95 GMC Yukon Intake Backfire

[ezusmanis]

I have a 95 GMC Yukon with 160K miles to which I have recently made the following mods:

Edelbrock 60859 heads (got great deal on mildly used heads)
Edelbrock 37004 intake manifold
Comp 1416 Roller tip rockers
+.1 pushrods
TBI spacer
New thermostat, wires, distributor, cap, rotor, water pump
Switched to Champion RC12YC plugs per Edelbrock recommendation
40kV ignition coil
Shorty headers
New O2 sender, oil pressure sender and both temp senders
Cold air intake
Gibson exhaust

It idles good, but sometimes bogs on throttle and then backfires through the intake. It very rarely occurs when cold but increases in frequency when in closed loop. Once it backfires, it's very sluggish for a while (MAP catching up from being clobbered?). My suspicion is that it's going lean on throttle.

I've tried the following to diagnose the problem:

Replaced coil, distributor, cap/rotor, wires when mods
Checked order of plug wires maybe 25 times
Disconnected battery for about 5 minutes about 25 times
Checked timing - Basic mechanical and running (EST disconnected) - Base timing at 0
Tried to advance timing - helped with backfire but knock sensor went wild
Checked fuel pressure with T-fitting on gas line to TBI - 13.5 PSI (constant under load)
Tried increasing pressure and 65 lb injectors - Basically ran too rich but still backfired
Check compression - 145 - 160 across cylinders
Vacuum steady at around 18
Sprayed carb cleaner around intake and all vacuum lines - no change in engine speed
SES not on and no codes stored
Ran TunerPro and verified proper inputs from O2 sensor, TPS, MAP, knock sensor and CTS
Using TunerPro looked at BLM relative to MAP/RPM - generally wants to lean out.

The only thing I'm thinking is that the basic mapping isn't proper for the mods I made. I'm not a modder or tuner so I may be totally off base.

I'm really at a loss for ideas. If it helps, I have data logs off of TunerPro.

 

[ezusmanis]

Solution to hesitation

When I swapped out the Edelbrock intake for the OEM intake, I move the EGR valve over with no changes.

It turns out that the new setup requires a smaller orifice diameter than the stock one. I kept reducing the size until the hesitation went away!

Still backfires through intake...

 

[retorq]

Recheck timing properly, get rid of the .1+ pushrods and adjust the rockers properly.

 

[ezusmanis]

I rechecked timing with the brown wire under the glove box disconnected and it is still 0 degrees. I put in +.1 pushrods (7.9 instead of 7.795) per the head installation instructions for my 60859 Edelbrocks. I would assume it's best to stay with their recommendation. The roller tip rockers are running the standard 1.5 ratio.

I adjusted the rockers with the engine running to avoid overtightening since the lifters have some miles on them. Eliminated noise, then 1/2 turn more.

 

[SunlitComet]

adjust them per a service manuals instructions. people really need to stop adjusting these engines by running and listening to the engine sounds.

 

[ezusmanis]

Found the problem to be timing. With the changes I made, I had to advance the timing 10-14 degrees to get it to run right. I used TunerPro RT, advanced the timing until knock was detected and then backed down by the last advance amount.

Rungs great now!

 

[SunlitComet]

honestly, even with the heads, intake, rockers and pushrod changes i am not seeing it needing to change unless edelbrock did some crazy rework from the stock form and required it. are you absolutely sure the dizzy or any other removed or disconnected component has been going in correctly? the rest of your list is kind of irrelevant in this matter it seems.