3500rpm mfi problems help

[my72911t/rs]

Ive been tring to get my new motor dialed in to no avail... The motor was upgraded to 2.7 with euro pistons solex cams sport exhaust..Put the motor in broke the cams in.Got the mfi idle settings good. All throttle rods adjusted. The motor runs great down low. When on the gas 3500 rpm the motor falls on its face?
I tried to adjust the pump richer a half turn to left. There is no difference. No black smoke out the back when on the gas? The motor wont rev past 4000rpm under a load. The motor revs fine with no load. Does anybody have any ideas I can try... Thanks frustrated Matt

[pwd72s]

Matt? Don't laugh now...but what muffler are you running? MFI has a reputation for developing "flat spots" if the muffler isn't stock, or doesn't have stock back pressure. Yeah, I know it sounds weird, but not enough muffler back pressure can actually hurt the performance of MFI cars. The smaller the displacement, the more noticeable this phenomena is. 3500 is about where it happened to me, back in '75, running a sport muffler that was actually a stock muffler modified as described in a PANO article. I went back to a stock muffler, and the flat spot vanished. My ears have been happier since, too

[Randy W]

Paul is absolutely right regarding mufflers. Besides the stock muffler, the only one I know that works with MFI is the factory sport muffler. In addition, you must have the correct distributor curve and MFI space cam for your particular engine to begin with. IMO, the absolute best way to dial in an MFI 911 engine is on a dyno with an exhaust gas analyzer (lambda measurement). It is very easy to get fooled into making incorrect MFI adjustments without one. I gained 10HP and 10 ft lb of torque by doing just this on my 2.7RS spec engine recently.

Randy Wells

[tobluforu]

Besides the muffler, did you, as randy stated, have your pump re-calibrated to match the pistons and cams. Or, are you using the T pump? If not, your car will never run right.
On my 72 w\rs pistons, I got my car to run great with the stock muffler, the bursch triple muffler-to loud and lost a little low end and the newly created-My own design muffler. Would love to do what randy did with a dyno to see if the new muffler is actually doing a damn thing or hurting the motor. Only the dyno really knows.

[Matt Smith]

Make sure your correlation is absolutely correct. Then start dialing in more fuel on the main mixture until the flat spot no longer exists. Back it off lean again to the point just before the flat spot occurs. Sounds to me like you need to get a baseline mixture setting started.
Just remember how many clicks and in which direction you are changing things; this will let you get back to where yopu are now if everything goes pear shaped.
Muffler willmake a bit of a flat spot, but you should be able to compensate with a bit of fuel; adjustment. Don't go out and buy a new muffler as yet.