Quick Fuel 750DP Tuning Part 3

Continuation of this thread




Quick recap, I've done a fair bit of tuning with my Quick Fuel 750 mech secondary, annular booster carb.

Stock settings

Primary Jet: 72
Secondary Jet: 82
IFR: Unknown - suspect its .032 or .033
IAB: 72
MAB: 33
PVCR Unknown - estimate somewhere around 0.055" to 0.060"


Revised Settings

Primary Jet: 68
Secondary Jet: 82 - reduced to 78 after very short test drive
IFR: .030
IAB/MAB unchanged
PVCR 0.040"


I was hoping that the revised settings would be perfect however after a few miles I think I either went too aggressive on the IFR or need to look at the accelerator pump. On very light acceleration, there is a brief lean spike and a slight stumble. It sounds like classic accelerator pump but I'm not entirely sure as it certainly wasn't a problem when I first got the carb (before I started making changes).

I suspect that because the original settings were so incredibly rich it was covering the brief lean hole so I have gone back to 0.032 IFR to see if it makes a difference short term and have a larger accel pump shooter on order which will deliver a faster squirt on acceleration.

Below is a snippet of data - the yellow line is manifold vacuum (0 is full acceleration) and the white is AFR.

Just for interest, here's a similar plot of gear changes with my Edelbrock carb which had nasty lean hesitation on quick throttle openings that I could never tune out:

Anyway, back to the top graph. the dips in the yellow line are gear changes. The data shows me accelerating at about half throttle in 2nd to begin with (on the left) then foot flat in 3rd, 4th and 5th after that. I was exiting a roundabout hence part throttle in 2nd and rolled fairly gently into 3rd gear to be super safe in a straight line before nailing it properly. You can see from the later gear changes (shifting to 4th then 5th) that there's clearly nothing wrong with the accelerator pump, the pump volume is adequate and there is no lean spike on shifting. If anything the pump shot is a tad too much. You can also see the AFR leans out at high RPM. I haven't got RPM plotting working yet as I think I messed up the tacho feed wire but I was shifting at around 5000rpm. An adjustment to the high speed air bleed should help reduce the top end lean out.

 The red highlighted area is a lean spike going into 3rd. It wasn't lean enough to feel a hesitation so I've never noticed it before until I looked in detail at the data. I'm a little confused as already discussed - the other gear changes the pump shot is definitely sufficient. More learning required!