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Discussion Starter · #1 ·
im new here.i have a 74 model 350 bored .30 over 882 heads .125 dome pistons a weind stealth intake computer compatible cam throttle bodie adapter plate all of wich is in a 94 s 10 it starts right up with a choppy idle but its flooding and im not sure why.when i did the break in on the motor all was well untill it got to running temp then the idle went bad.it would drop down to about 200 rpms and then up 600 back and fourth so im not sure whats goin on . any advice would be appreciated
 

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Discussion Starter · #3 ·
thanks for the input.I just dont understand why it would start perfect idle fine real crisp and clean throttle response and just idle like crap after it gets up to running temp.And even then it still has great throttle response
 

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tbi355 said:
im new here.i have a 74 model 350 bored .30 over 882 heads .125 dome pistons a weind stealth intake computer compatible cam throttle bodie adapter plate all of wich is in a 94 s 10 it starts right up with a choppy idle but its flooding and im not sure why.when i did the break in on the motor all was well untill it got to running temp then the idle went bad.it would drop down to about 200 rpms and then up 600 back and fourth so im not sure whats goin on . any advice would be appreciated
More definition, the title refers to a "TBI355", then you describe something else built on a 74 block. So what exactly is this thing?

The body is a 94 S10, what came in it?

Apparently you've tried to take that TBI and use it on a 355. is this correct?

Since 94's at best had a 4.3 V6 the leap to a 5.7 plus V8 is a big one and the chip doesn't understand what happened. These computers are really stupid, they understand engine sensor inputs and the size of the original engine. As long as sensor inputs of RPM, throttle blade position, and manifold vacuum (what GM is pleased to call Manifold Absolute Pressure (MAP) for the engine size, transmission type and overall gear ratio (read that parts with gears plus tire size), and road speed are for the original truck configuration the Block Code Number will be wrong. The sensor data is integrated into a Block Code Number for the vehicle type in the computer chip. The computer's chip takes the Block Code Number and looks for it as close as it can come to on a map for fuel flow and ignition timing for the original vehicle. An aside as people get all excited about this, the computer has a Block Code Learn capability thru the O2 sensor to make some corrections, but this is quite limited in scope and was only intended to allow correction in the mixture and timing for production tolerances and local environmental condiitons and fuel quality variations. So this doesn't have the scope necessary to fix big increases in displacement, volumetric efficiency, or changes in vehicle type, or gearing.

If you change the engine size, and or the cam timing, and or anything else that upsets the volumetric efficiency of the engine, and or changes the transmission or overall gear ratio; the sensors are now sending data that generates the wrong Block Code Number for the conditions. The computer takes that incorrect number and pulls down the fuel and ignition maps for that number. It makes no difference to the computer if that number is wrong for the real conditions 'cause it just responds to the integrated sensor data for the vehicle it was originally programmed to.

So when the engine is bigger the total mass flow is increased, the sensors aren't even looking for that, they compute it based on manifold vacuum, throttle position, and RPM. If a bigger cam is put in the manifold vacuum goes down without a change in throttle position and may, or not, have a change in RPM. Better flowing heads, intake, and exhaust increase the mass flow thru the engine but by themselves don't change the throttle position or RPMs. Changing overall gear ratios change the mechanical advantage the engine has on the load it has to move. This changes the throttle blade position, which in turn changes the relationship of manifold vacuum for the RPMs, vehicle speed, and throttle blade position. You should get the idea of what you're up against by this point.

More likely than not is you're set up is under fueling, it runs nice when cold because the computer is in cold start rich, when the engine warms the choke enrichment stops with the resulting mixture being too lean for the mass flow change. You'll at lest need a new chip. But you need to come clean about all you did so we can see if that's the only thing you'll need as there are subtle changes in sensors and data forms from year to year and some combinations of parts don't work together.

Bogie
 

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Discussion Starter · #5 ·
truck had tbi 4.3 stock.computer throttle bodie and all sensors are off of a 94 1500 350.tranny is stock...Its running rich a start up.Also i wander if i need to run like a 160 thermostat?We have gotten alot of rain here so road conditions are bad so i have not been able to get the truck to the exhaust shop so i dont have my o2 sensor installed and i know this will have somthing to do with the engines running habbits.Rearend is also stock
 

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tbi355 said:
truck had tbi 4.3 stock.computer throttle bodie and all sensors are off of a 94 1500 350.tranny is stock...Its running rich a start up.Also i wander if i need to run like a 160 thermostat?We have gotten alot of rain here so road conditions are bad so i have not been able to get the truck to the exhaust shop so i dont have my o2 sensor installed and i know this will have somthing to do with the engines running habbits.Rearend is also stock
There ya' go, the 4.3 computer is not programmed to feed a 350. The sensors only feed a set of conditions, actually the sensors on a 4.3 TBI are the same ones on a 305, 350, or 454 TBI. The program that makes each computer unique for the engine is on a PROM (Programmable Read Only Memory) chip inside the computer's case. The chip needs to also know the tranny and the gear ratios in the rear end and tire size. You might be able to get away with a 350 TBI computer, but given the total changes you really should have a custom chip made. Lot of places do this, here's some good ones:

http://www.fastchip.com/

http://www.tbichips.com/

http://www.superchips.com/

http://www.tpis.com/

160 degree thermostat is really hard on the engine especially when you live in a cooler climate, yes guys who want to cheap it out up front use it to force the computer to stay in choke mode making the engine run rich. Problem with this is too much fuel gets blown into the crankcase, this is really hard on cylinder-walls and piston rings and since the oil never gets hot enough to boil the waters of combustion out that also get in there with the blow by, the oil turns acid quite quickly and starts to attack the softer metal parts like bearings and pistons.

Right now with out the O2 sensor the computer is running open loop instead of closed loop. The only difference this makes is the Block Learn and the correction for mixture errors once the engine is warmed up is not happening, it's running on the default program which is always running in the background when the engine is operating, the O2 is just a trim-it-up nice and clean function.

Bogie
 
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