MS460 carb on an 039

[Redfin]

The idea was to bolster fuel delivery via the L circuit to increase torque when revs dropped

Is it strictly the position of the plate that dictates the draw of fuel from the different orrifices or is it air velocity through the carb? Does that makes sense?

 

[MustangMike]

FYI, I have ZAMA carbs on my 2 044s (one original, one AM), and Walbro on my 2 046s. They all work just fine!

 

[Terry Syd]

Here's something I wrote over on AS when Brewz and I were talking about the metering spring. There is a bunch of other information in the post that may help explain the concepts.

"The stiffer spring only affects the 'ramp' (or slope) of the fuel coming in from the low speed circuit. The spring affects the 'ramping' in of that low speed circuit fuel. If the diaphragm section (diaphragm, metering orifice and valve and the spring) weren't in the carb - then the low speed circuit would be full on as soon as you started the saw. You need to 'ramp' in the fuel slowly as the revs rise so the engine doesn't flood. Think of the slope of a ramp for something like a loading dock, some ramps are steep and others are much shallower - that's what the different springs and such are doing, they are changing the slope of the ramp.

If the ramp is too steep, you will get too much fuel too quick and the saw will bog with a rich mixture. If it is too slow, then the saw will hesitate with a lean bog.

The whole diaphragm section of the carb is for IDLING AND TRANSITION. If you want to prove that, pull the pump diaphragm out of the carb and start the engine with the choke on. It will start, but don't let the revs drop too low or it will stall.

However, there is a limit to how much flow there is in the low speed circuit. In the Zama it is the two jets, in the Walbro it is the low speed needle. The ramping in of the low speed circuit will continue until you have opened up the metering orifice to the same size as the jets in the Zama or the low speed needle in the Walbro. After that the flow in the low speed circuit is determined by the pressure differential caused by the air flow. Since the low speed circuit is essentially a fixed jet, the flow will start dropping off fairly quickly - but by then the venturi in the high speed circuit should be coming in to pick up the slack.

We're trying to get the best air/fuel mixture we can for power no matter what the revs are.

If you go back through the links I posted, you will find some air/fuel mixture charts that show the power available at different air/fuel mixtures.

I used to make my own jets, needles and slides for Mikuni carbs. Figuring out these diaphragm carbs was a COMPLETELY different experience. I'm surprised that such a simple device can be made to work as well as it does."

 

[TJ the Chainsaw Mechanic]

Hmmm....Maybe I'll figure out a way to put an MS880 carb on my MS460. lol! Great read up folks.

 

[Adirondackstihl]

FYI, I have ZAMA carbs on my 2 044s (one original, one AM), and Walbro on my 2 046s. They all work just fine!

That's because of the HD2 filter, DP muffler and re-tune

 

[MustangMike]

They all got that too! Plus I gave 044#1 (my only unported saw) a timing advance, and just deleted the base gasket. Now I got to put it in some real wood and see how she does, feels great in my practice wood!

 

[Deets066]

Here's something I wrote over on AS when Brewz and I were talking about the metering spring. There is a bunch of other information in the post that may help explain the concepts.

"The stiffer spring only affects the 'ramp' (or slope) of the fuel coming in from the low speed circuit. The spring affects the 'ramping' in of that low speed circuit fuel. If the diaphragm section (diaphragm, metering orifice and valve and the spring) weren't in the carb - then the low speed circuit would be full on as soon as you started the saw. You need to 'ramp' in the fuel slowly as the revs rise so the engine doesn't flood. Think of the slope of a ramp for something like a loading dock, some ramps are steep and others are much shallower - that's what the different springs and such are doing, they are changing the slope of the ramp.

If the ramp is too steep, you will get too much fuel too quick and the saw will bog with a rich mixture. If it is too slow, then the saw will hesitate with a lean bog.

The whole diaphragm section of the carb is for IDLING AND TRANSITION. If you want to prove that, pull the pump diaphragm out of the carb and start the engine with the choke on. It will start, but don't let the revs drop too low or it will stall.

However, there is a limit to how much flow there is in the low speed circuit. In the Zama it is the two jets, in the Walbro it is the low speed needle. The ramping in of the low speed circuit will continue until you have opened up the metering orifice to the same size as the jets in the Zama or the low speed needle in the Walbro. After that the flow in the low speed circuit is determined by the pressure differential caused by the air flow. Since the low speed circuit is essentially a fixed jet, the flow will start dropping off fairly quickly - but by then the venturi in the high speed circuit should be coming in to pick up the slack.

