Adjusting my QFT...

[Austria_Cobra]

Hello
Actually I am beginning to adjust my QFT on my Cobra. I would like to ask you a view things, to improve my knowledge of carburetors work.

351W stroked to 427cui
650QFT (I know it is a bit to small)
MSD Ignition 83541 Ready-to-Run, Coil Blaster, Ford Racing spark wires, Champion RC12YC, limiter to 5.500rpm

Basic setup on QFT 650 HR mechanical sec. (E-choke)
- primary throttle plates on square of transfer slot
- secondary throttle plates nearly closed
- primary main jet 72, secondary main jet 82
- idle air bleeds 70, main air bleeds 28
- idle restriction jet 28
- idle mixture screws at 1.5 turn
- idle speed screw 1/4 turn to square transfer slot
- fuel level at half of glas at 6.5psi (electric Fuel Pump with regulator)

startup of engine. worm up.
-> to get to an idle(850-900rpm) with the most inHg(10) on main manifold, the idle mixture screws ar at 0.5 turns, 18° ignition, idle speed screw at 0.5 turn.

test drive
-> on thrust load and part load the engine starts shaking, is not running smooth. Feels like one cylinder does not have an ignition. idle mixture screws to 0.6 turns made it quite better, but still not fine.
-> after pushing the gas pedal 2mm the engine runs great until limiter with much power

OK, this is the situation right now.

First thing I would like to know is, what are the steps to do, to get out the idle mixture screws to 1.5 turns (is there a difference on small adjustments with 0.5 turn and 1.5 turns ?), or should I leave it as it is.

I just got an AFR gauge and this will be the next step for me to find out if the engine leans or richens on thrust load and part load. (should lean)

The next thing will be adjusting idle and part load. Therefore I have to adjust the idle air bleeds and restriction jets.
My actual thinking about that is, that I have to adjust that for part load. Because the mixture is coming out of transfer slot and no other adjustment can be done. The idle mixture screws have to be adjusted after, to get back to a good idle. I am correct ?
This will also take affect that my idle speed screw can go back to 1/4 turn, correct ?
I think of a lean mixture, that needs to richen up (air bleed smaller, restrictor bigger), means also that my idle mixture screw has to go in, but I am at 0.5 turns...?
Because of this fix situation to transfer slot, I do not see a way to get the idle mixture screws to 1.5 turn out, wrong?

Thx
Richard

[Hotfingrs]

You don't say what size squirter you have, but sounds like you need a bigger one. Usually something in the 32-36 range is in order.

[Austria_Cobra]

Originally Posted by Hotfingrs

You don't say what size squirter you have, but sounds like you need a bigger one. Usually something in the 32-36 range is in order.

Hi
Squirter at idle, thrust and part load ?
Squirter is for pushing the gas pedal and works with the acc. pump.
Regards.

[Hotfingrs]

Originally Posted by Austria_Cobra

Hi
Squirter at idle, thrust and part load ?
Squirter is for pushing the gas pedal and works with the acc. pump.
Regards.

Squirter is located by the vent tube with the phillips head. Helps on thrust and part load by giving a little extra gas. Could also be to much slack in the acc pump lever.

[Austria_Cobra]

Originally Posted by Hotfingrs

Squirter is located by the vent tube with the phillips head. Helps on thrust and part load by giving a little extra gas. Could also be to much slack in the acc pump lever.

OK, then I was talking about an other part ??
But do not get the point you mean, could you please explain.
THX

[Hotfingrs]

When the acc pump lever is pushed, the gas comes out the squirter. You could need a larger amount of gas for the transition, so you would need a larger squirter size.

The accelerator pump nozzels or “squirters” are sized according to the size of the carburetor. Different intake and cylinder head combinations may require more or less pump shot. If the engine seems to have an off-idle stumble, changing the size of the squirter itself can fix this issue. The first thing to do is determine whether it’s a lean or a rich stumble.

If the engine seems to take a deep breath and then accelerate quickly (almost like the iginition cuts off for a split second), it’s generally a lean stumble. Check the size of the squirter on the primary side and install the next largest size. If that seems to help, but not completely eliminate it, try increasing the size of the secondary squirter.
If the engine seems to have a “blubber” or is lazy to accelerate then cleans up as it builds RPM, it’s generally a rich stumble. Check the size of the squirter on the primary side and install the next smallest size. If that seems to help, but not completely eliminate it, try decreasing the size of the secondary squirter.

[Austria_Cobra]

OK, we are talking about the same part, but the acc. pump including squirter is not active on my described problem in #1.
I do not pusch the pedal, do not need a pump shot.
The thing is that at thrust load and the rpm go down the engine starts shaking.
Also when I hold the gas pedal just 1mm the engine shakes. Thats the mixture beween idle and part load.
The other thing is that my idle mixture screw is at 0,6 turns. Would like to get it more out (1,5 turns).

[Wbulk]

The Quick Fuels are a good carb and have a lot of adjustability, but they are sensitive to temperature. Meaning the AF ratio will change with the temp of the engine and the outside temp. You did not say what altitude you are at. It does not matter if you are .5 or 1.5 or turns out. Cubic inches and altitude have a lot to do with it. Set the idle AF mixture screws for highest vacuum. What you are reading about the screws being out about 1.5 turns is mostly for the guys that live at sea level. I live at 5,000 ft and mine are .5 turns out. If you have a wide band O2 sensor you need to get it hooked up. Without it you will not be able to tell if the idle transfer circuit, main metering or power valve delivery is correct. They are all adjustable on this carb.

