 oddity

Ron Brown /

Bryan,

I had a problem similar to this. It was caused by the air cleaner being too close to the carb intake.

As I tuned the ped for more speed, when the rpms passed a certain point, I would get "blowback" from the carb, no doubt because my porting, exhaust etc. were not designed for higher rpm. This would soak the air cleaner in fuel/oil causing four cycling which slowed the ped down, stopped the blowback and allowed the intake air to dry the air cleaner, which allowed me to go faster....

Given that this started all by its self without you changing anything, I would make sure the air cleaner is very clean and that the carb needle and float are clean and working well.

Ron

Re: Yamaha QT 50 oddity

I just found out yesterday what to do if you have purchased an unregistered QT50. I bought mine from an Indiana resident who never had it registered because in Indiana you don't have to. However, I live in Ohio and it has to be registered. So I called the Ohio BMV and they informed me that I would need a bill of sale from the previous owner with their name and address on it, the serial number and the amount you paid for it. Then I have to go the title office and get an "out of state inspection." This may not apply to you so check with your state BMV. Hope this helps.

Re: Yamaha QT 50 oddity

Hi,Bryan!You might laugh when you see what caused my Motomarina to have the same symptoms.Ron was trying to help me ,too,and I always appreciate it.My problem turned out to be the points were set too wide,but I don't think you have points,do you?So I would make sure the air filter is unobstructed,and put in a fresh spark plug gapped correctly and make sure the exhaust port isn't carbonned up and the muffler doesn't need de-carbonizing.Hope you fix it !

Re: Yamaha QT 50 oddity

Update:

same damn problem only worse. its acts flooded just about all the time now but still only at full throttle....

Im getting about 20 MPH on it now...

I took apart the carb, and it looked fine... and I removed the exhaust (loud as hell!!) but it didnt seem any better... so one would assume that that is not plugged.

WHat else? the air cleaner is good, but old. I didn't check the intake manifold yet... anything in there? the service manual I have says a reed valve that can go bad... what is one of those used for? Its been too long since Auto mechanics in high school... :)

Re: Yamaha QT 50 oddity

check this out...

I really do't remember taking this out, so I wonder if this isn't just missing... and maybe part of the problem?

see this photos on this auction.

http://cgi.ebay.com/ebaymotors/aw-cgi/eBayISAPI.dll?ViewItem&item=593301921

(address is all one word)

see the idle adjustment screw? the philips in the center of the photo. Note the brass slotted screw below and to the left slightly. I don't have that one... hmmm...

Re: Yamaha QT 50 oddity

Hi,Bryan! I don't think the moped would run at all if a major adjustment screw were missing.That looks like a cover screw for an orifice or jet of some kind.You might look down in there and see if maybe some small passage is plugged,i guess.Remember earlier I said to clean the exhaust port?Did you do that? It doesn't matter that the muffler is clear if the port at the exhaust opening is restricted.BYE!

Re: Yamaha QT 50 oddity

DIdn't try that... Next I may take the intake manifold off and look at that too...

Re: Yamaha QT 50 oddity

Ron Brown /

Bryan,

Unless you think you have a leak or some other problem, forget the intake manifold. The exhaust port plugs up because of carbon accumulation, intakes do not have this problem. Just pull the exhaust and look toward the piston, if the port needs cleaning, it is easier to pull the cylinder. If you are careful, you can scrape most of the carbon with the piston over the port, then finish with the piston down. If you can hanh the ped with the exhaust port pointed down it would help.

Ron

Re: Yamaha QT 50 oddity

I took the carb off again and had it blown out at a friends who has a air compressor... drained the gas out of the thank, and took out and looked at the petcock... the upper screen was plugged with a bunch of that looked like dirt, or rust flakes...

put everything back together, and it still runs like crap, and I can only get 20 out of it...

Loosened the exhaust and it makes no diference.

No im really stumped... one mentioned the exhaust port being plugged. the spark plug is clean, so what could be plugging up the exhaust port? soot from the burning oil?

Oh well... I might try a new plug anyway, but after that.... it might make a good boat anchor...

Re: Yamaha QT 50 oddity

Ron Brown /

Bryan,

How many miles on this ped and have you tried running without the air cleaner?

Ron

Re: Yamaha QT 50 oddity

almost 1800 miles.

I think I remember running it with out the air cleaner... not sure it was right after the problem started... Maybe I'll try again.

