MS460 carb on an 039

[Brewz]

Having recently rebuilt my Stihl 039 from the ground up and dual porting the muffler, It was suggested to me that fitting an MS460 carb to the saw would help deliver more fuel in the cut to match the increased airflow, increasing torque and reducing bog.

By feel, it did just that!

The carb is just a $20 Chinese ebay special. It is physically identical to the stock carb and just bolts on. Only physical change I needed to make externally was to swap the air filter compensation top for the stock Walbro plate top as my air compensation hole is full of silicon.



The venturi is 17.45mm rather than 16.67mm in the stock carb. Also, the air bleed holes are larger and allow more fuel through the low speed circuit. This keeps plenty of fuel supplied to the saw when you load it up and speed drops.

Now it wasn't all strait forward and some simple mods are needed to make it work.

First up I drilled out the tiny little 0.25mm hole in the brass jet (bottom left) which is an EPA restriction to reduce emissions (lower power) on the L circuit. A 1 mm drill bit just gently twisted through by hand did the job

Before:


After:


The result I wanted was a richer L circuit with the remainder made up by the H circuit at WOT, the idea being a richer loaded speed to keep torque up in the cut and reduce bog. To get this, the carb supplied a LOT of fuel at idle. In fact it ran blue smoke every time I hit the throttle off idle. It also supplied so much fuel on the L circuit at WOT that I couldn't get the revs over about 11000 RPM with the H screw all the way in, and it guzzled fuel.

It had not worked but I felt it could.

I took it home and after some discussion with a member of another forum, I completely blocked one of the jets supplying the H circuit (top left) by screwing it out and cutting a small round piece of old carb gasket and screwing the jet back down on it to plug the hole. I also filled the hole with some Motoseal 2 to make the change easily reversible but solder would be better now that I know what I know.

I also used a round file to make a small air bleed on the butterfly on the oposite side of the standard one. Its only just visible but lets a little bit of air through, reducing the amount of air traveling through the factor idle air bleed. This works to reduce the amount of fuel sucked at idle by air moving over the fuel jet, alowing me to tune the L circuit richer to supply more fuel at high revs, but lean the saw out at idle giving better throttle response. If you do this, remember that you can always take some more material out, but you cant put it back in as easy if you go to far.

My small air bleed is on the oposite side to the factory one seen below. I only really relieved the thickness of the butterfly edge, kind of beveling it to bleed a tiny bit more air. No large grooves.


Bolted the carb back on and after a bit of back and forth with the screws, got it to fire up.
It now ran great. I have full control over idle speed using the L screw with 2 turns of adjustment and can lean it out to the clean scream and bring it back to 4 stroke at about 12700 RPM.

I took it out to run it in some timber and bolted on my 30" bar with 3/8 full comp full chisel chain to really test it.
In 2 foot thick timber it ran the 30" bar fine and held far better revs in a light pressure cut. Previously I had to almost hold it up in the cut to keep it cutting at a happy rate. I could still bog it with a lot of pressure.
I removed the 30" bar and bolted on the more practical (for this saw) 20" bar running 3/8 full comp semi chisel chain.
With this bar on, it is an animal. I can dig in the spikes and really pull on it hard before it bogs.
It hasn't turned it into an 066 but it feels a lot stronger than it was.

I will be getting some timber tomorrow and plan to keep some of it as big as I can to use as cookie testers.
I still have the stock carb set tuned and plan to do a couple timed cuts with both to really test it out.

It feels stronger but does it cut faster...... that is to be seen

 

[Deets066]

I got all of it except where you took the removable jet out and plugged it with a piece of gasket? How does that work?

 

[Brewz]

You can see there is a flat blade screwdriver slot in it. It just screws out like a grub screw.
I just put a round bit of gasket material in the hole and screwed the jet back in which clamped down on the gasket material and sealed the jet, stopping all fuel flow through this part of the carb.

 

[CR888]

I got all of it except where you took the removable jet out and plugged it with a piece of gasket? How does that work?

Plugs it off rendering it inoperable, this is an interesting thread and show whats possible when ya hit a brick wall & end up with something good.

 

[Deets066]

You can see there is a flat blade screwdriver slot in it. It just screws out like a grub screw.
I just put a round bit of gasket material in the hole and screwed the jet back in which clamped down on the gasket material and sealed the jet, stopping all fuel flow through this part of the carb.

I would have guessed that was the high side jet.

 

[czar800]

Great read! Your knowledge on carburetors is amazing!

 

[Deets066]

I don't knows great deal about these carbs yet though, I'm learnin

 

[Brewz]

Oh I am learning too
Most of my work here can be attributed to the knowledge of a member of another forum who has spent countless hours, and carbs, experimenting to learn.

Once you get your head around how the carb delivers fuel, modifying them becomes a bit easier.

Then you get the differences between the different brands that will fit a certain saw..... they all work different and the best thing you can do is find some cross section flow diagrams........... which is not easy for older carbs

 

[Brewz]

I would have guessed that was the high side jet.

Edit: late night brain fart correction, it is a 2nd H side jet

 

[Brewz]

The H circuit is just a simple hole that feeds fuel to the main jet.
The main jet is the brass thing that pokes out into the carb venturi.
The amount of fuel is regulated by moving the H screw in and out. It uses vacuum in the main jet created by air moving through the carb over the jet to suck the fuel out and into the motor.
The main jet has a larger hole so the vacuum drops off fast as air speed drops when revs drop. This means the H circuit supplies lots of fuel at high revs but the amount of fuel drops off very fast as revs drop.
If we can supply more fuel via the L circuit which uses smaller holes which will keep supplying fuel at lower RPM's, we can reduce the amount the H circuit delivers at WOT, making up the loss with the L circuit, and the L circuit will then continue to pump in lots of fuel as revs drip under load in the cut.

