I have a 97 lt1 in a 98 gmc sonoma. It is a 383 with trickflow heads ported by LE,2.08/1.65 valves,psi springs and titianum retainters,custom cam by Brett B,42lb racetronic lucus injectors,1 3/4 long tube headers.Every since I have put the motor back together with these parts,(heads and cam,short block probalby has 5000 miles on it) The engine will start to backfire around 5500 rpms and most of the time will not rev much over that. Its not a constant backfire,but you can tell when it backfires every few seconds.While its not revving correclty It has a weird vibration to it. I held it in 1st gear the other night for about 5 seconds and it finally built up enough rpms to turn the 6500rpm shift light on. Im not seeing anything weird with freescan and neither did Bryan at pcmforless.

I have bypassed my msd digital 6,
replaced stock opti with msd billet unit(no change what so ever)
replaced spark plugs
replaced otvc taylor spark plug wires,
removed the msd coil and tried a stock coil
readjusted all rockers,1/8 turn past 0 last( old style comp r lifters)
ck both filters before and after the a1000 fuel pump,pressure at 44wot.
ran it will maf unplugged,ran it with 02 sensors unplugged.

The engine runs and drives normal at all other rpms,it revs fine till it hits that ~5500 rpm range.It will do it at 1/4 throttle or WOT

 

You might have a leak on your intake manifold. This caused a misfire on my little brothers rebuilt H/c motor.
P.s. I want to see that truck at ennis sometime. Do you still have the draglites on it? Are you having trouble getting it to hook?
Steve
Edit: I just watched your videos and found out you are having no problems getting that truck to hook 1.4 60 ft. <--- that is awesome!!

 

Sure you did the valve lash right? The other day someone said something on here about 1/8 of a turn being too much on Comp R's ... But I think it was for the new style. Not sure if there is a difference in lash adjustment between the two.

Fuel pump? How many PSI does it have at full throttle? If its the stock pump, thats gonna be pushing it for that kinda power ...

 

I was supposed to go to the pinks all out race sept 2 ,but im not going to make it at this rate.
Ive checked for vacuum leaks,unless is sucking from the bottom,its all good.

 

Ive set them the same way every time and never had this problem.
fuel pump is aeromotive a1000,ive got plenty pressure and flow.

 

I will gladly paypal someone some money for a correct diag......

 

could an icm problem cause this?

 

well i guess im going to pick up and icm and see what does.anyone have any other ideals?

 

sounds almost like you have some plug wires mixed up.

 

Download Datamaster and record some data and post it up. You sure you have the correct springs to turn the motor that hard?

 

It ran the same way before and after i replaced all wires,they are most definately in the correct location.

 

The heads where set up by Trevor Johnson,Lloyd Elliot uses him for all of his valve jobs and he had all the specs for the cam so I can only assume they are correct.I have had a engine that floated the valves and this is nothing like that.I also have an afr rev kit in the motor.

Downloading datamaster now.Is datamaster any better than free scan?

 

Well I downloaded datamaster and could not figure it out,I was not getting it to communicate.So I just did a few runs on freescan and uploaded them.

If anyone sees a problem ,please let me know.

Here are the links,you will probably need to right click and save them.

http://www.slammed98gmc.com/testrun.csv

http://www.slammed98gmc.com/testrun2.csv

 

do not pay attention to the speed due to the fact that it is not correct,both runs where from around 20 mph through first and second,I probably let off at around 95mph

 

Noone?