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Old 11-03-2012, 04:16 PM
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Carb set up help.

So continuing my budget beater 350 vette build, as a couple people had suggested I picked up a used Performer rpm for 50$ a couple weeks ago, yest I got a used b&m 20413 holeshot converter from a friend that is stepping up to a 4500 so having been in his car I know it works well. And today I picked up a used 750dp, 4779. I had a 650 dp and traded it for the 750 as the guy is class racing 358 dirt modified and the 750 isn't legal.
So what I'm trying to determine is a decent baseline for the Carb, squirter size etc. It has 30cc accel pumps now, 70/80 jets, and 28/37! Squirters.
Its going on

350, stock bottom end, 4 valve relief flat tops
416 heads, 1094 headgaskets
Lunati 224/234 , 465/488
Performer rpm
Headers to 3.5spiral flows as sidepipes, 3" all the way down the sides
TH350, 3000 stall
411 gears
Timing at 18 initial, 38 total, vacuum advance hooked up to manifold.
Runs on 94 exclusively with zero pinging
Lumpy 77 Vette.
Have a qjet on it now, but when i install the converter and intake this week I will be switching to the Holley so looking for a good baseline and i will creep up on it till the snow flies.
Thanks
Dave

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Old 11-03-2012, 07:58 PM
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Lunati 224/234 , 465/488 Thats not the same cam you said you had before...
why the different specs now? what is the Lunati cam part number..
(nothing wrong with it, just wondering)

4779 750carb. 70 pri and 80sec jets are fine. (this will probabily be your best jetting in the mid hot summer Ottawa weather.
carb jetting range will be 70 to 74 primary and 80 to 85 sec. Get some
rear jet extensions.and the matching notched rear float . track testing will tell you what the best jetting will be (track MPH)
.031 pri and .028 to .031 sec shooters is good. The .037's are probabily too big.
the correct accelorator pump cams are the most critical thing.

If that is the actual cam specs, recurve the distributor for 24-26 deg initial base at idle (no vacuum advance)
and 34 to 36deg total max advance.. (a 10deg mech advance curve) then use ported vacuum advance.

If thats the cam you really got you should have gone for the 3500 stall converter...but what is done is done.

unless the balancer timing tab location is fubared 38deg is a hair too much total timing. track testing will show you.
usually its 34 to 36deg for best power and track MPH with those heads.
It will run best with a bit more inital base and abit less total (shorter mech adv curve length)

Last edited by F-BIRD'88; 11-03-2012 at 08:12 PM.
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Old 11-03-2012, 08:00 PM
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When I used a 750 I think my jets were 71/79,power valve was changed to match vacuum. squirters were stock.It worked quite well.I had an open plenum manifold.
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Old 11-03-2012, 08:39 PM
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When using the RPM hi rise maniifold on a corvette you want to use this
OEM corvette air cleaner. Best under hood clearance.
When using this air cleaner base and a 14x3 " element and a holley 4779 carb
the carb choke horn gets too close to the air cleaner top.
The cure is to remove the choke horn. (the factory OEM holleys had down leg boosters
and a shorter choke horn. This is not an issue with a Qjet carb , just the aftermarket holleys
with straight leg style boosters and the taller choke.
The Holley Proform HP 750 style carb body kit is a great up grade.
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Old 11-03-2012, 09:54 PM
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Your completely right, In the garage now actually, lunati 10001, 214/223, 443/465, oops.....
Does that make my current timing curve a little more realistic?
I'm running an after market L88 hood and have lots of Clearance, as it sits now with the performer, the qjet, 1" Carb spacer, and a non dropped base air cleaner I still have at least an 1.5" clearance.
Will def order squirters this week, I have a jet kit and a bunch of Holley stuff laying around, just no squirters for some reason.
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Old 11-03-2012, 10:28 PM
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Quote:
Originally Posted by bygddy View Post
Your completely right, In the garage now actually, lunati 10001, 214/223, 443/465, oops.....
Does that make my current timing curve a little more realistic?
Ya just the 38deg is a bit too much. ( assuming accurate timing tab)

I'm running an after market L88 hood and have lots of Clearance, as it sits now with the performer, the qjet, 1" Carb spacer, and a non dropped base air cleaner I still have at least an 1.5" clearance.
Will def order squirters this week, I have a jet kit and a bunch of Holley stuff laying around, just no squirters for some reason.
Get the accellerator pump cams set.

