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Hot Rods 94's A.F.R from Idle to W.O.T.

Discussion in 'The Hokey Ass Message Board' started by 19Eddy30, Apr 11, 2016.

  1. 19Eddy30
    Joined: Mar 27, 2011
    Posts: 1,361

    19Eddy30
    Member
    from VA

    I am confused on my set up , I have Read several pages , hear on the HAMB ,FordBarn & several other places on the web,
    I am running Two 94 , Offy intake ,on my 39 Flat Head , It was rebuilt But not by me ,
    I change oil every 2000 miles with no Oil used,
    When I first purchased the car it would pull good threw 1st 3,500 rpm's
    2nd it would pull to around 2,500 and start hesitating / Breaking down ,same in 3rd
    I did a few thing wires ,cap ,Rotor, looked at plugs looked to show little rich ,
    Did a few other things , Still same problem.
    Still scratching my head ,I put the car in the air ,drilled a hole in exhaust pipe ,welded in a bung , with in 40 mints , I was on a test drive !!
    Dam , Lean , So I Took a look at jet's 50s , so I installed 52s , test drive Night & day !!!
    Pulls 4000 no problem,
    Now this is where I am confused!!
    I have 7.5 pv's
    It pulls 19 inches of vacuum @ 750 RPM's (with this much Vac ,I'm sure stock cam )
    @ idle with 14 .4 AFR ,
    Cruise @ 50 mph , 2,200 rpm's with 19 inches Vac , AFR 10.5-11.5 ( rich)
    Cruise @65 mph 3,000 rpm's With 19 inches vac , AFR 10.5-11.5 ( rich)
    Hit a hill vac drops down to 8-9 inches. AFR still about the same
    Wot Its Lean , AFR is 14.5 Lean ,
    I have read that a 2x2 94 ( tandem set up ) should need 3.5-4.5 power vale with stock jetting .49-.51

    But wright now It's @ a good lean @ idle , Rich @ cruise , Lean @wot ,

    So if I use ,let's say 18 inches of Vac divided by 2 , I would need 9.5 power Vale's,
    I would think with the 9.5 Pv , when I hit a hill & drop below 10 inches of vac
    it would richen up until the vac goes over 10 inches, & WOT
    I would be rich with about 12.5-13.5 AFR ,

    But wright now with 7.5 pv's & 52 jets , I would think I should be Rich from
    Cruise to Wot ,
    Has any one hear tried to get a correct AFR from Idle to WOT ,
    Looking at plug's with the crap unleaded gas will Give you false reading's ,
    I have alway's tune with shiny plug's
    My vac gauge is a 5 inch face that read 0 -30 inch in 1/2 inch increments,
    & Pro Auto Meter wide Band 02
     
    Last edited: Apr 12, 2016
  2. clem
    Joined: Dec 20, 2006
    Posts: 3,511

    clem
    Member

    Aren't you supposed to halve vacuum to get power valve size, and approx halve again for twin carbs
    So 19 vacuum, equals 9&1/2 pv for single carb
    Equals 4&1/2 pv for twin carbs approx
    Or am I wrong ?
    (My single carb 1&1/16 -57 jets on 8ba had afr of 12.9 sitting in garage )
     
    Last edited: Apr 12, 2016
  3. CheatersPete
    Joined: Sep 25, 2002
    Posts: 1,279

    CheatersPete
    Member

    I agree, IMO you should first ad 3.4 or 4.5 PV with original jets, then read AFR and change jets if needed...
     
  4. flatjack
    Joined: Feb 13, 2007
    Posts: 945

    flatjack
    Member

    I'm not believing your vacuum gauge. You can't have 19" at 2200 rpm and still have 19" at 3000 rpm. What rear axle ratio are you running?
     

  5. 19Eddy30
    Joined: Mar 27, 2011
    Posts: 1,361

    19Eddy30
    Member
    from VA

    Do not know the exact gear , 28 tall tire ,and allso verify Speedo was correct ,
    Gps was Plus 1 mph
    Why is 18-19 inches hard to beleave? Flat road .
    I have verify (Vac gauge )With other test Equipment /tools . Not a cheap $30-$40 Gauge ,
    About the same price as 1 new Firestone drag cheater , It reads correct .
    It's possible That tach could be off a few hundreds after 1,500 rpm's .
    I have not realy payed that close attention above 1000 rpm , At idle ,750 on tach & like 760 on timing light , but again it may be off a few 100's

    I'm going to try 4.5 &5.5 pv's
     
    Last edited: Apr 12, 2016
  6. Turnaround
    Joined: Dec 21, 2007
    Posts: 64

    Turnaround
    Member

    Your (and most others) carb has five circuits. (1) idle for sitting there running when you are out of the car.
    (2) off idle for a tremendous amount of the driving we all do, ESPECIALLY with two carbs. (3) transition, where some of the "off idle" and "the beginning of the main jets" are both delivering fuel.
    (4) the main jets only. (5) the main jets plus the power valve controlled two high speed jets for a total of four jets per two barrels.

    Lean over and look down the air cleaner area. You can rev it pretty high before you see fuel spraying our of the venturie boosters. All the fuel delivered before you see the venturie spray, is controlled by the off idle jetting and air bleeds. The off idle fuel delivery is affected (not controlled) by the main jets, so always start tuning by getting your engine load at a point (typically up a 35 mph hill but light enough throttle to still have high enough intake manifold vacuum to guaranty that the power valves are CLOSED, meaning you are running on just the main jets and no power valve controlled additional enrichment.
     
    Dan Timberlake likes this.
  7. Turnaround
    Joined: Dec 21, 2007
    Posts: 64

    Turnaround
    Member

    Only after the main jet selection has been finalized, you can choose to tune either WOT up a hill, by changing the jet sizes under the power valves. Either drill or insert wires, Or you can tinker with the off idle (and before the jets spray through the venturie boosters) circuits. Again, because the off idle jets (normally fixed sizes and requiring creative mods to richen or lean as do the power valve circuit jet) the off idle jets get their fuel from the main jets, so bigger main jets will cause the idle jets to get richer just like increasing line pressure in any hydraulic circuit. Think drilling a hole at the bottom of the Hoover dam as compared to drilling a hole just below the water line.
    If you mess with it long enough you can get pretty the same performance out of a lean mixture and a lot of timing, or a rich mixture and a lesser amount of timing. Knock or Ping will guide you to the combo for your driving style.
     
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  8. Turnaround
    Joined: Dec 21, 2007
    Posts: 64

    Turnaround
    Member

    You have to realize that the different vacuum number power valve don't change WOT air fuel ratios. All power valves deliver the same amount of fuel at WOT. The different number just open and close sooner or later. There are jets built into the carb that control how much fuel the power valves deliver. The power valves are as their name says, valves not jets.
     
    Dan Timberlake likes this.
  9. Turnaround
    Joined: Dec 21, 2007
    Posts: 64

    Turnaround
    Member

    Cruise @ 50 mph , 2,200 rpm's with 19 inches Vac , AFR 10.5-11.5 ( rich)
    Cruise @65 mph 3,000 rpm's With 19 inches vac , AFR 10.5-11.5 ( rich)
    Hit a hill vac drops down to 8-9 inches. AFR still lean.

    Here is a perfect case of main jet too large. Drilled "jets" in the power valve circuit too small.
    To fix the problems. Decrease the jet sizes till will go up a hill with 11 - 13" vacuum and give you a decent air fuel ratio. Leave that jet selection alone forever more. Now attack the hill at WOT and increase the drilled restrictors in the carb body ( down stream of the power valve) until you get your A/F ratio and performance acceptable. Good time to duplicate runs with more or less timing (block vacuum advance for WOT runs.
    Later, fine tune you off idle and transition circuits by inserting small wire or by drilling. Again tinker with timing. You can go up and down on Power Valve numbers until you hit the sweet spot for throttle response at highway speeds. Massive amounts of vacuum advance and less initial advance will give you the best gas mileage, but you might die of old age before you get the exact combination. That is why there are computers and fuel injections.
     
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  10. 19Eddy30
    Joined: Mar 27, 2011
    Posts: 1,361

    19Eddy30
    Member
    from VA

    Thanks Turnaround,

    Yr Quote,,
    Here is a perfect case of main jet too large. Drilled "jets" in the power valve circuit too small.
    You are referring to the Power Vale Restricter, ( holes under the power vale )
     
  11. Truck64
    Joined: Oct 18, 2015
    Posts: 5,237

    Truck64
    Member
    from Ioway

    There's the "half of idle vacuum" methodology, but there's also (I think) the "subtract two" from the cruising vacuum.

    It may because rumpety-bumpety engines don't idle well to begin with because of the cam, and don't pull much idle either. Maybe they need to divide by two, from what little idle they pull, and because it will fluctuate. This causes the wrong value power valve to flutter at idle. One reason to hook up racing engines distributor to manifold vacuum, lots of advance at idle too.

    Civilians just want to find out what cruising vacuum is, and install a power valve ("economiser") that is close to that figure. Testing is the only way to do it. But one that is very close to the cruising vacuum should be very responsive when the go pedal gets pushed.

    This is all altitude dependent btw. Remember going up Pike's Peak and seeing cars with a stream of gas dripping out the tailpipe? That was a flatlander at 10,000 feet. Power valve open all the time.
     
    clem likes this.
  12. Automotive Stud
    Joined: Sep 26, 2004
    Posts: 4,178

    Automotive Stud
    Member

    I am running a similar setup and I agree with above, you want something closer to a 4.5 power valve in each, and probably a smaller size jet, closer to 46.
     
  13. CheatersPete
    Joined: Sep 25, 2002
    Posts: 1,279

    CheatersPete
    Member

    On my french flathead 255ci, full ported, with literio l100 cam, I run dual 94, with 4.5 PV and 46 jets, AFR is from 13 to 14 all the time
     

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