Trying to dial in my QFT 750

[straightlinespeed]

I've been reading articles, I've got a wideband A/F gauge and been reading my plugs. Im stumped honestly as to which way to go with my carb at this point. Im trying to get maximum efficiency and everything I've been reading says I should be in the 14's idling and cruising, WOT I should be around 11.7. Im running 91, non oxy gas.

Engine specs and carb set up.

LA 360, Magnum heads, XE268H cam, Air Gap, QFT Slayer 750, TTI's, 727 with 2800 stall, 3.73 gears, 15in vacuum.

The QFT has 72 mains, 74 IAB, 6.5 PV

The A/F gauge says its running rich at idle when warming up its 11.3-12.5, It is rough idling on initial start up, for the first couple minutes and the gauge occasionally will hit 10.2. When warmed up at idle its 12.7-13.5. In gear 12.9-13.8.

Light cruise around town 45mph or less it is around 13.1-14.5

While cruising down the road at 60 and 3000 rpm its 10.8 to 12.7.

WOT 10.0

If I let the car sit and it cools down a little, and I start it back up. It idles very rough, the A/F gauge is all over the spectrum, from rich in the 11's to very lean in the 17's. I have to feather it, and rev it a few times to clear it out only after a couple minutes does it idle smooth.

I've increased the IAB's from 70's to the 74's per QFT's recommendation because it was carbon fouling the plugs, earlier this spring.

I've tried setting the idle mixture screws for highest vacuum reading, but that throws the A/F guage into a rich condition. To get the A/F as close to 14 as possible Im actually dropping the vacuum reading usually about 1 to 1.5 points. I've checked for vacuum leaks and have none, I've disconnected my PCV and that makes no difference.

I have not done the WOT shut down to check the plugs, but just a overall. I figure they should be that nice golden brown/tan color all the time. I've put about 150 miles on the plugs. The ceramic is bright white almost like new and the electrode is grey, all the plugs look exactly the same. Im running Champion RC12LC4, stock plugs for magnum heads. This says lean condition all the way, but the gauge is saying otherwise. Im confused!!!!

View attachment IMG_20150621_171210604.jpg

Oh and the marks on the ceramic is anti-seize.

I would appreciate any help as to what to do.

 

[Woods74]

I have the same carb and have worked it out for the most part, first off, have you read this?

http://ls1tech.com/forums/carburete...t-level-holley-tuning-serious-ocd-tuners.html

http://www.burtonmachine.com/documentationtechnical/

That power valve calibration sounds rich, 12.8:1 sounds more like it, it comes down to what your engine pulls best at. Idle A/F ratio is the same, wherever it want's to be smoothest at.

Have you bought a box of brass set screws? 6-32 8-32 and 10-24 from McMaster/Carr? 81-60 drill index with a pin vice? (Amazon) This way you can calibrate inexpensively.

If you haven't, do that first.

I'd start here, if you need help I can guide you. Start with the stock holley calibration from the 60's, go from there.




I had to dial back my idle feed quite a bit, pretty much EVERYTHING including airbleeds quite a bit. These carbs are setup for healthy big blocks as far as I'm concerned. Rich as hell!!!

 

[Woods74]

My QFT Slayer 750 calibration looks like this:

360 Magnum stock bottom end, RHS heads

Cam is .525/.525 lift, 226 @ .050 duration, 112 LSA. 18" Vacuum.

Crosswind intake.

Headers, 2.5" exhaust crossover pipe, duals.

Idle Jet: .028 (6-32 tap/set screw)
Main: 70
Secondary: 76
Main IAB: .028 (10-32)
Secondary IAB: .035
PVCR's: .028? (6-8-32) about to get smaller
9.5" PV
Vacuum secondary spring: Plain steel
.028 Squirter (brass tips)
Blue cam #1 step.

Idles at 12:1
Cruises at 14.5-15:5:1
PV and WOT at 12.8:1

Cleanly flawless transitions... Hope this helps get an idea where you should be.

 

[straightlinespeed]

I have the same carb and have worked it out for the most part, first off, have you read this?

http://ls1tech.com/forums/carburete...t-level-holley-tuning-serious-ocd-tuners.html

http://www.burtonmachine.com/documentationtechnical/

That power valve calibration sounds rich, 12.8:1 sounds more like it, it comes down to what your engine pulls best at. Idle A/F ratio is the same, wherever it want's to be smoothest at.

Have you bought a box of brass set screws? 6-32 8-32 and 10-24? 81-60 drill index with a pin vice? This way you can calibrate inexpensively.

If you haven't, do that first.