Ron Brown wrote:

>

> Bryan,

>

> How many miles on this ped and have you tried running without

> the air cleaner?

>

> Ron

Re: Yamaha QT 50 oddity

yep... no air cleaner makes no difference...

compression is fine too... got a new spark plug, and still no better...

im 'bout ready to take it down to the Yamaha dealer... and ask if they want to buy a boat anchor...

Re: Yamaha QT 50 oddity

Ron Brown /

Bryan,

Time to check the exhaust port.

Ron

Re: Yamaha QT 50 oddity

could it be getting too much oil from the oil pump? I noticed A LOT of blue smoke recently as well... It didn't used to do that either...

Re: Yamaha QT 50 oddity

Ron Brown /

Bryan

I just went back and read most of this thread.

Get focused and complete, or respond to one thing at a time before we all go nuts.

This thread started with your ped running too rich and displaying all the classic symptoms. The following are some questions asked by you, me or someone else which do not seem to have good answers yet, at least in the thread.

Are you sure your needle valve and float are good and there is no gas in the float causing your fuel level to be too high?

Did you determine that the screw in the side of the carb was not supposed to be there?

Have you checked the exhaust port for carbon build up?

Do you have a reed valve and if so, is it ok?

Ther may be more, but for running rich, these will do for now.

Ron

Re: Yamaha QT 50 oddity

thanks for you persisent interest!!

>Ron Brown wrote:

>

> Bryan

>

> I just went back and read most of this thread.

>

> Get focused and complete, or respond to one thing at a time

> before we all go nuts.

>

> This thread started with your ped running too rich and

> displaying all the classic symptoms. The following are some

> questions asked by you, me or someone else which do not seem

> to have good answers yet, at least in the thread.

>

> Are you sure your needle valve and float are good and there

> is no gas in the float causing your fuel level to be too high?

THe float seemed fine the second time I took the carb apart at a friends place with an air compressor... it floated in the gas tank of his Harley Davidson. the needle valve seems fine.

>

> Did you determine that the screw in the side of the carb was

> not supposed to be there?

THis one is tough to answer. there are no threads in the hole there... and there appears to be a (factory)plug inside that hole. the manual I have is for a QT 50 F. Im not sure what suffix mine has.... its a 1980... the namual also doesn't show the idle mixture screw, just the idle speed screw....

>

> Have you checked the exhaust port for carbon build up?

thats on today's agenda, along with checking the oil pump in some way, shape or form...

>

> Do you have a reed valve and if so, is it ok?

I guess I could look at that too... My 'Harley', Allen didn't think it would be of any benefit, but your right, I haven't looked at it... looks like bitch to get at though...

>

> Ther may be more, but for running rich, these will do for now.

>

> Ron

Re: Yamaha QT 50 oddity

Ron Brown /

Bryan,

Its only an engine : )

Good on the missing screw, I am sure it is a fixed jet in your carb.

Assuming the float is hollow, did you actually shake it to make sure there was no gas in it? This would still float but raise the fuel level in the carb.

I am not sure and I would expect it to screw up the idle, but if your reed valve leaks, some of the mixture gets blown back into the carb where it picks up more fuel.

If you have been playing with the throttle cable, you may have changed the oil pump adjustment. It is worth checking but unlikely to cause the problem.

I don't remember us ever discussing the choke, what kind do you have and are you sure it is completely off?

Ron

Re: Yamaha QT 50 oddity

PROGRESS!!! read on...

> Assuming the float is hollow, did you actually shake it to

> make sure there was no gas in it? This would still float but

> raise the fuel level in the carb.

I don't remember any. my friend Allen was pretty experienced withthe float bowl, as he just went through a incident on his Honda Accord, where the floats were bad... so it was fresh in his mind to check it...

>

> I am not sure and I would expect it to screw up the idle, but

> if your reed valve leaks, some of the mixture gets blown back

> into the carb where it picks up more fuel.

That makes sense. I read something somewhere (probably this thread) where a guy was having the same _orginal_ problem after he hopped up the bike, and it would blowback into the carb, and even get the air filter wet, and suck it into the carb...

>

> If you have been playing with the throttle cable, you may

> have changed the oil pump adjustment. It is worth checking

> but unlikely to cause the problem.

