Tuning with tachometer

[chainsawMal]

Why is a Tachometer (rev counter) needed for tuning?

 

When tuning with a tachometer, am i right that the manufactures recommended high revs should not be exceeded, even if the engine sounds better?

 

I have a cheap tachometer ordered and on its way!!

[blazer]

I will try to answer best I can. The problem with 2T motors is the oil is with the fuel so, if a 4T runs weak it's not a problem for the piston ( will burn valves plugs etc) as the engine oil is a constant - but with a 2T motor weak fuel equals less lub on the piston = possible siezure. Most 2T motors will rev well if a bit weak and will 4 stroke if a bit rich, so the idea is to set the H screw so at max revs the engine will start to 4 stroke, setting the max revs and this way you know oil is getting to the piston.

 

For example my MS260 will over rev a bit +14k but the H screw won't open any more so I need to remove the limiter cap to open a bit more.

 

Example 2, I fitted a twin port exhaust to my MS460 (at last) so I needed to remove the limited caps to set the revs down to 13.5k before it started to 4 stroke but under load working it stops 4 stroking and runs smooth again. This is because it needs more fuel under load, so I assume it's about right but I need to run it hot for a while and cut the motor under load and check the plug colour - it can change colour as it drops back to tickover.

 

Hope it helps:thumbup1:

[rxe]

Yup, don't exceed max revs, or it will probably melt. To be safe, I'm always at least 500 - 1000 under max revs.

 

One trick to be aware of with a tach: they are often slow to update. It is quite possible to have a saw that is running a bit lean, nail it, and by the time the tach has got the reading, it is already knackered. If you are going to try tuning, set the saw massively rich, get the first reading - it will be 8000 rpm or so, and there will be smoke everywhere. Give it a tiny tweak on the H screw and do it again - and it will creep up to 9000 - keep going like this until you're in right place.

 

Modern saws with rev limited coils can be really deceptive - lean as you like, but the revs stay the same. I'm glad most of my stuff dates from the 90s....

[chainsawMal]

So what does 4 stroking sound like as oppossed to 2 stroke. I guess your taking about 4 stroke engine sound for a car?

[spudulike]

The tech in all of this is you are using your carb settings as a rev limiter, when the saw gets up to near max revs, you want the carb to over fuel the engine and cause it to only fire every other stroke, hence the term "FOUR STROKE":thumbup: When you cut wood, the saw is under power and the four stroking clears and produces power.

 

It sounds like a flubbing noise or burbling, trouble is on the later saws is that they rev so damn high and also when fitted with blue limited coils, without a tach, they are damn difficult to tune.

 

The tach will allow you to tune the saw to a known level -I am usually 500 - 1000rpm down when I tune by ear, this equates to chain speed and a slower cut.

 

Refresh should be 0.5 sec or lower, 1 sec refresh is far too slow, there is a real trick to getting the tune correct on a limited coil machine - ask Terry Tibbs how his 372 is after I reset the carb from fresh:thumbup:

[chainsawMal]

The tech in all of this is you are using your carb settings as a rev limiter, when the saw gets up to near max revs, you want the carb to over fuel the engine and cause it to only fire every other stroke, hence the term "FOUR STROKE":thumbup: When you cut wood, the saw is under power and the four stroking clears and produces power.

 

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So if your not doing big cuts all the time, then the above is not a good idea?

[GardenKit]

So if your not doing big cuts all the time, then the above is not a good idea?

Wrong, any cutting, not just big cuts, loads it enough to bring it back to two stroking.

[spudulike]

Wrong, any cutting, not just big cuts, loads it enough to bring it back to two stroking.

 

Thanks Barrie - yup, my description is the gospel according to Spud, abandon all who deviate from it as your pistons will fry:thumbdown:

[GardenKit]

I expect someone wll be along to disagree shortly Spud.

[spudulike]

I expect someone wll be along to disagree shortly Spud.

 

Yes, that will be typical but what do they know:001_rolleyes: