Edelbrock 1904 QuadraJet Rebuild and tuning

[86chevk20]

Hello,
     I have a 1986 Chevrolet K20 Pickup with a 4 speed manual transmission. The engine is very mild built 350. Flat top Pistons, Comp Cams 4X4 Cam, nothing major. When I bought the truck from my father it had an Edelbrock SP2P Aluminum spread bore intake manifold. From what I have read online that was a real "Fuel Saver" intake manifold. Sitting on top of that was a fairly low mile Edelbrock 1904 Quadrajet carb on it...it lists as being a 795cfm carb. The truck ran good and was very reliable but didnt make the power I felt the motor was capable of making.

Shortly after buying the truck I purchased a Edelbrock Performer RPM square bore intake manifold and a Holley 600cfm carb.  I have since had that on it for the past 6 years. While it ran decent and made alot more power, I was constantly fooling with it and adjusting it, the typical Holley BS...At this time the Holley needs rebuilt and I am kinda just done with it and want to go back to a Quadrajet for reliability. 

I have the original Rochester Quadrajet for this truck which I plan on sending in to be rebuilt by Cliff's in the off season. For now I want to get the truck back up and running for the summer so I am playing with the idea of rebuilding the Edelbrock and running that on my current intake with an adapter. My question is; the 1904 seemed to be way to big for my engine, it used to have a terrible flat spot in 2nd gear until the 4 barrel would start to open up, then it would start to "pull" good and take off. Is there a way to tune this down a bit to run better on the engine? Right now the jets in it are stamped "73" each. Should I rebuild it and put it on and see how it runs with this intake manifold? Or is there something I should try while I have it apart?

Also one final twist into this is that since switching to the Holley, the gear ratio of the truck has changed, I went from 3.42 gears down to 4.10s ....Which kind of makes me think i should just rebuild it and put it back on and see how it does??

[Cliff Ruggles]

The SP2P intake is a complete POS and waste of time to even mess with.

The RPM is too big for what you are doing and will actually hurt power some (low and mid-range) but not really enough to worry about.   The best choice would be a factory iron intake.  Improved thermal efficiency and excellent design for engines that need a smooth/flat power curve and tons of low and mid-range power.

No mention that I saw of what cylinder heads were used but GM made some real "home-runs" and some real "turds" from 1974 to around 1990. 

The worst of the worst will be the 882 castings followed closely by the early "Vortec" with the cast iron "swirl" in them in the intake runners at and past the short turns.  The cylinder heads more than any other single item will have the biggest impact on power production across the load/speed range.

In any case the 1904 is EXCELLET and will easily outrun the 600 Holley at every level without having to mess with it every time the wind blows.  Those are also the EXACT same castings as the 1910 "850" cfm units with just a slightly longer stop on the secondary airflap shaft to limit cfm just a tad.

It's not "too big" for what you are doing.  The engine will only use what it needs from it another big plus for the spread bore design using secondary airflaps that are spring loaded and adjustable.

The 1904 doesn't have the "generous" idle and off-idle fuel delivery that the 1910 has, but will be an excellent choice for your engine with just a few minor modifications and some good internals. 

If you want to get it up to par call me at the shop and I will put together a custom rebuild kit with tuning parts so it's basically "plug and play"....tks.....Cliff

[86chevk20]

Cliff,
  I got my rebuild kit today and are starting to put the carb back together. Everything is going good, but before I button it up, and I don't remember from our phone conversation if you had said to leave the APT setting at the stock setting of 2 turns out, or change it? I wrote down 3.5- 4 turns out but pretty sure that was for the idle mixture adjustment.

Thank you!
Andrew Miller

[Cliff Ruggles]

The idle mixture screws are fine pitch metric threads.  Not uncommon to have to come out quite a ways with them to bring in adequate idle fuel.  I usually start with those at 4.5 to 5.5 turns out from gently seated.

Set the APT at 3.5 turns up from seated.  That puts the tapered section of the rods close to middle of their travel. 

Always spend the time needed to make sure the power piston hanger arms are level and EXACTLY even.  I go thru this all the time with customers, they just glance at the hanger and thinks it's not bent up or down and 9 out of 10 times it's bent some and the rods are NOT even.......

[86chevk20]

Cliff,
   Got my 1904 rebuilt and on the truck. Overall I am very happy with it. Got it dialed in pretty good. I dialed the idle mixture in with a vacuum gauge. I ended up having to take the APT in to about 1.5 up from seated to get rid of a off idle stumble.  Right now idle is great, normal driving around town and highway cruising is good as well but where the carb seems to be struggling is if I am cruising at about 1/4-1/2 throttle (2k-3k rpm) and then pound down on it.  It cuts out (almost like a cough) for a split second, comes back, stumbles very lightly until the secondaries start to open then it takes off.  This only happens if I pound on it quickly. If I feather it to WOT it does not stumble or cut out at all....

[Cliff Ruggles]

Check the accl pump shot to make sure it's quick and solid stream with any quick movements of the throttle.

Also make sure the ignition timing is advancing when the RPM's increase.....