I found a few adjustments in the shop manual I have. Only one of them I was able to understand/do without special tools. It just entailed adjusting the slack in the oil throttle cable. Apparently there is a little screw thats supposed to line up with a make on the oil pump, and it wasn't... So lined it up, and it runs much better now...

>

> I don't remember us ever discussing the choke, what kind do

> you have and are you sure it is completely off?

I would say so, yeah... the choke lever is pulled all the way to off, and the choke cable seems all the way down. (moving it up, kills the engine)

I also took the exhaust off and tried cleaning the exhaust port. it wasn't plugged, but the whole thing, and the pipe is caked on the inside walls. I scraped what I could/reach....

So now im back to the orginal problem of stalling/flooding/cutting out/ drop in power (whatever you want to call it) at about 24mph.... Getting over 20 now, but still not getting what I used to....

Re: Yamaha QT 50 oddity

Ron Brown /

Bryan,

Sounds good, if your choke is the shutter or flapper type that closes the air intake, you can see if it is all the way off. If it is an enrichener type, it has a rubber tip which allows more gas to flow and can leak even when closed.

If the float is easy to get to, you may want to give it a shake just to make sure it is empty, also, make sure the float is not loose on the needle, allowing it to rise up on the needle.

Bye the way, I don't remember from the original thread if this problem came on all at once or was a gradual thing.

A good quick test on whether you are really fighting a too rich problem is to get up to max speed, then turn off the gas, as the level drops in the carb, you will accellerate a bit, just before it dies, if you are too rich.

It's only an engine : )

Ron

Re: Yamaha QT 50 oddity

Ron Brown wrote:

>

> Bryan,

>

> Sounds good, if your choke is the shutter or flapper type

> that closes the air intake, you can see if it is all the way

> off. If it is an enrichener type, it has a rubber tip which

> allows more gas to flow and can leak even when closed.

I would guess the later. Its got rummer on the end of a little plunger looking think, but its not a tip, not pointy anyway, but just a rubber end to it... Its adjusted all the way down... but I'll keepthat in mind tomorrow, when next I plan to look at it...

>

> If the float is easy to get to, you may want to give it a

> shake just to make sure it is empty, also, make sure the

> float is not loose on the needle, allowing it to rise up on

> the needle.

>

It all seemed to be in factory condition...

> Bye the way, I don't remember from the original thread if

> this problem came on all at once or was a gradual thing.

kinda both. THis intermittant loss of power at 24 mph was kinda gradual, but the loss of continous power at 20 was all of the sudden. Thats been taken care of for the most part. Ive been thinking of adjusting the oil pump further in the same direction to see if its gets better or not... but then you have to wonder if the motor is getting enough oil... Maybe I have the Yamaha shop give it the once over...

>

> A good quick test on whether you are really fighting a too

> rich problem is to get up to max speed, then turn off the

> gas, as the level drops in the carb, you will accellerate a

> bit, just before it dies, if you are too rich.

>

I wich I had more straigh leve ground by my house to try this... but its all hills, and busy streets here... maybe I'll try messing with it some more at a friends place...

It is drivable now, in fact I had a steady run of 25 mph for about 6-7 blocks tonight... but like I said I used to hit 27 or so... and I would like to top out at 30 so Ican handle the bigger streets on the north end of town....

Re: Yamaha QT 50 oddity

Ron Brown /

Bryan,

I would advise leaving the oil pump alone unless it seems to burn a lot of oil. The adjustment is fairly straight forward and if you followed the rules it should be ok.

At this point, you are close enough to your target that setting the timing exactly and leaning out a little will most likely fix it.

Try the gas off test.

Ron

Re: Yamaha QT 50 oddity

Ron Brown wrote:

>

> Bryan,

>

> I would advise leaving the oil pump alone unless it seems to

> burn a lot of oil. The adjustment is fairly straight forward

> and if you followed the rules it should be ok.

>

> At this point, you are close enough to your target that

> setting the timing exactly and leaning out a little will most

> likely fix it.

>

> Try the gas off test.

I'll try it... but last night when I forgot to turn it back on, and it ran out of gas, I don't remember a blatent surge... but I wasn't looking for either...

Re: Yamaha QT 50 oddity

Ron Brown wrote:

>

>

> Try the gas off test.

