Quickfuel carb tuning

[MEV]

Put in a 4.5 holley power valve

Light cruise 2300 rpm Vac -15 AFR 10.5

Highway cruise 2700 rpm Vac -22.5 AFR 10.4

Moderate acceleration AFR 11.8

I talked to quickfuel tech today. After changing out the power valve and basically seeing no difference. He said put the 6.5 back in, and jet down the mains 2 sizes, which i did, i saw an incremental increase in AFR, i went down 2 additional sizes to a 74 and re-ran my test loop, the numbers are as follows:

65 mph steady cruise 11.1 AFR -19 VAC

35 mph steady cruise 12.2 AFR

WOT 13.4 0 VAC

 

[65sporty]

Better, but still room for improvement during part throttle. I wouldn't want the WOT any leaner.

 

[70bigblockdodge]

You are probably going to end up with a strange combo. Seems like something is wrong if you have to go less than 72 on the primary side. I would definitely keep going down on primaries till cruise comes up to 14 or better. Be weary of WOT if it continues to lean your getting into a dangerous area up in the 13s.
It's working in the right direction now. I would separate the primaries and secondaries and do the primary side first.

 

[Knebel]

Go to 68s in the primaries and up the secondaries 4 jet sizes at least at the same time. see wherethe afr goes. I mean... I said no WOT til you got the cruise dialed in but you can do it that way too if you want, it just complicates things more because you don't exactly know which circuit you are on.

I can tell you whats going on with the WOT most likely. Your power valve orfices will be too small. Easy to tune if they are of the tapped jetted kind, little more tricky but not horrible if you have to drill them. But we cross that bridge when we get there, it requires more testing with the secondaries wired shut. Have you done that?

 

[70bigblockdodge]

Go to 68s in the primaries and up the secondaries 4 jet sizes at least at the same time. see wherethe afr goes. I mean... I said no WOT til you got the cruise dialed in but you can do it that way too if you want, it just complicates things more because you don't exactly know which circuit you are on.

I can tell you whats going on with the WOT most likely. Your power valve orfices will be too small. Easy to tune if they are of the tapped jetted kind, little more tricky but not horrible if you have to drill them. But we cross that bridge when we get there, it requires more testing with the secondaries wired shut. Have you done that?

That carb is a double pumper style so I really think it needs separated. A quick fuel has screw in power valve restrictors so if he would need that extra he could change them to larger (not there yet). The issue I have with not separating the halves is, it's a mechanical secondary linkage and a extra accelerator pump circuit.

 

[Knebel]

That carb is a double pumper style so I really think it needs separated. A quick fuel has screw in power valve restrictors so if he would need that extra he could change them to larger (not there yet). The issue I have with not separating the halves is, it's a mechanical secondary linkage and a extra accelerator pump circuit.

Oh yeah I haven't even thought about the double pumper.... I guess that still would be easiest to disconnect the secondaries then. Also what kind of linkage is on it? progressive or 1:1?

Without seperating the bores i think I'd do the same jets all around to get the cruise right and then finetune the WOT with the powervalve restrictions to get it to 12.7. But its its really easier when separating the primary from the secondary side.

I had tuned a holley pretty perfect with an AFR gauge and drilling the powervalve orfices and such, that is so easy to get adjusted right when you have the screw in restrictions! Also, to go 1AFR point richer you need 25% more fuel. Same with going leaner. you can also do the math to calculate orfice area and then think about where you gotta be to get 25..50% or whatever more ore less you need. Its tedious but puts you right in the ballpark.

 

[Knebel]

Okay here we go. I did some Math for you.

Put in 68s in the primaries and 74s in the Secondaries. That reduces "100%" Fuel on Primaries and "100%" on the Secondaries. for a total of 200%. 100% sounds like all the fuel lol...but its not, its relative to the area of the Jets. so a change of 25% drops 1AFR... 200% drops 4AFR. That would mean you go from a 10.5:1 cruise to a 14.5:1 cruise. This can be a tad variable due to fuel flow and such but I really expect it to be in the high 14s.

While you are at it, look at both metering blocks and tell me the number for the Powervalve restrictor sizes.

Check the cruise AFR, I bet it will be around 14.1. You gotta remember that cruise AFR could go as lean as 16:1. It just depends on if your Motor will be happy with it. But if we get it in the 14s you should be just fine.

Do one more test if the jetting is okay and go up a slight incline, then slowly put your foot down and watch vacuum and AFR when you hit the threshold where the Powervalve opens (6.5?). What will happen as you do this, the AFR will go leaner before that valve opens, so dont be surprised! If you do this test and the engine starts to go lean and stumble and cut out and then suddenly the Powervalve opens and the AFR goes richer, then we need a valve that opens sooner! Report the AFR number

If you decide to do WOT, stop instantly if you see it go above 13:1. Tell us the number you saw and ill help address that.


BTW, you went from the 78 jets to 74s. That gave a 0.6 AFR change (from 10.5:1 to 11.1:1). The total Area of a 78 Jet is 4775,94. 25% less (or 1 AFR point) would be a 1193.99 Area decrease (so..597 reduction per Jet). The 74 Jet area is 4298.66. That is a difference of 477 per jet! See how that is not quite 25% less fuel and only made a 0.6AFR difference? Mind you this is not linear because Jets flow different but I bet that with the suggested Jet numbers you will be in the Ballpark for fine tuning. The tricky part is that you cant just go off of one Jet but gotta balance 2 jets. Actually 4. It sounds kind of stupid but maybe the Carb is actually a little on the large side for your engine?

Let us know if you notice any stumble.



EDIT: @70bigblockdodge. The accel pump circuit would not have an effect on the cruise but rather just on initial acceleration. He could see that if it goes real lean or real rich momentarily when opening the throttle. Thats another area to fine tune but as long as there is no bogging down or backfire it should be okay!?

 

[70bigblockdodge]

. Also what kind of linkage is on it? progressive or 1:1?

It is progressive, starts about 1/3 open on the primaries.

While you are at it, look at both metering blocks and tell me the number for the Powervalve restrictor sizes.

No power valve in rear metering block. That is another reason to separate and make sure the secondary is not adding to the criuse because it has no lean out at higher vacuum.

 

[Knebel]

here is the thing. According to the calculation, if you had only the primaries it gave me a 55 Jet size. Thats way small and I would not do that.

If its progressive then maybe staying a little bigger than 74s would be good to try out. Maybe trying 68s in the primaries and leaving the secondaries alone for now since they should just come on at a little higher flow or rpm. I'm just guessing with that now because I only have tuned vacuum secondaries and wired them shut and then adjusted the opening point, and then the jetting...