Los of room for sure, with a L88 hood.

The holley and RPM manifold should run pretty good,
Getting the amount of exhaust heat riser flow volume up to the
manifold plenum to heat the holley carb just right is critical.
Full open unrestricted flow is too hot for Hot August Ottawa cruising. Makes the carb too hot.
full blocked is too cold for cold Ottawa winter driving ( if quick winter warm up matters)

One side of the intake manifold heat passage blocked off and one side restricted with a 1/4 to 1/2" hole at the intake gasket, is a good compromise for summer/winter driving. Getting this just right makes a good difference and takes some work.
Full blocked off heat riser passages is best for hot summer and max power.
(I prefer to fill the {twinned} exhaust heat /egr passages in the exhaust ports of the
416 heads with molten aluminum (old melted down pistons)

You can try the Qjet carb with an adapter, on the rpm intake too.
I recomend a divided/split style plenum Qjet adapter/spacer
(Not 4 hole) Now you will have two good carbs for your RPM manifold.
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Old 11-03-2012, 10:34 PM
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Sometimes the accelerator pump linkage arm gets bent , on used double pumpers.
Needs to be bent back to shape to get proper fuel shot action.
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Old 11-03-2012, 11:13 PM
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Fbird is right I bought used double pumper both the arms were bent had fix that and another thing u need do is look at the air gap intake and u will see where they have cut small slot out of the divider on the rpm it is best to do that to it will make little more power
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Old 11-03-2012, 11:31 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
Get the accellerator pump cams set.

Los of room for sure, with a L88 hood.

The holley and RPM manifold should run pretty good,
Getting the amount of exhaust heat riser flow volume up to the
manifold plenum to heat the holley carb just right is critical.
Full open unrestricted flow is too hot for Hot August Ottawa cruising. Makes the carb too hot.
full blocked is too cold for cold Ottawa winter driving ( if quick winter warm up matters)

One side of the intake manifold heat passage blocked off and one side restricted with a 1/4 to 1/2" hole at the intake gasket, is a good compromise for summer/winter driving. Getting this just right makes a good difference and takes some work.
Full blocked off heat riser passages is best for hot summer and max power.
(I prefer to fill the {twinned} exhaust heat /egr passages in the exhaust ports of the
416 heads with molten aluminum (old melted down pistons)

You can try the Qjet carb with an adapter, on the rpm intake too.
I recomend a divided/split style plenum Qjet adapter/spacer
(Not 4 hole) Now you will have two good carbs for your RPM manifold.
The felpro 1205 intake gaskets have the heatriser blocked off, that's what I'm using now,, Is this good enough or should I do more? Not worried about cold weather really, its a little cranky when its below zero like this morn, but it warms up OK and then is fine. And as soon as snow hits it will get parked and taken apart till springtime for paint.
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Old 11-03-2012, 11:33 PM
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Quote:
Originally Posted by F-BIRD'88 View Post
Sometimes the accelerator pump linkage arm gets bent , on used double pumpers.
Needs to be bent back to shape to get proper fuel shot action.
It seems as the second the throttle moves the pump arm moves so i belive its ok,what color cam do you reccomend?
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Old 11-04-2012, 08:24 AM
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Quote:
Originally Posted by bygddy View Post
It seems as the second the throttle moves the pump arm moves so i belive its ok,what color cam do you reccomend?
I recomend you get the kit with the different colours and test to find the
ones that work. Every car is different. There is like 6 or 8 pump cams
each has two or three mounting positions.
You can work with the .037" shooter by solding it up and drill by hand with a
1/32" drill (.031") ... if the .037 proves too big, (too fat a shot all at once)
(I think it will be)
From that point you can hone by hand with the drill to increase the shooter
.032 .034" etc. Usually the cam has more to do with it than the shooter size
itself. .028-.031 range.
A afr gauge is nice to use. (even a simple heated narrow band is good) lots of driving to evaluate. Engine must be warmed up.
correct plugs and gap. Need traction... the car will show you what it likes.
The 3000 high stall makes it easy. So does agressive initial base timing.
Use ported vacuum... you will see why, when you get into it.
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Old 11-04-2012, 08:33 AM
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The blocked riser is a ok for summer driving when its hot.
Incorrect manifold temp ( too cold for the day) will effect throttle response
and best jetting. so..... its up you you.
cold manifold cold day wants richer jets and more shot as the fuel vapourizes slower. (or a hotter manifold)
hot is opposite. (leaner jets smaller shot) Its up to you to realize this and tune, retune for the day
or find the happy average. (one side with limited flow, other side blocked seems to work, ok)
Its all about fuel vapourization....liquid fuel does not burn.

