Rebuilt engine has a miss

Hello All,
We just rebuilt the engine in my 89 SC and put on a 94/95 S-flow SC. Once we got the firing order figured out(stupid Haynes manual), the car runs pretty good except for one thing. Acceleration at low RPM's causes a slight miss, but it isn't evident at higher rpms. It acts like it's fuel starving, or some vacuum problem. We didn't put in bigger injectors, but I thought these would do up to 300hp. The only thing I can think of is that there was an extra vacuum line that looked like it should go on the plenum. Is there a difference in the # of lines on the old and new superchargers? Any advice would be appreciated. Thanks.

300 ponies on stock injectors or the injectors you had on there before? I dont know about that (if they are stock) How is your intake? (MAF, TB, Plenum?) If you got the S-port i think you might be fuel starving.

Al

The plenum is the one that comes with the new SC, other than that, all intake and injectors are stock. I'm somewhat sure that when I replaced the fuel tank(in order to put in true dual exhaust) a few years ago, I put in a bigger pump. My only question is why wouldn't it miss at higher rpms, or is it just being covered up?

These cars do weird things at lower rpms than higher. Mine actually detonated at lower loads than at WOT. What all did you change in the enginefor your new rebuild?

Al

It isn't so much that the car misses at light load as compared to WOT, it's just at low rpm's, say stomping it down when cruising at 60mph in 5th. If the engine is revved up, the miss is either gone or just covered up. We didn't really add any new things(other than the new SC), except for head components, new plugs and wires, machined everything, new rings and gaskets, etc.

RE: Miss

Is is possible that maybe the miss is simply the computer trying to compensate for you getting on it at low RPM? Maybe what you're feeling is the knock sensor trying to retard the timing as you boost it at low RPM.

It seems to me there's more load on the motor at low RPM and low speed, than at WOT at high RPM because the motor has to work harder against weight and friction.

I have always chalked a small miss on near stock SC to be fouled wire or plug. Even new plugs foul up in a new motor. In my old 3.8 I had to replace the plugs after it developed a miss at just 5K miles! New plugs and it was fine. Now with my 4.2 I'm seeing a small miss too with barely 700 miles on the motor.

However I'm not sure in your case, as generally as you all say, a fouled plug usually presents itself under WOT, not partial throttle until the plug wears even further. I drove on fouled plugs once for a month. As long as I stayed out of boost it was fine. It took two additional months for the car to become undriveable. Of course we all know that's no substitute for the true answer: FIX IT!!!!!