[dv657172]

Installed an Innovate data collection system for AFR vs. RPM and discovered the car is running lean.

Despite having an average AFR of 12.87 on a Mustang Dyno, at the track my AFR in 1st gear was approximately just over 13 on average, 2nd approximately 13.5 and 3rd gear roughly 14.

Its a Holley 750 DP (4779-6) with 67/77 jets and a 5.5 Primary Side PV, none on Secondary side. Car has 14 inches of vacuum in Park, 12.5 inches at idle.

I am planning on increasing the jet sizes by 2 or 3 on both Primary and Secondary side to improve the AFR across the run.

Any reason to move the PV up (i.e. 6.5 for example) based on the old Holley rule of thumb or should the jet change be enough?

Thanks for any thoughts
DV

[johnta1]

Why no secondary PV?
The 5.5 is the vacuum point it opens up.
If you never get to 5.5 it will stay closed.
What is the vacuum reading at wide open throttle?

The primary looks really lean as it is. The 77 isn't bad except with no PV in secondary I'd think that would be bigger.

I'd put a 6.5 rated PV in both sides and see what it does. Then maybe richen jets up some?

[dv657172]

I'm no carb expert by any means but if you put a PV on both the primary and secondary side I would think you would probably want to have the same jet sizes on both sides albeit maybe 70 squared as they say.

My understanding is that you have the bigger jet (i.e. +10 or so) on the secondary side because you have a blocked PV.

Setup from the factory on the carb is apparently 70/80 with a 6.5 Primary Side PV and blocked Secondary Side PV. Just to be cleared mine is currently 67/77 with 5.5 Primary Side PV and blocked Secondary Side.

[johnta1]

My 4779 ( although not as new as yours) had both sides with PV and around 72 square. (been around 25 years since I ran it)


But the reason to take rear PV out is because on launch the gas flows away from PV and has big lean out.


Then it is jetted up around 10 sizes on rear jets.

[dv657172]

Yes I understand the reasoning for the larger jet sizing on the Secondary side.

While I would think increasing my jets t by 2 to 3 sizes would move the AFR down into a better range across the run, wondering if the PV is likely closing and thats why its leaner in 2nd and 3rd gear or if thats just normally how the AFR changes on a run (richer early in the run and leaner later) and you have to shoot for an acceptable middle ground?

[johnta1]

Well the AFR sounds like it's way lean so need more fuel somehow?

The secondary is way lean I think with no PV in it.
The primary is lean in my opinion also, but don't know the calibrations they use on the newer ones. Mine was when they were for racing mostly, full throttle, not much idling.

Were your AFR readings for full throttle or just cruising?

[steve25]

Always keep in mind that with regards to jets and making jetting changes that due to the tolerance that jets have if you need to try going richer going up by 2 numbers is the minimum to see a change taking place the like you want.

I always go up by 4 numbers so as to see a definative rich condition.

[dv657172]

Johntal1 - Car is dedicated 65 GTO drag car so all readings at full throttle over 1/8th mile

[Formulas]

4779-6 750 (F)71,(R)80 6.5 (F)

A quick look on the interwebs

[dv657172]

Thanks for the correction Formulas, in my original post and in my notes I had 70 as the Primary Jet size from the factory but as you stated it is actually 71.

I have increased the jets by 3 to 70 and 80 currently. As just a benchmark it richened the AFR by 0.53 at 3,000 rpm with no load. Hoping to get to the track at the end of the month for testing.

Still curious how much increasing the PV from 5.5 to 6.5 would impact the AFR and on what portion of a run? 1/8th mile track - leave on a transbrake at 4,000 rpm

[johnta1]

I think that mine I had rejetted it to around 75/80 with no rear PV.
The main thing that helped me was bigger squirters in it?
I actually went to the 4780 (800cfm) and raced it till I went with the Toilet FI. (needed more cfm flow, didn't want a Dominator)

[Half-Inch Stud]

Here's a thought; jet up to obtain rich readings, then back down into desired reading.

My 472 used 98 jets on the 1050.

[dv657172]

When I bought the car the engine had 74/84 with a 5.5 PV and no PCV plumbing or plug on the PCV carb nipple, just a big vacuum leak although it ran ok setup that way.

Installed the stock PCV plumbing and put car on a Mustang dyno where the avg AFR was 10.8. Operator jetted it down to 67/77, keeping the 5.5 PV, to get an average AFR of 12.8.

Installed a data collection system and at the track it appeared a bit lean as mentioned in my original post. I think the plugs also showed that to a small degree although I don't have a lot of experience reading plugs yet.

[Formulas]

Power valve effects... Primary side.. At steady flat cruise you need xx fuel .. At WOT you need xxxxx fuel.. To get these two values you have to chop off or provide some fuel supply at a proper time... And the way this is done is by having two paths of fuel, main jet and power circuit,

Main jet you adjust to get best MPH in 1/4 = highest HP. .. Power valve comes into play flat steady cruise and you start giving it more throttle at a certain point the extra air will make you go lean until the power valve kicks in and enrichens the circuit, The timing of the power circuit is based on the vacuum you tailor the circuit to keep yourself out of a unproductive lean spike

Now on a dedicated drag car your mostly sitting staged with an elevated vacuum reading with a quick transition to WOT near zero vacuum so the power valve is less of a player at this point you just want the power valve to be a couple HG. under idle or stage vacuum whichever is lower.

BASIC explaination your mileage may vary

[dv657172]

Thanks Formulas. Will have to check my stage vacuum.

[Skip Fix]

You can also play with Power Valve orifice sizes also and enlarge them.

[dv657172]

Got to the track this weekend and increasing the Primary and Secondary Jets by +3 took lowered my average AFR from 13 (1st), 13.75 (2nd) and 14 (3rd) to 12.50, 13, and 13. Plugs looked better than prior outing. Still have a 5.5 PV installed but hoping to connect a MAP sensor by our next outing in late Sept to see what vacuum looks like from launch and throughout the run and go from there in regards to PV changes. New personal best by a little but likely more weather related (2,100 DA vs. 2,900 DA last time out)