We're trying to get the best air/fuel mixture we can for power no matter what the revs are.

If you go back through the links I posted, you will find some air/fuel mixture charts that show the power available at different air/fuel mixtures.

I used to make my own jets, needles and slides for Mikuni carbs. Figuring out these diaphragm carbs was a COMPLETELY different experience. I'm surprised that such a simple device can be made to work as well as it does."

So, if I read it right the same can be accomplished in a Walbro with the needle adjustment?

 

[tree monkey]

depends on the carb at hand. the had 199 has an acc pump the completely closes the low circuit at full throttle

 

[tree monkey]

never seen a hd carb that had a c clip holding the main jet in place. is it after market?

my understanding is that the hole you drilled is a mid range circuit of it's own

 

[RU NUTZ TOO]

I can respectfully disagree with anyone. I really don't care to be right. If I'm never wrong how do I learn anything? I really want to know more and just stating what I know. I have my own opinions and I fully expect others to have their's. This is really something that should be studied more so we can have the most accurate info in these threads. I don't recall many disrespectful disagreements on AS either. I did have this one guy who wanted me to send a chain for a race. I disagreed and he really didn't like that. He showed me disrespect, I showed him disrespect but it's done. Hopefully he can put it behind him like I have. I might have a walbro with that jet out. I'll get a pic later to show that it feeds the low speed needle. Just for info in the thread as it is a good one.

Yup, fatbastard90 thinks he can change his name, join a different forum and then act like he is buddies with all the guys he had trash talked over the years on AS.

He now doesn't feel like he needs to be accountable for everything he has said elsewhere.

You sure are good at looking innocent. Many have probably seen your BS on both other forums over the years. I won't hold it against you. I don't recall ever once calling you names but I fully expect you to resort to that. Just saw the face behind the name. My nephews (15) look as old as you. They Don't look like a little rag doll either. Was that fair game? Now let's be men genius and save the *s-word talking for when we're face to face. Just so you know too, I changed my name for a change. I literally made it known in another thread and everyone who didn't see it can ask if they want. I don't care.

 

[RU NUTZ TOO]

Here's something I wrote over on AS when Brewz and I were talking about the metering spring. There is a bunch of other information in the post that may help explain the concepts.

"The stiffer spring only affects the 'ramp' (or slope) of the fuel coming in from the low speed circuit. The spring affects the 'ramping' in of that low speed circuit fuel. If the diaphragm section (diaphragm, metering orifice and valve and the spring) weren't in the carb - then the low speed circuit would be full on as soon as you started the saw. You need to 'ramp' in the fuel slowly as the revs rise so the engine doesn't flood. Think of the slope of a ramp for something like a loading dock, some ramps are steep and others are much shallower - that's what the different springs and such are doing, they are changing the slope of the ramp.

If the ramp is too steep, you will get too much fuel too quick and the saw will bog with a rich mixture. If it is too slow, then the saw will hesitate with a lean bog.

The whole diaphragm section of the carb is for IDLING AND TRANSITION. If you want to prove that, pull the pump diaphragm out of the carb and start the engine with the choke on. It will start, but don't let the revs drop too low or it will stall.

However, there is a limit to how much flow there is in the low speed circuit. In the Zama it is the two jets, in the Walbro it is the low speed needle. The ramping in of the low speed circuit will continue until you have opened up the metering orifice to the same size as the jets in the Zama or the low speed needle in the Walbro. After that the flow in the low speed circuit is determined by the pressure differential caused by the air flow. Since the low speed circuit is essentially a fixed jet, the flow will start dropping off fairly quickly - but by then the venturi in the high speed circuit should be coming in to pick up the slack.

We're trying to get the best air/fuel mixture we can for power no matter what the revs are.

If you go back through the links I posted, you will find some air/fuel mixture charts that show the power available at different air/fuel mixtures.

I used to make my own jets, needles and slides for Mikuni carbs. Figuring out these diaphragm carbs was a COMPLETELY different experience. I'm surprised that such a simple device can be made to work as well as it does."

