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Me and my dad had just gotten done rebuilding my Chevy 350. I had invested the money in buying a comp cams, thumpr cam. When tearing into the engine I had to have a valve job done on the stock cast iron heads. I did install a new Mellings high pressure high volume oil pump to help out with the cam. The truck also has stock pistons, distributor, crank, and tbi. We have had my truck running quite a few times before, but every time it had been unsuccessful in its idle. We have tried adjusting the distributor to set the idle but it seems to have no affect. My dad had suggested that I make the switch to an HEI distributor to eliminate the computer crap. I was wanting to know if the HEI distributor would be able to help my idle issues, or if I would have to also switch to a different tbi. The truck runs good while driving it, but once I let off of the gas pedal, it wants to bog down and die. I guess the other question would be if I had to adjust the idle on my stock tbi.
 

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Me and my dad had just gotten done rebuilding my Chevy 350. I had invested the money in buying a comp cams, thumpr cam. When tearing into the engine I had to have a valve job done on the stock cast iron heads. I did install a new Mellings high pressure high volume oil pump to help out with the cam. The truck also has stock pistons, distributor, crank, and tbi. We have had my truck running quite a few times before, but every time it had been unsuccessful in its idle. We have tried adjusting the distributor to set the idle but it seems to have no affect. My dad had suggested that I make the switch to an HEI distributor to eliminate the computer crap. I was wanting to know if the HEI distributor would be able to help my idle issues, or if I would have to also switch to a different tbi. The truck runs good while driving it, but once I let off of the gas pedal, it wants to bog down and die. I guess the other question would be if I had to adjust the idle on my stock tbi.
u need to have the ecm tunned completely or it will never run right

check out thirdgen.org/techboard and look in the diy prom section for more info

if thats beyonjd what u can do theres a few members there that run tuning services for the older gm efi stuff that can help u out
 

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Me and my dad had just gotten done rebuilding my Chevy 350. I had invested the money in buying a comp cams, thumpr cam. When tearing into the engine I had to have a valve job done on the stock cast iron heads. I did install a new Mellings high pressure high volume oil pump to help out with the cam. The truck also has stock pistons, distributor, crank, and tbi. We have had my truck running quite a few times before, but every time it had been unsuccessful in its idle. We have tried adjusting the distributor to set the idle but it seems to have no affect. My dad had suggested that I make the switch to an HEI distributor to eliminate the computer crap. I was wanting to know if the HEI distributor would be able to help my idle issues, or if I would have to also switch to a different tbi. The truck runs good while driving it, but once I let off of the gas pedal, it wants to bog down and die. I guess the other question would be if I had to adjust the idle on my stock tbi.
The TBI and the ignition are under computer control. The system uses a programming formate caller Manifold Absolute Pressure or (MAP). It computes fuel delivery based on relationships of manfold pressure (which you can think of as manifold vacuum) to RPM, to throttle positIon. It takes sensor data from these three primary sensors along with several others that are secondary fuel trimming sensors from which it computes an entry number. With the entry number it then looks at table called a map to get a number that is then referenced to a fuel delivery rate and an ignition advance. What the computer is doing for the fuel delivery is calculating the air flow into the engine, it does not read air flow directly. There are direct reading systems which use the term Mass Air Flow or (MAF), TBI engines do not use MAF sensing it is an expensive systems largley restriced more costly and/or performance oriented vehicles of that time period.

Anything you do that affects mass air flow ability of the engine will to some extent throw MAP, TBI systems out of adjustment for a little to a lot. Headers, or dual exhaust, changes in compression, a cam with more than about 210 degrees duration measured at .050 inch lift, changes to valve lift due to cam or rocker changes, changes in thermostat temperature, addition of cold air induction, changes to or replacement of the cylinder heads, or elimination of EGR all will upset the TBI computing system.

The biggest thing is the cam change. Since these engine use a very mild cam (about 170 degrees of duration at .050 lift and less than .4 inch of valve lift) which produces a high manifold vacuum; changing the cam, changes the manifold vacuum in relation to the throttle position and RPM. This causes the computer to calculate a map lookup number that is wrong for the current operating condition of the engine. The only solutions are to either put it back the way it came from the factory, or to put a custom reprogrammed chip in the computer. If you go with a custom chip this guy can help a lot <<< TBICHIPS Home | Harris Performance, INC >>>. Before you do that you may want to consider L31 Vortec heads, the Swirl Port heads that cam on that engine are very resistant to power improvements anywhere else in the engine. They are intended for a working low RPM truck engine to where the vane in the valve pocket pretty much restricts the RPM limit to about 4500.

Bogie
 

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It actually has hei, just not the big cap dis. You can't run a different distributor with factory tbi, your injectors will not fuel if it does not have a signal coming off your distributor.

Also since it is computer controlled, unless the timing is were it needs to be stock, the computer will try and compensate to an extent, it's expensive to make any real power with factory tbi, the only way to really take the computer out of the equation is to go carb
 

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I have considered going carb, but my dad says that tbi is the way to go. My truck will not only be a daily driver, but I want to put it through mud runs. I feel that it will be better running tbi over carb due to a carbs bowl and tbi being able to go through without relying on a float to tell its fuel level.
 

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Well tbi and efi make the tuning aspect much simpler, they adjest themselves to the conditions automatically, no worries about running rich or lean and having to rejet or adjust floats and whatnot. You could check out some of the links these guys put up and i believe holley makes aftermarket throttle bodys and the intakes for them that are supposed supposed to hook into your factory harness. A little pricey, nut it all depends on how far you want to go with it as to how much your going to spend, that's why alot of people go with a carb and dump the tbi.
But like these guys said, those truck heads aren't worth much of anything, youll get a good bang for your buck throwing on some vortec heads.
What cam did you get specifically?
 

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Discussion Starter #7
cam

I got the Thumpr, the little cam. Not the muthr thumpr, or big muthr thumpr. Just the basic choppy idle and street performance cam. It is in a 1990 350 engine block, with tbi obviously.
Or do you want the specs?
 
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