I'd start here, if you need help I can guide you. Start with the stock holley calibration from the 60's, go from there.

I had to dial back my idle feed quite a bit, pretty much EVERYTHING including airbleeds quite a bit. These carbs are setup for healthy big blocks as far as I'm concerned. Rich as hell!!!

I forgot that you are running the same carb. I've read the LS1 article and got into the Yellowbullet one, but thought that was a little to advanced for what I needed so I stopped, but I can continue. I was on page 3 of that one.

I was thinking that I need to increase my PV to a 7.5 and downsize my main jets.

For the idle air should I increase it for maximum vacuum or just see where it runs the smoothest at?

No I have not bought the set screws, drill index or pin vises yet? Like I said I thought that was to advanced for what I needed, but I most certainly can.

Thanks for the offer to help set up it up. When I get the stuff I'll get back to you.

 

[Woods74]

I can condense then information, really, it WAS a long read, but the basics are easy, I can walk you through it.

I'd wait on ordering a power valve until you can get a good baseline going with the idle A/F. Since I can pull 18" I'm pretty sure you can with a smaller cam.

Shopping list is:

McMaster-Carr

Brass cup point set screws:

http://www.mcmaster.com/ 6-32 x 1/...www.amazon.com/dp/B000VNPRNQ/?tag=joeychgo-20

OR something similar

You:

A chuck/chuck-less drill of some sort.

PM me when you get all that, we'll get her tuned up!

 

[straightlinespeed]

Thanks man! I already have the drill bits on order, I can get the set screws and everything else with in the next few days!

 

[Woods74]

Excellent, I'm going to be doing another round of tuning real soon so it will be fresh on my mind!

You may also need a small assortment of drill bits, namely something that is around .040 to drill out the e-bleeds and IFR channel for tapping. Same goes for the PVCR's. They might be a little bigger....

 

[crackedback]

The Burton write up is very good. Great info.

 

[jefflock]

I take it you drill holes in the brass set screws to change the air bleeds sizes instead of purchasing them? The air bleed kit is pretty pricey.

 

[rmchrgr]

Sounds like you might want to try and go back to a baseline. Not sure if you have you changed jets/air bleeds? I'd put the original jets, air bleeds and P.V. back in. Then, verify everything - transfer slots, floats, linkage. Start with the mixture screws 1 1/2 turns out and adjust idle speed to where you want it.

Also, 33 squirter and blue pump cam might be too much for that gear and converter. Maybe go to a smaller nozzle like a 28-30? And maybe a different pump cam that does not dump all the fuel so fast. You can play with the holes on the pump cam too if needed.

What is your timing set at? (initial and total). Obviously, if it's not dialed in right you will be chasing your tail. Maybe try and verify initial with a vacuum gauge - advance until vacuum stops increasing then back it off 1 degree. Might have to adjust your total depending where you end up. Vacuum advance plugged of course.

I believe you are getting ahead of yourself with jets and air bleeds for idle and part throttle driving. Air bleeds are for fine tuning when you have it in the ball park, you need to make some more coarse adjustments first. Jets are for WOT for the most part which it sounds like you have not messed with too much.

Once you get it to idle where you want it, drive it around with a vacuum gauge which will help verify which power valve you need. It will be especially useful with part throttle driving. The A/F gauge will corroborate the vacuum gauge.

 

[TrailBeast]

And one other thing.
In my own experience, trying to get it to 14 to 1 on the A/F gauge will cause it to be to lean to actually run good.
I run mine at about 12.5 or 13 to 1 for a smoother idle and mid range, and about 12.5 -13 to one WOT

 

[Abodybomber]

My QFT Slayer 750 calibration looks like this:

360 Magnum stock bottom end, RHS heads

Cam is .525/.525 lift, 226 @ .050 duration, 112 LSA.

Crosswind intake.

Headers, 2.5" exhaust crossover pipe, duals.

Idle Jet: .028 (6-32 tap/set screw)
Main: 70
Secondary: 76
Main IAB: .028 (10-32)
Secondary IAB: .035
PVCR's: .028? (6-8-32) about to get smaller
9.5" PV
Vacuum secondary spring: Plain steel
.33 Squirter (brass tips)
Blue cam #1 step.

Idles at 12:1
Cruises at 14.5-15:5:1
PV and WOT at 12.8:1

Cleanly flawless transitions... Hope this helps get an idea where you should be.

Thanks,Poison... Have the same basic combination, same carb. Some great ideas,for future references.

 

[RustyRatRod]

Flame away, but IMO your 750 is way too much carb. That's why you're having so much trouble. An out of the box Eddy 650 would be beautiful. Bolt it on and hold on.

