carb to big ?
Moderator: Team
-
- Guru
- Posts: 1608
- Joined: Sat Dec 24, 2011 10:02 am
- Location: melfort saskatchewan Canada
carb to big ?
What happens when a carb is just to big for the engine ? Does it run real rich ? Even if the carb is factory calibrated to supply the amount of fuel needed for the amount of air its supposed to deliver ?
thanks
Brian
thanks
Brian
The Word of God is quick and powerfull
www.therocketshop.blogspot.com
www.therocketshop.blogspot.com
- mt-engines
- Expert
- Posts: 879
- Joined: Fri Sep 27, 2019 12:35 pm
- Location: MN
Re: carb to big ?
A good carb runs just fine. One that sucks will be leanprairiehotrodder wrote: ↑Thu Jun 01, 2023 3:17 pm What happens when a carb is just to big for the engine ? Does it run real rich ? Even if the carb is factory calibrated to supply the amount of fuel needed for the amount of air its supposed to deliver ?
thanks
Brian
Re: carb to big ?
Yeah I got one of those custom lean sucks carbs currentlymt-engines wrote: ↑Thu Jun 01, 2023 3:27 pmA good carb runs just fine. One that sucks will be leanprairiehotrodder wrote: ↑Thu Jun 01, 2023 3:17 pm What happens when a carb is just to big for the engine ? Does it run real rich ? Even if the carb is factory calibrated to supply the amount of fuel needed for the amount of air its supposed to deliver ?
thanks
Brian
Re: carb to big ?
pretty much any carb no matter how large can be tuned to run on an engine. will require extensive tuning, and likely wont have very good throttle response. wont run rich, or lean just because its on a too small of engine. it will run rich or lean due to how its set up.
-
- Guru
- Posts: 4607
- Joined: Sun Sep 18, 2011 11:31 am
- Location: Heading for a bang up with Andromeda as we all are.
Re: carb to big ?
One can’t deny that big throttle bores require big shots of accelerator pump and that in and of itself can make for rich running just off idle with a small engine until that clears out.
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
Re: carb to big ?
No it does not nessessarily run "rich" by default.
A carb that is just too big will show low speed drivability issues low rpm throttle responce as it lacks air velocity at throttle input. Low air velocity equals poor fuel metering response.
But the jetting can be lean or rich ir right on the money.
The idle/ off idle transition can be sensetive.
Dependas what you bolt it onto.
A carb that is just too big will show low speed drivability issues low rpm throttle responce as it lacks air velocity at throttle input. Low air velocity equals poor fuel metering response.
But the jetting can be lean or rich ir right on the money.
The idle/ off idle transition can be sensetive.
Dependas what you bolt it onto.
-
- Guru
- Posts: 1608
- Joined: Sat Dec 24, 2011 10:02 am
- Location: melfort saskatchewan Canada
Re: carb to big ?
Thanks guys, I'm still helping my friend with his 423 SBC drag car. He has a QFT 1050 (4150 base) that wants to be leaned from the factory jetting alot for best ET and MPH. Doesn't seem right to me. Factory jetting is 86/93 and it wants 77 / 84 if i remember correctly. Whenever I see a carb want to go that far away from the base calibration i think something is wrong.
Brian
Brian
The Word of God is quick and powerfull
www.therocketshop.blogspot.com
www.therocketshop.blogspot.com
Re: carb to big ?
Annulars or downleg?prairiehotrodder wrote: ↑Thu Jun 01, 2023 6:59 pm Thanks guys, I'm still helping my friend with his 423 SBC drag car. He has a QFT 1050 (4150 base) that wants to be leaned from the factory jetting alot for best ET and MPH. Doesn't seem right to me. Factory jetting is 86/93 and it wants 77 / 84 if i remember correctly. Whenever I see a carb want to go that far away from the base calibration i think something is wrong.
Brian
Either way might be worth checking mab sizes in case there’s a factory slip up. .028” if downleg, .033” if annulars. That or a lack of emulsion holes in the metering block.
I’m a Street/Strip guy..... like to think outside the quadrilateral parallelogram.
Re: carb to big ?
Most QF carburetors I've had were all to fat out of the box, most recently my good Stocker friend had one of the QF 950's on his car and after dropping the rear jet size from 86 to 80 the car ran 3 tenths quicker that night. Quite a surprise for him on that one. He now has JL work over his NHRA Holley's and they are spot on.
