350 TBI being a pig from 2k-3k RPM

[shortboxchevy77]

Hey all, new to this forum and I'm looking for some help with my 350 TBI that I have swapped into a 77 chevy K10. Here's the rundown...

77 K10, 31 inch tires, 3.73 gears, 700R4 trans. Low mile new GM TBI 350 using a stock 87 suburban ECM and a painless wiring harness.
Mods to engine include shorty headers, ultimate mods done to the TB, power bowl on top of TB and air cleaner base cut to match the power bowl to allow maximum airflow. Fuel pressure has been bumped from stock, to 15psi, to 17psi and this has helped with the way it runs. Timing has also been bumped to 4 degrees throughout the testing process.

New parts that have been installed since engine install include a full tune up, new dizzy, knock sensor, MAP sensor, o2 sensor.

The engine runs like a swiss watch and is extremely responsive, in town all it takes is a blip of the throttle to lite the tires up. Unfortunately it seems to struggle in the 2k to 3k rpm range, just like you cut the power way back. This problem is very apparent when in OD on the highway trying to push this brick of a truck at 2200rpm. It seems to run really well cold which led me to think it needed more fuel which is why I have bumped my pressure.

I'm beginning to wonder if for some reason the ECM isn't pulling timing on me and will probably use a resistor on the knock sensor wire and go to ground which will make the ECM see zero knocks as a way to test this idea. Other than this I am at a loss with why this engine doesn't seem to be performing the way I think it should be, its also burning fuel like a pig. Any help is greatly appreciated, I am starting to get frustrated with this setup considering the time I have into it.

 

[IOWNJUNK]

Midrange flat spot sounds like timing. Did you replace cam? Maybe too much lift and knock sensor is retarding timing?

Revs through range without load?

How does spray pattern look on TB?

Low fuel mileage gets another finger pointed at timing.

 

[K15 Blazer]

second the timing... the distributor...
if it is starting and driving fine then its probably not "timing" in terms of rotating the distributor... but if it is having a hard time keeping power and vacuum while driving then yeah, its probably ignition/ timing advance within the distributor itself....

have you checked vacuum on it and checked the plugs yet?

 

[shortboxchevy77]

Stock cam, this is a GM TBI new crate motor. perfect cone shaped spray pattern. This is a TBI engine with all TBI components therefore it does not use a vacuum advancing dizzy. Dizzy is new and the truck was acting the same before replacement. Rev's fine in park and is okay in gear, it doesn't stall or sputter it just sorta flattens out in the 2-3k rpm range which unfortunately is where its running in overdrive.

 

[IOWNJUNK]

Off a tooth installing distributor?
Timing set correctly?
Im trying to think of something besides timing but I keep coming back to a timing related issue.

 

[shortboxchevy77]

Off a tooth installing distributor?
Timing set correctly?
Im trying to think of something besides timing but I keep coming back to a timing related issue.

Timing checked, checked, and rechecked. Its always dead on, currently running 4 degrees advanced. Truck seems to like it a little better but advancing from zero didn't solve the problem.

 

[IOWNJUNK]

Does it advance with rpm?

Just throwin stuff at you man.

 

[shortboxchevy77]

Seems to when its sitting, I just put in a 160 degree thermostat so I'll check it again while the hoods up. I'm beginning to wonder if its for some reason picking up ghost knocks and retarding the timing on me, I read in another forum that headers can cause issues like this. This is why I'm thinking I'll try the trick of wiring a resistor to the knock sensor wire and then go to ground trick, this will fool the ECM into thinking everything is fine and it will read zero knocks. I read in a Z28 forum where a few did this with great improvements in the way the engine ran because the knock sensor was just too sensitive to the engine after mods were performed.

 

[IOWNJUNK]

Disconnected wire from distributor when checking timing? It should be at zero but you can run 4-8 deg of advance and be ok.

 

[shortboxchevy77]

Yes, of course.