Mods make a saw run richer not leaner

[blsnelling]

Brad, what kind of R's where you getting with the Redmax with that high speed clear in. Just dosent sound rite to me

14,800 and still getting a little 4-stroke. I'm thinking this carb must be kind of like my old Homelite SXL-925. The screw is all the way in on it as well and still 4-strokes if not under enough load. This is per the manual as well. What must be happening is that there is a fixed jet that flows most of the fuel and the H only meters the last little bit of fuel.

 

[blsnelling]

It likely your saws were too rich prior to modding. The whole intent and purpose of modifying the cylinder on a modern chainsaw is to 1.- Increase flow, getting more air and fuel to the top of the cylinder before compression, and 2.- Increase compression to enhance combustion and have a higher post-combustion pressure in the cylinder.

Any time you increase the flow of air, you need to increase the flow of fuel. Every large modern saw that I've ported I've needed to get more fuel to the engine as I've dramatically increased the amount of air moving through that engine.

I don't disagree with you at all. I'm simply stating that I'm getting that fuel with less turn on the H needle.

 

[Orange Hill]

I believe it is increase intake velocity at work which increases the venturi effect drawing in more fuel. Or it could be less turbulence in the intake again increasing the venturi effect. I have a KX250 that I had bored and replated to a 310 by Eric Gore and I ending up with jetting in the same ball park because of the increase of intake velocity even though I have 60cc's more. I did go from a #52 to a #55 on the power jet, and down from a#158 to a#162 on the main. I went to a more tapered needle that was also raised and a larger pilot because of the need for more fuel to start the bike when intake velocities are low.

It depends on how much excess capacity (head room) your stock carb has because there comes a point where turbulence from increased velocities start to decrease draw efficiencies. I work with industrial air venturi vacuum pumps on our equipment and the pumps have a air pressure to vacuum efficiency chart. The chart shows a smooth steady rise in volume with 87 psi being peak and then it tappers off from there and gets choppy with a down ward slope. I have the hardest time convincing customers that more is not always better.

Or I could be 180 degrees out from the real answer.

opcorn:

 

[blsnelling]

I believe it is increase intake velocity at work which increases the venturi effect drawing in more fuel. Or it could be less turbulence in the intake again increasing the venturi effect.

You have described what I could not. This is what I believe to be the cause. I do have to go rather rich on my LA side like you mentioned as well.

Again, I'm not talking about an isolated case here. These are saws that have no running issues. They run well and run strong. I suppose I should have more supporting documentation before posting something like this. But I figured you smarter jaybirds out there could explain to me what's going on and I believe the post above does exactly that.

 

[Tzed250]

You'll have to explain that one for me.

In a two-stroke there are all kinds of pressure waves going back and forth. Changing the timing can cause a reversion of the intake flow. This reverse flow through the carb picks up more fuel. When this over rich mixture is then pulled back through the venturi, it picks up fuel again. This is where spitback comes from and can cause many tuning problems

 

[blsnelling]

Hmmm. So if this is about intake velocity and the demand for more fuel, then it sounds like this is proof of the need for a larger carb. A larger venturi would lower the intake velocity but increase the ability to flow more fuel at high RPMs. That greater volume of fuel is also going to increase base compression and help force more fuel into the combustion chamber. Thoughts?

 

[FATGUY]

OK Brad, here's why I think you're right. A carb is supposed to provied the engine with the correct stoichiometric ratio of gas mix to air in order to facilitate combustion. Under normal circumstances this doesn't change much ( within reason) If your modifications "pull" in more air, you must allow the carb to pull in more mix as the current settings restrict more mix from entering. I believe this was your intitial point and I see no flaw in it.

 

[041fboss]

after you port your chainsaws....they are extremely efficent therefore might not need as much fuel..

 

[yo2001]

It would depend on the carburetor's ability to meter fuel to begin with. On a lot of modern saws the carbs are choked down due to EPA restrictions. So on a saw like say, the MS-460, where the carb is already choked down, it may not be able to meter that much more fuel to keep up with the demands of the modifications done to the piston and cylinder. This is why builders pull the limiter caps and/or replace the fixed jet or drill it.

Didn't think of that. that makes sense also since my 2 044 with Zama carbs (old) and 026 with walbro with both adjustment needed less turn even ported and my newer MS290 needed more fuel with the adjustment tabs cut out.

 

[Lakeside53]

Every muffer mod I've ever done requires a richer mix both on L and H. On the other hand, I limit the Max rpm to something closer to factory WOT... and not "max 4-stroking".

 

[flyboy]

I wonder if it has to do with the velocity of the air through the carb....sort of like air over the top of an airplane wing. Planes fly because the air on top of the wing is going much faster than the air on the bottom of the wing. The air on the top of the wing is very low pressure and the air on the top of the wing is reletively high pressure....so the plane goes up.

I think you have significantly lowered the intake manifold air pressure by increasing the velocity of the air through the venturi. More vacuum will suck more fuel...so you have to lean it down to compensate.

Wouldn't a bigger carb have a larger diameter bore? That would effectively slow down the velocity of the air and increase intake manifold pressure. Then you would have to make it rich because the fuel is not being sucked out of the carb as hard.

I wonder if you could afix an intake manifold pressure guage on a saw for before and after readings.

Just my 2cents.

 

[teacherman]

The venturi effect makes sense to me, but I am wondering if the carb will eventually reach its limit of fuel flow, at which point one would need to install a larger carb. I hear Brad C. has an 026 that is modded to insane specs, and it has an 044 carb on it. I got to run my 044 today, and it just flat out rips through the wood like, well, like a chainsaw....I imagine the older carbs actually will allow some serious gas to flow thru them.