Re: carb to big ?
Does this carburetor have power valves front and rear with both jetting sets.Also you can tune the air bleeds 2 are 3 settings from the stock sizes and also those QF carburetor have adjustable emulsion bleeds that can be changed over the jetting sizes.
Re: carb to big ?
You are racing at altitude. (elevation). will want smaller jets.. especiallt hot humid day. Is that carb got pri and sec power valves?prairiehotrodder wrote: ↑Thu Jun 01, 2023 6:59 pm Thanks guys, I'm still helping my friend with his 423 SBC drag car. He has a QFT 1050 (4150 base) that wants to be leaned from the factory jetting alot for best ET and MPH. Doesn't seem right to me. Factory jetting is 86/93 and it wants 77 / 84 if i remember correctly. Whenever I see a carb want to go that far away from the base calibration i think something is wrong.
Brian
-
- Expert
- Posts: 994
- Joined: Sun Mar 21, 2010 9:48 pm
- Location: Somewhere, Alaska
- Contact:
Re: carb to big ?
I know people running 1600 cfm carbs on small blocks...and no, not 10k rpm either. Carbs these days are not like the carbs of old. Newer booster designs, better metering block design/calibration makes what was once not possible, routine....and no, you won't always loose fuel economy or low speed drivability with a very large carb. One thing Dave Braswell told me about carbs was that if you can measure ANY vacuum below the throttle plates at WOT, you're leaving power on the table. Carbs can start pulling fuel with a depression as low as .1" hg. Contrary to popular belief, it's not the depression below the carb that pulls fuel, its the airflow through the venturi that pulls it.....and as I stated, because of better booster design, carbs can be made HUGE and still pull fuel correctly.....the old adage of when in doubt use a smaller carb is pretty much useless anymore unless someone either can't tune a carb or doesn't want to. Just like the formula for calculating the "ideal" size of carb. Was outdated more than 25 years ago and inaccurate then. Even more so now......
Mark Goulette
Owner/Driver of the Livin' The Dream rear engine dragster
Speed kills but it's better than going slow!
http://www.livinthedreamracing.com
Authorized Amsoil Retailer
Owner/Driver of the Livin' The Dream rear engine dragster
Speed kills but it's better than going slow!
http://www.livinthedreamracing.com
Authorized Amsoil Retailer
Re: carb to big ?
Amen.Coloradoracer wrote: ↑Fri Jun 02, 2023 12:52 am I know people running 1600 cfm carbs on small blocks...and no, not 10k rpm either. Carbs these days are not like the carbs of old. Newer booster designs, better metering block design/calibration makes what was once not possible, routine....and no, you won't always loose fuel economy or low speed drivability with a very large carb. One thing Dave Braswell told me about carbs was that if you can measure ANY vacuum below the throttle plates at WOT, you're leaving power on the table. Carbs can start pulling fuel with a depression as low as .1" hg. Contrary to popular belief, it's not the depression below the carb that pulls fuel, its the airflow through the venturi that pulls it.....and as I stated, because of better booster design, carbs can be made HUGE and still pull fuel correctly.....the old adage of when in doubt use a smaller carb is pretty much useless anymore unless someone either can't tune a carb or doesn't want to. Just like the formula for calculating the "ideal" size of carb. Was outdated more than 25 years ago and inaccurate then. Even more so now......
Re: carb to big ?
For what it's worth-
I was at Westech a couple years back and had a conversation Steve about "Good carbs". They had a 7,000 rpm 572 BBC he and run with a 1,200 carb. It was tuned very well with perfect 12.75:1 AF . A week later a customer had a car on the chassis dyno with a 6,500 rpm 427 sm blk. It had an 850 carb and they wanted to see of the 850 was holding it back. They swapped the carbs and the 1,200 produced the near same perfect 12.75:1 AF across the rpm range.
As Steve said, " A well tuned carb will adjust. Carbs are very versatile"
I was at Westech a couple years back and had a conversation Steve about "Good carbs". They had a 7,000 rpm 572 BBC he and run with a 1,200 carb. It was tuned very well with perfect 12.75:1 AF . A week later a customer had a car on the chassis dyno with a 6,500 rpm 427 sm blk. It had an 850 carb and they wanted to see of the 850 was holding it back. They swapped the carbs and the 1,200 produced the near same perfect 12.75:1 AF across the rpm range.
As Steve said, " A well tuned carb will adjust. Carbs are very versatile"