Help with interpreting AFR gauge readings

saf

Petty Officer 3rd Class
Joined
Nov 22, 2019
Messages
76
So, I've been trying to tune my new Holley 4160 (600 cfm marine carburetor model 80551) on 5.7 block. Had to increase main jets from #63 to #71 and switch to black (strongest) vacuum secondaries spring. Still have hesitation around 3500 rpm before the surge at WOT (as secondaries open). I don't have a lot of experience with carb tuning, tried reading plugs with little success. Fearing lean condition and potential damage to the engine I bought a broadband AFR gauge to help with tuning. Installed the broadband O2 sensor in a spacer under port riser. The gauge worked flawlessly and I got readings from the entire RPM range (while moving, of cause). I am getting 12.5-13 AFR all the way from idle to about 3500rpm where the hesitation starts. When the hesitation starts, the AFR readings get sporadic jumping all over the place from 11 to 16 and sometimes showing blank. Once RPM goes over 4000 I feel the boost from the engine and AFR readings stabilize around 13 again. Does anybody here have any experience with tuning marine carburetors using AFR gauge? Can you help interpreting these results?

 

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
11,802
Holley.com has support forums, try posting up there, I think for sure someone will know.
It sounds like you're having trouble in the transition when the secondaries just start to open. Which power valve does this carb have? There's a procedure to determine what valve you should be using that is also on Holley's website.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,307
according to the Holley library, you should have a #25 power valve

600 CFM is a bit small for a 350

your running rich

go lighter on spring (not heavier), you want the secondaries to slowly open starting about 3000 RPM
back off on the jets - suggest #68
may need to go to #35 power valve

wish you had a 4150 so you could adjust the secondary jets
 
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