Lean Carb 2

[Buick 350]

I have a carb that I think is going lean much like Marx3's pinging problem. The carb is a 7042240, Cliff’s #2 recipe with 74 jets and 44 rods, Buick 350 with ported heads, big valves ect., 232/110 for the TA 310 cam installed at 4* advanced.
What’s happening is say cruising along at 2000 rpms in 1st(four speed car)open the throttle not even to the secondary’s and I get detonation. This will happen at basically any rpm. Once the split second of pinging is over it accelerates just fine. I do have control over the APT in the base plate but to make the problem (almost) go away I practically lift the rods out of the jets. I did go through the different power piston springs and installed the heaviest thinking that the vacuum was holding the piston down. So at this point I think it might need to go up in jet size. Any thoughts? Or am I running down the wrong path?

[Cliff Ruggles]

Once the split second of pinging is over it accelerates just fine.

Define this, maybe the vacuum advance is adding enough timing that it pings until you are heavy enough on the throttle that the vacuum to it lowers and the timing retards some?

NEVER tune the primary side of the carb based on detonation alone for symptoms.  Does the engine hesitate, surge, feel "flat", poor throttle response, or any other symptoms of being lean at part throttle?

At heavy throttle without the secondaries, does it have any of the symptoms mentioned above?

Have you done a "tip-in" test as described in our book?.....Cliff

[Buick 350]

I should have mentioned that the vac advance is currently plugged and I have set up the distributor to bone stock to rule out and timing issues and at one point ran 100 octane av gas, but have since switched back to premium since the symptoms did not change.

I see how I described my “lifting the rods out of the jets” as part of tuning; not very clear on my posting today. That was just a try it to see if I could figure out the cause.  I re adjusted the APT  so when  I slightly slow the engine down with the choke plate I can pull a vacuum line and the rpms just slightly increase. Is that correct “tip in”? Never could get it to slow down like the APT part of the procedure says.

This is a new motor with very little run time so I'm not beating up on it with WOT. Aceleration is fine, rpm's come up very quick and can feel the difference in the throttle position by the pedal. No surge, rolls out easily with hardly any gas pedal movment. I'm scratching my head on this one! Any rapid opening of the throttle at any rpm or gear gives me a second or maybe a few of pinging.

More throw needed for the accelerator pump?

[Cliff Ruggles]

Did you install aftermarket distributor springs into the distributor?  If so, go back to factory weights, springs and center cam, test it again.....Cliff

[Buick 350]

Been down that road! Even changed distributors.
I ran down the left side cylinders with a compression gauge. (As you know the right side with ac is not easy to get to) 190-200lbs plus a little more in one cylinder is some serious compression; so yes I did back way off the timing.
  No change in symptoms.

I posted this problem on the Buick forum and did receive many answers; several from those who have a similar set up. After the timing questions and answers; more fuel was the consensuses.

[Cliff Ruggles]

The engine doesn't have any symptoms of being "lean", lack of power, surging, flat on accelleration, etc.  The crainking pressure sounds extemely high for the combination of parts.

The cam being advanced may simply have the intake valve closing point too early in the cycle, making a lot of compression at lower rpm's....Cliff

[Buick 350]

Compression was the clue, and yes the cam was in the wrong position. Runs good now and will run better with a little tuning. thanks for  the help!

[Cliff Ruggles]

Good news!

I couldn't imagine that much cranking pressure in a 350 engine with a 232 degree @ .050 camshaft.

I have a roller cam in my 455, 11 to 1 compression, 236 @ .050", and it only squeezes 160psi on a cranking test.

The ICL is set at 109 degrees.........Cliff

[Buick 350]

Cliff, your telling me to gain access to the apt screw in the base plate was definitely good info! What a difference it makes in tuning the older carbs. Once I got the right combo of rods and jets the "tip in" procedure was just as described in the book. Recipe #2 with 74 jets and 44 rods, for this engine works well. Now I'm off to dial in the secondary's.

Thanks for sharing your knowledge,
Pete

[Cliff Ruggles]

Good news, and yes, it is a very good system if you can get it free'd up.  It was never meant to be "adjustable", just pre-set at the factory.  In most cases the screw will not move, and any attempts to turn it result in breaking it. 

I'm able to get about 80 percent of them out, the other 20 percent have to be drilled out and replaced.

They work best with "B" series metering rods, which have a slight taper on the upper section then taper down to .026" tips.....Cliff