Look out you will get hooked.
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?
C'mon Troll,
you know the reason they are going to AT/MT?
Keeps everything in check emissions wise.
.....
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?
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.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 suspect the big advantage of crankcase FI would be if it gets a competitor around Husky's patents on strato and AT.
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.
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.Potential downside of FI is having to pressurize the system before each start, which would seem like a step backwards to me.
Potential downside of FI is having to pressurize the system before each start, which would seem like a step backwards to me.
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.
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.
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.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.
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.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:
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