Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Preformer rpm
t!mortician:
So I now have some control over the idle screws but i can't seem to get the power I want or expect at any point in acceleration the only thing I have noticed is that the power piston won't seat until about 2000 rpm. I think I have the lightest spring already any suggestions 🤔
Cliff Ruggles:
The power piston is controlled by engine vacuum, so it is a load sensing device just like the vacuum advance in the distributor.
It will always be applied or in the down position at idle and "normal" driving at light throttle/light load.
If you aren't making enough vacuum at idle or when you are driving the vehicle gently it's there is something wrong someplace unrelated to the carburetor.
Most well thought out engines, even those that make a lot of power will make at least 10-12" vacuum at idle speed and a LOT more than that at light part throttle cruising. They will nearly bury the vacuum gauge coasting.
Take some vacuum readings at idle, and during "normal" driving scenarios and post the results......
old cars:
Curious, What type of cam that is. Roller or flat tappet. seems like a big spread between advertised duration and @.050 specs.
Assuming its an automatic trans, do you know your vacuum at idle in gear?
Cliff Ruggles:
Edelbrock doesn't make camshafts, they source the from CMC just like Speed Pro, Sealed Power, FM and about a dozen other companies have for decades.
The cam is most likely rated at .002" tappet lift instead of .006" like most Cam Companies rate them, so you're going to see a much wider spread between advertised and .050" duration numbers.
What the engine needs here is about one more full point of compression to be happy.
I have a 455 engine in my Ventura with CNC ported aluminum heads and 236/245/112 LSA cam with the ICL at 109, lift at the valves is .639", so a very similar build.
It idles just about smooth with a very slight hint of "attitude" making 13.5" vacuum at 750rpm's.
The BIG difference is that my engine has 11.3 to 1 compression instead of 9.5". It would NOT idle well clear down at 9.5 to 1. It would also have poor throttle response and lackluster performance until I got it past 3500rpms or so.
What is really needed here is more compression for that much camshaft in this particular build.....IMHO.......
old cars:
Too many variables here. Street vehicle or track warrior? Pistons,Rear axle ratio, stall converter, tire size?
If this is a true street vehicle that cam would be better suited to more cubic inches. If those are stock flat top pistons it might not be making even 9.5:1 compression.
A 454 (true street engine) could make More power, better drivability, and use less fuel with less duration and tighter LSA camshaft.
Edelbrock makes excellent intake manifolds and cylinder heads. There camshaft choices not so good. Low intensity (too slow lifting the valve) and wide LSA to get the engine to idle better with more duration. This especially does not favour low compression.
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