Qjet, Sec. throttle valve opening angle?

I have the Q-jet # 7040240 for my 70 455 out for adjustments.

I found that the Primary throttle valves only opens approx. 45 Degr, (or 5/16 opening gap), before the secondary throttle valves start opening.
A spare carb, (#17057250), does same at approx. 80 Degree, (or 15/32).

What should the angle of the primary valves be at, when the secondary should start opening?

I have a feeling that this early opening of the secondaries causes "sluggish" performance at cruising.

Per.

:beer

 

Per,

Your later style spare carb is designed to have more primary opening before getting into the secondaries mainly for fuel economy and emissions. In order to change the opening point while still achieving full throttle they had to change the geometry of the linkage. You will notice that the hole that the rod (the rod that links the primary and secondary shafts) goes through is farther from the centerline of the shaft on the primary side on your spare carb. This gives the rod more travel from the primary side since it travels in a larger arc because it is farther from the centerline. The other end of the rod is in the same position relative to the centerline of the secondary shaft.

Either set up should work fine. The thing is, even if you wanted to change your main carb to linkage like your spare carb, you would not be able to. To have a later opening point and still achieve full throttle with the secondaries you would need a larger arc for the rod to travel in on the primary side (hole is already at the end of the arm) and/or a smaller arc for the rod to travel in on the secondary side (hole is already at the bottom of this arm).

If tuned properly (jetting, rods, air valve tension, etc.), the factory linkage on your main carb would be the best in my opinion. I don't like the feel of linkage like your spare has because you practically have to have your foot on the floor to get into the secondaries. I actually changed my linkage on my street car to start opening the secondaries a little sooner. This works fine on the street as long as you have a decent stall converter and some decent gearing (higher numerically).

Hope this helps. Give me a call if you want to discuss further.

Dean Oliver
www.holsfan.com/deano

 

Thanks a lot Dean.

That's all I needed to know.
I got worried when I saw the huge difference in opening angle of the two carbs.
Anyway, fuel econemy is not the issue. This will never be a daily driver with a litre price for petrol of 1$.

Per.

:beer