Carb Tuning

General engine tech -- Drag Racing to Circle Track

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F-BIRD'88
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Re: Carb Tuning

Post by F-BIRD'88 » Tue Oct 02, 2018 4:35 pm

Is your 750 one of the 3310 versions.
It can be nicely hot rodded and upgraded into a "HP" vac sec carb with a proform 750 hp vac carb body kit and a bit of work and tweeks of the secondary metering plate to dial it in better than out of the box.
It will end up better overall than a 600 or a ”650 vac sec carb on this motor.

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Re: Carb Tuning

Post by BobbyB » Tue Oct 02, 2018 4:48 pm

Holley 750 Vac secondaries

# 3310-3
3387

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Re: Carb Tuning

Post by F-BIRD'88 » Tue Oct 02, 2018 4:53 pm

Likely the best overall choice of the 3 carbs you have there. It is a good start point. Gets much better too with the mentioned upgrades.
If you get the proper high stall torque converter you can use a 750hp double pumper carb to good effect on this car. Don't be shy on the converter choice if you want to go fast.
Is this 306w motor in a Mustang?

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Re: Carb Tuning

Post by BobbyB » Tue Oct 02, 2018 4:57 pm

Thanks

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Re: Carb Tuning

Post by F-BIRD'88 » Tue Oct 02, 2018 5:18 pm

Because of the stock converter a combo carb spacer or a 4 hole carb spacer will probabily work better than a open spacer on the rpm intake. Or no spacer..
A shear plate spacer plate in combo with the 2 types of carb spacers can be effective...
Header collector extensions can help too to boost low end torque to launch hard. This motor wants gears too.

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Re: Carb Tuning

Post by BobbyB » Tue Oct 02, 2018 7:41 pm

I just got back from a spirited ride on some twisty country NC roads in the car. It does pretty well as is, especially on such a beautiful fall evening.
There is a little hesitation from a stop sign, & it acts a little goofy at about 2000rpm just as the centrifugal advance starts to increase timing when vacuum is low.
I am looking forward to trying the locked out timing.
How should I go about testing to find the best timing setting with the timing locked out?

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Re: Carb Tuning

Post by F-BIRD'88 » Tue Oct 02, 2018 7:48 pm

1/4 mile acceleration passes looking for best mph.
Can be 1/8 mile.
A $100 radar gun (Bushnell) can test for best mph.

Then road test with the vacuum advance 12 ish max limited. And dial in the vac advance rate in and out.
Use ported on the carb.
The 750 will want 70 to 74 pri and 80 to 85 sec jets.
Last edited by F-BIRD'88 on Tue Oct 02, 2018 8:06 pm, edited 1 time in total.

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Re: Carb Tuning

Post by F-BIRD'88 » Tue Oct 02, 2018 7:50 pm

Verify the diz has acceptable cap rotor phasing.
Vacuum advance mechanizm setup mounting point effects this.

With the diz mech advance fixed locked out, if the motor pings on part throttle roll on from cruise to WOT reduce the vac adv rate, slow it down. If it persists reduce timig by 2 degress. Do not use junk gas.
I'd bet the best wot drag test power (mph)will be with 32-36 deg timing.
Do not over time it. If there is no real difference beteeen 32 and 34 deg, use 32 degrees timing setting.

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Re: Carb Tuning

Post by F-BIRD'88 » Tue Oct 02, 2018 8:03 pm

If the 600 vs holley is a #1850 carb then 64-66 pri and a # 21 sec metering plate will work well. (Rob the sec metering plate from a 3310 750 vs carb.)
The 750 carb will be the most powerfull.
And likely idle the best too.

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Re: Carb Tuning

Post by F-BIRD'88 » Tue Oct 02, 2018 8:13 pm

In the final tune set up u may nit end up with a fixed locked out mechanical timing curve but this is where to start. Start here and find the best wot acceleration power timing. And rough in the vacuum advance curve. Be sure it is limited to about 12 deg MAX.

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Re: Carb Tuning

Post by Tuner » Wed Oct 03, 2018 1:47 pm

BobbyB wrote:
Tue Oct 02, 2018 2:31 pm
Cam is:

229 deg at .050, .533" lift both intake & exhaust.

Events are:
11.5 deg IO
37.5 deg IC
47.5 deg EO
1.5 deg EC
13 deg overlap at .050 lift

Current set up uses 17 degrees initial with the lightest springs that Cranes send with the adjustable vac advance kit. This gives about: (I'm at work notes at home). Vac advance is using ported vacuum which is working better that full manifold vac FOR ME, (not saying this is always best). Vac advance is crane unit that is adjusted to never spark knock on small hills, not sure how many total degrees it adds with current setting, but it is all it can stand without spark knock.

650 rpm=17 deg BTDC
1000 rpm=17 deg BTDC
1500 rpm=17 deg BTDC
2000 rpm=20 deg BTDC
2500 rpm=25 deg BTDC
3000 rpm=30 deg BTDC
3100 to 6500 rpm=31 deg BTDC

I am told by cam guy that engine should need 29 to 32 degrees BTDc max.
Your advance curve ^^^^ is pretty good, but the fastburn chamber may be allergic to more than 24~26 deg. at 3000. Even if it doesn't knock, too much timing kills the torque. Locked advance is downright evil with efficient chambers like these because the electronic ignition slew rate causes the timing to retard as RPM climbs, instead of advancing as required for the best torque curve. These heads definitely will like an advance curve with less timing at 3000 than at 5-6000.

Is this a Ford/Motorcraft distributor? The Ford distributor is the easiest to work with to modify the advance curve.
Fbird & anybody else, please tell me the timing curve you suggest.

timing BTDC at 650 rpm in gear?
BTDC at 1000 rpm?
BTDC at 1500 rpm?
BTDC at 2000 rpm?
BTDC at 2500 rpm?
BTDC at 3000 rpm?
BTDC at 3500 rpm?

Thanks!
Your "afr165 heads" have a "fast burn" combustion chamber. I googled "afr165", is this the chamber in your heads?

Image

timing BTDC at 650 rpm in gear? 18
BTDC at 1000 rpm? 18
BTDC at 1500 rpm? 18~22
BTDC at 2000 rpm? 22~24
BTDC at 2500 rpm? 24~26
BTDC at 3000 rpm? 26
BTDC at 5000 rpm? 30
BTDC at 7~8000 rpm? 32

Vacuum advance
6" Hg 0 deg.
7" Hg 1
10" Hg 8~10
15" Hg 18

What are the list or part numbers of the carbs you have? Holley list number?

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Re: Carb Tuning

Post by BobbyB » Wed Oct 03, 2018 2:25 pm

My heads are older afr165s with 1.94" intake valves & the exhaust chamber modified. Shown before heads were milled .012"
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Re: Carb Tuning

Post by BobbyB » Wed Oct 03, 2018 2:26 pm

Distributor is a motorcraft duraspark with low milage.

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Re: Carb Tuning

Post by BobbyB » Wed Oct 03, 2018 3:18 pm

Holley 750 Vac secondaries

# 3310-3
3387

Quick fuel 650 brawler vac secondaries BR-67207 on car now.

Holley 600 vac secodaries has been on car and did ok. (at work carb list numbers at home)

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Re: Carb Tuning

Post by BobbyB » Wed Oct 03, 2018 3:20 pm

Chamber before exhaust mod.
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