So in a big gay build thread Sputnik40 made this IH8MUD.com Forum - View Single Post - Big Gay 2F Build Tech post. Here is what you need to know to get you up to speed.
--snip--
Joey G. - Another warm Mud welcome
. If you can take it, you're more than welcome to dish it! The SM420 problem is due to the wide spacing between 2nd and 3rd (I honestly forget right now). You have to rev out 2nd to a higher RPM in order to get any torque/hp (for lack of a better description) out of 3rd gear, otherwise it lugs (lower diff gears help, but not that much). This has been a well documented problem, since way back in the day when Mike Smythe (Spaceghost) and others were breaking ground on FI. I think the route you're going would be the most viable - but like I said - I like keeping it simple. I spend all my time chasing women, working, fishing, surfing, working on the house, Land Cruiser, Land Rover(s), or the boat, spending time with my 16 year old, and drinking beer with friends (and chasing women) - I don't have time to toy with computers and the more I age - the more I think I like Ludditism 
. If you are successful, and want to start a thread on this - I'd consider swapping someday when someone maps (no pun) the best and cheapest way to go MS.
Till then - it runs sideways and upside down, so no complaints!
Finally - nice to meet someone who types waaaaay more than me!
--snip--
OK I am by no means an expert here but I have a hunch the reason you run into this problem is because the GM ECU has AFR tables that are not right for you setup. I am guessing that your O2 sensor is a narrow band which only tells the ECU if it is above or below 14.7. Given the other inputs that the ECU is reading it is putting you in a bad place on the AFR table. I think that if you put a wide band O2 sensor in the exhaust and did some data logging you would find the AFR is way off when you are experiencing this issue. This is the Wide Band O2 setup I use with my Megasquirt. Innovate LC-1 Digital Wideband o2 Controller with Bosch LSU4.2 Sensor DIYAutoTune.com
The Megasquirt can adjust the AFR table on the fly when it is connected to a WB-O2 sensor. Not sure if the GM ECU can be connected to a WB-O2 sensor and if it will automagically adjust the AFR table on the fly. You would have to install the WB-O2 sensor next to the NB-O2, data log the AFR with a laptop, hopefully find someone who can use the data to make the necessary adjustments to your AFR table on the GM ECU. At 200 bucks for the WB-O2 and all the hassle you need to go through just to see if this might be the cause, it's not worth the effort.
On the plus side you already got 90% of the pieces in place so you can make the jump to the MS any time you want with not too much effort.
"the more I age - the more I think I like Ludditism" Same here man, same here.
--snip--
Joey G. - Another warm Mud welcome
. If you can take it, you're more than welcome to dish it! The SM420 problem is due to the wide spacing between 2nd and 3rd (I honestly forget right now). You have to rev out 2nd to a higher RPM in order to get any torque/hp (for lack of a better description) out of 3rd gear, otherwise it lugs (lower diff gears help, but not that much). This has been a well documented problem, since way back in the day when Mike Smythe (Spaceghost) and others were breaking ground on FI. I think the route you're going would be the most viable - but like I said - I like keeping it simple. I spend all my time chasing women, working, fishing, surfing, working on the house, Land Cruiser, Land Rover(s), or the boat, spending time with my 16 year old, and drinking beer with friends (and chasing women) - I don't have time to toy with computers and the more I age - the more I think I like Ludditism . If you are successful, and want to start a thread on this - I'd consider swapping someday when someone maps (no pun) the best and cheapest way to go MS.Till then - it runs sideways and upside down, so no complaints!
Finally - nice to meet someone who types waaaaay more than me!
--snip--
OK I am by no means an expert here but I have a hunch the reason you run into this problem is because the GM ECU has AFR tables that are not right for you setup. I am guessing that your O2 sensor is a narrow band which only tells the ECU if it is above or below 14.7. Given the other inputs that the ECU is reading it is putting you in a bad place on the AFR table. I think that if you put a wide band O2 sensor in the exhaust and did some data logging you would find the AFR is way off when you are experiencing this issue. This is the Wide Band O2 setup I use with my Megasquirt. Innovate LC-1 Digital Wideband o2 Controller with Bosch LSU4.2 Sensor DIYAutoTune.com
The Megasquirt can adjust the AFR table on the fly when it is connected to a WB-O2 sensor. Not sure if the GM ECU can be connected to a WB-O2 sensor and if it will automagically adjust the AFR table on the fly. You would have to install the WB-O2 sensor next to the NB-O2, data log the AFR with a laptop, hopefully find someone who can use the data to make the necessary adjustments to your AFR table on the GM ECU. At 200 bucks for the WB-O2 and all the hassle you need to go through just to see if this might be the cause, it's not worth the effort.
On the plus side you already got 90% of the pieces in place so you can make the jump to the MS any time you want with not too much effort.
"the more I age - the more I think I like Ludditism" Same here man, same here.
