carb to big ?

[prairiehotrodder]

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

[mt-engines]

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

A good carb runs just fine. One that sucks will be lean

[Elroy]

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

A good carb runs just fine. One that sucks will be lean

Yeah I got one of those custom lean sucks carbs currently

[lefty o]

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.

[mag2555]

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.

[F-BIRD'88]

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.

[prairiehotrodder]

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

[HQM383]

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

Annulars or downleg?

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.

[1980RS]

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.

[1972ho]

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.

[1972ho]

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.

https://www.quickfueltechnology.com.au/ ... s-AUST.pdf

[F-BIRD'88]

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

You are racing at altitude. (elevation). will want smaller jets.. especiallt hot humid day. Is that carb got pri and sec power valves?

[Alaskaracer]

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......

[289nate]

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......

Amen.

[JC565Ford]

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"