Sounds like an interesting situation, the specifics of which are way above my pay grade.

 

[excess650]

Brad, I wish that you had started this thread earlier, and I might have made notes. I swapped a CAT muffler for a non-CAT muffler on one saw, and opened the muffler(more) on another saw, and then tuned a third saw besides this weekend.

I can say that the previously muffler modded Makita DCS401, previously tach tuned, needed tuning again after further opening the muffler. What did surprise me was that it needed richened further on the low speed. I THINK that I did have to lean out the high speed on that saw after resetting the low speed and idle.

I was debating upon pulling the baffle, which has the screen, but opted to just further open the outlet. Threads here on AS indicate that just removing the bent tab on the recoil side of the outlet would "waken the saw". Well, it did, but it seemed to me the outlet was too small, and I'm not so sure that the baffle itself isn't a problem. I further opened the outlet, but its just slightly smaller now than the exhaust port piston side.

I DEFINITELY had to fatten the low speed to get rid of lazy acceleration, but I think that I leaned out the topend.

 

[Taxmantoo]

The two theories I like have both been covered already.

1. Moving more air than the carb was set up for, and the carb is acting like you've got a choke butterfly closed because of the excessive manifold vacuum.

2. Blowback through the carb, gets mixed with fuel going forwards, gets mixed with fuel again going backwards, then gets fueled a third time going forwards and on into the engine. Needless to say, doing this with a small percentage of the charge volume will make a noticeable difference in needle settings. Did you increase intake duration by lowering the cylinder?

 

[Tzed250]

I wonder if it has to do with the velocity of the air through the carb....sort of like air over the top of an airplane wing. Planes fly because the air on top of the wing is going much faster than the air on the bottom of the wing. The air on the top of the wing is very low pressure and the air on the top of the wing is reletively high pressure....so the plane goes up.

I think you have significantly lowered the intake manifold air pressure by increasing the velocity of the air through the venturi. More vacuum will suck more fuel...so you have to lean it down to compensate.

Wouldn't a bigger carb have a larger diameter bore? That would effectively slow down the velocity of the air and increase intake manifold pressure. Then you would have to make it rich because the fuel is not being sucked out of the carb as hard.

I wonder if you could afix an intake manifold pressure guage on a saw for before and after readings.

Just my 2cents.

This post brings to light that most folks have a mis-understanding about the way a carburetor works. The airflow through the venturi (essentially the top surface of an airfoil formed into a circle) does not "suck" fuel into the airstream. The fuel passes through a jet into the airflow because of a pressure differential in either a float bowl or metering chamber. A paint spray gun or airbrush works on the same principal.

 

[blsnelling]

I hear Brad C. has an 026 that is modded to insane specs, and it has an 044 carb on it.

I have a MS260 is a 044 carb as well. Before the 044 carb it ran very strong and turned 15,800 WOT. It pulled strong but fell off in bigger wood compared to a modded 346 I was running against. With the 044 carb it now turns around 17,400 and was the fastest 50cc saw at last weeks GTG. That carb made a HUGE difference in that saw. That was the only change I made.

This post brings to light that most folks have a mis-understanding about the way a carburetor works.

I'll be the first to admit that! All I do is clean and tune them.

 

[Andyshine77]

This post brings to light that most folks have a mis-understanding about the way a carburetor works. The airflow through the venturi (essentially the top surface of an airfoil formed into a circle) does not "suck" fuel into the airstream. The fuel passes through a jet into the airflow because of a pressure differential in either a float bowl or metering chamber. A paint spray gun or airbrush works on the same principal.

:agree2: That's how carburetors work. When air flows through the carb it drops the pressure lower than the ambient pressure. With the ambient pressure being grater it forces the fuel through the jet, just like Tzed said.

Brad when you raise the compression you're effectively decreasing the volume of combustion chamber. Correct??

If the combustion chamber is now smaller, I would think less fuel would be required for proper combustion. Also with higher compression you get a more complete aka efficient burn. This would allow the engine to require less fuel.

 

[Bowtie]

You have described what I could not. This is what I believe to be the cause. I do have to go rather rich on my LA side like you mentioned as well.

Again, I'm not talking about an isolated case here. These are saws that have no running issues. They run well and run strong. I suppose I should have more supporting documentation before posting something like this. But I figured you smarter jaybirds out there could explain to me what's going on and I believe the post above does exactly that.

Brad, Im with you. My 361 with the muffler mod the screws are under a full turn out both H and L. Saw runs perfect. I just finished the 036 Pro that has been maddening me for a while now (Thanks again Jacob). I ended up splitting cases and new seals bearings, etc...Anyway, back on topic. It has a brand new carb as well as everything else, and the best operation is under one turn out on both needles.

Before anyone says I have a bad ear at tuning, my 044 set at close to 1 1/4 turns is right, as well as the 064.

 

[Lakeside53]

:
If the combustion chamber is now smaller, I would think less fuel would be required for proper combustion. Also with higher compression you get a more complete aka efficient burn. This would allow the engine to require less fuel.

Not so sure about that... the ratio of oxygen to gas remains substantially the same even if you raise the compression.

 

[buzz sawyer]

Very interesting thread so far. One difference between an auto carb and saw carb - In a car, the HS jets are fixed and flow is controlled by needles which move in relation to manifold pressure or mechanical air valve position. I'm far from an expert but in a saw, the HS needle must be manually adjusted to compensate for increased air flow, right? I realize the increased intake air velocity would result in lower pressure but I'm not sure that would be enough to significantly increase fuel flow.

What effect if any does the mod have on the carb impulse pressure? Could that be affecting the diaphragm and causing the carb to flood slightly?