Wednesday, May 1, 2013

TBI Tuning: Spark

I've pretty well dialed in the VE (fuel) tables with average BLM numbers between 126 and 130 for most cells.

BLM is Block Learn Mode and basically indicates how much fuel the computer is adding or subtracting versus what's called for in the VE table. Low numbers suggest rich condition, high numbers suggest lean condition.

Earlier this week I worked on adjusting timing, starting with Bill USN-1's timing table on Binder Planet. After driving and minor tweaking, the part throttle timing appears to be pretty good, now.

Two issues remain. Sudden WOT from idle/stopped results in popping and stumble. I'm looking at a possible lean condition and/or timing issue.

Second, the vehicle has little power at 30mph (low rpm). Part of the problem is my transmission shift kit refusing to do part throttle downshift at this speed. Transmission linkage seems properly adjusted.

Using a '7747 ECM, $42 ASDU bin, there's the Main SA (Spark Advance) table, but it's modified by several things. There's a table for power enrichment spark advance which shouldn't be needed in my case and may be causing the first problem. There's also an accelerator enrichment (same function as a carburetor accelerator pump) which I've increased slightly to prevent lean out in hopes of preventing the first condition occurring anywhere else.

Finally, I've restored the Warm Bias setting. The Main SA table can be biased, that is, the zero point can be set to a positive number to allow negative SA numbers. The stock ASDU bin has this set to 9.84, in other words, that value is subtracted from any cell in the SA table to get the 'real' value. A value of 9.84 in the table is zero. A value of 19.84 is 10.00.

I had removed this bias and adjusted my timing table down by 9.84. Just in case this is causing problems elsewhere, I reversed this change, adding 9.84 to my SA table, and restoring the 9.84 warm bias value.

I haven't had a chance to test drive yet.

Due to personal reasons I need to take time off from the Jeep and the blog for several days.

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