Chainsaw Porting Theory

[Terry Syd]

I spent some time with Jerry Branch on the flowbench at Flowmetrics. The guy was a true artist - and genius. I remember him showing me a stock head off a V8 and then his 'ported' version. The stock head had almost a straight open runner to the valve pocket - the same runner on the 'ported' head was considerably restricted and no longer straight. Hell, I would have ground out all the material he had put in. - However, the flow bench doesn't lie, it just records the reality.

He did a funny demonstration with a very slick looking air horn that was an accessory for a motorcycle carburetor. He put the air horn on the bench and fired it up. I forget the exact numbers, but say it was flowing 600 cfm. He then took a small piece of clay (he used a lot of clay in his work), rolled it up into a ball about the size of a pea. He stuck the 'pea' on the edge of the air horn and the flow meter jumped up to 625 cfm. He then added another 'pea', up again it went to 650 cfm - he ended up with six of these 'peas' on the edge of the air horn and flow meter was showing close to 750 cfm.

I said, "it looks like the air horn may actually restrict the flow". He replied, "no, the carburetor it mounts on only flows 450 cfm". I then said, "so the air horn is completely useless". He laughed, "nope, it makes the owner FEEL BETTER about their bike". - (some porting jobs could fit that description)

As far as the carb question, yeah I think guys should spend more time trying to get the two circuits dialed in a bit better. There is a lot of extra torque and resistance to bogging that can be achieved by getting the fuel mixture closer to the optimum throughout the RPM range. Most people concentrate on the 'top end', up where the horsepower is, but that is only part of the powerband that a WORK saw utilizes. After dialing in the carb, the saw may not make anymore horsepower, but it will have more 'grunt' for when you need it.

As far a 'bigger' carbs, I wonder how many air filters are the real bottleneck in the intake. I expect it would be very informative to have access to a flowbench to run carbs and their airfilters. Although we don't have access to a flowbench, we can check the RPM with the filter on and with it off. I wonder how many guys do that before they start trying to cram a bigger carb on the saw...

 

[mdavlee]

Matching carbs is the most time consuming thing to do. I bet I tried 7 or 8 on the 046. 4 on the 665 until I got one running the way I want. The 372 is next for getting a carb dialed in.

 

[Chainsaw Jim]

After the hda carb experience I mentioned I've been wanting to try that same carb and mod on a 372.

 

[SawTroll]

What he's saying I understand but I don't. I've had "saw builders" tell me the same thing. It all goes back to what Jerry branch always preached. It's not about what you take out of the cylinder or do to it. It's more about what you pump into it. I know, I know some of you will say that we're talking two different animals here, but it's all relative in principle. There's much I don't know and understand but that's why I'll work my ass off to try to crack this riddle. There's always more to be had. But yes, please elaborate Saw Troll. I'm interested to hear your take on this. I'd love for @Terry Syd or @Poleman to contribute to this carb discussion as well.

Into it, and trough it - there has to be a balance that goes all the way. Also, it is the mean pressure below the piston that really counts (can't be measured, just approximately calculated), not the static combustion chamber compression (that can be measured).

 

[jmssaws]

When you need to port a saw everyday or two it becomes to time consuming to start piddling with the carb,on my saws I will but I have unlimited time,I hate to open a can of worms on a customer saw and spend a day trying to get it right again.

 

[Chainsaw Jim]

I'm my mind this is only a necessary concept for racing or show off saws.

 

[mdavlee]

I'm my mind this is only a necessary concept for racing or show off saws.

It makes some saws much more user friendly and widen the power and. The modded zama Rich does for the dolmar and Stihls is worth it. Made milling easier on my 046 and tuned just fine.

 

[idiotwithasaw]

When you need to port a saw everyday or two it becomes to time consuming to start piddling with the carb,on my saws I will but I have unlimited time,I hate to open a can of worms on a customer saw and spend a day trying to get it right again.

It would be like your porting once you have done a few you will have it down and just repeat it every time.

