Quadrajet 350SBC Tuning Advice

[willem445]

Hi all,

This is my first time really digging into carb tuning and looking for advice.

Engine:

Engine is a 350 SBC in a 1980 Chevy K20. Don't really know the details on engine. Dad said that a local guy in town installed an RV cam many years ago. It appears whoever did the cam, must have swapped some parts because EGR, EFE, etc. don't exist at all.

When I took over truck, I have installed headers, redone vacuum lines, had carb rebuilt (~ 8 years ago), new open air cleaner, Bosch Double Iridium spark plugs (bosch 9659), wires (duralast). Currently just have bolt on glasspacks but will switch to full exhuast w/ Borla Pro XS soon.

I recently cleaned up some vacuum lines so this is the current diagram:



I found a post I had from 9 years ago where I measured compression, not positive I did it right but here are the numbers (yes the firing order is labeled wrong):



Exhaust does not smoke at all. Usually smells rich.

Transmission is SM465 3 speed manual for reference.

Problem:

To start engine I usually have to pump gas a number of times, turn over for 30s, stop, pump some more, and then it will fire. Usually need to rev to 1500 rpm for a minute or two to get it to idle without dying. Engine runs & sounds great at idle once warmed up.

I recently pulled a trailer loaded with skidsteer and really noticed that I lacked power on the highway. Going up a small hill for example, I would have foot to the floor and truck still struggled. Truck sounded like it was hesitating and bogging.

I did some additional test runs. Truck runs good in 1st & 2nd when moderately accelerating (hardly enough gearing to even go WOT in 1/2 gear). In 3rd though I get lots of bogging and lack of power, even when foot to the floor.

Diagnosis This Far:

• I have confirmed that my rear pull off vacuum can is not working. It stays open always. Since it's open, the secondary air doors are not locked. I'm suspecting my sudden acceleration hesitation is caused by the air doors opening too fast. Unfortunately the replacement did not come this weekend so it will be a week before I can swap that and test.

• The spring adjustment on rear air door felt way too tight to me so I was also suspecting it wasn't opening at all. I loosened that up so that it still spring back but takes much less force to open. Today I confirmed that at WOT, the secondary air door opens as well as the secondary throttle plates. This was with no load & stationary.

• Today I rebuilt my distributor. New coil, cap, rotor, ignition module, and cleaned up mechanical advance (one weight has wear in pin hole so doesn't always totally snap closed). I added an adjustable vacuum advance, link to part.

• I also confirmed that my choke is adjusted properly & opens fully when warm (unlocking the secondary air doors)

Numbers:

I bought a vacuum gauge and timing light. Here is what I measured.

• Manifold vacuum @ idle: 18 in Hg
• Idle timing @ 854 rpm w/ no vacuum advance: 17 degrees
• Timing @ 2940 rpm w/ no vacuum advance: 35 degrees
• Idle timing @ 1000 rpm (idle went up) w/ vacuum advance: 37 degrees
• Timing @ 2830 rpm w/ vacuum advance: 54 degrees

The vacuum advance appears to be adding 20 degrees. According to reviews, the advance canister should only add 12-13 degrees. Also, when adjusting the internal set screw to supposedly adjust timing, I'm getting no changes. It seems mixed in the reviews as to whether this canister is supposed to adjust timing or just the rate.. I did confirm that at WOT the vacuum advance lets off and I return to ~35 degrees timing.

The advance without vacuum seems reasonable to me. I'm not so sure about the vacuum advance though. Thoughts on these numbers?

Further testing:

I took the truck out and did a couple test runs today. Unfortunately I did not take notes so I'm not confident in what gave the best results.

I for sure tried:

• 3rd gear hard acceleration w/ vacuum advance connected to manifold.
• 3rd gear hard acceleration w/ vacuum advance plugged.
• Maybe 3rd gear hard acceleration w/ vacuum advance to ported.
• Stationary revving w/ vacuum plugged, vacuum to manifold, and vacuum to ported.

Again, I'm not positive on the combination but one run actually felt pretty good. Had good acceleration. Still had hesitation on sudden WOT but I believe that is due to the rear pull off not "dampening" the air doors opening. I'm thinking this was the no vacuum advance run but it could have been the ported vacuum run. I will have to retest and take better notes once I get the rear pull off replaced.

Testing the 3 vacuum combos, I still seem to get some hesitation occasionally at idle when stationary when I blip the throttle.

Suspicions:

I still suspect the following as things potentially causing issues:

• Quadrajet accelerator pump. I have replacement but have not replaced yet.
• Fuel filter. Also have replacement, will replace if I do accelerator pump.
• Fuel pressure? I replaced the fuel pump probably 10 years ago now. Maybe I don't have enough pressure at load and heavy acceleration?
• AFR mixture adjustment. I didn't realize I need a special tool (double D) to adjust my Quadrajet AFR so I'll have to order that and adjust later.

Summary:

Any thoughts on anything I shared? Any advice or tips?