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?

[cadillac_al]

Well, I have a few minutes before the games start so i will throw some thoughts out there. When the truck sits for a while the carb goes dry and the fuel pump loses prime. People think they can cure it but they all do it. Pumping the gas while the carb is dry does nothing but wear out your accelerator pump. I crank it over until the oil pressure comes up, that's about the time the carb gets some fuel, then give it a couple pumps while still cranking and they all fire up the same way.

Now to make sure your truck is getting adequate fuel you should automatically replace the fuel filter. If there is a question about the fuel pump this is the time to test the fuel pressure. When you have adequate fuel pressure and a clean filter you can check you accelerator pump. After the engine is running (or long cranking) you should be able to look down the carb and pump the throttle and see a nice stream of fuel shooting out. Once you ensure everything fuel related is working normally you can move on to the tuning part. Maybe the idle mixture needs some tweaking but often times they do not.

I never had an engine sensitive enough to require an adjustable vacuum advance. I buy them from the parts store and they always used to be customized for each vehicle. I don't remember if that is still the case or not. There are rumors that penny pinching aftermarket supply companies may only be making a few different versions and using them for all vehicles. I did order a specific one for my K20 last year but I don't recall why. Usually a K20 vacuum can would give about 10 degrees advance and a Camaro vacuum can would give around 20 degrees and other vehicles were everywhere in between. I would keep it simple and buy the correct one. I don't want a Camaro vacuum can on my K20 and have it ping hauling a load up a hill. Good luck

[cadillac_al]

a couple more thoughts:

you need the vacuum pull off for the secondaries

your fuel pressure is definitely suspect and you should be able to test it with your vacuum gauge.

usually manifold vacuum to the distributor works best.

[willem445]

@cadillac_al Thank you very much for taking the time to reply!

Your notes on starting strategy are helpful, I will try that next time I go out tuning.

The K20 vacuum can giving 10* of advance is a great reference. I was looking for that number earlier with no avail.

I wish I would have measured timing before I rebuilt my distributor. My old vacuum can appears to be holding vacuum so I may swap that one back in and give it a whirl. Truck sounded fine with the 20* of advance though too.

[Dead Parrot]

Q-Jets have a known problem. The casting plugs under the jets will leak. Enough to cause the gas to leak out while parked overnight. Most rebuild kits include a rubber pad that doesn't work very long. Common fix uses some variation of epoxy to seal the plugs. But most epoxies fail under constant exposure to gas. Another place they sometimes leak is the plug on the front of the carb by the filter housing. Even more difficult to seal as it is under pressure.

What ratio are the differentials? If you have an ' fuel economy' set, towing a heavy load like a skid steer + trailer will be slow. There is no magic tune that will change that.

Compression #'s being close to each other is good.

There should be 2 holes on the accel pump lever. If push rod on the outer one, try moving to the inner one. Should shoot a bit more gas when you mash the pedal.

[willem445]

@Dead Parrot

Interesting that you mention that, I was just reading in my Quadrajet book about the epoxy sealing being a temporary fix & instead drilling and tapping a plug. Starting really isn't a big deal to me. Is it worth fixing the plugs?

Power train is 350 SBC with RV cam (supposedly), SM465, NP205, Dana 44 front & 14 bolt FF rear, 4.10 ratios.

Lever looks to be on inner already:

[willem445]

Decided to pull the carb and attempt to rebuild. First time rebuilding.









This may or may not be related, but I've also been noticing that the carb looks "wet", especially around the accelerator pump area. Leaking gasket perhaps?

Don't have a great pic but these may kind of show what I'm seeingL

[willem445]

Fuel filter, look dirty?

[Dead Parrot]

4.10 should be good for towing. I was worried it had something like 3.08 or even a 2.X set.

Check the float for being saturated with fuel. Sometimes one will start to soak up gas and quit floating. Result is a over full bowl.

Check the float level. Since you say it seems rich, might lower the level a bit. Hopefully, your book has the proper level.

Don't remember if the inside or outside is the 'dirty' side on a Q-jet filter. Since it is out, replace with new.

Your lever is in the inner hole. If all else fails, might try the outer hole. Sometimes less gas is the answer.

[LS short box]

If all else fails there is a guy over on Corvetteforums.com that goes buy "Lars". His is highly regarded as a rebuilder of Quadrajets.
Many many moons ago as chev line tech we rebuilt many quads. But we would take them apart in the morning or afternoon and then let them soak over night in the carb cleaner bucket. I kind of wonder if any shop uses that stuff anymore? It was nasty. Didn't want to get it on your skin as it might create a chemical burn.
The next morning rinse them in tap water and reassemble with the rebuild kit.
One more thing. Check to see if there is a bunch of play in the throttle shaft. It can create a vacuum leak if badly worn.

[burnin oil]

A couple of thoughts about this. First is if this is not a new problem the qjet may already be on the secondaries so it lacks that punch. This happens alot on low geared trucks since the primary are so small. Secondly double check that the choke linkages are working correctly. They may be keeping the air door from opening under load.
Your timing curve sounds pretty good. I second the manifold vacuum for the vac advance but you need to try both ways. My RV cammed 350 actually likes ported vacuum. I also agree that 20 degrees of vacuum is probably to much advance for a truck motor. Usually the adjustable advance cans adjust through the vac nipple and only control tension. This means it only adjusts the amount of vacuum needed to have vacuum added. These cans usually require a limiting tab to control the degrees of advance. The crane kit has decent directions on the usage.
A sticking timing wieght can cause all kinds of issues.

If you get a new fuel pump be sure to check the pressure. The last parts store stock replacement I bought had to much pressure and constantly lifted the needle of the seat causing it to flood. Needed a cheap regulator to fix that issue.

If the carb is good and just needs more fueling, you should be able to use secondary metering rods from an edelbrock carb since they are clones. Q jets are 750 cfm but the air door design only let's it flow what the truck needs. Yo may only be getting 500 cfms of air and need some richer rods. I have seen qjets run well on a mild 283 without being over carbed. Great carbs with airplane gears but can be a mismatch in a low geared heavy truck. My 1 ton with 5.14s is always on the secondaries and felt weak. Swapped a used edelbrock carter clone on after 1 trip. Way better manners. I hope all this made sense. My phones browser isn't letting reread this post for clarity.

As an afterthought. If ignition and carb tuning goes nowhere, pull the valve covers and check the cam. Intake lobes going flat will cause power loss under load but rum pretty good otherwise. Exhaust lobes going bad are the ones that cause back fires through the carb since the increased exhaust gas can't get out of the cylinder.