question for a mechanic about muff mods
- Thread starter MarcusB
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In a nut shell the saw is "breathing" better. Flowing more air, there for needing more fuel to burn in the air to fuel ratio.
nmurph
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It also has less back pressure so more of the incoming fuel/air charge escapes without being burned.
Unless ya happen to have an auto tune or mtronic.....
M
moody
Guest
Well if you're interested in gains and not losing the piston and cylinder I'd recommend not taking his advice. Remember you don't always need rpms to get gains from a saw.
fearofpavement
Trying them all
Well, while I don't agree that you "don't need to touch the carb" after a muff mod, it may not need to be turned richer. It does need to be checked. A carb mixes fuel into the air passing through it. More air passing through means it will pick up more fuel. Whether the extra fuel is "enough" is why the tune needs to be checked. If a saw is slightly four stroking before a muff mod, it will probably not be lean enough afterwards to cause a problem but certainly check it. I have a saw I'm going to do a muffler mod on tomorrow (hopefully) and will make a note of how much tuning it needs.
All of my stuff that was modded needed a little tweaking. It wasn't much, but was necessary. At idle the flow is low enough that the setting is close enough, unless it was ported.
F.W.P.T
ArboristSite Operative
I just recently did a muff mod on my 372xp x-torq and had to adjust both screws at least 1/2 a turn or more. The idle was ridiculously high and it was too lean. On the initial startup after mods, It almost ran on choke whereas before there was a faint "pop" or "pfft".
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fearofpavement
Trying them all
Ok, today I did a fairly aggressive muff mod on a Stihl 039 I'm building but there were other issues making the saw out of tune so I wasn't able to "measure" the amount of tuning required. (had a leak in the fuel hose to tank connection and some other issues. Made a noticeable difference.
However, I also had a dual port muffler cover that I had never installed on my MS660. I put that on and it required no richening of the H screw. However, the idle was much faster and I had to turn that down and readjust the L screw accordingly. I have not put it in wood yet but it is lightly four stroking at WOT just like it was before.
However, I also had a dual port muffler cover that I had never installed on my MS660. I put that on and it required no richening of the H screw. However, the idle was much faster and I had to turn that down and readjust the L screw accordingly. I have not put it in wood yet but it is lightly four stroking at WOT just like it was before.
Chris-PA
Where the Wild Things Are
A chainsaw carb responds to increased air flow (air velocity really) by delivering a richer fuel/air mixture. Just lifting in the cut raises the rpms and air velocity enough to make it so rich it misfires. At WOT the carb cannot tell the difference between higher rpms or a muffler mod - all it responds to is air velocity. Therefore if your saw was properly tuned before the mod, AND if the muffler mod was effective (i.e. it increased the air flow), then you will likely need to LEAN out the H needle to compensate.
If your carb has no accelerator pump (most), then you may need to richen up the L needle. Without a pump the L circuit must provide the extra fuel for acceleration (which is why you need to set the idle richer than optimum), and with the improved air flow you may need more for proper throttle response.
Strato engines do not seem to be as sensitive.
If your carb has no accelerator pump (most), then you may need to richen up the L needle. Without a pump the L circuit must provide the extra fuel for acceleration (which is why you need to set the idle richer than optimum), and with the improved air flow you may need more for proper throttle response.
Strato engines do not seem to be as sensitive.
nmurph
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Moving more air without an corresponding increase in the amount of fuel a needle allows to pass will result in a lean carb.
Stihl working hard
Is it Friday yet
If you listen to your friend you run a very high risk of a seizure or at the very least a big end bearing like stated in another thread power from worked saws doesn't always come from high rpm
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Chris-PA
Where the Wild Things Are
I know that gets repeated here a lot, but it's not how carbs work. Even if it were a normal carb with accurate mixture control, then more air would pick up proportionately more fuel and the fuel/air ratio would stay constant. These carbs will pick up way more than a proportionate amount of fuel with increased air velocity, making it rich. That is what causes "4-stroking".
chadihman
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Anybody who owns a saw should get familiar with tuning a saw or at least know the difference between lean and fat. My younger brother has no mechanical ability and I had to tell him to shut his saw down because I could hear something wasn't right with it. Smooth running at 16000 rpms flips a switch in my brain. Every MM I did needed some tweaking.
nmurph
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Doing a muffler mod increases the WOT RPM's if there isn't a change in the needle settings. A typical 346 muffler mod will increase WOT RPM about 3000 if the H needle isn't adjusted. In a two stroke, an increase in RPM's is caused by one thing...a decrease in the amount of fuel in a given intake charge. A saw that is pulling extra fuel- i.e. four stroking, will have some cushion that can accomodate a minor muffler mod. The stronger the four stroke, the larger the cushion. But a muffler mod large enough to produce a noticeable bump in power will increase the amount of air the saw pumps by a significant amount and will outstrip this reserve unless the carb was tuned extra rich to begin with. If a carb had no limits on the amount of fuel it could pass through it (passage size, length of time the passage is open, pressure of the fuel in the carb), then yes, it would pick up an increased amount of fuel, the ratio would stay constant, and the saw would not need to be retuned. But changing the air flow only affects the pressure differential bw the venturi and the fuel passage. It doesn't change the size of the passage or how long it is open. The increase in pressure will deliver a limited amount of additional fuel, not proportinal to the additional air flowing through the saw, and WILL result in a leaner tune.
