Mods make a saw run richer not leaner

[blsnelling]

OK. That statement is intented as a point of discusion not as a fact. I've hesitated to post my findings for a long time simply because it flies in the face of what we read around here.

To get straight to the point, I find that modding a saw requires me to lean out the carb rather than richen it up. I'm guessing that this has to do with flow through the carb. If more air is flowing through the carb then it's drawing more fuel as well.

Case in point, my little Redmax has the H needle turned all the way in. Yes, you read that right, all the way in, and I fine tune it with the L. Fortunately, it idles and has good throttle response there. And no, there's nothing wrong with the saw.

I've had similiar experiences with other saws. I can't say this is always the case because it's just something I've been slowly coming to realize as I mod more saws.

What have ye to say on this topic?

 

[superfire]

intresting point

please explain more about what you have discovered. i have heard of this on cars but chainsaws are a different animal. it sounds correct that the fuel would be drawn in faster then in restricted engine set up.


opcorn:

 

[blsnelling]

I can't explain my findings technically. All I know is what I have to do to tune a saw correctly after porting them.

 

[sachsmo]

Makes some sense to me. With the exhaust all choked up there has to be reversion which would dilute the incoming fuel charge. Free up the exhaust and you would have an undiluted intake charge. Which should help with the diaphragm pulse. .................................Or not!

 

[fourfivefour]

Lean is mean, Fat is where it's at!!
Well that's how it's with car engines............
Chainsaws, run'em too lean and you'll melt'em down...........454

 

[blsnelling]

Lean is mean, Fat is where it's at!!
Well that's how it's with car engines............
Chainsaws, run'em too lean and you'll melt'em down...........454

True, but that has nothing to do with this discusion.

 

[056kid]

tuning the transfers/milling the head on a bike wants you to put a bigger carb on it.


interesting B

 

[yo2001]

I think it makes sense. The carb is able to take in more air with mods which allow the carb to feed more fuel even with the jet set leaner than stock. The mixture is not any leaner but the jetting is no needed as much if that makes any sense. Seems to me my H jet is always little leaner than 1 full turn after porting. Even though it drink gas alot faster than stock saw.

I think it's got something to do with the fact the carb is velocity driven. If the saw can taken in more air, the metering goes up (more impulse via rpm) on and on.... I'm sure someone can explain this better.

 

[blsnelling]

tuning the transfers/milling the head on a bike wants you to put a bigger carb on it.


interesting B

I think it makes sense. The carb is able to take in more air with mods which allow the carb to feed more fuel even with the jet set leaner than stock. Seems to me my H jet is always little leaner than 1 full turn after porting. Even though it drink gas alot faster than stock saw.

I think both quotes drive at the same point. A ported saw will drink more fuel. It has the ability to use the fuel from a larger carb. But I think that's a seperate discussion as well. I'm not trying to be a jerk here. I'm just trying to keep the discussion on topic and drill down on this thing. It's the total opposite of what we usually read here. I'm curious what the case would be with only modding the muffler? Most of my saws are ported so I don't know.

 

[nmurph]

but the fuel flow is restricted by the opening of the jet regardless of how much additional air can be pumped. i am sure that with a given jet opening that there is some additional fuel pumped through the carb bc of the increase in presure, but i would think that point of diminished returns would be reached very quickly.
Brad, could there be something else at work? have you had similar experiences with other saws? would you have to increase fuel flow if you restricted a muffler?

 

[CentaurG2]

Are you trying to say that a modified chainsaw with factory settings on the carburetor will run too rich and needs to be leaned out??How are you determining what is too rich and what is too lean for a modded saw?? Is this just based on RPM or timed cuts or do you run the saw till it melts down?? Is your redmax running at factory recommended RPM, modified, with the H needle all the way in its seat? Sorry for all the questions but I am not sure what you are trying to say or get at.

 

[blsnelling]

I've found this to be true on multiple saws. I can't say whether all saws I port are this way or not. I'd have to see where the screws are.

 

[yo2001]

Just muffler mod seems to always want to be richer than stock 1 turn on the H jet. (in case of my MS290) but every other saw, less than 1 turn on the H side jet. I guess it's got something to do with the increase of VE of the engine when you port a saw.

Samething on the car engine as well. High compression, high VE less fuel needed to be stoic.

 

[Mitymouse]

I agree with Brad

On all saws I have modified, everything from muffler mod to porting and assembling with no base gasket all needed to be set leaner or at a minimum the same. I have never had to richen them up. One possible question that came to my mind is that they could have all been too rich prior to modding. Also is it possible after the mods the efficiency of combustion has increased thus no need to richen the H.

 

[Tzed250]

I've found this to be true on multiple saws. I can't say whether all saws I port are this way or not. I'd have to see where the screws are.

Maybe you have changed the nodes enough that the engine is double-triple carbureting....

 

[TwinTurboRam]

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

 

[blsnelling]

Maybe you have changed the nodes enough that the engine is double-triple carbureting....

You'll have to explain that one for me.

 

[Jacob J.]

On all saws I have modified, everything from muffler mod to porting and assembling with no base gasket all needed to be set leaner or at a minimum the same. I have never had to richen them up. One possible question that came to my mind is that they could have all been too rich prior to modding. Also is it possible after the mods the efficiency of combustion has increased thus no need to richen the H.

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.

 

[TwinTurboRam]

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.

+1

Well put Jacob

 

[Jacob J.]

I think it makes sense. The carb is able to take in more air with mods which allow the carb to feed more fuel even with the jet set leaner than stock. The mixture is not any leaner but the jetting is no needed as much if that makes any sense. Seems to me my H jet is always little leaner than 1 full turn after porting. Even though it drink gas alot faster than stock saw.

I think it's got something to do with the fact the carb is velocity driven. If the saw can taken in more air, the metering goes up (more impulse via rpm) on and on.... I'm sure someone can explain this better.

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.