Question on BLM and INT values when engine is idling.

I have a 1978 chevy k20 with the 350. I just put a melling ccs-2 camshaft into it. Duration at 050 inch Lift:194 int./202 exh. Anyway when the engine is first started and warming up the throttle response is great. Once its warmed up and in closed loop it stumbles when I give it gas. If im slow on the throttle I can get it to rev up but if I press the throttle with any speed it bogs and stumbles. I assume this is because it need more fuel. When idling hot the INT is 128 but the BLM is 155. Right now it has a bone stock AFBW bin with only the egr turned off. What can I initially change so the engine runs better so I can start getting it on the road for tuning. Right now it barely moves when in gear when giving it gas.

BLM's above 128 are lean. BLM's below 128 are rich.

I'm guessing the TBI computer 1227747?

Base Pulse Width (BPW) is a very important parameter. For example, increasing BPW from 135 to 137 will lower BLM's from 155 to a number a little closer to 128.

dave w

Should i mess with the BPW or should i start trying to tune the fuel tables and leave the BPW alone?

BPW is course tune. Ideally adjusting the BPW so some fuel table cells are rich and some fuel table cells are lean is a good plan.

Ok. I will alter the BPW to try and get some rich and lean fueling then start messing with ve and spark.

I'd agree, start with BPW until the BLM are closer to 128 then start fine tuning.

You may need to mess with the accel enrich vs TPS too once you start getting the BLM dialed in. This gives it a "pump shot" as the throttle is pressed.

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Originally Posted by lionelhutz

I'd agree, start with BPW until the BLM are closer to 128 then start fine tuning.

You may need to mess with the accel enrich vs TPS too once you start getting the BLM dialed in. This gives it a "pump shot" as the throttle is pressed.

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ok. thanks

Alright. I changed the spark table to a starter table more appropriate for my heads. I changed the BPW to 155 with the engine all warmed up and in closed loop. This got my BLM and INT right at 128 idling. I also tripled the AE-TPS PW and the AE-MAP PW. The engine is very responsive now and I can mash the gas and it revs right up. My concern is that according to the formula a BPW of 135 should be appropriate for a 5.7l engine. Why do i have to set my BPW so much higher at 155 to get my BLM to sit at 128? Moving up 20 values seems like a lot but maybe its not. Im just wondering if its a fueling issue. I made my pressure regulator adjustable and last I checked it was holding a steady 14psi but this was probably years ago. Im just concerned moving the BLM to 155 might point to a fuel pressure issue?

to ensure you have appropriate fuel flow, you should verify your fuel pressure is totally unchanged (with no vacuum reference) or does not decrease as load increases (with vacuum reference) when you are at heavy load and high RPM. you definitely don't want to tune around a fuel supply issue.

you also need to verify that your injectors are clean, your map sensor, coolant temp sensor, and tps sensor read correctly, your supply voltages to the ECM, coil, and injector are good, you have a good strong spark, your base timing is correct, your o2 sensor and its wiring is in good condition, and that there are no exhaust leaks, your compression and cam timing are okay, there are zero vacuum leaks, your air filter is in perfect condition.

if you aren't sure of all of those things you could be tuning around a problem, and when you correct the problem (or the problem suddenly corrects itself) your tune will be totally lunched

if you're sure of all of those things, and you need to crank your BPW up to get things in line, then that's not a cause for concern at all.

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Originally Posted by steveo

to ensure you have appropriate fuel flow, you should verify your fuel pressure is totally unchanged (with no vacuum reference) or does not decrease as load increases (with vacuum reference) when you are at heavy load and high RPM. you definitely don't want to tune around a fuel supply issue.

you also need to verify that your injectors are clean, your map sensor, coolant temp sensor, and tps sensor read correctly, your supply voltages to the ECM, coil, and injector are good, you have a good strong spark, your base timing is correct, your o2 sensor and its wiring is in good condition, and that there are no exhaust leaks, your compression and cam timing are okay, there are zero vacuum leaks, your air filter is in perfect condition.

if you aren't sure of all of those things you could be tuning around a problem, and when you correct the problem (or the problem suddenly corrects itself) your tune will be totally lunched

if you're sure of all of those things, and you need to crank your BPW up to get things in line, then that's not a cause for concern at all.

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Understood. Most things are new cts, injectors, heated 02, tps, new entire tbi wiring harness. verified base timing is set at zero degrees. Im not going to get crazy with the tune as I have another motor for the truck. The truck is mainly used as a yard truck and to plow my driveway. With the engine that's in it now it is good for me to get familiar with tuning and learn the different parameters but im not concerned with getting it running perfect. Thanks for the info.

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Originally Posted by joegreen

Understood. Most things are new cts, injectors, heated 02, tps, new entire tbi wiring harness. verified base timing is set at zero degrees. Im not going to get crazy with the tune as I have another motor for the truck. The truck is mainly used as a yard truck and to plow my driveway. With the engine that's in it now it is good for me to get familiar with tuning and learn the different parameters but im not concerned with getting it running perfect. Thanks for the info.

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As a wild guess . . .maybe the new injectors flow 54 lbs / hr = BPW 153 (if installed on a 5.7 liter). Hmmm some 5.0 liter injectors are rated at 54 lb / hr.

dave w

Quote:

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Originally Posted by dave w

As a wild guess . . .maybe the new injectors flow 54 lbs / hr = BPW 153 (if installed on a 5.7 liter). Hmmm some 5.0 liter injectors are rated at 54 lb / hr.

dave w

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I believe these are the injectors i got from autozone. GP Sorensen Fuel Injector 800-1807N