In general my car has been running strong with no problems for a while now. The only issue I have is taking off from a stop and going to WOT or just puching it, I get a hesitation or flat spot for about half a second. It seems to be between 1200-2000 rpm then picks back up. If I launch at a higher rpm instead of idle-900 rpm, I usually can bypass the flat spot but I cant do that all the time.

I've read and used all Lars' papers but cant find an obvious cause for this (lean/rich). I'm thinking it must be with the a-pump but mine is new. I'm sure my jetting isn't the problem.

This should be a common problem that should have an easy answer/fix.........right?

 

Ill be watching this post. I seem to have the same prob...

 

Have you adjusted the secondary air valve spring? Sometimes if it's too loose, the air valves on the secondaries open up too fast and cause the motor to bog down.

 

What is the initial advance set at?

BigBlockk

Later.....

 

Secondary A-valve is set perfect to 3/4 turn.

Initial is set to 11 deg BTD.

 

What carb number do you have and how is it jetted?
Lars

 

Lars knows...pay attention....BUT, I find the very best setting for the secondary air valve spring to be tighter than what you would normally think....about a slight twist less than binding on full open....
do not be shy over this....don't break the spring either, but the more the delay the better the car runs.....I think it's the sheer size differential from Primary to Secondaries....

 

Carb No is 7029251. It's from an Olds. I dont have the jet numbers with me but I know I went 2 sizes up from the stock for that specific carb.

I'll get the numbers later.

 

OK, found the jets. For that carb stock is (70 49B AU) 1.962/3.317

I have in it (68 46B DA) 1.969/3.100

 

I thought you said you went up in jet size. A 68 is smaller than a 70. At least it was the last time I looked.

How are the idle mixture screws set. Maybe you could open them up some and see what that does.

BigBlockk

Later.....

 

I thought I went up too. Looking at my jet sheet I had tried many combos, but then I was having a soot problem and had to back down the cruise area which was over 2 something.

The jetting can get confusing since the carb is from an Olds and going on a 350. The stock vette 350 carb uses smaller area's so in a way I have upped the jets. I dont see how the jets would actually cause the stumble.

 

This is where we get into all the crazy ness of these old carbs....

you MAY have smaller jets, but you sure they not been drilled in the past....and you also have many differant sizes and slopes to the primary and even secondary metering rods....differant lengths even, and then differant springs for the primary rods.....

I nearly went nutzo over my Q for a 455 Pontiac, some years ago....

sent it off for a good PRO to look at, and it came back 110% fine....

all sorts of problems in them....one if the machine work necessary to alleviate primary throttle blade/shaft bores wearing oval with age...and so extra air getting in there messing up the calibrations....that's assuming all i'ts not been messed up over time....

A chebby Q has the fuel going straight in up center, a BPO has it going in the pass side....I dunno why, just was.....Caddy was like chebby, the one's I seen anyway....

send it to Lars, or someone who really knows, maybe if he can't do it...he knows someone who actually CAN.....

I remember years ago having a whole lot of parts for Q's....and the count rivaled that of a Carter AFB strip kit....serious....

so much of this stuff is so far outta production these daze, I not sure you will ever find the proper setup, as parts are getting stupid allready....been what?? 25 years since the last one left the GM lines....

 

You're jetted extremely lean. I'm surprised it runs at all... try a 74/43 jet/rod combo as a starting baseline and see how it responds. Then tune and jet from there.
Lars

 

Sounds good Lars. I'll try that combo.

Remember also, this car runs strong with the above combo. I'm just looking for some tweaks to get rid of the off idle stumble.

If it resolves the issue I'll post an update. THX

 

Lars I'm confused by your comments.

You always say jet the carb to THAT carb stock specs first which in this case has a cruise metering area of 1.962 (.001").

My new combo has a cruise area of 1.969 (.001") which is almost the same.

A stock 69 350 vette carb 7029203 has metering cruise area of 2.391, so I can see if we are comparing to that it looks lean. I did jet this carb up to that area once and I got a sooting problem.

Why do you say you're surprised it even runs?

 

What do you do if you already have #"77" jets?
There isn't any higher is there.
I need more on take off what can be done?

 

Main metering jets DO NOT control the cruise metering. You do not change jets to alter cruise metering you change the rods. On a Quadrajet carburetor you change jets to alter WOT mixture. When the engine is cruising the rods restrict the main jets thereby taking control of the mixture.

We really don't know where this carburetor is at at WOT as far as jetting goes. You need to start at the beginning. Put a 72 or 73 main jet in it, lockout the secondaries and make some timed runs. Change the jets one size at a time up or down until you get the best time. Then you can set the cruise metering with the rods knowing that the jets are right.

Remember, the jets control the mixture at WOT, the rods control it at cruise.

BigBlockk

Later.....

 

Then I automatically want some smaller rods!!!!!!!!!!

 

I never said I was changing main jets for cruising area only. I'm changing both to optimize both WOT and cruise. Cruise equals main minus the rod, I get it.
I was asking help on solving a stumble. The car runs better than fine so I'm wondering why we're stuck on the jetting?

 

Lars, are you still out there?