Quadrajet lean spot.

[rfoll]

I took a carb that was working well on my stock 305 and installed it with an engine change to a fresh 350. The 350 has a static compression in the low 9s and requires the initial timing to be pulled back to 4 degrees initial with the heavy springs in the MSD distributor to run on regular. Vacuum levels are in the low 20s. The 350 runs fine, and is fairly snappy, but when it's cold, it appears to lean out hard in the 1500-2000 rpm range under normal acceleration. Its runs fine when up to temperature, although I can tell it's still a little sluggish under load at the aforementioned rpm. My thinking is that I need to step up the jet size and run the apt down in the jet further to re-establish cruise afr. Have I thought this out correctly?

[dannobee]

Verify that the choke pull-offs are working and adjusted correctly. There are a few special tools that are necessary for the adjustments.

[CGT]

I would inspect the secondary pull off.

[Tuner]

What is the manifold vacuum when this occurs?

[turbo camino]

As a point of reference, I have a 10:1 350 (though with aluminum heads, and a very short camshaft) that really likes 20* initial. 4* is miserably low. If the only reason to run so little initial is that the total ends up too high you need to limit the total by a bunch.

[rfoll]

The choke pull off is new, I adjusted it both directions, and it clearly favors having the choke partially closed when cold. Vacuum is down in the 5 range, I would think the power piston should be up at that vacuum range. If I lean into the throttle harder it picks up better, but it still lays down some when it shifts. The carb is a 17057204, Federal application for all models with a 350 engine. I converted it to electric choke, and I doubt if I changed the factory 41 jet and 72 needle. The engine combo is here viewtopic.php?f=1&t=55245&hilit=el+camino The 305 I used this carb on was stone stock except for the electronic carb and distributor swap. I measured the cam when I removed it and it was the smallest I have ever seen. 176/194 112 with 351/385 lift.

[rfoll]

As a point of reference, I have a 10:1 350 (though with aluminum heads, and a very short camshaft) that really likes 20* initial. 4* is miserably low. If the only reason to run so little initial is that the total ends up too high you need to limit the total by a bunch.

When it's up to temperature, You would never guess the timing is pulled so far back. It has a ton of low end torque, barks the tires when it shifts under half throttle acceleration, and revs freely past 5000 rpm. I know I need to change the distributor bushing for more total timing with the small initial setting, but I would like to get a good fuel economy run,(this weekend), before I go much further. One of the things dictating timing is the lockup converter. It is wired to lockup when the engine shifts into high gear, and it tends to ping severely under acceleration if I advance even 2 more degrees. I do have a switch to interrupt the converter, but I usually just tap the brake pedal as necessary.

[bill jones]

---back in the 1970's and 80's--Gm has an emission system that was actually a neat deal--exactly fixes your problem.

---this was called TVS---"Trapped Vacuum Spark"

--basically there was temperature controlled sensor on the thermostat housing with 3 vacuum ports---and one vacuum hose went to the vacuum advance and worked normal once the temperature got up to about 165F.

---the other hose also went to the vacuum advance with a one way check valve in the hose---so when you first started the engine it pulls the vacuum advance to max advance----and holds it there while the engine temp is less than around 165F---until the temp switch opens and everything works normal.

---I rigged it up on my El Caminos and used it on maybe 4 different SBC engines I had in that era and even later on.

---it would take about 10 blocks or so max to get the sensor to open so it only works when the engine is dead cold---and as soon as it starts to get some hot water flowing it opens up.

---seems like I also found similar sensors on some Fords at a wrecking yard that had different temps settings.

[turbo camino]

Well that's weird, cause the chambers on your heads are very nearly identical to mine ('113 L98s). Mine is happy with 20* initial, 32* total, HEI, still using the ported vacuum advance and however much that's adding - not 'optimized' but it doesn't do anything wrong timing-related, so haven't felt the need to change it. I have more static but they're aluminum, so the end result is probably pretty similar.

The one big difference is I've got a (too) loose converter from back when it had a big nasty cam so it doesn't ever get loaded at low RPM, it pretty much is already at full mechanical advance by the time it gets into any load, and the initial is only in effect at idle speeds.

[dannobee]

Which SP-2P manifold? I think there were 4 different styles, depending on 2 or 4bbl and EGR or not. If it had an EGR valve originally and it's been removed, it's gonna ping pretty good. And the fuel economy will drop by 2mpg.

[cjperformance]

What is the manifold vacuum when this occurs?

^^^ did you miss this Q? Answer this and good info will come your way!

[77cruiser]

Vacuum is down in the 5 range, I would think the power piston should be up at that vacuum range. If I lean into the throttle harder it picks up better, but it still lays down some when it shifts.

It was kind of hiding.

[rfoll]

What is the manifold vacuum when this occurs?

^^^ did you miss this Q? Answer this and good info will come your way!

Yes I did answer, in a previous post. Right at 5 inches it falls flat on it's face. After my last test drive, with the engine warmed up I tried replicating it by power braking at a dead stop. Same thing, and if I pressed harder in the pedal, when it got to 3 inches it picked up strongly. I believe the secondary's were opening, felt and sounded like it. I suspect the secondary lockout isn't working because they will open when the engine is stone cold. What kind of vacuum does it take to keep the power piston and needles pulled down? It occurs to me It might be sticking in the bore, I can pull the plug and check it. Hard to watch it function running down the road. For info the carb number is 17057204

[77cruiser]

Depends on what spring you but I think 8-10" would be close. You can push the piston down with small screwdriver through the vent. Something light like a straw watch it move.

[chevyfreak]

I had a similar problem with pinging. Turned out the msd dizzy were the problem. The reluctor points were not lining up with rotor. Once i sorted that all were good.
This was on a 327 with 882 heads with a qjet and crane 260 cam.
Mech adv were limited to 10deg. Normal it were at 21deg.
Static were at 26 deg. Really woke up the responce on this motor.

On the qjet if you change main jet you alter idle and entire main circuit. A needle change only changes cruise metering.

Chevyfreak.