 

[Chainsaw Jim]

It makes some saws much more user friendly and widen the power and. The modded zama Rich does for the dolmar and Stihls is worth it. Made milling easier on my 046 and tuned just fine.

There's the obvious exceptions but I was coming from the mindset of problem solving to get that extra bit that is only necessary if timed cuts matter.

 

[mdavlee]

There's the obvious exceptions but I was coming from the mindset of problem solving to get that extra bit that is only necessary if timed cuts matter.

Yeah pretty much. I will spend time trying different modded ones from Rich on a saw to get the best running one.

 

[Terry Syd]

Rich (Poleman) has done a fair amount of work with the Zamas and can quickly set up a carb for an application. That's the time consuming part, figuring out what needs to be done.

However, once you have the 'recipe', then it can be done quite quickly. For a Walbro it may be just a change in the metering spring and then reset the mixture screws. There is a metering spring kit that is available that has a bunch of different springs in it. If someone went through the springs on a particular modded saw, then they would know the proper spring to pop in the carb on the next job.

 

[jmssaws]

I mostly port 064's and 066's and I haven't seen the need for a different or modded carb
I have no doubt that there is saws that need carb help but the ones I port don't.

 

[mdavlee]

I bet the 1122 would gain with a little bigger carb. It's running the same size as the 70cc saws. There's just no drop in carb that I've found.

 

[Poleman]

Terry and Mike are right on with getting the recipe....each saw is different with different caritaristics, saw to saw brand to brand. Once something is figured out and you have a base line it's a quick process. I like to build in adjustability so a particular carb will fit a given application but with room for adjustment because of the differences in saws, their numbers and caritaristics.
Working with Mike has really helped me build and understand what is needed for different applications as he gives tremendous feedback along with RedElm(Vaughn) I've been able to make suttle changes and make things better.

My thoughts on BIGGER is NOT always better. I've bored Zamas with success but there is a lot of tinkering involved after doing it to get them to run right. I can be done and it does help but it's almost a tailor made carb for a particular saw, as putting it on another saw you need to start over again because of suttle differences in like saws. I really can' recommend this method to performance.

I believe with increased jet size the saw itself with altered caritaristics will feed itself by increased intake duration, you just need to supply enough fuel as more air will be introduced at the same time drawing through the same carb size.

Terry is on to something stating the restrictions may not be the carb but the air filter, air filter feed tube and just lay out. Something to look at and explore also.

 

[Stump Shot]

Very interesting discussion about the carburetors here. First I'd like to say that in my opinion everyone that weighed in on this is right. That said these are my thoughts: To start with carburetors are never 100% perfectly tuned in at all levels of throttle, there is only the best tune, so gains are always possible. The metering spring is the master of the fuel circuit only letting in what its pressure will allow. Seeing as the carburetor works off of differences in pressure, this is a critical point in tuning. A weaker spring allowing more fuel in, stiffer spring less. So if "pop off" pressure is checked with a pressure tester and put to a value in PSI instead of spring x,y,z and so on, that way when you go from carb A to carb B, you don't have to start all over again with x,y and z. It should eventually become apparent at what pressure level the metering circuit should be at, thus allowing the carburetor to be bench set to a pop off pressure before it is ever installed ending the spring roulette game. This of course would still be just a generalization, as these little engines are sometimes quite the unique individuals, but a good generalization none the less. Once again very interesting, and happy tuning!

 

[mdavlee]

Air filters are over rated any way.

 

[jmssaws]

Air filters are over rated any way.

Who needs filtered air? I don't

 

[Poleman]

Lol....wouldn't fine wood partials just add to the ignition in the combustion chamber???? Kinda like solid rocket fuel.....

 

[SawTroll]

Lol....wouldn't fine wood partials just add to the ignition in the combustion chamber???? Kinda like solid rocket fuel.....

Likely not, but a small amount of fine wood dust doesn't hurt much either. Some people just are too fanatic about air filtration.

 

[mdavlee]

Milling yes. Firewood panty hose is probably sufficient