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 » Wed Oct 03, 2018 3:50 pm

The camshaft overlap and thus the exhaust gas remix "EGR" effect at idle and low rpm is what effects the combustion speed . This is why the motor wants more idle timing and more timing at low speed. Thats why locking out the mech curve is not evil.
This may not be the final setup thst is optimun but it is where you need to start at to get the carb dialed in..Then it may want a very short mech curve with just 10 deg (or less) of advance travel. Eg: 22 to 26 initial base +10 deg mech advance for 32 to 36 total wot advance.
Limit the vacuum advance to 10-12 max limit and Start tuning process with the mech curve locked out.
It will want all the idle and low speed timing you can throw at it. Start there.

The curve you have now is more suited to a manual trans car. And is not a good curve for a Auto trans car, with a cam like yours. Lock the mech curve out and start tuning from there. It will be very close to optimum"98% there"

Right now you are a mile away.

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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 4:02 pm

And I'd say that your chamber mod is ADDing to the exhaust re mix charge dilution EGR effect at idle and low speed, thus slowing combustion even more. At those driving mode points. Locking out the diz mech curve will help. It will improve and STABLIZE the idle and enhance low spped torque and throttle responce.
Exhaust gas does not burn thus dilutes the a/f mix and slows down combustion speed. Thus more timing is needed at idle and low speed.

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Re: Carb Tuning

Post by BobbyB » Wed Oct 03, 2018 4:09 pm

Based on what I have seen with the car, trying what Fbird says makes good sense to me.

With the static time set at 15 degrees, at 1000rpms in park, 11 inches of vacuum yielded about 25 degrees total timing...UNTIL THE TRANS WAS SHIFTED INTO GEAR. In gear the vacuum dropped far enough that little if any vacuum advance was added to the base timing.

Hey, if it does not work I will switch it back.

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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 4:22 pm

Not very familier with your 650 "Brawler" carb but just loocing at the out of the box setup I would change the power valve to a 3.5" or 4.5" pv. The primary idle air bleed (.070") may well need tuning. Get a spare and solder up the hole and re drill 1/16" .062" Now the idle transition will shift rich on throttle progession, instead of probable too lean with the 70 bleed.
Then hand hone the 062" bleed a little at a time with the drill bit and test the off idle afr shift effect till you get it balanced (does not shift too rich ir too lean on light throttle openings). Use the afr gauge for this.
It will want to be some where between .062 and .070.
Mild hand honing of the .062 bleed a bit at a time has a big effect on the idle bleeds afr shift effect.

The main jetting looks ok as a start point. Optimum wot final jetting should be close to that +/-4 jet sizes.
Do not tune to a specific afr on the afr gauge. Give it the jetting it wants to perform best, but use the sfr gauge and plug appearance to keep you from getting TOO LEAn in your jet play , looking fr best mph on aceleration test.

The air cleaner has a big effect on air bleed tuning.
Install the air cleaner to drive test.

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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 4:23 pm

This car is going to want a agressive converter and gears. 4.10++ 3500 rpm ++

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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 4:29 pm

The timing at idle when put in gear must stay high.
Especially with a stock converter with this sgressive cam in your lil302. Thus the locked out mech curve and ported vac adv. Now the timing stays up when put in gear and idle quality is much better and throttle responce is much better coming off idle as it "gets on the cam".
The locked out timing will alow you to get the idle air bleed tuning correct. It is not evil.
Do not tune on junk gas. This 10.4:1 motor needs the best premimum gas you can buy. 92+ octane.

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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 4:35 pm

Wot test blasts looking for best mph to find best carb main jetting is best done at the Dragstrip. Next best is on a chassis dyno. Then verified by drive testing.
A $100 Bushnell radar gun can be a big help.

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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 4:44 pm

Do not look at the afr gauge when setting the base idle fuel mix screws. Set for best idle quality and manifold vavuum. In gear. The curb idle afr will be richer than 14.7:1 Like in the 12.5 to 13.5:1 zone.

The afr gauge is used to then dial in the off idle transition
By fine tuning the idle air bleeds.
It is then also used to see the afr at part throttle cruise when on the primary main jets (cruising at hiway speeds +/-…).
When tuning for wot jetting the gauge is only used as a you're now too lean" warning device. To keep u out of trouble.

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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 4:46 pm

Any all exhaust system leaks like header gasket leaks will make the afr guage show false readings.

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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 5:01 pm

When you do lock out the diz you will now be able to close up the throttle setting at idlevso you have correct trsnsition slot exposure on BOTH the pri and sec throttles. You want it set at about .020" of the slot edge showing at idle and more or less balanced pri and sec.
As a start point. Then minor adjust +/- to get it right (idle rpm in gear) and balanced.
Holleys idle on all 4 bbls.
Auxillary idle air is supplied thru the pcv valve at idle.
Not all pcv valves are the same and tweeking the pcv valve is critical to getting it right too.
Do nit drill throttle plates. Dial in the pcv valve flow rate.

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Re: Carb Tuning

Post by BobbyB » Wed Oct 03, 2018 6:40 pm

How do I dial in the pvc?

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Re: Carb Tuning

Post by BobbyB » Wed Oct 03, 2018 6:41 pm

I meant pcv

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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 6:44 pm

You can try different ones from different oem cars.
You can take one apart and modify it or swap parts etc.
You can buy an adjustable pcv intended for hot rodded engines like yours.
Some of the old 60's era ford pcvs are 2 piece screw together type that allow easy disassembly for cleaning or modifying.. Many of the gm type are easy to cut open and re-seal with heat shrink sleeves. Or soldering.

Google search for modifying a pcv valve.

A gm truck LS motor pcv is a simple fixed orrifice type and thus easy to modify the internal flow orrifice to change the airflow rate. Do not bugger up the anti backfire valve function.

If you have aftermarket valve covers be sure they have functional pcv valve breather baffles. To stop oil getting sucked up the pcv valve.

Your carb is setup and calibrated to WORK WITH a pcv valve. Its up to you to dial that in on cammed up non oem engines. By fine tuning the pcv valve flow rate and function.

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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 7:03 pm

Does your Brawler 650 carb have screw in airbleeds?
I would have just upgraded your 3310 750 carb with a Proform HP carb body kit.
This car can easily use a 750 carb.

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Re: Carb Tuning

Post by BobbyB » Wed Oct 03, 2018 7:07 pm

Yes my quick fuel carb does have screw n air bleeds.
My current pcv came from a 74 Torino 302 Automatic. Can you suggest the next pcv to try?

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