Before you install the carb, flip it over and preset the pri and sec throttles idle position/ transfer slot exposure evenly
Throttle response and "drivability has a ton to do with the idle --off idle transition circuit.
If this is out of wack no amount of fooling with jets and shooters will get it right.
This where the proform-holley 750 HP carb body upgrade comes in, with its swappable tunable idle and high speed air bleeds
as well as smooth air flow design.
Its all about dialing it in for YOUR MOTOR. These 750 carbs can be dialed in very very sharp.
Its up to you to do that.

Last edited by F-BIRD'88; 11-04-2012 at 09:03 AM.
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Old 11-04-2012, 09:11 AM
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Like I said , if you are willing to tune,, you will end up with two really good carbs for this manifold.
the holley 750 and the Qjet.

The split plenum Qjet to square bore carb adapter is rare.
You can make a custom one starting with the common 4 hole type Q jet to square bore carb adapter.
Just carve it into a split design. now you can mix and match the split with a open spacer combo.
to get that Qjet just right on the RPM manifold.
What colour is the Vette going to be? I used to restore those cars for a living.
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Old 11-04-2012, 09:18 AM
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Quote:
Originally Posted by F-BIRD'88 View Post
The blocked riser is a ok for summer driving when its hot.
Incorrect manifold temp ( too cold for the day) will effect throttle response
and best jetting. so..... its up you you.
cold manifold cold day wants richer jets and more shot as the fuel vapourizes slower. (or a hotter manifold)
hot is opposite. (leaner jets smaller shot) Its up to you to realize this and tune, retune for the day
or find the happy average. (one side with limited flow, other side blocked seems to work, ok)
Its all about fuel vapourization....liquid fuel does not burn.

Before you install the carb, flip it over and preset the pri and sec throttles idle position/ transfer slot exposure evenly
Throttle response and "drivability has a ton to do with the idle --off idle transition circuit.
If this is out of wack no amount of fooling with jets and shooters will get it right.
This where the proform-holley 750 HP carb body upgrade comes in, with its swappable tunable idle and high speed air bleeds
as well as smooth air flow design.
Its all about dialing it in for YOUR MOTOR. These 750 carbs can be dialed in very very sharp.
Its up to you to do that.
The squirters are fairly inexpensive so I'm going to order a 31 and see how that works. Currently the primary transfer slot is squared up, and the secondary is closed up. I will be pulling the HEI apart and limiting advance as you suggested to more of a 15-16 curve so i can get a liitle more aggresive with my initial, then switching to ported source for my vacuum so things don't get real stupid at idle. With such a mild cam and more initial I hope I will be OK running on the primary idle circuit and leaving the secondary closed up?
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Old 11-04-2012, 09:33 AM
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With such a mild cam and more initial I hope I will be OK running on the primary idle circuit and leaving the secondary closed up?

This is wrong. The holley carb idles using both the primary and seconday throttles.
It idles on all 4 barrels at once.


set it up so all 4 are even. You want even "4 corner idle" Thats the beauti of a holley. Don;t fugg it up by closing the sec throttles at idle.
That is not how a holley works. Don't shoot your self in the foot.

From that point the idle speed is fine adjusted by base idle timing, PCV system air flow (PCV valve choice) and fine adjustment of the throttle(S) position at idle.
Closed sec throttles will create the primarys to be too far open.
You want balanced 4 corner flow at idle.

recurve the distributor after you find the best idle timing, throttle position
PCV flow balance.
The throttles ALL 4 throttles must be in the sweet spot at idle for the idle and idle transition to work correctly and smoothly.

99% of the holley problems are tuner error, not carb design problems
That goes equal for Qjets and Edelbrocks too.

Q jets and edelbrocks are different.
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