Mastermind Revisits The MS261CM Type II

[SOS Ridgerider]

Never wanna do that to a cylinder.
But I have done it to an intake port.
Sorta like a golf ball. Creates less drag.

I wouldn't do that either.
They used to do it to the pistons, not in the cylinder.

I've thought about the dimple theory a few times for saws.
There's a lot of aerodynamics involved in it.

 

[paragonbuilder]

I think it's best left as is. Period. The divider is not a restriction in any way, so why make it an issue?

Why does it matter if I was gutting them?

Think it over.....

Ok....and the divider changes that in what way?

If everything after the carb is gutted, then how is a divider in the carb changing or not changing anything? Plenty of time for mixing in the intake, jug, transfers, crankcase. And, the divider is no thicker than the throttle plates and shafts. So it's not a restriction. But, removing it throws the calibration off so much that it won't run well at all.

My point is, why make a issue out of something that is not an issue in any way?

There is only an issue in my mind. When you said it runs worse with the divider out, I want to understand how to correct it. On this Saw it may not matter, but if learning helps me diagnose issues in the future it's a win for me.

Even the smallest changes in taking material away or changing angles in the ports effect power, so I'm certain that air flow is different with the divider removed. Hell it might improve airflow, I don't know. But it's worth considering in my mind.

I see air traveling down 2 tunnels and then joining into one, whereas on a standard carb the air hits two knives, the choke and throttle plate that separates the flow and then it can rejoin before being sliced again. This would perform differently on a flow bench I would think.

It may be the position of the jets due to the divider compared to a carb with none that is causing the issues

All thoughts running through my brain.

 

[Mastermind]

When you said it runs worse with the divider out, I want to understand how to correct it

The reason it runs worse is because the low pressure signal changes dramatically when the divider is removed. The whole system is calibration around that signal.

Your trying to understand how to fix a problem that didnt exist until you created it by messing with the carb.

 

[David Young]

kinda like fix it till its broke...

 

[MustangMike]

Randy, does that provide a signal to the M Tronic system?

 

[paragonbuilder]

The reason it runs worse is because the low pressure signal changes dramatically when the divider is removed. The whole system is calibration around that signal.

Your trying to understand how to fix a problem that didnt exist until you created it by messing with the carb.

Yes. That's correct. [emoji41]
I'm always searching for other ways to achieve my goals. Often I learn that the original way was indeed better. But on occasion I learn something new!

Carry on , derail over...

 

[Mastermind]

Randy, does that provide a signal to the M Tronic system?

Yes sir. The whole system is calibrated around that pressure signal.

 

[Adirondackstihl]

Yes sir. The whole system is calibrated around that pressure signal.

Ahhhhhhhhhhhhh

 

[blsnelling]

The carb does not actually send a signal to the MTronic system, rather the MTronic system sends a signal to the carb. By removing the divider you're asking the carb to operate outside the parameters in which it was designed to operate. It simply won't work.

 

[Mastermind]

Yes. That's correct. [emoji41]
I'm always searching for other ways to achieve my goals. Often I learn that the original way was indeed better. But on occasion I learn something new!

Carry on , derail over...

Learning is a great thing.....and you guys have taught me a lot.

Let's kick this around a little more.

Do you understand why removing that divider screws the M-Tronic system up so badly?

Once I realized how that works, I was able to troubleshoot carb issues better.

The smaller the venturi, the stronger the low pressure signal is. So, more fuel will be forced thru the same size jet because the weight of atmospheric pressure is exerting more pressure on the fuel.

So, if we could increase the throttle plate and intake tract size, but keep the venturi the same size, what would happen to the air fuel mixture? Would more air also be pulled thru the venturi?

 

[MustangMike]

Would more air also be pulled thru the venturi?

Don't know, but it would create less vacuum.

 

[MustangMike]

Yes sir. The whole system is calibrated around that pressure signal.

First time I've heard about this!!!

 

[Mastermind]

The carb does not actually send a signal to the MTronic system, rather the MTronic system sends a signal to the carb. By removing the divider you're asking the carb to operate outside the parameters in which it was designed to operate. It simply won't work.

I'm not sure exactly how the system picks up its feedback from the changes in air fuel mixture. If I understand it correctly, it alters mixture, then checks rpm in relation to that change. How it prevents lean out is the part I don't completely understand. I suppose if it's too lean it will lose power and rpm?

 

[Mastermind]

Don't know, but it would create less vacuum.

I think vacuum would be higher. At least at the jet. In the intake tract it may be lower.

 

[blsnelling]

RPM, RPM, RPM. The way I understand it, it tunes for max RPM, whether it be lean or rich. The only feedback it has is RPM.

 

[MustangMike]

Can't be that simple, as a small lean would make it run stronger.

My post was in response to this from Randy: How it prevents lean out is the part I don't completely understand. I suppose if it's too lean it will lose power and rpm?

 

[blsnelling]

I THINK....By removing the divider, you're causing less fuel to be drawn through the nozzle. MTronic does not have a wide enough operating range to be able to compensate.

 

[blsnelling]

Can't be that simple, as a small lean would make it run stronger.

Read the Stihl literature. That's exactly how it's explained.

 

[MustangMike]

I think vacuum would be higher. At least at the jet. In the intake tract it may be lower.

A lot of factors at play, but the way I was thinking of it, if total volume is the same, or almost the same, velocity would be reduced which would reduce vacuum at the venture. The jet size would need to be increased correspondingly.

 

[blsnelling]

FYI, you can remove the divider from the carb of a non-MTronic saw and still tune it to run properly. However, you end up with the H needle much further out. MTronic just can't compensate that much.