361 tuning tach/rpm question

[wvengineer]

Hello all first post, and a quick question.

Anyways MS 361 stock saw. Chain runs at idle. I initially thought it might be the clutch springs, but then I started thinking that the idle was too high. I have also been feeling like the rpm's have been too high at WOT. I'm using an automotive tach set to 4-cyl. The instrument I'm using is meant to be connected to the coil. So for a 4 cyl I'm thinking that whatever it's reading it's dividing it by two. If that's the case then my saw is way over. The idle is like 5k and WOT is around 20k or so. I know it seems like half of that would be about normal. Something's wrong here. And just to be sure we're on the same page I'll explain why I'm doubling my rpm reading for a 2-stroke. For a 4 cylinder every 180 degrees of crank rotation get's a spark on one of the cylinders. Since this thing is made to measure from the - side of the coil then it's reading all cylinders. It would be dividing the reading by two because it would read spark on at least two cylinders for every 360 degrees of rotation. The 2-stroke is one for one. One spark for every 360 degrees of rotation. So where did I go wrong in my thinking? If my thinking is right then I'm in trouble because I'm going off of the service manual and turning the LA screw all the way down will stall way before it reaches 2800. Again is my logic just wrong?

Thanks for the help.

 

[breymeyerfam]

first of all, welcome to AS! put your tach down and do it by ear. read this:http://madsens1.com/saw carb tune.htm.

 

[MotorSeven]

Wait...
Do yourself a favor and get a tach or borrow one. Even a dealer would most likely check it for you FOC. Next, did you take out the limiter caps on the carb before trying to tune it? Be very careful, as improper tuning can cause your belved 361 to melt itself....not good.
Something is wrong, because it will not turn 20K. The clutch springs may look good & still be worn out(sprung). Replace them anyway as they are cheap.

Oh and Welcome to AS!

RD

 

[ironman_gq]

Are you sure a 4cyl fires every 180 degrees or is it every 90 degrees. I would take it to your dealer and have them check it. or set idle by turning the idle speed up until the chain just starts turning then back it off till it just stops and then 1/3 turn more so the chain doesnt run at idle. as for WOT tune it so it four strokes slightly but clears up when in the wood

 

[J.W Younger]

Are you sure a 4cyl fires every 180 degrees or is it every 90 degrees. I would take it to your dealer and have them check it. or set idle by turning the idle speed up until the chain just starts turning then back it off till it just stops and then 1/3 turn more so the chain doesnt run at idle. as for WOT tune it so it four strokes slightly but clears up when in the wood

4stroke 4 cly will have a power stroke every 180*. 2 stroke 1 cyl every 360

 

[wvengineer]

Thanks for the replies everyone.

Breymeyerfam: I've read that site before. Found it searching this site. I understand that, but I'm having a little trouble hearing this saw four stroke. I would still like to use the tach just out of principle now. It should atleast agree that I'm in the ballpark if I tune by ear. Right?

MotorSeven: I have a tach. It's just not configured for a 2-stroke motor. It shouldn't matter other than what I was discussing above...just a math thing. All of them just count pulses over time. Oh I don't know if I mentioned it but I'm measuring across the control switch. Also it's a pain for me to go to the dealer between work schedule and distance. I would assume that it wasn't turning 20k or it would of ate itself right there. I also realize that if I run it too lean it'll eat itself also. I have a new clutch assembly coming my way in the mail. I haven't removed the limiters yet. The only adjustments I've done: 1. Turned LA CCW to try to get the chain to quit running (it stalls before that). 2. Turned H CCW to stop (with limiters...out of the service manual). 3. Turned L CW to stop and then back 1/4 turn (with limiters...out of the service manual). I pretty much quit here because my readings didn't make sense. Again I'll replace the clutch either way. You know the sound you get when you're running out of gas and it leans out? Right before the saw cuts off. It started sounding like that all the time and with the chain running so that's why I'm messing with it.

ironman_gq: Every crank I've ever seen has them 180 degrees. An example would be for a 4 cyl. One cyl is at TDC on compression stroke. Another one would be TDC right after it pushed the exhaust out. Another one would be at BDC at the end of the power stroke, and the last one would be at BDC after pulling in new air/fuel mix. It's a pain for me to go to the dealer, and it won't idle low enough to stop the chain (new clutch on the way).


J.W Younger: I agree.

I'm still wondering why my tach won't work. It works on 4-strokes...I've used it too many times. Now I'm just wanting to know.

 

[albert]

With a stock muffler, the 361 is hard to tell when it 4 strokes, I think your tach would have to have a 2 cylinder 4 stroke mode capable of 15,000 rpm

 

[MotorSeven]

A whole clutch assembly or just the springs? In order to properly tune it the limiter caps have to come out. Use a drywall screw to get them out, cut off the "wings" & put them back in. Then you can turn them all the way down(be easy because you can damage the carb by cranking them down too hard) and then back out 1 turn. It's a good starting place to begin tuning.
I'm like you, I had a hard time hearing the 4stroke burble, so I bought a tach. Now that i've done it a few times I am better at hearing it, but I double check everything with the tach. Do a search here and you will find many posts about tuning that will help.

RD

 

[wvengineer]

With a stock muffler, the 361 is hard to tell when it 4 strokes, I think your tach would have to have a 2 cylinder 4 stroke mode capable of 15,000 rpm

I haven't been able to hear it. I used to have an 034 that I could, but not this one. I just don't see the difference in the tach. It's counting a pulse. In this case it's dividing by two since it thinks it's a four cylinder. If I turn it from 4 to 8 cylinder it cuts it in half. That makes sense to me since there would be 4 firings per revolution instead of two like the 4-cyl. I see this relationship the same as between a 4-cyl and a 2-stroke. HHHhhhmmm I can't figure it out.

 

[J.W Younger]

I haven't been able to hear it. I used to have an 034 that I could, but not this one. I just don't see the difference in the tach. It's counting a pulse. In this case it's dividing by two since it thinks it's a four cylinder. If I turn it from 4 to 8 cylinder it cuts it in half. That makes sense to me since there would be 4 firings per revolution instead of two like the 4-cyl. I see this relationship the same as between a 4-cyl and a 2-stroke. HHHhhhmmm I can't figure it out.

Any tack is going to have a deg of accuracy. Most automotive style tachs are designed for a range of 500-7000 or there abouts,so accuracy may be poor at 27000.

 

[blsnelling]

first of all, welcome to AS! put your tach down and do it by ear. read this:http://madsens1.com/saw carb tune.htm.

With a stock muffler, the 361 is hard to tell when it 4 strokes, I think your tach would have to have a 2 cylinder 4 stroke mode capable of 15,000 rpm

If totally stock, it will be very difficult to tune a 361 by ear. The mufflers are so restrictive that it even leads to flooded saws since the "pop" is hard to hear when starting cold. Mod the muffler and you'll be good to go.

 

[wvengineer]

Any tack is going to have a deg of accuracy. Most automotive style tachs are designed for a range of 500-7000 or there abouts,so accuracy may be poor at 27000.

That is a thought, but at idle it shouldn't be an issue. I should be reading 1400 for the idle and say less than 7000 for WOT. Even ignoring WOT it should be fine for idle. The tolerance on the meter is +/- 3%. I do have a small engine shop (lawnmowers and such) down the street from me they probably have a tach designed for 2-strokes I could check mine against theirs.

 

[Paul001]

Thanks for the replies everyone.

Breymeyerfam: I've read that site before. Found it searching this site. I understand that, but I'm having a little trouble hearing this saw four stroke. I would still like to use the tach just out of principle now. It should atleast agree that I'm in the ballpark if I tune by ear. Right?

MotorSeven: I have a tach. It's just not configured for a 2-stroke motor. It shouldn't matter other than what I was discussing above...just a math thing. All of them just count pulses over time. Oh I don't know if I mentioned it but I'm measuring across the control switch. Also it's a pain for me to go to the dealer between work schedule and distance. I would assume that it wasn't turning 20k or it would of ate itself right there. I also realize that if I run it too lean it'll eat itself also. I have a new clutch assembly coming my way in the mail. I haven't removed the limiters yet. The only adjustments I've done: 1. Turned LA CCW to try to get the chain to quit running (it stalls before that). 2. Turned H CCW to stop (with limiters...out of the service manual). 3. Turned L CW to stop and then back 1/4 turn (with limiters...out of the service manual). I pretty much quit here because my readings didn't make sense. Again I'll replace the clutch either way. You know the sound you get when you're running out of gas and it leans out? Right before the saw cuts off. It started sounding like that all the time and with the chain running so that's why I'm messing with it.

ironman_gq: Every crank I've ever seen has them 180 degrees. An example would be for a 4 cyl. One cyl is at TDC on compression stroke. Another one would be TDC right after it pushed the exhaust out. Another one would be at BDC at the end of the power stroke, and the last one would be at BDC after pulling in new air/fuel mix. It's a pain for me to go to the dealer, and it won't idle low enough to stop the chain (new clutch on the way).


J.W Younger: I agree.

I'm still wondering why my tach won't work. It works on 4-strokes...I've used it too many times. Now I'm just wanting to know.

Stop. Do not fire it up, cut with it, try to adjust it, etc until you've done a pressure/vac test. See my sig for how.

You have an air leak someplace, fix it, then worry about making carb adjustments.

BTW, as has been stated, it's damn near impossible for someone to have enough time around chainsaws and be able to hear well enough to adjust by ear.

 

[wvengineer]

Stop. Do not fire it up, cut with it, try to adjust it, etc until you've done a pressure/vac test. See my sig for how.

This kind of sounds like a leak down test in reverse. Am I wrong? If not then I already have what I need for that.

 

[Paul001]

This kind of sounds like a leak down test in reverse. Am I wrong? If not then I already have what I need for that.

Pretty much. I'd guess that the clutch side crank seal is the issue.

Do not forget to pressure check the fuel pick up as well. Clamp off the fuel line inside the tank with hemostats, disconnect at the carb and pressurize the line.

Also check your AV mounts, I've seen two 361's where they are shot and the additional flex is enough to tear the carb boot.

 

[wvengineer]

Pretty much. I'd guess that the clutch side crank seal is the issue.

Do not forget to pressure check the fuel pick up as well. Clamp off the fuel line inside the tank with hemostats, disconnect at the carb and pressurize the line.

Also check your AV mounts, I've seen two 361's where they are shot and the additional flex is enough to tear the carb boot.

I'd just hate to think I'd be having these kinds of problems with a gently used saw that I bought new a year ago.

 

[Paul001]

I'd just hate to think I'd be having these kinds of problems with a gently used saw that I bought new a year ago.

Not so much about how gentle you ran the saw. Hell I've got customers who in general beat the snot out of their saws, and wonder how they survive for months, let alone the years they get.

You seem to be fairly well versed with things mechanical. As such you know, sometimes #### just happens.

 

[J.W Younger]

That is a thought, but at idle it shouldn't be an issue. I should be reading 1400 for the idle and say less than 7000 for WOT. Even ignoring WOT it should be fine for idle. The tolerance on the meter is +/- 3%. I do have a small engine shop (lawnmowers and such) down the street from me they probably have a tach designed for 2-strokes I could check mine against theirs.

don't know what I was thinking, deserve the dumb ass award for sure. I was x your readings when I should have devided by 2.
anyway,was thinkin that the normal place to connect tach leads is on the neg primary coil terminal which is dc on a car but prob ac on a chainsaw. this is one of them days I don,t trust my thinking.

 

[wvengineer]

anyway,was thinkin that the normal place to connect tach leads is on the neg primary coil terminal which is dc on a car but prob ac on a chainsaw.

That's not a bad thought. I tell you I messed with it a little again this evening. I swear I think this thing is reading on without dividing by two. Right now the idle sounds right when I set it at 2800 per the meter. At this base setting per the "book" at WOT it pegs right around 14k +/- about 30 rpm. Now I feel rather confident with my logic on the meter set to 4 cyl. So I'm just stumped about that. To sum up no dividing by two seems to be on. It starts quick...acceleration is smooth and quick...but the chain still runs at idle. I have the new clutch assembly (I bought the whole assembly online seemed cheap enough). I'll probably replace that tomorrow. Thanks for all the help and I'll update this if I figure anything else out.

 

[wvengineer]

Well I replaced the clutch this morning. The chain isn't running anymore with the idle @ 2800. I still don't understand why the meter appears to be on without multiplying by two. I may never understand that. Anyways everything seems right with it...runs like a top. Also when I had the spark plug out it didn't show that it's been running too lean. Thanks for all the help everyone.