Throttle Response

[Ketchup]

For clarity sake only (not trying to be a dick), no coil can advance timing. They can retard timing and then go back to a normal non-retard timing. So they are in fact only “advancing” timing by not retarding it at higher than idle rpm.

It’s effectively the same as advance for arguments sake, but for clarity with engine theory it’s actually as described above.

The dual magnet Flywheels you see on the newer computer saws have me wondering what’s possible. When a chip comes into the equation, and then the electronics to drive fuel injection…

What’s the verdict on 500i? Does it have good snappies?

 

[Al Smith]

I have no idea about how things are done on a so called modern saw .However on a modern automobile engine the pulse rate and length on a fuel injector and timing on individual spark plugs can be changing 20 times a second .It takes specialized equipment to even work on them and before too long I think it will be so on small engine equipment .

 

[drf256]

The dual magnet Flywheels you see on the newer computer saws have me wondering what’s possible. When a chip comes into the equation, and then the electronics to drive fuel injection…

What’s the verdict on 500i? Does it have good snappies?

Good point. Once there is another magnet there for power generation anything is possible. But in general, a coil can’t fire before the magnet gets to it.

 

[slackinoff]

Are there any saws that stick out that have fast throttle response right out of the gate?


My clone 440 I built has much better throttle response than most of the saws I have tried. I haven't ever timed the saw, but the compression has never been great at 155psi with base gasket delete. Also it doesn't care if it has the pencil exit 460 muffler on or the wide open dual port 440 muffler. It's just fast and is a strong cutting saw.

 

[huskyboy]

What’s the verdict on 500i? Does it have good snappies?

Imo the throttle response on a 500 is no better than a 462. At least with both stock/muffler modded.

 

[slackinoff]

Imo the throttle response on a 500 is no better than a 462. At least with both stock/muffler modded.

Yea those don't even sound like they have a chain on them, at least the videos I have seen. After work today I am going to finish advancing the ignition timing on my closed port 61 build to see if that will make a difference on response off idle.

 

[slackinoff]

I only got as far as popping the flywheel off. Darn thing was stuck, had to build a puller. Gonna have to finish Monday night.

 

[slackinoff]

Found some time before work and cut the key. I over shot it. It's measuring 9 degrees of rotational advance. Key was .1150, I cut the slot to .0750 so .040.


I'm still going to try it, but I have another key ordered. I will update more towards Monday when I am back in town.

 

[davidwyby]

My 2156 by @Benwa is snappy. So is my @Mastermind 6100. Stock 266s and 288s are snappy imo.


Aside from the porting science, I’m wondering about carbs/stock/carb work. I was trying to think how one might affect snappiness…it occurred to me that if the low circuit is not properly atomizing/vaporizing fuel (dribbling, etc.) it could still idle but maybe not snap well….?

 

[Woodslasher]

My 2156 by @Benwa is snappy. So is my @Mastermind 6100. Stock 266s and 288s are snappy imo.


Aside from the porting science, I’m wondering about carbs/stock/carb work. I was trying to think how one might affect snappiness…it occurred to me that if the low circuit is not properly atomizing/vaporizing fuel (dribbling, etc.) it could still idle but maybe not snap well….?

Idk a lot about carbs, but the non-muff modded 395 I've run was a bit slow off the get-go while the one with the gutted muff had far snappier response. That being said, the non-modded one was a wee bit torquier once it'd spooled up.

With the carb stuff, I know if your low end is too lean the saw will fall on it's face because it won't get enough fuel and if it's too fat it'll stumble and blubber because you're essentially choking it to death, so I could see "thicker" fuel affecting throttle response. Honestly I'd guess that throttle response is dictated by the carb's ability to flow the right amount of fuel to the cylinder at the right time, the transfer's ability to feed the right amount of fuel in the right amount of time, and the exhaust's ability to blow the burned fuel out as quickly and efficiently as possible. Pretty broad based, I know, but I'd say all of those factors play into the other. If you make the carb dump more fuel in but don't change the flow rates of the transfers/exhaust (assuming they were set correctly initially), you'll create a "too rich" sort of problem instead of improving the response.

 

[Michael Rupley]

You want a snappy saw? Increase compression, a lot.

 

[davidwyby]

On the idle atomization thing, I have seen it with 4 stroke engines…but there isn’t much of a way to see it on saws…maybe rig it up with the intake elbow off…hmm.

@Woodslasher my 395 was stock and even low compression was pretty snappy. Ported, it still is, or would be if it wasn’t in pieces in a box

@Michael Rupley that seems to be the case with the 288 I just put together stock other than BGD.

 

[Stump Shot]

could still idle but maybe not snap well….?

Yes, under certain conditions, lets just say the first low idle circuit is working perfectly and the second or third is obstructed. The saw will idle perfectly but fall on it's face when the throttle is depressed from that circuits failure to add to the mix more fuel as more air enters.

 

[davidwyby]

@Stump Shot I think that quote got a little mangled, but I follow, thanks. I need to become more of a student of 2S carbs. I hadn’t noticed they have progressive or multiple circuits. Not as simple as I thought.

 

[Stump Shot]

@Stump Shot I think that quote got a little mangled, but I follow, thanks. I need to become more of a student of 2S carbs. I hadn’t noticed they have progressive or multiple circuits. Not as simple as I thought.

They're staggered in a row, so as the throttle plate opens more the more lo speed jets open up. Different amounts by carburetor model, usually there's three, could be two or four as well depending.

Look at a throttle side of a carburetor and open up the throttle and notice the little holes in the bottom of the body that start to appear.

 

[davidwyby]

@Stump Shot yeah, I have seen them 4s carbs which I have worked on a fair bit over the years (“AMC/Jeep 258s run forever-terribly” unless you got good with the carb) but I hadn’t noticed them on 2s carbs, or didn’t remember…

 

[kneedeepinsaws]

IMO a properly tuned, functioning and good model carb is most responsible for throttle response. There are some stock saws with no port work that have instant throttle response, in which I don’t think It could get any better. Namely in my experience the 395xp and 3120 have both shown me perfect throttle response in stock form

 

[Al Smith]

In a lot of cases just a tad more rich on the low jet will improve the spool up .It's kind of a fine line. Too rich it will load up and too lean it falls a little until the high speed circuit kicks in .My 024 is just a bit too rich but it runs so well otherwise I'm not going to fool with it .Gutted muffler while it is a improvement can cause a little annoyance now and again .

 

[pbillyi69]

so when you port a saw and it needs carb work to take advantage of the porting. what work do you do to the stock carb? its not like you cand change the jet like on bike carbs. I know there is a lot of thing that affect overall performance. but on the small saw engines without swapping to a bigger carb, what do you do to them? drill the jets?

 

[huskihl]

so when you port a saw and it needs carb work to take advantage of the porting. what work do you do to the stock carb? its not like you cand change the jet like on bike carbs. I know there is a lot of thing that affect overall performance. but on the small saw engines without swapping to a bigger carb, what do you do to them? drill the jets?

Most carbs have adjustable H and L jets. Sometimes they need to be richened up, sometimes they need to be leaned out