Gene,
If you aren't getting enough ignition advance (either mechanical or vacuum advance), you will lose power at full throttle. But it just doesn't "feel" like an advance problem. As if I can tell how it "feels"! But to be sure, you can check it by borrowing (or renting) a timing light with a timing advance dial indicator feature. The specs for the various years are listed in the FAQ section (thanks to RalllyBob) under:
http://www.opelgt.com/forums/showthread.php?s=&threadid=360
Just check the mechanical advance at various rpm without the vacuum advance connected, then re-connect the vacuum advance, and check it again at full throttle at 2500 rpm, which will give you total advance. If you have access to a vacuum guage and pump, you can actually check the vacuum advance separately from the mechanical advance. My '71 and '72 manuals state that the centrifugal advance should start at 1100-1200 rpm, be 7.5 - 15 degrees at 1400 rpm, and be at a maximum of 28 - 32 degrees at 3600 rpm. Vacuum advance should start at 5 inches of vacuum, and be 7 - 10 degrees at 5.6 - 6.4 inches of vacuum, with a total advance of 43 - 55 degrees at 2500 rpm.
I would also look for a couple of very different possible problems.
The first is ignition. A weak spark (bad plug, overly resistive wires, carbon-tracked cap, arc'd points or bad coil) will often manifest itself in a similar way. Works fine as long as you don't give it too much load, but open up the throttle, and the increased ionization load will cause one of the components noted above to cause a mis-fire. Or a LACK of fire.
Or, for something completely different, check out your fuel pump. At less than full throttle, everything is fine. But try to deliver more fuel to the engine than the pump is putting out, and the bowl level drops and the carb leans out. I don't know what the flow spec for a stock Opel mechanical fuel pump is, but according to the manual, the outlet pressure is a good indicator of pump condition. And it should be 3.1 to 3.7 psi, which you can check with a vacuum/pressure guage at the carb inlet.
HTH
If you aren't getting enough ignition advance (either mechanical or vacuum advance), you will lose power at full throttle. But it just doesn't "feel" like an advance problem. As if I can tell how it "feels"! But to be sure, you can check it by borrowing (or renting) a timing light with a timing advance dial indicator feature. The specs for the various years are listed in the FAQ section (thanks to RalllyBob) under:
http://www.opelgt.com/forums/showthread.php?s=&threadid=360
Just check the mechanical advance at various rpm without the vacuum advance connected, then re-connect the vacuum advance, and check it again at full throttle at 2500 rpm, which will give you total advance. If you have access to a vacuum guage and pump, you can actually check the vacuum advance separately from the mechanical advance. My '71 and '72 manuals state that the centrifugal advance should start at 1100-1200 rpm, be 7.5 - 15 degrees at 1400 rpm, and be at a maximum of 28 - 32 degrees at 3600 rpm. Vacuum advance should start at 5 inches of vacuum, and be 7 - 10 degrees at 5.6 - 6.4 inches of vacuum, with a total advance of 43 - 55 degrees at 2500 rpm.
I would also look for a couple of very different possible problems.
The first is ignition. A weak spark (bad plug, overly resistive wires, carbon-tracked cap, arc'd points or bad coil) will often manifest itself in a similar way. Works fine as long as you don't give it too much load, but open up the throttle, and the increased ionization load will cause one of the components noted above to cause a mis-fire. Or a LACK of fire.
Or, for something completely different, check out your fuel pump. At less than full throttle, everything is fine. But try to deliver more fuel to the engine than the pump is putting out, and the bowl level drops and the carb leans out. I don't know what the flow spec for a stock Opel mechanical fuel pump is, but according to the manual, the outlet pressure is a good indicator of pump condition. And it should be 3.1 to 3.7 psi, which you can check with a vacuum/pressure guage at the carb inlet.
HTH