I wouldda throwed that 750 as far as possible by now.

 

[straightlinespeed]

The Burton write up is very good. Great info.

I read thru this article last night! Damn good info

Sounds like you might want to try and go back to a baseline. Not sure if you have you changed jets/air bleeds? I'd put the original jets, air bleeds and P.V. back in. Then, verify everything - transfer slots, floats, linkage. Start with the mixture screws 1 1/2 turns out and adjust idle speed to where you want it.

Also, 33 squirter and blue pump cam might be too much for that gear and converter. Maybe go to a smaller nozzle like a 28-30? And maybe a different pump cam that does not dump all the fuel so fast. You can play with the holes on the pump cam too if needed.

What is your timing set at? (initial and total). Obviously, if it's not dialed in right you will be chasing your tail. Maybe try and verify initial with a vacuum gauge - advance until vacuum stops increasing then back it off 1 degree. Might have to adjust your total depending where you end up. Vacuum advance plugged of course.

I believe you are getting ahead of yourself with jets and air bleeds for idle and part throttle driving. Air bleeds are for fine tuning when you have it in the ball park, you need to make some more coarse adjustments first. Jets are for WOT for the most part which it sounds like you have not messed with too much.

Once you get it to idle where you want it, drive it around with a vacuum gauge which will help verify which power valve you need. It will be especially useful with part throttle driving. The A/F gauge will corroborate the vacuum gauge.

The carb is at baseline, the only thing I changed was the IAB's 4 sizes larger to help with the carbon fouling of the plugs. I looked at the tslots yesterday before I took it for a drive and I squared them up. The one thing I did notice is 1 slot was longer than the other, so I kind of averaged it for the two. This carb came with the orange pump cam, not the blue. Timing is 18 initial and 35 total, I have not checked it with a vacuum gauge, but that is a simple check to do.

I think Im going to take that Burton article to the garage with me and go thru it, step by step to try to get it close.

And one other thing.
In my own experience, trying to get it to 14 to 1 on the A/F gauge will cause it to be to lean to actually run good.
I run mine at about 12.5 or 13 to 1 for a smoother idle and mid range, and about 12.5 -13 to one WOT

Thanks for the info!

Flame away, but IMO your 750 is way too much carb. That's why you're having so much trouble. An out of the box Eddy 650 would be beautiful. Bolt it on and hold on.

I wouldda throwed that 750 as far as possible by now.

I know we have talked about it before, but sadly I just dont have the money to throw towards a new carb. Besides I like pulling my hair out LOL

 

[rmchrgr]

The carb is at baseline, the only thing I changed was the IAB's 4 sizes larger to help with the carbon fouling of the plugs. I looked at the tslots yesterday before I took it for a drive and I squared them up. The one thing I did notice is 1 slot was longer than the other, so I kind of averaged it for the two. This carb came with the orange pump cam, not the blue. Timing is 18 initial and 35 total, I have not checked it with a vacuum gauge, but that is a simple check to do.

I maintain that fouling is generally more of a timing issue than carb related. 18 initial seems like a lot for that combo. My last combo was much gnarlier than yours is (idled at 1,100) and that's pretty much where my timing was set. Can I suggest backing it down a few degrees then re-set the idle to where you want it? See if that may give you an inch or two more vacuum. I have a hunch it might be easier to tune that way, you can always go back to where it was if it doesn't do anything. Are your ignition components in good shape? Maybe you need a one step hotter plug? Did you check the gaps, etc?

I also believe your part throttle issues are power valve related. That's where driving with the vacuum gauge comes in handy, you can see what vacuum you are cruising at and whether the power valve is dumping fuel unnecessarily.

Did you take the carb off to check the t-slots? They should be square under the throttle blades. .020" is the generally accepted measurement.

Do you have a fuel pressure gauge? If it's too high then you could be pushing fuel past the needle and seat but I doubt that here.

Where are your floats set? Does this carb have clear sight glasses? On a double pumper, Quick Fuel wants the floats set to be halfway up the glass, not just sloshing out the bottom when you shake the car like on an older Holley. Not sure what they would say about this carb though, maybe lower it a tad and see if it does anything.

And again, the blue pump cam and 33 nozzle seems like overkill. You didn't mention any bog off the line. I'd go back to the orange or maybe even a pink one if you have an assortment. The cams all deliver the same volume of fuel but over a different amount of time. Again, the blue is like quickly dumping the whole shot all at once while the other cams are more of a steady stream over the opening of the throttle.