I have a 440 with a QF 850 mechanical.
When I let off the throttle, it leans out from 12-13.5 to 19.0-23.0. Occasionally stalling the motor.
Much worse when it is below normal operating temps.
Right now my idle is 13.0 on the recently calibrated AFR.
Cruise 13.5 - 14.5
Acell 10.5 - 12.0
For all you carb tuning guru's, how do i fix this? My QF 850 is very adjustable. I have a selection of jets and air bleeds and cams. I have played around with it and accel and cruise are great. But what do I do for the throttle "let" off lean out?
:dunno:
Maybe try smaller on the high speed air bleeds?
https://www.holley.com/blog/post/carb_class_air_bleed_basics/#:~:text=Idle%20Air%20Bleed%3A&text=Air%20to%20be%20mixed%20with,due%20an%20incorrect%20idle%20bleed.
You might check the throttle blade settings against the slot and assuming that is good, if Brad's idea did not work, I think I would try a bit smaller on the idle air bleeds. Sounds like it is over transitioning to the idle state when you let off.... maybe :D
Ok. A couple questions to go with the air bleed suggestion. How many numbers at a time? A couple? And should I expect to have to play with the idle mixture screws after each air bleed change?
Quote from: Strawdawg on May 04, 2021, 03:41:44 PM
Sounds like it is over transitioning to the idle state when you let off.... maybe :D
If I back off the throttle nice and easy, it goes back to a nice idle no problems. If I just lift my foot quickly, that's when I have issues. Is that 'over transitioning'?
I would say if you change the idle air bleeds then yes you will need to adjust the mixture screws. The high speed ones I don't think so.
What size does it come with " stock" ? 30ish?
https://www.summitracing.com/parts/qft-br-67245?seid=srese1&gclid=Cj0KCQjw4cOEBhDMARIsAA3XDRjnWKnLJGoXC18-Cj0F_dR4CfDg0fC0VFk2BV2YnU01gPZEJaiv0C4aAr89EALw_wcB
I'm cheap and just bought blanks and drilled them out myself. It takes a lot of time I found. Love the QF carbs though. I opened up the "bleeds"? behind the power valve when playing with mine . I could see the difference with each change on the A/F ratio gauge , sure couldn't do that as easy on a standard Holley.....
Of course weather conditions changes things, I found I needed to fully warm up the engine before I made any changes.... :alan2cents:
Mine is the opposite of yours .... goes rich when I let off.... still not totally happy with mine yet but would rather drive it than mess with it more. Free time to play seems to be in short supply these days....
That was my definition... But might not be the correct term. Might just try closing the needles some and see if it changes anything
Specs
Quote from: Brads70 on May 04, 2021, 03:58:08 PM
but would rather drive it than mess with it more. Free time to play seems to be in short supply these days....
Last year I would agree with you and have lived with my situation, tinkering with it every now and then..
But...
I retired three days ago and plan to do more 'messing with it' and driving it. :burnout:
Might as well...I can't go anywhere..lol
Quote from: dodj on May 04, 2021, 04:14:23 PM
Quote from: Brads70 on May 04, 2021, 03:58:08 PM
but would rather drive it than mess with it more. Free time to play seems to be in short supply these days....
Last year I would agree with you and have lived with my situation, tinkering with it every now and then..
But...
I retired three days ago and plan to do more 'messing with it' and driving it. :burnout:
Might as well...I can't go anywhere..lol
Congrats on retirement! New project car on the horizon? :Stirring: :D
Just looked up mine to compare specs. Close on air bleed size. Crazy how much they have gone up in price ! Wow! I think I paid about $600 USD 7-8 years ago....
https://www.summitracing.com/parts/qft-q-850-an
Quote from: Brads70 on May 04, 2021, 04:21:38 PM
Crazy how much they have gone up in price ! Wow! I think I paid about $600 USD 7-8 years ago....
Ya, I think I paid around $600. The price now would make a FI setup much more likely...
Quote from: dodj on May 05, 2021, 10:04:47 AM
Quote from: Brads70 on May 04, 2021, 04:21:38 PM
Crazy how much they have gone up in price ! Wow! I think I paid about $600 USD 7-8 years ago....
Ya, I think I paid around $600. The price now would make a FI setup much more likely...
:iagree:
That's now in the ballpark for a Holly Stealth 4150 EFI
https://www.holley.com/products/fuel_systems/fuel_injection/sniper_efi/sniper_efi_stealth_4150/sniper_efi_stealth_4150/parts/550-872
Makes you wonder if they are trying to steer more customer's to EFI.
Good luck on your carb tuning and congrats on your retirement.
Personal opinion but yeah, I know I'll never buy a new carburator again, ever.
I would make a significant change to make sure it helps so maybe decrease the size by 4-5 on the idle bleed .so the vacuum has to pull in more fuel . Basically I agree with all of the advice above .
Congrats on retirement a lot of us are getting to that age now .
Quote from: RUNCHARGER on May 05, 2021, 05:33:00 PM
Personal opinion but yeah, I know I'll never buy a new carburator again, ever.
I bought mine about the same time as Brad. I had a 3310 on there before. FI wasn't on my radar. Then some time after I bought the QF850 I saw a post on FITech. I remember then thinking 'maybe I should have spent the extra for FI...'
Oh well, it's a hobby right?
Quote from: Chryco Psycho on May 05, 2021, 09:56:05 PM
I would make a significant change to make sure it helps so maybe decrease the size by 4-5 on the idle bleed
Ok. Thanks
Dropped three sizes in idle bleeds. Initial testing is encouraging. Idle afr is in the mid 12s, but maybe that's what it has to have?
Did a couple things that would have stalled it before, and it kept going, so I think we're on the right track!
Haven't gone for a long drive yet.
Thanks all for suggestions
Glad to hear it is going in the right direction. Makes me wonder if a fuel passage is partially stopped altho it seems to have richened up. I guess there could have been some human error during assembly.
My experience with engines with decent cams in them is that the idle needs to be closer to 13-1 than 14-1 to get a good constant idle.
You all are right about the price of carbs. The less common, more performance oriented ones have shot way up in price for sure. At first glance, FI does seem cheaper, but to do it right, takes a in the tank pump (imo) and the plumbing, etc can add a lot of money.
Quote from: Strawdawg on May 14, 2021, 10:23:12 AM
My experience with engines with decent cams in them is that the idle needs to be closer to 13-1 than 14-1 to get a good constant idle.
:iagree: mine likes 12.5-12.9
Quote from: Strawdawg on May 14, 2021, 10:23:12 AM
My experience with engines with decent cams in them is that the idle needs to be closer to 13-1 than 14-1 to get a good constant idle.
It's a lunati 703. Installed it a few years ago when cc.c was still going strong. You helped me with degreeing and the prw rocker setup.
:bigthumb:
I remember that, faintly. But then, these days, I remember most things faintly....except for the '40's and 50's, I remember them pretty well! LOL
I assume you love the cam , it is a great choice !
Quote from: Chryco Psycho on May 14, 2021, 06:39:41 PM
I assume you love the cam , it is a great choice !
Yes!
It was your recommendation. :bigthumb: