Follow up to carburetor stumble

[Aron Gleason]

So at cruise I can stab the throttle I'll stay in it and it will flutter for about 3 seconds then it will catch up. Afr gauge will go up to 17.0 then when the carburetor catches up it will drop. So switching to the blue pump cam did help alot. I guess now I need to go bigger on the power valve jets.

 

[Geoff 2]

.....Which was always the problem. Glad you are now going in the right direction....

People mistakenly think that the acc pump is the first port of call for every stumble. It is not. Correct tuning will result in the acc pump making little difference, enough to cover a change in carb cct. Same people need to think how carbs that do not have acc pumps perform flawlessly.

 

[Aron Gleason]

.....Which was always the problem. Glad you are now going in the right direction....

People mistakenly think that the acc pump is the first port of call for every stumble. It is not. Correct tuning will result in the acc pump making little difference, enough to cover a change in carb cct. Same people need to think how carbs that do not have acc pumps perform flawlessly.

Hopefully I'm not going to big on the power valve jets. A local carburetor guy was asking me what size pvcr I had. And when I told him they're .067 he said way too small. So he sold me .078 pvcr's.

 

[Aron Gleason]

And then he said to go from the white cam to the orange cam on the secondary accelerator pump.

 

[Geoff 2]

Aron,
You are also using a single plane intake with spacer. And not just any single plane, but a M1. I have measured the plenum volumes [ PV ] of the Victor, Torker 1 & 2, & M1.
The M1 has by far the largest PV, 1.33L. More than double the T1.!!!
What does that mean for jetting? A huge slug of air that absorbs carb signal. Single plane intakes usually require richer jetting because of this. So be prepared for a large increase in PVCR jet size.
The only relevant info I can find is in the MP notes for a 383 with a Holley intake:
PVCR 0.086" t/side & 0.073" driver side.

Leave the acc pump cams in the drawer for now. Get the jetting correct, & THEN fine tune the acc pump.

 

[Aron Gleason]

Aron,
You are also using a single plane intake with spacer. And not just any single plane, but a M1. I have measured the plenum volumes [ PV ] of the Victor, Torker 1 & 2, & M1.
The M1 has by far the largest PV, 1.33L. More than double the T1.!!!
What does that mean for jetting? A huge slug of air that absorbs carb signal. Single plane intakes usually require richer jetting because of this. So be prepared for a large increase in PVCR jet size.
The only relevant info I can find is in the MP notes for a 383 with a Holley intake:
PVCR 0.086" t/side & 0.073" driver side.

Leave the acc pump cams in the drawer for now. Get the jetting correct, & THEN fine tune the acc pump.

Thank you for that bit of information.

 

[Sweet5ltr]

So at cruise I can stab the throttle I'll stay in it and it will flutter for about 3 seconds then it will catch up. Afr gauge will go up to 17.0 then when the carburetor catches up it will drop. So switching to the blue pump cam did help alot. I guess now I need to go bigger on the power valve jets.

I would change the squirter, two to three sizes larger (try a 35). That will clear the initial hesitation or lean spike.

If the carburetor is tuned well, other than the initial hit, why make any other changes?

Takes all of five minutes.

 

[Geoff 2]

Any updates?

 

[Aron Gleason]

Not yet. Haven't had a chance to get the car out after putting in bigger pvcr's