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help me pick jets

Old 05-29-2021, 08:11 AM
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Default help me pick jets

Chevy 383, 10.3:1, Quickfuel Marine 750 which can be treated like a Holley double pumper with 4 corner idle. Has very similar design and many parts interchange.
I ran the boat yesterday, cooler temperatures, roughly 60 degrees, 500 ft above sea level. OK 504 feet.
I saw AFR 14 around the docks, 12.3 to 12.5 accelerating, 13.5 to 13.7 at high'ish cruise, like 3600 to 4000. Then at larger throttle openings it gets real happy and goes 12.3 or 12.5.
I'm sorry to say I forget what is in there. I suppose I could inventory my jet box and find the missing sizes to get it. But at this point I will just look at it as jets plus or minus.
I plan to increase the AFR in the primary side. I wonder how many sizes to go? 4? 6? And ought I take the same amount of jet out of my secondaries?

Thanks
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flashgordon (05-29-2021)
Old 05-29-2021, 08:54 AM
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Pretty close for out of the box.

Small steps, so Id+2 primary and -2secondary if you are saying your fast cruise does not have secondaries open at all.
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Old 05-29-2021, 09:16 AM
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I made note of the throttle position where the AFRs go to the 12's. I'll have to put it there and look at the bores. it's mechanical secondary so it should open them.
Either way it would be good to know how the stick works the throttles.
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Old 05-29-2021, 11:56 AM
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#1 - get a notebook
list all baseline parameters, list results
every change, list it and list results.

Put a throttle stop right before the secondaries start to open.
Then go jet the primaries to the afr you want.
After you get "primary wide open" jetted to suit you, make sure you like your idle transition. Do normal idle to cruise acceleration and see if the power valve is opening around the right spot to grunt you onto plane in the right zone.
After that remove the throttle stop and do a wot run and jet the secondary where you want it.
Then come back and creep into secondaries and watch the primary/secondary transition - the secondary pv might need tweaked.
Once you get those big parameters close,, you can start playing with air bleeds.
keep a notebook - each change will affect not only the part of the curve you are focused on, but will also affect wider change that might need addressed..
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flashgordon (05-29-2021)
Old 05-29-2021, 01:15 PM
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Originally Posted by mcollinstn View Post
#1 - get a notebook
list all baseline parameters, list results
every change, list it and list results.

Put a throttle stop right before the secondaries start to open.
Then go jet the primaries to the afr you want.
After you get "primary wide open" jetted to suit you, make sure you like your idle transition. Do normal idle to cruise acceleration and see if the power valve is opening around the right spot to grunt you onto plane in the right zone.
After that remove the throttle stop and do a wot run and jet the secondary where you want it.
Then come back and creep into secondaries and watch the primary/secondary transition - the secondary pv might need tweaked.
Once you get those big parameters close,, you can start playing with air bleeds.
keep a notebook - each change will affect not only the part of the curve you are focused on, but will also affect wider change that might need addressed..
if this isn't a +1 for efi. hook up afr meter,,tap some keys and off you go.
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Old 05-29-2021, 01:25 PM
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Originally Posted by sutphen 30 View Post
if this isn't a +1 for efi. hook up afr meter,,tap some keys and off you go.
EFI Snob
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Old 05-29-2021, 02:20 PM
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Too late, I already have the carburetor, t shirt, and hat.

Maybe the next engine I do...
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