Advanced Carburetor Tuning Question

[mattr66]

Alright guys, I've searched around and I can't find this exact issue and I am experienced in tuning (bigger stuff - no chainsaws just Fords). I put a genuine Walboro Wt215 on my stock MS180. I cannot for the life of me make the carburetor perform like it should. Is there a way to richen the transition between the high and low? If I get a nice and clean idle, the transition is way too lean with a properly adjusted H needle. If I get it too rich at idle where I really have to give it more idle air to stay running, the response is okay but still not a nice sharp response. How can you add a little more fuel between the High and Low portions of the carburetor? I did not transfer the snorkle from the old carburetor on the new one either because I read that it creates issues.

Thanks,
Matt

 

[stihl86]

What does the snorkel affect ? Does it restrict air flow and make it run a little richer to help with acceleration?

 

[StihlSolo]

The airbox for the snorkeled carb has this port, at the arrow, that has a small opening into the airduct leading to the carb. If you use a carb without snorkel, then unfiltered air gets into the airduct leading to the carb.Try plugging the port.

There are older airboxes without the port. I built an 018 with two-needle carb, and no snorkel, with one of those older airboxes.

 

[StihlSolo]

What does the snorkel affect ? Does it restrict air flow and make it run a little richer to help with acceleration?

The snorkel allows air into the diaphram chamber. Older carbs just have a hole in the chamber cover.

In this case the snorkel draws air from the airduct leading to the carb.

 

[Loghead91]

Unrelated but do u work for gearhead tuning? If so you wrote my 7.3 and 6.4 tunes.

 

[CausticUC]

So..... at idle you are using the standard bypass and part of the low, mid throttle is controlled fully by the low screw, full throttle by the high. You already know this.

Unfortunately a carb this small is amplified in its adjustments; you will need to run rich on the low to get what you are looking for.

 

[Thommo]

You could try running a lighter spring under the metering lever in the carb but you really need to know what the blow off pressure is before doing this. If the blow off pressure is too low your saw will flood when you switch it off when hot with fuel in the tank. I dont like to go below 10Psi blow off pressure and have seen new carbs hold 40 Psi which seem to run lean at part throttle. You could also try filing a bit off the top of the throttle butterfly to let more air through at idle , so you can richen the low speed mixture

 

[stihl86]

The snorkel allows air into the diaphram chamber. Older carbs just have a hole in the chamber cover.

In this case the snorkel draws air from the airduct leading to the carb.

Yes. I know. It's called a compensator. Why is it creating a problem? It doesn't matter where the metering chamber gets air from. As long as it can create a balance to atmospheric pressure. The only time I've ever heard of it causing a problem is in a model plane.
Anyway, take a close look at the inlet needle lever (as posted). Try making small adjustments and test.
The fuel pump diagrams are available in two different materials. One rubber and one a Teflon type material.
One is for a machined surface, one isn't. But, I've been told the rubber material will pump fuel better, but not last as long. I've found that poor acceleration can be linked to lever height. Quite often. Sometimes the material helps. Small engines can be very finicky. Sometimes extremely annoying.

 

[Maintenance supervisor]

Ok so when your talking about "transition" from low to high you might be thinking about the slot in front of a 4 barrel holley where as the throttle plate/ butterfly progressively uncovers the slot introducing the fuel from idle ,transfer, main venturi.
Well these little saw carbs have progressive holes that are exposed in the throttle bore in a similar fashion. I don't know where they are in a wt215 but in the first picture I have they are under the Welch plug I removed ( I smeared 1 with the punch removing the plug and redrilled it) the second picture shows 3 little holes in a channel of the carb body that are covered with a metal plate.
I have enlarged these transition holes on some carbs that have a stumble, but be aware that they are TINY, VERY TINY! I used some bits I had from a friend who worked at NASA.

 

[Cliff R]

I work with tiny precision drill bits daily. You will need a small pin vise to hold them.

What you are looking for is a numbered bit set #61-80. They will run from .0135 to .039". Don't even waste your time with any sets in that range that are imported, they will not be sized correctly and break if you stare at them too long!

I use and prefer HUOT, but any good USA made set will work equally as well.......Cliff

 

[drf255]

Did you modify your muffler?

Is a 215 a bigger than standard carb on the MS180? I know they came on the MS250’s.

There are some carbs, like the HDA 199, that have an accelerator pump on them like an auto carb. With the others, you’re forced to live with what you got.

Make sure everything else is right before you monkey with the carb. Mod the muff for some more flow, pull the screen and put a coarser mesh in if needed . Advance the timing .025 off key or roughly 1/4” of flywheel travel on its perimeter.

If it comes down to the carb, it may just be too big. You can try to adjust the transition circuit. There are generally 3 tiny holes in the bore besides the main nozzle. They will be behind the notch in the throttle butterfly. The one closest to the butterfly is the idle hole and the other 2 are transition holes. You have to remove the Welch plug and progressively drill them bigger. I‘d probable get a China knockoff to experiment with, because you can’t go backwards with the mod. You’ll need a rebuild kit because the welch plug is a one time use item, it gets destroyed when you remove it.