>

> Ron

I thik nwe might be onto something here... I tired this test you described on the way home today. THere is a stretch of street that is about 7 blocks long, with no cross streets, soI can open it up for a extended period of time. I shut the fuel off just after entering the strech. It stuttered with random intermittance over 20 MPH like it has been, and I got about 24 mph out if it. As the fuel got lower, and shortly before it ran out, I noticed that it wasn't stuttering, and I was going 27 mph!!

I did this 3 times in a row.

Now what? new float?

Re: Yamaha QT 50 oddity

Ron Brown /

Bryan,

Great!!!

Unless the float has gas in it, a one size smaller main jet is most likely the cure.

A friend of mine who has been racing 2 cycles almost since all motorcycles were mopeds tells me that the older the engine gets, the more sensetive it is to a too rich mixture.

Ron

Re: Yamaha QT 50 oddity

However, it has NEVER down this going up hill..... I wonder just because it never gets to 25 going uphill?

Also would it not do this at all speeds, and not just as it reaches 24-25, if it was a float problem?

Re: Yamaha QT 50 oddity

Ron Brown /

Bryan,

Some of this is conjecture and some is empirical from working on my own peds.

If your carb is like mine, it does not have a tapered needle extending from the bottom of the slide to the "needle jet", the opening in the venturi which allows gas/gas air mix into the air stream. In the absence of a needle, I do not know what to call this jet, so I will call it an atomizer which is what it would be called in a "butterfly" type carb such as in a car.

The purpose of this tapered needle is to regulate the available gas/gas air for mixing with the incoming air during intermediate throttle openings.

By the way, I use "gas/gas air" because many carbs have an emulsifying tube which mixes a small amount of air with the gas as it travels from the main jet to the atomizer. This assists in atomizing the fuel into the intake air stream.

If you are not thoroughly confused yet, consider the operation of the carb at full throttle. The venturi size is fixed by the diameter of the passage through the carb. The main jet and air jet to the emulsifier, if any, are also fixed. At some rpm, the velocity of the air through the venturi will create enough of a pressure drop to draw just enough fuel through the main jet/atomizer to give you a perfect mixture. If the rpm increases, the air will flow faster, creating a greater pressure drop in the venturi, drawing more fuel through the atomizer. This sounds perfect except that if diameter of the carb restricts air flow at all, because it was designed for optimum flow at lower rpm, then the pressure will drop even more at the atomizer. This causes too much gas to be mixed with the fuel, essentially a "choke" effect. The air cleaner/air box can also contribute to this.

The reason you never have the problem goin up hill is because you never attain this "excessive" rpm range.

If you are still with me, the contributing factors to this equation are:

Diameter of the carb venturi.

Main jet size.

Emulsifier air jet size.

Atomizer jet size.

Float bowl fuel level.

Any restrictions to air intake, air box, filter, air turbulence around the air box.

Any restrictions to gas flow, dirty jets, plugged filters, float bowl and gas tank vents.

Any of the components which are sized to regulate flow can be adversely affected by over agressive cleaning with hard wire, drill bits etc.

Any air leak in a position to allow more air into the mixture, carb to manifold, manifold to block, crankshaft seals..

Any fuel leak which allows more fuel into the mixture, leaky float needle, leaky, stuck or misadjusted enrichener (choke), loose main jet.

As you can see, the opportunities for error are many. Some cause the mixture to be more rich, some more lean.

As we know that you have a too rich problem, there are only so many things to look at which we can do something about. Main jet size, float level and free air flow are the simplest, although it is worth verifying that the enrichener is ok and the air passages to the emulsifier are clear. You have allready determined that the air filter/air box make no difference, although this surprises me. This leaves you with the float level and main jet size to check/play with.

It occurs to me that you should also check that the throttle slide is opening all the way when you twist the throttle.

If you are still awake, I hope this helps.

Ron

Re: Yamaha QT 50 oddity

Ron Brown wrote:

>

> Bryan,

>

> Some of this is conjecture and some is empirical from working

> on my own peds.

>

> If your carb is like mine, it does not have a tapered needle

> extending from the bottom of the slide to the "needle jet",

> the opening in the venturi which allows gas/gas air mix into

> the air stream. In the absence of a needle, I do not know

> what to call this jet, so I will call it an atomizer which is

> what it would be called in a "butterfly" type carb such as in

> a car.

the throttle cable comes into the carb on the top. 2 cables actually, one being the choke. THe throttle cable is connected to a slider, (that word seems right, but I actually have no idea what it is...) about the size of a quarter roll of dimes. its hollow, and inside, and sticking out the bottom is a long narrow needle valve. There is a compression 'spring' that holds the needle in the slider. This spring is compressed against the inside surface of the slider. I seem to remember this needle sitting at the bottom of the slider, but it seems possible for it to be set anywhere along the height of the slider, with only a small degree of an ability to stay there, so I set it at the bottom...

does this change your response?

