I am in tuning my pair of qft 850 carb on my blower engine. The stock jeting is complenty off, 78 primary with power valve boost refenrence, 88 secondary no power valve , a way too rich in bottom and too lean at wot.

I have try diferent jeting setup to try to have a good afr at every part of throtle. To get a good afr at wot i raise the secondary main jet to #104 and #75 with leanner pcvr restrictor in the primary to having a good result at cruise speed. But with that setup i still have a lean spot that i dont like at 5000rpm 3/4 throtle like 12,2 afr at 5.5 - 6 boost.

I finaly blank all the 4 stage emulsion hole in the primary and the secondary change main jet to # 94 at the secondary same thing on the primary side and now i have no lean spot at 3/4 throtle and the afr is very stable under boost. I will probably get the same result by changing the air bleed .010'' smaller from what i have.

Anyone can explaine me more about how the emulsion hole channel is work and for what reason i have need to blank it all to get what i want?

If there was not any emulsion circuit/channels a carburetor would continue to meter more fuel (proportionally) as mass air flow increases. The emulsion circuit counter acts this by introducing air into the fuel stream at mass flow increases. The lower positions in the channels are for higher flow rates and higher postions for lower flow rates (main circuit operation only). Most newer Holley (and derivatives) carbs have too much emulsion circuit bleed area and too large of primary air bleeds thus causing what you where witnessing (rich low flow and lean higher; again main circuit only). Your issue is only excacerbated due to be force inducted.

Since you are not running a secondary power valve make sure that your primary jet area + PVCR area = secondary jet area so you have even distribution under high load/throttle.

If you still have issues with high flow lean out your main well and booster channels need to be opened up assuming your float bowls are staying full.

Im no pro but I have a lot of experience with blow through carbs and E85 blow through carbs in automotive applications. Stick with it you will get it.

What are you seeing for AFR say from 3000 to WOT?

I wonder if you were better off playing with power valve opening rates before getting into the bleeds. In other words, say you were rich at 3000RPM and below. Maybe switching to a 2.5 PV to delay the enrichment until you get into boost, cleaning up the low end. Then running a little more main jet in the primary, which plays a part in WOT also not just cruise.

Typically, a PV add's 8-10 jet sizes. So with your stock 78-88, you were more like 88-88 when PV was open. Which proves that's too lean based on your wot afr. Now, I probably would have stepped up to 82-92 main jet, to get WOT AFR closer, but delayed the PV opening point, so that its not opening until you're into boost.

I think the reason your seeing the AFR numbers your liking by blocking the emulsion holes is your simply giving the engine more fuel, which it probably needs at wot because of the small main jet you have (75). Your compensating for the small main jets basically.

Your linkage setup may also be an issue. I don't know how your propped, or what wot rpm is, etc, but is it possible the lean spot at 5000RPM is occurring during the transition from primary to secondary opening?

AFR At 3000rpm 12,5-12,8, 3500-3700 rpm (cruise speed) 0 vacuum 11-11,3 4000-4300 10,5-10,7 4500-5000 11,5 and 5000-5500 11,3 (WOT 5500-5600). This AFR # is with 75 primary jet 1.0 primary power valve all emulsion hole open in primary metering block #25 air bleed, in secondary jet # 94 all emulsion hole blank no power valve and #31 air bleed.

The lean spot at 5000 rpm is not at opening of the secondary its open at 3800rpm, I have corect the lean spot by installing smaller # air bleed i have pass from #31 to #25 (starting the main circuit sooner)

The carb linkage is progresive, can the 1 to 1 carb linkage is better on blower engine?
I think my afr is not too bad with the last result but i am trying to make it more consistant , dont want an afr @ 10.5 at 4000-4300 rpm.

I dont have experience with the 1:1 linkages, so I am no good to you there. I do think however it would be worth a shot considering how cheap and easy it is to convert. If you go this route just make sure your carb is "squared", all jets, bleeds, PVCR, etc is the same corner to corner, front to back.

Depending on where your secondaries are (angle) in the overly rich area you describe above it may be worth opening back up an emulsion bleed on your secondary block (or middle of the primary if your rear mains have not started) relatively high in the channel. Height of the bleed (emulsion) changes the timing of the effect, the size of the bleed changes the magnitude of the effect.

Thanks all for your help I will make some other carb test with the faster opening linkage or with the 1 to 1 secondary opening linkage