You lost me. I'll have to read through this better on the computer in a bit. Where is the ramp you speak of? Are we talking about the metering chamber? The metering chamber is basically a regulator. It is always full of fuel and the circuits pull from it. A way to look at it is the metering chamber along with the pump side and everything have absolutely nothing to do with feeding the saw. The job of them is the make sure the metering chamber has sufficient fuel to feed the jets so they can feed the saw. When the jets pull fuel the pressure drop pulls the metering diaphragm down into the lever which opens the needle to refill the chamber. The lever height is literally a regulator setting. The higher it is the earlier it will open the needle to refill the chamber. At high speed the needle will pretty well always be open as it has to fill the chamber as fast as the jets can pull from it. to high of a lever will result in the pump overfeeding the chamber which will force feed the jets which makes that rich running condition we all know of when the lever is set to high. At full throttle the low speed circuit is inop because the velocity through the Venturi is pulling through the H. This hold back fuel from the low in the chamber. I've bored a lot of carbs and avoid it for the most part now as the slowed velocity in the Venturi can create other running issues. Nothing wrong with a larger carb though. When you tune a low lean you can get it to the point where it will idle fine but not throttle up. This is cause the idle jet is fed but there isn't enough to feed the transition jet. same thing happens when the metering lever is to low. starves the jets. The carb does not know what load the saw is under. all it knows is that air is being sucked through it. This is why we tune to hear 4 stroking. This way you know you have enough fuel to run the saw under load. The way I've heard it is the carb will add fuel through the low when under load. This is not the case at all. I may have to Reread everything though lol good thread.

 

[RU NUTZ TOO]

What? You don't believe the tuning concepts in the Mikuni diaphragm carb manual? I can kinda, you know, understand your dismissal of the results that guys like me, Brewz and Poleman have had tuning diaphragm carbs on saws, but to dismiss MIKUNI?

I thought this forum was going to be different than AS.

dang, i jumped on here on the phone and missed some posts. sorry man. just adding the the discussion. quoted you and missed your link to and that is a great link! saved for future reference out of all carburetors mikuni are by far my favorite. wish they made carbs for saws. i'm gonna grab a pic in a bit as i'm pretty sure i have one with the small jet removed. my thinking is that if that jet were drilled it would way overfeed the low speed if it was not controlled by a needle. also i understand completely what you are trying to do. i believe this is what poleman does the the zamas. i don't know of a walbro that needs the same treatment. i have drilled that jet out a little larger on misbehaving HD12's. seems it's the only thing that fixes them. i've damaged a carb getting that jet out before just cause i wanted to see what was underneith it. i might still have that carb.

I had a look in the stock 039 carb and the fixed H jet is not threaded. Its held in with a small cir-clip so they cant be swapped.

must be some kind of aftermarket thing? all the 39 carbs i've seen had a threaded fixed jet IIRC. i have one here but i believe i butchered it for some kind of testing.

 

[Brewz]

My manuals say it should be a HD-19b
It has a Walbro and Stihl end covers and HD stamped on it but the numbers under that are 5 and then ( from memory) 704

I will take some pictures when I get home from work

 

[Terry Syd]

This carb mod had me baffled too, if you were running out of fuel while you were really cutting hard with the saw then why not open the the H jet instead of the L? I kinda understand your theory that if the L is larger it will draw a little bit more fuel through at WFO. But aren't they both regulated by needles and won't flow anymore than what those needles allow?

The carburettor is used to meter the fuel according to the air flow through it, that's the basic function of the carburettor. However, an engine will run within a wide range of air/fuel mixtures. Although the engine will run within a wide range of fuel mixtures, there is an OPTIMUM air/fuel mixture that will give the best power. An engine can run with a fairly lean mixture, but it won't make the power it can with a richer mixture.

When a two-stroke is running at a lower RPM there is greater time for the fuel mixture to escape the cylinder. The manufactures appear to have got around the EPA guidelines by LEANING out the mixtures at lower RPMs.

In answer to your question, there are TWO fuel curves in the carb. One fuel curve is the low speed circuit from the straight section of the carburettor. The other fuel curve, the high speed circuit, is from the main discharge nozzle in the venturi section. What we are doing is BLENDING the two fuel curves together to get the best air/fuel mixture we can for whatever the air flow is in the carb.

If you just open up the high speed needle, then the carb will run too rich higher in the RPM range.

By increasing the flow in the low speed circuit we can increase the torque of the engine as the RPM drops. However, we have to be able to meter the low speed circuit flow in order to get the saw to idle and to transition smoothly. The idle and transition is regulated by the metering diaphragm components.

