Chainsaw Porting Theory

[Terry Syd]

The metering spring is the master of the fuel circuit only letting in what its pressure will allow.

Stump shot, this is a common misconception about these diaphragm carbs. The metering spring does not set pressure, in fact there is no pressure.

I read everything I could find on how diaphragm carbs worked and found that EVERYTHING I read was BS. Even the stuff by EC Birt had errors. It took me 4 months before I got it worked out with empirical testing. The only thing I hadn't figured out was why there were different size metering holes under the metering valve. That was cleared up when I read the Mikuni manual on tuning their diaphragm carbs.

There is a thread on tuning the diaphragm carbs somewhere on this site that has lots of discussion and links (like to the Mikuni manual). Perhaps someone knows where it is and can link it.

 

[Terry Syd]

I can't find the thread with all the discussion of various concepts, but I did find a link to the Mikuni manual - http://www.mikuni.com/pdf/sbn_manual.pdf

 

[paragonbuilder]

I can't find the thread with all the discussion of various concepts, but I did find a link to the Mikuni manual - http://www.mikuni.com/pdf/sbn_manual.pdf

Terry, I think that thread was on AS.


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[jmssaws]

What's a good mod for a wj?

 

[Stump Shot]

Terry Syd,
I'm not sure if you understand that on the metering diaphragm that pushes the inlet lever controlled by the spring one half (fuel side) has a negative pressure from the air drawn through the carburetor, the other side with ambient pressure (the air all around us) gives the diaphragm a push thus overcoming the spring pressure letting more fuel back in, and starts all over again. So there is positive, negative and ambient pressures all coming into play to make everything work properly. Put too heavy a spring into the mix and these pressures will not be able to fully overcome the springs pressure causing a lean condition, too light a spring it will be rich.
I'll give an example, a Tillotson HR carburetor was rebuilt from an aftermarket kit with a replacement metering spring. As the picture shows with not so good results. After the engine had a complete rebuild, and a little more closer attention was payed after fire up, it was running hot and lean and was shut down. With some bit of detective work it was finally ascertained that the "pop off" pressure(the amount of pressure it takes to push off the needle valve from its seat with a pressure testing tool) of the replacement spring was at 35 PSI, proper replacements were obtained from Tillotson in Ireland, those springs, some other old ones I was able to conjure up and one sent to me from the aftermarket manufacturer tested at 11 PSI. This was installed and ran fine the engine tuned in properly and ran cool, and continued to run properly for two seasons now. So there was no doubt as to the cause of the lean out.
So that being said, it would seam reasonable that a built engine that is now fuel starved should have a bit lighter spring installed allowing all of those pressures to overcome it more easily letting more fuel in that the built engine requires. Heck, just having the inlet lever a little high or low will make an engine rich/lean. Or putting the gasket on the wrong side of the diaphragm will cause the engine not to run at all. I'm sure a few of us can attest to that myself included.
As far as Mikuni carburetors go I'm only familiar with their slide carburetors.
Not sure that I'm the best at explaining things especially while typing. Hope this is a little clearer for every one. If I botched a part let me know, I'm getting tired now.
I've also included some reading from Tillotson and Zama to look at as well, tried one from Walbro but it was too large.

Picture of 294 Rotax engine failure from lean out.

 

[GoBigBlue1984]

Mind blown......keep this discussion going. It's absolutely phenomenal. I'm trying to soak as much of this in as I can.

 

[Terry Syd]

The two manuals you posted are basic repair and basic tuning (fuel screw) manuals, they are not 'modding' manuals. That is, how to change the fuel curves in the carb.

The metering spring works in conjunction with the metering valve seat to 'ramp' in the fuel. If the circuit did not exist the engine would flood at low speed. The pump diaphragm system is only for the idle and transition phase of carburetion. It only works (depending upon spring and orifice) up to about 1/4 throttle (on chainsaws). However, as indicated in the Mikuni manual you can change the components so the circuit will work much higher (good for part throttle work, especially out of corners) Once the metering valve is full open the wet side of the carb draws fuel from the tank just like a float bowl carb.

If you do not believe what I say, try this experiment. Take the pump diaphragm out of the carb. Start the engine with the choke on to get enough fuel for starting - if you keep the revs up you will be able to cut wood, if you let the revs drop down too low the engine will stall.