>

> The purpose of this tapered needle is to regulate the

> available gas/gas air for mixing with the incoming air during

> intermediate throttle openings.

>

> By the way, I use "gas/gas air" because many carbs have an

> emulsifying tube which mixes a small amount of air with the

> gas as it travels from the main jet to the atomizer. This

> assists in atomizing the fuel into the intake air stream.

>

> If you are not thoroughly confused yet, consider the

> operation of the carb at full throttle. The venturi size is

> fixed by the diameter of the passage through the carb. The

> main jet and air jet to the emulsifier, if any, are also

> fixed. At some rpm, the velocity of the air through the

> venturi will create enough of a pressure drop to draw just

> enough fuel through the main jet/atomizer to give you a

> perfect mixture. If the rpm increases, the air will flow

> faster, creating a greater pressure drop in the venturi,

> drawing more fuel through the atomizer. This sounds perfect

> except that if diameter of the carb restricts air flow at

> all, because it was designed for optimum flow at lower rpm,

> then the pressure will drop even more at the atomizer. This

> causes too much gas to be mixed with the fuel, essentially a

> "choke" effect. The air cleaner/air box can also contribute

> to this.

>

> The reason you never have the problem goin up hill is because

> you never attain this "excessive" rpm range.

>

> If you are still with me, the contributing factors to this

> equation are:

>

> Diameter of the carb venturi.

> Main jet size.

> Emulsifier air jet size.

> Atomizer jet size.

> Float bowl fuel level.

> Any restrictions to air intake, air box, filter, air

> turbulence around the air box.

> Any restrictions to gas flow, dirty jets, plugged filters,

> float bowl and gas tank vents.

> Any of the components which are sized to regulate flow can be

> adversely affected by over agressive cleaning with hard wire,

> drill bits etc.

> Any air leak in a position to allow more air into the

> mixture, carb to manifold, manifold to block, crankshaft

> seals..

> Any fuel leak which allows more fuel into the mixture, leaky

> float needle, leaky, stuck or misadjusted enrichener (choke),

> loose main jet.

>

> As you can see, the opportunities for error are many. Some

> cause the mixture to be more rich, some more lean.

>

> As we know that you have a too rich problem, there are only

> so many things to look at which we can do something about.

> Main jet size, float level and free air flow are the

> simplest, although it is worth verifying that the enrichener

> is ok and the air passages to the emulsifier are clear. You

> have allready determined that the air filter/air box make no

> difference, although this surprises me. This leaves you with

> the float level and main jet size to check/play with.

>

> It occurs to me that you should also check that the throttle

> slide is opening all the way when you twist the throttle.

I think so... I remember palying with it when I took the carb apart the first time, and was trying to see how the damn thing worked...

>

> If you are still awake, I hope this helps.

Ahhhh Auto mechanics in high school comes rushing back.... I rebuilt a carb for my '74 Subaru wagon then... what a mess...this is far simpler!

Re: Yamaha QT 50 oddity

Ron Brown /

Bryan,

Damned clever, these Japanese! I should have guessed that they would use a needle, most of the euro peds seem to live withou and and depend on air box restriction to enrich the mixture at wider throttle openings.

With a needle, you have the option of moving the clip on the needle to a different groove, normally there are 5 grooves. The top groove is the leanest setting and the bottom groove, closest to the point, is the richest setting.

Unfortunately, your problem is at full throttle, when the needle is typically all the way out of the needle jet. You may want to verify this when you check that the slide rises up all the way. If the needle can be set low enough, you may be able to lean the mixture at full throttle, otherwise, the main jet and float level are the only things you can change. If your carb has a hinged float, it is fairly simple to lower the float level.

The normal carb adjustments, in order, are:

Float level, to establish the correct fuel supply.

Main jet, to control wide open throttle mixture.

Needle position, to correct mid-range mixture.

Idle mixture and speed adjustment.

There you go, have fun.

Ron

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