The air bleed/transition holes provide air into the mixture at idle. The larger the holes are, the leaner the idle mixture is. On the Walbro, if you open up the low speed needle to get more flow from the low speed circuit, the idle mixture will also get richer, which can cause idling problems. However, you can lean out the idle mixture by increasing the size of one or both the air bleed/transition holes (thus the use of the 460 carb on the 039 - it has larger air bleed/transition holes than the stock carb). You can also use a much stronger metering spring to lean out the idle.

The use of the stronger metering spring should be the first approach to cleaning up the idle and transition. A Walbro metering spring kit has a good selection of springs to work from. If someone wants to try increasing the flow from the low speed circuit on their saw with a Walbro, I'd recommend they get a good selection of springs. They could always just start with the strongest spring in the kit and work down to see what spring will give them the best powerband with a good idle and transition. If the strongest spring doesn't allow them to open the low speed needle up far enough, then they will have to go into the carb, pull the welch plug and drill an air bleed/transition hole a bit larger. Then they will have to start over with testing the springs.

 

[RU NUTZ TOO]

what are the air bleed transition holes your talking about? if they are the holes drilled behind the butterfly how do you lean out a mixture drilling them larger when they are literally fed by a chamber of fuel lol. i'm trying to get where what your saying is coming from man. i just can't grasp it. these carbs are designed to be all or nothing. they have transition jets to smooth out the running from low to high speed. i can't think of any reason why a guy would want to try dump more fuel through the low speed or the transition jets. when at full throttle the nozzle is pulling so much from the chamber with enough suction from the velocity in the venturi that it keeps fuel from being fed out the low speed ports. a guy might be able to get them to dribble a WOT by really drilling them out but at that point it would screw up the mid range transistion, and all metering accuracy would be gone. especially if that drilled out jet was not restricted by the needle. it would act like a vent in the chamber. regardless of any load or RPM on the engine, pretty well all fuel will be coming from the nozzle with the throttle wide open. this is really where my confusion with what your saying comes from. why try add fuel where it really doesn't matter. i totally agree with this carb swap although i think all that was needed is the fixed high speed jet should have been downsized to that of the stock jet. then of course, go cut wood!

 

[Deets066]

what are the air bleed transition holes your talking about? if they are the holes drilled behind the butterfly how do you lean out a mixture drilling them larger when they are literally fed by a chamber of fuel lol. i'm trying to get where what your saying is coming from man. i just can't grasp it. these carbs are designed to be all or nothing. they have transition jets to smooth out the running from low to high speed. i can't think of any reason why a guy would want to try dump more fuel through the low speed or the transition jets. when at full throttle the nozzle is pulling so much from the chamber with enough suction from the velocity in the venturi that it keeps fuel from being fed out the low speed ports. a guy might be able to get them to dribble a WOT by really drilling them out but at that point it would screw up the mid range transistion, and all metering accuracy would be gone. especially if that drilled out jet was not restricted by the needle. it would act like a vent in the chamber. regardless of any load or RPM on the engine, pretty well all fuel will be coming from the nozzle with the throttle wide open. this is really where my confusion with what your saying comes from. why try add fuel where it really doesn't matter. i totally agree with this carb swap although i think all that was needed is the fixed high speed jet should have been downsized to that of the stock jet. then of course, go cut wood!

I smell what your steppin in, but he may be on to something. He obviously has done this mod before with gains otherwise he wouldn't be so confident.

 

[RU NUTZ TOO]

I smell what your steppin in, but he may be on to something. He obviously has done this mod before with gains otherwise he wouldn't be so confident.

this has been the best thread i've seen in a long time. i wish i could run a saw with this done without doing it to one of my own lol

 

[tree monkey]

the fuel coming out of the low will not atomize as well as it will from the high nozzle. won't burn as clean.

 

[RU NUTZ TOO]

the fuel coming out of the low will not atomize as well as it will from the high nozzle. won't burn as clean.

lol scott, gotta love your posts man. it's like you know the answer to all of this but you sit back watching us hash it out. just pop in the post a little fuel to the fire lol

 

[Deets066]

this has been the best thread i've seen in a long time. i wish i could run a saw with this done without doing it to one of my own lol

I do simple carb mods that don't require a flow chart or a Phd. I try to bring in more air and more fuel at the H jet. That's why this thread really got my attention