Idle problem

[dlphil10]

Background:
I have recently rebuilt a 350 Pontiac motor. The specs are as follows:

350 with 0.030" Overbore
061 heads which are supposedly 72cc heads but published values are known to be a little off
Stainless steel valves: 2.11" Intake 1.77" Exhaust
Crower 60420 Cam: 270/276, 210/221 @ 0.050" lift, 0.421"/0.446" lift, 112LSA
Automatic Car (TH400)

Carburetor: 7029262 which from a 428 motor
The carburetor was rebuild using cliffs kit that had the most components and the throttle shafts were bushed using components from this site.
I believe the primary jets are 72s and the metering rods are 46s

I have just broke in the cam with 40 minutes total run time at 2500 rpm +/- 500 rpm. Brad penn break in oil was used and only the outer valve springs were used to aid in cam break in.

Problem:
Whenever I was breaking in the cam the car had a very slight stumble but it didnt seem too regular. I was told to proceed with breaking in the cam, then tune the car afterwards. When I went to tune the setup, the idle was a bit rough(in park). The timing was set around 10-12* and the idle was ~550rpm-600rpm. I got the vacuum at idle to be around 15inHg by backing out the idle screws, but now I think that my expectations of a higher idle vacuum were incorrect and misguided me into possibly leaning out the idle. Whenever I was breaking in the cam I did notice I had around 20-21inHg of vacuum strong and steady.

I dont think my vacuum advance is engaging at idle, but could that cause this issue (there is that slight stumble at high vacuum cruise scenario). I ordered a crower adjustable unit just in case.

Could an incorrect power piston spring cause this problem? (I was expecting a lot more vacuum at idle and chose the spring that seemed most like the stock spring in length and tension)

I know thats a lot of info, so let me know if you have other questions.

Thanks!

[dlphil10]

Also note that the spark plugs are slightly tan, not as tan as I have normally seen, but I attributed that to the cam break in with little load. Is that an incorrect assumption?

[dlphil10]

It should also be noted this casting has idle bypass. I couldn't imagine that the build would use the same air as a 428 car, so I set the primary throttles so the slots aren't exposed at idle.

[Cliff Ruggles]

Pretty lean jet/rod combination for what you are doing.  For sure I would install smaller primary metering rods to add some fuel to the part throttle part of the curve.  The idle system should be adequate for that engine combination.  Make sure to remove the triangular metal cover over the vent tube, there is a tiny screw under that cover that control idle air to the upper bleeds, which can be used to fine tune the idle system as well.....Cliff

[dlphil10]

Thanks Cliff! What would you suggest for the primaries? Should I go down to about a 42?

[dlphil10]

The screw I believe you are talking about is fully seated (the small screw behind the tube)

[dlphil10]

I apologize for all of the additional posts...

I just test drove the car and the carb seems to be fair. The only big issue is a bog upon part throttle. It doesn't have as much power as I believe it should have either. Could this be due to the lean jet/rod combo?

[dlphil10]

I found two quadrajets in my dads stash that are in need of rebuilding. One is a 67 and the other is a 69 or 70. Both are chevy quadrajets he has acquired. One pair of rods are stamped 44B, and the others are stamped 42B. The rods stamped 42B are stamped in the middle of the rods while the rods stamped 44B are stamped towards the tips of the rods.

Would these rods be suitable candidates for my build?

[Cliff Ruggles]

I don't like using used jets and metering rods for custom tuning.  Very few used jets will pass a pin gauge test, they are almost always "egg" shaped and worn some.  Same with used metering rods. 

I would call the shop when you get time and get new jets and metering rods for it and PP spring for what you are doing.  Since you already have good rebuild parts in it should be good to go and nothing else needed to get it dialed in....Cliff

[dlphil10]

Primary rods, jets, and secondary rods are ordered. In the mean time, do you think its an issue that the screw (Idle air bleed screw im assuming) is fully seated?

[dlphil10]

Jets and rods won't affect my idle correct? The idle still runs pretty rough lIke a misfire. If I put my hand over the air horn primaries it smooths up. More fuel? Turn out that air bleed screw on top of the airhorn?

[Cliff Ruggles]

If it smooths out when you cover the top some, it wants more idle fuel.  Turn the bleed screw all the way in.  Fine tune the mixture screws.  If it still want more increase idle tube size and DCR's a small amount.....Cliff

[dlphil10]

I had a chance to install the new 72 jets, new 42 rods, and new DA rods shipped from Cliff. The power difference is night and day, and the setup runs a lot more smoothly.

I replaced the idle tubes and I think it has helped some. The idle mixture screws are backed out around 3.5 -4 turns currently and the screw at the top of the air horn is seated all the way.

There still is a slight "Flub in the idle" and the off idle hesitation is barely there. It chokes down real quick and then picks back up quickly. I think it might still need more fuel, or vacuum advance tuned. The crane vacuum advance is in the second slot before being all the way locked out. I should also note that I have not drilled out the mixture screw openings, and I only have aorund 0.040" idle bypass. Should I follow the example in the book and drill the idle mixture screw holes out to 0.090"?

[77cruiser]

Did you open up the DRC's any?

[dlphil10]

No I have tried to avoid modifying things if I can help it but it seems that might be unavoidable. I never would have thought the 350 would be as demanding as it is, especially since the carb was made for a 428.

I didnt measure the DCRs because I didnt have anything to measure them that well. It should be a stock setup for the idle circuit, besides changing out the idle tubes and mixture screws.