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Discussion Starter #1
Hi, I have a pump gas 548 BBC 725hp with a quickfuel FX-4712 1250 dominator carb. Powerglide trans and loose converter.3000lbs.
The carb ran good on the dyno, idle and WOT afr is good. On the street it goes rich rolling away from idle and bottoms at 10-1 afr at 3100 rpm. After that it cleans back up and rips. I need this rpm area cleaner to drive the car.
The carb has been molested before and has the int. bleeds removed-wide open holes. It has no power valve capability.
I noticed it has multiple linkage settings to bring in the secondaries. I was going to play with that and see if it helps.
The long game will be efi but id like to get a little use out of this fuel system if possible.

Thanks for any thoughts
 

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I run a QFX 1050 dominator on a 540 bbc on the street. Had same issues. Switched to 2 circuit metering blocks with powervalve in the front.
Few jetting and air bleed changes and it runs awesome now.
 

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Discussion Starter #3
I run a QFX 1050 dominator on a 540 bbc on the street. Had same issues. Switched to 2 circuit metering blocks with powervalve in the front.
Few jetting and air bleed changes and it runs awesome now.
Do you remember what p/n blocks?
 

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Discussion Starter #5
Update, I installed BLP 2 circuit metering blocks with a powervalve in the front. The idle is very lean with the screws backed out. I closed up the IAB with no change and drilled the primary IFRs a little with no change. It will run at 17-1 afr so I drove it and it shows good potential. The mains come in at 3200 rpm and it pulls hard until it goes lean again up top. Thanks for any ideas.
 

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If it isn't too much trouble, post the size of all the orifices, venturi diameter, MJ, MAB, E-bleeds (how many-how big?), IJ, IAB, T-slot jet, size of hole the point on the idle adjust needle passes through. Just the little stuff.
 

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Did you check the spark plugs? Mine will show 16-17 AFR on the O2 gauge at idle, but the plugs will darken up if I leave it idle for 5-10 minutes.
My thoughts are the O2 doesnt read correctly at idle because of reversion and small leaks in the slip together headers. If there is no stumble off idle I figure it is fine and tune off the O2 readings at part and full throttle.
 

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Discussion Starter #9
Thanks 67Mike for the baseline and Yeti for the interest.

Rookie mistake I failed to move the idle bleeds into the intermediate location when I converted to 2 circuit blocks.
Now it idles at 14.5-15 afr with 2 turns out on the screws.67Mike it does seem to respond correctly with full exhaust on the car. Open header it acted like you described.
Off idle it goes 11-12-10 afr through the rpm until the mains come in and it cleans up until 5500 or so and leans towards 15-1.
Even rich, it drives better than any dominator I've run on the street. The 2 circuit tuneup is way smoother than the original QFX.

BLP 2 circuit metering blocks
4.5 Powervalve in the front, rear blocked
.070 PVCR
.042 IFR front and rear
Pri jet 88
Sec jet 98
Emulsions
1 .028
2 plugged
3 .028
4 plugged
5 plugged

High speed air bleed .028
Idle Air Bleed .065

I'm not sure on the T slot and I didn't put the jets in the main body yet. I have no vacuum port so it was a guess on the powervalve.Maybe open at cruise?

I figure at least 4 numbers on the mains to get the WOT afr set.

One mystery on the metering blocks was the openings directly below the stack of emulsion jets. Kinda figure 8 shaped passage with a threaded spot for a jet.

Thanks again for all the ideas, I never thought a dominator could be happy on the street.
 

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a 4150 carb will not only do everything better on the street, it will run quicker on the track too than that way too big dominator atop this 725HP engine.
 

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One mystery on the metering blocks was the openings directly below the stack of emulsion jets. Kinda figure 8 shaped passage with a threaded spot for a jet.
I believe what you are describing is the drilled/threaded passage for the IFR if you choose to move it from the top of the block down to the bottom.

A lot of carb tuners will recommend an IFR below the float level, the "standard" position in Holley blocks is the top. From what I recall, the bottom location is supposed to provide more consistent metering since it is submerged and not effected by pressure changes in the intake tract.


