Modifying M-Tronics or Autotune saws...

[XSKIER]

I thought that I had read on here that someone did bore an AutoTune carb? Something about it not running right afterwards, and ultimately being replaced. Can anyone else remember?

 

[MCW]

Look out you will get hooked.

Already own an M-Tronic Sorta hooked already.
Can't wait for a 3120XP-AT...

 

[mweba]

I thought that I had read on here that someone did bore an AutoTune carb? Something about it not running right afterwards, and ultimately being replaced. Can anyone else remember?

All of mine are "bored" with shafts and screws thinned.


Been playing with the timing and flow allowed through the secondaries on the "beater" 62. Not sure there is anything there but its fun to see how it reacts :msp_confused:

 

[SawTroll]

C'mon Troll,

you know the reason they are going to AT/MT?

Keeps everything in check emissions wise.

.....

Of course that is the main reason - in the long run it is getting rather pathethic with "adjustable" carbs that are set lean from the factory, and they have to do their best to keep people from actually adjusting.

As a bonus fewer costumers are likely to fry the engines, and fewer costumers will be unhappy because they are running saws that are not adjusted right.

The downside is of course the introduction of more electhronics (that can fail) - and (so far at least) the compatibility towards modifications....

 

[SawTroll]

I thought that I had read on here that someone did bore an AutoTune carb? Something about it not running right afterwards, and ultimately being replaced. Can anyone else remember?

Yes, Brad did that - but of course we don't know exactly what went wrong (or why), so it is a bit early to conclude on the matter - I believe.....

 

[Chris-PA]

One drawback would be the characteristics of the carburetor itself, Venturi and bore size. The original 365 zama could feed my saw just fine. Perhaps the walbro hd12 even better. The tillotson from a 385 maybe better. A walbro off a 394xp.... Etc etc etc. it changes the way the fuel is delivered, the effort required and whole bunch of other stuff I don't really know about. The smaller carb sure can supply it. But it runs better with a bigger carb on there. Can't do that with the AT and Mtronic. Perhaps modify the carb, but not replace it.

I'd say it's all about cfm, just like on any engine. If the venturi can flow the air the engine can pump then it is not a restriction, but how many have flow benches to test that out? Modders are left to guess, and just like with cars I bet a lot of folks put on way too big a carb. I don't know if anyone has cfm numbers for various venturi bore diameters for carbs of this size.

 

[Philip Wheelock]

I suspect the big advantage of crankcase FI would be if it gets a competitor around Husky's patents on strato and AT.

Potential downside of FI is having to pressurize the system before each start, which would seem like a step backwards to me.

 

[SawTroll]

I suspect the big advantage of crankcase FI would be if it gets a competitor around Husky's patents on strato and AT. I doubt it would work much different.

It wouldn't surprice me at all if that is what it really is about - I have been playing with the same though, as I doubt Stihl is very happy with the current situation.

 

[Chris-PA]

Potential downside of FI is having to pressurize the system before each start, which would seem like a step backwards to me.

I suppose if you could time the injection for after the air has started moving up the transfers then it might work a lot like strato, and of course you could run some similar feedback control, maybe also with a lean-out test system. But you would need more mechanical complexity and higher pressures. A venturi is actually a very simple and effective device - an elegant design solution that is hard to beat. Maybe they could make the pump and injector very simple and get the costs below what they have to pay in royalties.

 

[TK]

Potential downside of FI is having to pressurize the system before each start, which would seem like a step backwards to me.

If that's all you can reach for as a negative against FI then it sounds like a great system to me. Couple primer pumps and away you go.

I'd say it's all about cfm, just like on any engine. If the venturi can flow the air the engine can pump then it is not a restriction, but how many have flow benches to test that out? Modders are left to guess, and just like with cars I bet a lot of folks put on way too big a carb. I don't know if anyone has cfm numbers for various venturi bore diameters for carbs of this size.

There definitely comes a point where the carburetor is just too big, but from a modding perspective the stock carb is typically smaller than ideal.

I had not thought of that - with traditional saw carbs the mixture will get so rich it limits the max no load WOT rpm. With MT/AT they really needed some other way to limit the rpm.

AT/MT carbs can get so rich it will slow the saw down. It's no different than the non-AT saws out there with limited coils. Those carbs will go so rich that it will prevent the saw from hitting the limiter. But most saws with the limited coils when set properly will hit the limiter, just part of the parameters.

 

[Chris-PA]

AT/MT carbs can get so rich it will slow the saw down. It's no different than the non-AT saws out there with limited coils. Those carbs will go so rich that it will prevent the saw from hitting the limiter. But most saws with the limited coils when set properly will hit the limiter, just part of the parameters.

That is not making sense to me. A traditional carb will give an increasingly rich mixture with increasing WOT rpm, to the point where the mixture is so rich the saw will not rev any higher.

The saw manufacturer works out a good mixture setting under load, and then sees what no-load WOT rpms that results in. Then they tell you what that number is to use as a setting - not because it is a useful setting in itself, but because it corresponds to some particular fuel/air mixture under load. It's just a way to duplicate their setting under load.

