Carb Tuning

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F-BIRD'88
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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 7:14 pm

Not specifly , no. Good time to go junkyard shopping at your local pick A part yard. There are 100’'s of different pcv valves.

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Re: Carb Tuning

Post by F-BIRD'88 » Wed Oct 03, 2018 7:17 pm

Get your diz locked out and limit the vac adv travel and fix the carb throttle position t slot exposure First.
Worry about tweeking the pcv valve latter.

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Re: Carb Tuning

Post by BobbyB » Fri Oct 05, 2018 4:38 pm

Pulled plugs & found the ones fed from the passenger side of the carb were rich. The plugs fed from the drivers side looked good.

Locked out timing at 30deg.

Did not remove 650 quick fuel carb to check transfer slots. Adjusted idle speed in park to 1000 rpm. Leaned pass side mixture screw to get 11” hg. In park, 650 rpm & 6 to 7 in hg in gear.

Could only limit vac advance to about 15 degrees using a crane adjustable unit. (Will try for more soon).

Drove about 10 miles and it drove very well. Flat spot from stop sign is gone. Drove great. Did not test WOT yet.

Brought it home and turned it off for a few minutes & it restarted fine. Drove about 10 more miles & heard one spark knock on a hill at 30 or 40 mph. Just a tiny spark knock.

Stopped and bought gas (93 non Ethanol). Took a few minutes to go inside & pay etc. On restart I could tell it was harder on the starter than previous timing settings. I don’t think I want to try higher base timing.

Remember I have 5 degrees of start retard built in my ignition.

Why not try Based timing at 27 degrees & 5 degrees of centrifugal advance coming in by 3000 rpm?

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Re: Carb Tuning

Post by BobbyB » Sat Oct 06, 2018 2:48 pm

Drove again today. Drove very well.
WOT did very well.
WOT from dead stop without foot brake gave a faint rattle between 1000 to 1500 rpm.
Fbird, what would you do next?

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Re: Carb Tuning

Post by F-BIRD'88 » Sat Oct 06, 2018 3:53 pm

Minor brief detonation pinging as you rug it :s not serious.
It is likely slightly lean jetting or slightly lack of pump shot or pump cam adjustment or slight lean as the secondaries crack open. Or the vacuum advance is a hair too agressive or you need a slightly colder spark plug.
It is not seriius as long as it disappears.
You can likely tune it out. The converter is tight.
Whats gears?

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Re: Carb Tuning

Post by F-BIRD'88 » Sat Oct 06, 2018 3:55 pm

You can also employ a ignition power interupt switch to aid hot starting with locked timing.

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Re: Carb Tuning

Post by F-BIRD'88 » Sat Oct 06, 2018 3:57 pm

Uneven mix screw adj will maje it knock on throttle.

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Re: Carb Tuning

Post by Tuner » Sat Oct 06, 2018 4:41 pm

50 years ago, one of my first experiences with an "all in at 2500" advance curve causing a previously fine running engine to act like the brakes were dragging was with a '66 Mustang 289. There have been several, before and since, but this one stood out because the effect of the quick curve was so obvious and so profoundly negative. FoMoCo advice was 26 deg. at 3000, 30 deg. at 5000, 32 at 7000, and that was the setup in the original 289 distributor. Magazines said 'more is better' so some light springs Isky sent in cam kits were the hot ticket. A quick trip around the block proved otherwise. Since a few years before that experience I've had the pleasure of spending several nights at the Holiday Inn in a room full of distributor machines, and every time (so far) a SBF like yours, with cylinder heads having ports and chambers like yours, with compression ratio and cam similar to yours, responded with a wider, flatter, better, torque curve and more power over a wider RPM range when the distributor was set up like this. (your mileage may vary)

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

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Re: Carb Tuning

Post by F-BIRD'88 » Sat Oct 06, 2018 7:14 pm

The 66 mustang in your 50 year old storyvwas most likely a manual trans car, not an auto trans car.

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Re: Carb Tuning

Post by F-BIRD'88 » Sat Oct 06, 2018 7:18 pm

Assuming you don't now have the ;dle mix screws set un even, ..and there is no man;fold/carb vacuum LEAK, it is probaily needing a slight tweek of the pri idle air bleeds and or a slight tweek of the pri accel pump or a slight tweek of the primary main jets.
You want to try and tune this out first, with the timing locked, as is, for now.
Does disabiling the vacuum advance change this?
It may be a hair too hot.. Needs slowing down the rate.
Power valve needs to be a 4.5" rated.

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Re: Carb Tuning

Post by F-BIRD'88 » Sat Oct 06, 2018 7:23 pm

Pri and sec throttle T slot reveal needs to be checked and adjusted.

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Re: Carb Tuning

Post by F-BIRD'88 » Sat Oct 06, 2018 7:28 pm

Do you have a afr meter/guage?

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Re: Carb Tuning

Post by BobbyB » Sat Oct 06, 2018 9:25 pm

Yes I have AFR gauge.
Power valve in 650 quick fuel is 3.5
Driver side mix screw out 3/4 turn
Passenger side idle mix screw out 5/8 turn
3.55 gears 235-50-15 tires.

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Re: Carb Tuning

Post by Tuner » Sat Oct 06, 2018 11:01 pm

F-BIRD'88 wrote:
Sat Oct 06, 2018 7:14 pm
The 66 mustang in your 50 year old storyvwas most likely a manual trans car, not an auto trans car.
Oh gosh, you might be right (or you might not). Why would anybody want the widest possible torque curve with an automatic? The pre=smog 289-302 chambers were fast-burn similar to the new efficient heads like the OP's. The "all in at xx00" curves will get by in smog open chamber heads, but an efficient fast-burn head like OE and nearly all aftermarket heads are nowadays benefits dramatically with the correct advance curve. Did you learn anything 50 years ago?

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Re: Carb Tuning

Post by F-BIRD'88 » Sun Oct 07, 2018 12:48 am

Yes, and I still am. Lets help the OP work through and finish the tuning process. He will likely not end up with just a locked out diz mech curve. There ia a method to my madness. Your expertize in holley carb fine tuning IMHO will be of good value down the road, here.

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