Again, this could simply be like putting a 1000 cfm Dominator on a stock 283 Chevy small block. No matter what you do it won’t run right.

 

[Maintenance supervisor]

Did you modify your muffler?

Is a 215 a bigger than standard carb on the MS180? I know they came on the MS250’s.

There are some carbs, like the HDA 199, that have an accelerator pump on them like an auto carb. With the others, you’re forced to live with what you got.

Make sure everything else is right before you monkey with the carb. Mod the muff for some more flow, pull the screen and put a coarser mesh in if needed . Advance the timing .025 off key or roughly 1/4” of flywheel travel on its perimeter.

If it comes down to the carb, it may just be too big. You can try to adjust the transition circuit. There are generally 3 tiny holes in the bore besides the main nozzle. They will be behind the notch in the throttle butterfly. The one closest to the butterfly is the idle hole and the other 2 are transition holes. You have to remove the Welch plug and progressively drill them bigger. I‘d probable get a China knockoff to experiment with, because you can’t go backwards with the mod. You’ll need a rebuild kit because the welch plug is a one time use item, it gets destroyed when you remove it.

Again, this could simply be like putting a 1000 cfm Dominator on a stock 283 Chevy small block. No matter what you do it won’t run right.

This is a pretty common modification for the ms180 so I think it would have been poopooed on by now if it didn't work?
Maybe more like a 750 cfm on a 283 rather than a 1000 , that would be a HT style carb.
I do agree with that you should follow up with the other modifications with he carburetor swap.

 

[mattr66]

The airbox for the snorkeled carb has this port, at the arrow, that has a small opening into the airduct leading to the carb. If you use a carb without snorkel, then unfiltered air gets into the airduct leading to the carb.Try plugging the port.
View attachment 866575
There are older airboxes without the port. I built an 018 with two-needle carb, and no snorkel, with one of those older airboxes.

Took care of that with some gray RTV

Unrelated but do u work for gearhead tuning? If so you wrote my 7.3 and 6.4 tunes.

Owner

View attachment 866628
View attachment 866630
Ok so when your talking about "transition" from low to high you might be thinking about the slot in front of a 4 barrel holley where as the throttle plate/ butterfly progressively uncovers the slot introducing the fuel from idle ,transfer, main venturi.
Well these little saw carbs have progressive holes that are exposed in the throttle bore in a similar fashion. I don't know where they are in a wt215 but in the first picture I have they are under the Welch plug I removed ( I smeared 1 with the punch removing the plug and redrilled it) the second picture shows 3 little holes in a channel of the carb body that are covered with a metal plate.
I have enlarged these transition holes on some carbs that have a stumble, but be aware that they are TINY, VERY TINY! I used some bits I had from a friend who worked at NASA.

This is exactly the info I was looking for... Thank you.

 

[mattr66]

I'll set the carb for best idle, pull it off and look where the throttle plate is in reference to the transition holes and make adjustments where necessary. I did fix the identical transition circuit for demon carburetors back in 2000, so this concept is very familiar to me. Thanks guys.

 

[CausticUC]

Just have to keep in mind that you can not increase fuel pressure easily on a 2 stroke. (I imagine that pumps are now starting to show up on the injected saws)

With an external pump you can usually manipulate that parameter, with most saws you are more or less stuck with how the pump diaphragm in the carb is performing... for better or worse.

No slapping a mikuni racing carb on a saw where the extra 3rd and 4th paths are available to tune the curve or the ability to modify the advance in real time... yet.

 

[farmguywithasaw]

Just have to keep in mind that you can not increase fuel pressure easily on a 2 stroke. (I imagine that pumps are now starting to show up on the injected saws)

With an external pump you can usually manipulate that parameter, with most saws you are more or less stuck with how the pump diaphragm in the carb is performing... for better or worse.

No slapping a mikuni racing carb on a saw where the extra 3rd and 4th paths are available to tune the curve or the ability to modify the advance in real time... yet.

Just put a 2300 sires holley on it

 

[CausticUC]

Just put a 2300 sires holley on it

Hotsaw time?

 

[BradSt]

Yes. I know. It's called a compensator. Why is it creating a problem? It doesn't matter where the metering chamber gets air from. As long as it can create a balance to atmospheric pressure. The only time I've ever heard of it causing a problem is in a model plane.

Ideally, it should get its reference signal from the same place the carb is taking air in from (behind the air filter). Pressure behind the filter is slightly lower than atmospheric (at larger throttle openings), and can be significantly lower if the filter is restrictive. I believe this is part of Stihls "Intellicarb" setup. It essentially leans the fuel/air mix as the filter gets dirty.

 

[mattr66]

Fixed the problem completely.... After multiple times drilling the transfers (even had 70% of the WOT fuel coming from the L system at one point), adjusting the tab, etc the problem was fixed by using an MS250 carburetor. The main venturi is closer to the center of the carburetor and the main circuit comes in much faster than the walboro. Very sharp throttle response now and leaner idle that I was shooting for. The linkage is identical to the WT carb so no changes needed there. The L mixture screw position and length of the H screw required more changes to the airbox with the dremel but worth the work 100%. I wish there was a better air filter setup for these though. This is my limb saw and I love it.