The stronger spring you had in that carburetor likely leaned out the low speed circuit at part throttle. It did not affect the mixture at high speed. At high speed the needle was all the way off the seat and the fuel mixture was being determined by the jets in the carb.

EDIT: I suppose with an excessively strong spring the metering orifice could be partially blocked causing a lean mixture at high speed, but that should have been cured by a tweaking of the high speed needle to compensate and cause more demand. If so, a high speed plug chop should have caught the lean mixture.

 

[Terry Syd]

Terry, I think that thread was on AS.

I thought I brought some of that information over to this forum. Crikey, I wouldn't want to have to go back through all the discussions all over again.

 

[Stump Shot]

The two manuals show in clear detail and contrasting colors the pressures you said do not exist in a diaphragm carburetor.

The pump diaphragm is on the other end of the carb and not part of the discussion. If you had actually read that manual a little further you could have learned that on pages 12-14 all about pop-off pressure and could have contributed to the discussion.

"If you do not believe what I say" - Our first agreement

The engine seizure happened at WOT

Carburetor fitted with a fixed High speed, but you should have saw that with the computer app you brought over from AS.

Any of this reference material listed can help someone with the desire to tinker with their carbs to tune for more power, all the reference material is out there. It's fun tuning engines for more power, being on an argumentative thread is not. So I humbly bid this thread farewell.

 

[Deets066]

Great stuff here fellas, this carb stuff is usually what kicks my ass. I know there is more potential in my saws and the carb is a big factor of what's left. Most I do is thin butterflies and shafts and drill a jet if necessary

 

[Deets066]

The two manuals show in clear detail and contrasting colors the pressures you said do not exist in a diaphragm carburetor.

The pump diaphragm is on the other end of the carb and not part of the discussion. If you had actually read that manual a little further you could have learned that on pages 12-14 all about pop-off pressure and could have contributed to the discussion.

"If you do not believe what I say" - Our first agreement

The engine seizure happened at WOT

Carburetor fitted with a fixed High speed, but you should have saw that with the computer app you brought over from AS.

Any of this reference material listed can help someone with the desire to tinker with their carbs to tune for more power, all the reference material is out there. It's fun tuning engines for more power, being on an argumentative thread is not. So I humbly bid this thread farewell.

No reason for butt hurt, disagreeing isn't arguing

 

[Terry Syd]

Good luck on your future tuning.

EDIT: I just checked the pages 12-14 of the Zama manual you referred to. I think I spotted the sentence that sent you up the wrong track, their description is worded incorrectly. I'll let you figure it out, it's more fun this way.

 

[drf256]

Great discussion. This ain't the AS, so let's not get insulted if opinions are incongruent. That's what a fair discussion is about and how we learn together.

 

[drf256]

So, my understanding is that fuel gets drawn into the pump side of the carb, flows down to the wet side through the metering seat, and sits above the metering diaphragm.

From there, it fills the area under the Welch plug for the idle and transition holes (by the throttle blade in the bore) and the main jet base is immersed in it. When fuel is pulled into the carb bore by Venturi effect, the negative pressure of the FUEL pulls up on the metering diaphragm to allow more fuel entry.

It holds back pump pressure till it's needed based on fuel flow.

Do I have this wrong?

 

[Terry Syd]

You can remove the pump diaphragm and the engine will run - so where does the "pump pressure" you refer to come from?

 

[drf256]

Not diaphragm pressure. Pump pressure.

Doesn't the pump side of the carb pressurize the fuel to between 5-7psi over the needle?

 

[drf256]

How is fuel drawn into the carburetor in any tank position? Not trying to argue here, trying to learn.

 

[Terry Syd]

How is fuel drawn into the carburetor in any tank position?

That's the job of that long bit of hose with the filter on the end of it inside the tank, it drops down with the fuel when you change position.

 

[beaglebriar]

I believe the engine is the pump and the carburetor is a slave to it. You must have suction pressure from the engine in order for the carb to work. So when Terry says that "the carb doesn't pump anything" that makes perfect sense to me .

 

[Terry Syd]

You must have suction pressure from the engine in order for the carb to work.

Got it! It is the low pressure drop inside the carb that sends a signal to PULL fuel out of the wetside of the carb. The diaphragm moves inward and LIFTS the metering valve to open up the flow from the tank.