Here's some good reading on the subject:

http://speedtalk.com/forum/viewtopic.php?p=452681




Cole
 

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Discussion Starter #12
I believe what you are describing is the drilled/threaded passage for the IFR if you choose to move it from the top of the block down to the bottom.

A lot of carb tuners will recommend an IFR below the float level, the "standard" position in Holley blocks is the top. From what I recall, the bottom location is supposed to provide more consistent metering since it is submerged and not effected by pressure changes in the intake tract.


Here's some good reading on the subject:

http://speedtalk.com/forum/viewtopic.php?p=452681




Cole
Thanks Cole. I did that mod years ago on a 4150. I'll try moving them down.
 

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Discussion Starter #13
a 4150 carb will not only do everything better on the street, it will run quicker on the track too than that way too big dominator atop this 725HP engine.
You might be right. I'm just working with what I have. It was a 900hp nitrous piece detuned with compression and cam removed. Not sure of its future.
I have 4150 carbs but they all took plenty of time and effort to get good AFR numbers even though they felt good on the street and ran decent numbers.
 

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without question I am right and it doesn't matter what this engine was, all that matters is what it is now, a 725Hp pump gas engine with a carb that is way too big for the RPM range this street cars operates in.
 

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without question I am right and it doesn't matter what this engine was, all that matters is what it is now, a 725Hp pump gas engine with a carb that is way too big for the RPM range this street cars operates in.
Without question you are an idiot and should shut up. Don't judge others by your own limitations because you don't understand engines and carburetors. Do yourself a favor, you have some loose rim screws.


On the other hand, if you are suggesting a QJet is the ideal carb for the street, you are absolutely correct. ;)
 

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Thanks 67Mike for the baseline and Yeti for the interest.

Rookie mistake I failed to move the idle bleeds into the intermediate location when I converted to 2 circuit blocks.
Now it idles at 14.5-15 afr with 2 turns out on the screws.67Mike it does seem to respond correctly with full exhaust on the car. Open header it acted like you described.
Off idle it goes 11-12-10 afr through the rpm until the mains come in and it cleans up until 5500 or so and leans towards 15-1.
Even rich, it drives better than any dominator I've run on the street. The 2 circuit tuneup is way smoother than the original QFX.

BLP 2 circuit metering blocks
4.5 Powervalve in the front, rear blocked
.070 PVCR
.042 IFR front and rear
Pri jet 88
Sec jet 98
Emulsions
1 .028
2 plugged
3 .028
4 plugged
5 plugged

High speed air bleed .028
Idle Air Bleed .065

I'm not sure on the T slot and I didn't put the jets in the main body yet. I have no vacuum port so it was a guess on the powervalve.Maybe open at cruise?

I figure at least 4 numbers on the mains to get the WOT afr set.

One mystery on the metering blocks was the openings directly below the stack of emulsion jets. Kinda figure 8 shaped passage with a threaded spot for a jet.

Thanks again for all the ideas, I never thought a dominator could be happy on the street.
You are on the hunt. You need to restrict the passage to the T-slot in the gasket face of the main body. .052" is a good start. The object is to progressively lean out the off-idle range that is currently too rich until it becomes so lean it balks and then go back richer until it runs well. You need to move the idle jet to the low position.
 

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yeti's advice is pretty good.

whatever you do, DO NOT waste your money on a pro-systems carb. you'll be back here asking how to get it to work.
 

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Discussion Starter #20
My Pro-Systems 1050 works very well on the street!
Only change I made was restricting T-slot.
I did the t-slot restrictors last night and saw a huge difference but haven't driven it yet. I specified street use on a custom build Pro systems dominator and it was garbage. Until now, out of 6 dominators I played with on the street the best was a 1050 and I think it just worked better because it was the smallest. As everybody stated the idle and transition circuits need work. Not that hard, just tedious.
I'm working the mains at the same time so its pretty damn fun. It goes faster and drives better every time I hit the street!!!
 
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