I suspect that the need for limiting coils prior to AT/MT was because they were setting mixtures leaner than before, which could have resulted in dangerously high no-load WOT rpms, but then I'm not sure how you set those.

With AT/MT the mixture should be controlled to a relatively constant fuel/air mix regardless of rpm, so the rich mixture will not limit rpms. Hence the need for the limiting coils.

 

[TK]

That is not making sense to me. A traditional carb will give an increasingly rich mixture with increasing WOT rpm, to the point where the mixture is so rich the saw will not rev any higher.

The saw manufacturer works out a good mixture setting under load, and then sees what no-load WOT rpms that results in. Then they tell you what that number is to use as a setting - not because it is a useful setting in itself, but because it corresponds to some particular fuel/air mixture under load. It's just a way to duplicate their setting under load.

I suspect that the need for limiting coils prior to AT/MT was because they were setting mixtures leaner than before, which could have resulted in dangerously high no-load WOT rpms, but then I'm not sure how you set those.

With AT/MT the mixture should be controlled to a relatively constant fuel/air mix regardless of rpm, so the rich mixture will not limit rpms. Hence the need for the limiting coils.

I can see where you're coming from and understand your point - I think. I was referring to the capability of the AT carb saws to go so rich that it would reach max RPM then start to decline if enrichened further. Not that it will actually happen, but the carb can support the fuel needs.

As far as dangerously high no load speeds with a non-limited coil, I don't think there's any big concern there. For a non AT saw it's easy to see the difference. Take a saw that will max out WOT at let's say 15k rpm on a non-limited coil. Pop a limited coil on it and with the same exact carb settings it's going to hit the limiter and not reach that same RPM, but the saw isn't going to be run at any dangerous speeds with or without a limiter when tuned properly. The in the cut tuning is the same for each coil. The same thing applies for a AT saw. If it were even possible to put a non-limited coil on a AT saw, you'd find that it would max out with a rich 4stroke scenario out of the wood at a designated RPM, probably no higher than a non-AT saw.

It sounds to me like you're implying that if these saws were not limited via the coil that they would rev out to 18k rpm out of the wood or something. That would be way too lean and in the cut it would suffer from a lack of fuel. I don't think you'll find a saw that is capable of 18k rpm that loses half it's speed when a load is applied with a proper tune. Maybe I'm not understanding you correctly but that's how I'm reading it.

 

[Chris-PA]

It sounds to me like you're implying that if these saws were not limited via the coil that they would rev out to 18k rpm out of the wood or something. That would be way too lean and in the cut it would suffer from a lack of fuel. I don't think you'll find a saw that is capable of 18k rpm that loses half it's speed when a load is applied with a proper tune. Maybe I'm not understanding you correctly but that's how I'm reading it.

Well, kind of - of course the ignition and the control for the AT are not really separate and it won't let it rev like that, but the part I bolded above is not really true for AT/MT. That is what a traditional carb with its air-flow dependent poor mixture control would do, but one of the main purposes of these systems is to get away from that major defect and give a mixture that is constant with air-flow / load. So the mixture does not get richer at no-load and leaner in the cut - it stays the same (and better than that it stays optimal) due to feedback.

So in a sense, these systems are different from traditional saw carbs in two ways - one is a carb that can hold a constant mixture (which chainsaws have never had, unlike lawn mowers), and the other is feedback system to keep the mixture optimal.

Once you move to a carb (even non-feedback) that can hold a constant fuel/air mixture, then there is nothing to limit no-load WOT rpm anymore - except maybe venturi/bore size, and that's not much of a limit. That's why no other engines do no-load WOT tuning - they would explode.

Anyway, I lost the thread and don't remember why the heck I was even on that train of thought.:msp_unsure:

 

[TK]

Well, kind of - of course the ignition and the control for the AT are not really separate and it won't let it rev like that, but the part I bolded above is not really true for AT/MT. That is what a traditional carb with its air-flow dependent poor mixture control would do, but one of the main purposes of these systems is to get away from that major defect and give a mixture that is constant with air-flow / load. So the mixture does not get richer at no-load and leaner in the cut - it stays the same (and better than that it stays optimal) due to feedback.

So in a sense, these systems are different from traditional saw carbs in two ways - one is a carb that can hold a constant mixture (which chainsaws have never had, unlike lawn mowers), and the other is feedback system to keep the mixture optimal.

Once you move to a carb (even non-feedback) that can hold a constant fuel/air mixture, then there is nothing to limit no-load WOT rpm anymore - except maybe venturi/bore size, and that's not much of a limit. That's why no other engines do no-load WOT tuning - they would explode.

Anyway, I lost the thread and don't remember why the heck I was even on that train of thought.:msp_unsure:

Ok I see where you're coming from now. That makes sense. I was going at it from the opposite direction, just had to flip my thinking around. Don't worry what I said makes sense in my head lol.