Throttle Response

[pbillyi69]

i understand that. what i am talking about is when those arent enough. say the port work is more than the high jet will allow for the most performance.

 

[huskihl]

i understand that. what i am talking about is when those arent enough. say the port work is more than the high jet will allow for the most performance.

That “usually” doesn’t happen. Faster flowing air through the venturi pulls more fuel through the nozzle. But occasionally jets need to be drilled out larger or filled in and drilled smaller

 

[huskihl]

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

And some carburetors have numbered fixed mains that flow a certain percentage of fuel regardless of the adjustable jet settings. Some of those fixed mains can be replaced with a larger or smaller jet if they are still available

 

[pbillyi69]

ok thanks for the response.

 

[Sloughfoot]

A lot of good info in this old throttle response thread. My newly ported saw gained a huge amount of torque and rpm but lacks blip. Intake timing moved 8 degrees and exhaust widened a little on the piston side and a lot on the muffler side. I didn't spend much time with idle and low jet because it didn't seem to care. You all have me thinking I should try going rich as it can stand with the low jet. I'll be removing the cylinder after I've run it a little to replace the oldish oem piston with a new oem piston and rings. Also want to make the smooth exhaust port shiny before calling it done. Anything else i should do while the cylinders off to get thrilling throttle response if low jet tuning doesn't do it?

 

[Stump Shot]

A lot of good info in this old throttle response thread. My newly ported saw gained a huge amount of torque and rpm but lacks blip. Intake timing moved 8 degrees and exhaust widened a little on the piston side and a lot on the muffler side. I didn't spend much time with idle and low jet because it didn't seem to care. You all have me thinking I should try going rich as it can stand with the low jet. I'll be removing the cylinder after I've run it a little to replace the oldish oem piston with a new oem piston and rings. Also want to make the smooth exhaust port shiny before calling it done. Anything else i should do while the cylinders off to get thrilling throttle response if low jet tuning doesn't do it?

One nice thing from running the low-speed mixture a frog hair to the rich side is after running hard in a cut, it gives the motor a little bit of cool down as well as lubricating everything inside good. As long as you don't go too far so as to have a mosquito fogger when you go to cut again, it should be good. I especially like doing this to a saw I've intentionally leaned out, like for a GTG saw race. Otherwise for working I like to hear it four stroke but good the instant the load is taken off the motor. That seems to be as much work power as can be had and your motor running with the correct fuel to air ratio, to not be either rich or lean. It's also a very simple and super easy thing to check from day to day, all you have to do is lift up from a cut and go back into it.

 

[Sloughfoot]

One nice thing from running the low-speed mixture a frog hair to the rich side is after running hard in a cut, it gives the motor a little bit of cool down as well as lubricating everything inside good. As long as you don't go too far so as to have a mosquito fogger when you go to cut again, it should be good. I especially like doing this to a saw I've intentionally leaned out, like for a GTG saw race. Otherwise for working I like to hear it four stroke but good the instant the load is taken off the motor. That seems to be as much work power as can be had and your motor running with the correct fuel to air ratio, to not be either rich or lean. It's also a very simple and super easy thing to check from day to day, all you have to do is lift up from a cut and go back into it.

Those are some good indicators I haven't heard before. I'll give them a go soon as it sounds like the neighbors are done with their coffees. I've found out the low and high jet tune has a wider range on a ported saw. It'd die before going lean enough with the low to engage the clutch and with the high adjustment it'd sound good in the cut but had to be very rich to 4 stroke out of it. Stock, it had a more definite point with both.
Would widening the exhaust a bit more help or hurt? There's room for it.

 

[Stump Shot]

Those are some good indicators I haven't heard before. I'll give them a go soon as it sounds like the neighbors are done with their coffees. I've found out the low and high jet tune has a wider range on a ported saw. It'd die before going lean enough with the low to engage the clutch and with the high adjustment it'd sound good in the cut but had to be very rich to 4 stroke out of it. Stock, it had a more definite point with both.
Would widening the exhaust a bit more help or hurt? There's room for it.

Widening the exhaust port helps with performance, four stroking will be the same pretty much.

 

[mainer_in_ak]

Nother variable on throttle response:
In the wood can be different than out of the wood.

I have a 590 that comes on step in the cut quicker than my 620pw. But doesn't get up to as high of rpms as the 620. I realized this only recently, stopping in stump cuts, to place wedges. Bar was burried on both saws. Was running both saws back to back.

Out of the cut just standing there bliping the throttle like a dumb ass 12 yr old boy, my 620 has better throttle response.

The exuahst roof is much lower on the 590, as is the upper transfers. The carb is smaller. It has more compression than my 620.

 

[Sloughfoot]

Nother variable on throttle response:
In the wood can be different than out of the wood.

I have a 590 that comes on step in the cut quicker than my 620pw. But doesn't get up to as high of rpms as the 620. I realized this only recently, stopping in stump cuts, to place wedges. Bar was burried on both saws. Was running both saws back to back.

Out of the cut just standing there bliping the throttle like a dumb ass 12 yr old boy, my 620 has better throttle response.

The exuahst roof is much lower on the 590, as is the upper transfers. The carb is smaller. It has more compression than my 620.

I'll test this one in the cut and out. Do you remember the numbers on your exhaust and transfer roofs? This one is at 104ish and 124ish.

 

[mainer_in_ak]

No idea on the 590 numbers:
base gasket delete/ 170 psi of compression
Muffler opened up.
Walbro 86-587-1 nozzle
Ignition adavnced.

Tunes up ah smidgen rich 13,200 rpms.

620:
Stock, muffler opened up. 160 psi compression.
Tunes up ah smidgen rich at 13600 rpms

 

[Sloughfoot]

No idea on the 590 numbers:
base gasket delete/ 170 psi of compression
Muffler opened up.
Walbro 86-587-1 nozzle
Ignition adavnced.

Tunes up ah smidgen rich 13,200 rpms.

620:
Stock, muffler opened up. 160 psi compression.
Tunes up ah smidgen rich at 13600 rpms

You reckon it's the lower exhaust and transfer roofs and higher compression getting the quicker response with the 590 in the cut? I got 160 compression with a base gasket delete with this one. Now it's showing 170 with .030 off the squish band and the base cut to get back to .020 squish. Maybe go up 5 to 10 more, I think. I didn't measure max rpm but it has to be well over 14k, maybe over 15k judging by how it sounded at 12.5k in the cut.
May need a longer bar to tame her down a little. I'll make that purchase when accounting has her guard down.