[Austria_Cobra]

Good Morning
I live in Austria (Mid of Europe) at about 400m above sea level. Temperatur is about 15-30°C.

[ha_pe_1968]

Hi Richard!

Had the same problem with my Roush 427R and QFT750. Changing the inital timing, Powervalve and increasing the mainjet was my first way to success. I still have a small lean spot between 1.400 and 1.600 rpm at cruising but think (and hope) i will solve it with the squirters. Have more or less the same sea level like you but different engine, carb size and carb type - so sharing my jetting would not help you. Had an emission test with CO and HC (AFR) that helps a lot.
If you are a member from ÖAMTC you will have access to an emission test.

Greetings from the other end from Austria

Peter

[Cheesehead]

If your engine starts shaking it could also mean that your ignition is too advanced at that rpm. Somewhere in your post I read 18 degrees. That seems much to me. I have also a 351W stroked to 427. My engine likes 14 degrees idle advance. Total advance is 33 degrees at 2500 rpm.
From 850 rpm upwards it starts advancing. Usually this number is about 100 to 150 rpm above idle. My idle is 700 rpm.

[Austria_Cobra]

Hi Peter
I have a MTX-L now(not installed jet) and will get a full mobile measuring tool (specially made with all you need: AFR/Lambda, preasure, exhaust temp, angle of throttle plate, knock sensor...) in July.
Hope I can get some results in a view days.
Waiting (for months) for a view parts to my distributor tester, then the MSD will be setted up completely (indoor).
Actually the ignition timing is 18° to 34° (at about 3000U/min). Can is disconnected.
My Hermann MHC 222(4-Gas) is not operational, needs to be repaird and a service.
So.. a good time to go to have all together..
Now I am looking for theoretical input to improve my knowledge about carbs.
Regards from Uppper Austria (Wels)

[ha_pe_1968]

Hi Richard!
Timing sounds good to me, take a look at the distributor springs too - normally "heavy" ones are installed....this means additional timing comes very slowly....this was another problem at mine.
Seems you are well organized :-)

Greetings from Vorarlberg
Peter

[Austria_Cobra]

Hi
Distributor is out of the box, that means conservative setup. Think it's good enough for just run the engine. Optimizing time will be in July with the needed parts for my tester.
Actually the engine shaking is at thrust and part load (1mm on gas pedal).

We also found a major problem on the gas pedal mount ! It has a major design error. A friends gas pedal (1608) broke and mine (618) has a clearance of 2cm. Look at yours! Should not have a clearance, if it has, your throttels will not open complete

[ha_pe_1968]

Hi Richard!

I know about the gas pedal issue. Mine is great till now (2234) but do not know if they made some improvements over the years. At Craigs Cobras there is a nice fix for this problem :-)
Had the same lean spot on part load (1-2mm travel) was solved with Powervalve and main jets.

Peter

[Austria_Cobra]

Hi Peter
Craig did it, but there are other more fine ways to repair.
Powervalve, I did not get in 100% until now how the valve works, does it open on preasure (mine 45=4,5inHg) ? Do you know any printings and descriptions ?
Richard

[ha_pe_1968]

Hi Richard!

Yep.. the valve opens on pressure....you will find lots of infos on the QFT or Holley website. But lot's of mysteries around...some says the right one is half the vacuum....others says 0.7-0.8 x vacuum at idle is the right one.
However mostly 6.5 is delivered with the carburator...for a heavy streetfriendly car it will go more in 8.0 direction for something with a more aggresive cam it will go more in 4.5 direction.

Greetings
Peter

[Austria_Cobra]

Hi Peter
Yes, there are different meanings about how to choose the number of the PV. That's why I am not 100% fit on this part. But will be shown on tests.
My 4.5 seam to be good for my vacuum, but at 1mm gas pedal the PV is not active. The vac is over 10inHg. So it has no effect, also it works in the main circuit. Only when the vac drops down, by pushing the pedal to the metal the PV opens and adds additional fuel to main circuit.
Regards
Richard

[jhv48]

Throw away the vacuum gauge and adjust the four idle screws so that you get the highest idle. Once you have determined this point, drive the car. If it still bucks on light throttle tip in and/or decelleration, open the idle screws another quarter turn and test drive (opening them is making the mixture richer). Keep doing this until it smooths out.

Unless your choke is slow to open, or remaining partly closed, idle screws that are only open 1/2 turn are quite lean (perhaps compensating for high altitude or sticking choke).

[Wbulk]

The best thing you can do is watch Summit Racing's Quick Flicks for Holley Carb tuning on YouTube. Holley also has some. Your Quick Fuel has more adjustment than most Holley's. In addition to the air bleeds, and main jets, you can also change the idle circuit restrictors and power valve restrictors to get it right on. Many guys that live at sea level have no need or experience in doing this. The idle circuit restrictors control the amount of fuel to the idle and idle transfer circuits. Usually the benefit is greater in the idle transfer circuit where most driving mileage is realized. The power valve restrictors allow you to tailor the amount of fuel delivered when the power valve opens. This is why you need to hook up your O2 sensor.

I have been working on carbs since the mid 60s and have seen getting accurate information on a forum is very difficult, and can be way off base to dead on, but how would you know until you get some valid education. Watch the videos and then if you have question call Quick Fuels tech line.