In a stock saw, the saw leans out as revs drop. This is done by manufactures to conform with environmental emissions regulations. 99.9% of saw users will be happy with that but not us.......... right
We want fuel in the cut to make power to make it cut faster.

That's the theory behind the mod.

 

[Redfin]

Bear with me because im mildly confused. You opened one of the L jets but you then had to block another because there is to much fuel at wot? Knowing this, would you have drilled the L to begin with?

 

[Brewz]

Bear with me because im mildly confused. You opened one of the L jets but you then had to block another because there is to much fuel at wot? Knowing this, would you have drilled the L to begin with?

Redfin, well picked up!
It was late and my brain was cooked from a long day at work.

I have edited the text above to fix my small stuff up

The removable jet I blocked was a 2nd H circuit jet.
Once the L circuit was opened up to feed more fuel, the H circuit needed to be choked back to ballance the fuel delivery.

This carb is built for s saw 10 to 12cc's larger thT revs 1000+ rpm harder at WOT so it was always going to deliver more fuel than needed.

What has been accomplished by this mod is the ability to supply more fuel via the L circuit across a wider RPM band than was possible with the stock carb.

 

[Terry Syd]

Hi Brewz, I made it over to the forum. Poleman also gave me a 'heads-up'.

On my carb I haven't filed the throttle plate to let some of air bypass the idle discharge hole. My carb works, but with that extra mod to the throttle plate, you are running a bit more fuel through the low speed circuit than I am. I might give it a try and see if I pick up any more torque down low. Although, it is already pulling strong enough, but maybe it might pick up some more.

As I recall the 460 is 12cc larger than the 64cc 390 jug, I was surprised that the air bleed holes are such good match for the 64cc engine. I originally bought the carb to use on my BB365 (77cc), but decided to give it a go on my modded 029 before I started swapping linkages around. The carb has been on the 029 ever since. A cheap and easy mod for that engine.

The BB365 has a modded Zama that works just fine. It's drilled .35/.35 with the .55 air bleed hole.

Some guys that have the MS460 should pull their carb and drill that EPA low speed limiter out. Crikey, if it works on a 64cc engine, it should make a bigger difference on a 76cc engine.

 

[Brewz]

Hey Terry, glad you made it over.

I cut a trailer load of big timber today and plan to use a couple of the half rounds to test cut speeds with this mod with both carbs

 

[Deets066]

Hey Terry, glad you made it over.

I cut a trailer load of big timber today and plan to use a couple of the half rounds to test cut speeds with this mod with both carbs

I would be interested to see how the modified carb runs against a stock 460 on your 039. Is that what you had planned or are you goin to run it against your 039 carb?

 

[RU NUTZ TOO]

the jet you drilled out feeds the low speed so you are correct it would allow you to add more fuel on the low but you are still able to restrict it. with the throttle open the low speed becomes inop for the most part. alot of people get confused with carbs thinking when you lug a saw down the low speed is what feeds the saw. this is not the case at all. if the throttle butterfly is open regardless of what RPM or load the saw is at the majority of the fuel is coming from the main nozzle which is fed by the high speed. the only carbs that have needles that are heavily dependant on each other are carbs without nozzles. the problem you ran into is actually the fact that the fixed H jet was meant to feed a larger saw so was to much for the 39 even with the needle closed. the open hole without a jet feeds the high speed for fine adjustment while the fixed jet feeds the majority. i don't know who you talked to about it but it's sounding like he specializes in zama carbs. these walbro's are very capable of overfeeding on the low speed so that large drilled out jet really does nothing especially if it was already sized for a 460. i mean sure, now you can add more fuel at low speed but the carb was already capable of feeding to much to begin with i bet your high speed needle is out further then normal right now and it's because you took that fixed jet fuel away. you can just swap that jet out for the one out of your stock 39 carb and it would tune similar to the stock carb. even the stock HD on the 39 is very capable of feeding a 39. the benefit here is the larger venturi.

 

[Poleman]

GREAT READ GUYS!!!! Brewz great job on this!!! You explained everything very well!!!

Terry, good to see you!!!!

 

[Brewz]

Deets, planning to run the stock 039 carb against the 460 carb

Nutz, thank you for your input. The more info the better as we are all learning.

The reason we drilled the low speed jet is because the tiny little 0.25mm hole was smaller than the ports in front of and behind the butterfly in the carb throat that spray the fuel into the saw.

I would love to see some fuel curves relative to throttle position

I just picked out a big hard half round that I can noodle into cookies to load the saw up and test cut times.
I had to pull a nail out of it and when i belt it with the hammer it doesn't dint it. It's hard!

It didn't seem to bother my 066 this morning though. JHC that thing is a weapon. Can't wait to port it.

I am stuffed and its stinking hot so the test will have to wait

 

[MustangMike]

I understand how a larger CFM carb can help a saw, but on my 044s and 046s, if they already get enough fuel, why would I want to drill anything out???

Would be different if I could increase CFMs.

 

[Brewz]

I am however, going to get a cold beer and check out the jet in the stock carb.
You have me thinking now