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  •  » Timing Curve and of IDLE Bog holley 750 DP

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    5/07/2020 7:03 AM  #1

    Mach173
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    From: Sweden
    Registered: 8/08/2018
    Posts: 186

    Timing Curve and of IDLE Bog holley 750 DP

    Okay 

    Here are the Specs :-)
    351c over 0.30  ARP Bolts etc
    Howard flat tappet and roller rockers
    Lift 542" / 542" dur 281* / 227* .050" LSA 108* ICL 104*
    Compression 10.1
    2v CHI Heads flows 300 cfmEdelbrock airgap with 1 inch spacer
    Holley DP 750 jetting pri 70 sec 80 powervalve 8.5  squirters 40 Mechanical secondaries, the are opened a bit. Transfer slots are just right.
    MSD Probillet Dizzy with 6AL box special stop bushing advances 14degrees 
    Initial timing set at 18 + 14 = 32 total
    Timing curve A 2 silver springs
    4r70w tranny with US shift controller
    3.5 gearing and 17 inch tyres
    IDLE set at 800 rpm vacuum is 10
    Innovate lambda gauge

    Sometimes I have the classical of IDLE Bog :-), I can see in the AFR that its going lean for split second 17-18 on the gauge. What should I change ? Bigger squirters ? Im having 40 now, bigger requires drilled screw I think. or the cam for acc pump ? Car runs fine and the spark plugs are as they should be. Changing the curve in the Dizzy for faster how will it affect ? Is it a good idea ? what curves are you running ?



     

    Last edited by Mach173 (5/07/2020 7:04 AM)

     

    5/07/2020 7:16 AM  #2

    TKOPerformance
    Hyper FYIFORD ADDICT

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    From: Newark, DE
    Registered: 12/15/2014
    Posts: 7,511

    Re: Timing Curve and of IDLE Bog holley 750 DP

    With a 6AL you won't be able to read the spark plugs.  Spark voltage over 40,000 volts rendered reading plugs obsolete.  The spark is so hot it burns the plugs clean almost totally regardless of mixture. 

    I would start with changing accelerator pump cam.  17-18 is extremely lean, even for a split second.

    As far a timing curve goes I suggest having total timing in by 3,000 RPM.  You may need more than 32 degrees total timing.  How much compression are you running?  Spark lead is a measure of relative inefficiency.  The more you need, the less efficient the combination.  Typically a head that flows that much cfm is going to be lazy at low RPM and therefore inefficient.  If its not pinging I'd bump timing in 2 degree increments not to exceed 36 degrees total.  If pining is encountered back it off 2 degrees.  This isn't going to help an off idle lean condition, but may help power once the fueling is sorted out. 

     

    5/07/2020 7:51 AM  #3

    red351
    I FYI 24-7

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    From: Carlisle, PA
    Registered: 5/31/2018
    Posts: 445

    Re: Timing Curve and of IDLE Bog holley 750 DP

    The squirters are plenty big enough.
    Is the bog off idle to part throttle   or   to full throttle.
    Next question is the bog the very second you hit the throttle or a split sec after you hit the throttle. I know these question sound stupid, but there is a difference as to what may be the problem.

    Both clevelands we have are built the same specs. and the same max timing (36 deg).,but one is more sensitive to octane ping with the cast exhaust manifolds than the other with shorties

     

    5/07/2020 8:43 AM  #4

    Mach173
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    From: Sweden
    Registered: 8/08/2018
    Posts: 186

    Re: Timing Curve and of IDLE Bog holley 750 DP

    The Bog is at wot , and kan be off idle and something when cruising. It just when I floor it hard.
    Im running 10.1 compression. Its aluminum heads from CHI they dont recommend more then 28-30 total timing. Thats why I use the special weight to have the timing at IDLE around 18-19. Having the MSD original blak weight I get 18+18=36 total timing. When going below 17-18 initial timing my 351c just doesnt run good at all. It wanna be around 18-19 :-)

    Ohh the Bog its a the second I hit the throttle then it picks up and revs like crazy :-)

    As reference one 1 ago I had the Sniper EFI on it with timing and without timing control , and it run perfectly. No bogs what so over.  After 2 years the ECU gave up and I went back to Carb. 

    If the 6AL cleans out the plugs I could run a richer condition on IDLE and maybe the transfer will be better without bog ?  Now AFR is about 12.8 going down to 12 plugs are getting fouled.

    Last edited by Mach173 (5/07/2020 8:59 AM)

       •Thread Starter
     

    5/07/2020 9:13 AM  #5

    BobE
    Senior 24-7 FYI Addict

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    From: White Plains, NY
    Registered: 4/04/2013
    Posts: 2,665

    Re: Timing Curve and of IDLE Bog holley 750 DP

    I have MSD distributor, w/the 6AL box, on my 351W and found the advance springs of 'blue' and 'blue' working best for me and eliminated some bog that existed mainly coming off idle.  Initial timing is set at 12-14 degrees.
    Just something to consider.

    65 Fastback, 351W, 5-speed, 4 wheel discs, 9" rear,  R&C Front End.
     

    5/07/2020 9:21 AM  #6

    TKOPerformance
    Hyper FYIFORD ADDICT

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    From: Newark, DE
    Registered: 12/15/2014
    Posts: 7,511

    Re: Timing Curve and of IDLE Bog holley 750 DP

    Are you running vacuum advance?

    Are you sure TDC on the balancer is really TDC #1?  Timing pointers and balancers are often off. 

    At what RPM do you see full mechanical advance?

    I'm wondering if your advance springs are too light...

     

    5/07/2020 6:10 PM  #7

    6sally6
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    From: Surfside Beach,SC
    Registered: 4/02/2013
    Posts: 6,863

    Re: Timing Curve and of IDLE Bog holley 750 DP

    You're gonna get A LOT of opinions on here..........and none are worng!  Since we're dealing with mechanical stuff.. there is no absolute right/worng way.
    I have found with lumpy camshafts and snotty idle  they seem to 'clean-up' with a LOT of initial timing BUTT.......limit the total to about 32 (in your case)
    Verify your TDC (piston stop)
    Use a timing tape (or graduated dampner)
    Of course a timing light!
    The more initial you run the LESS curb idle you will need (better transition slot position)
    Double pumpers/manual secondaries take a lotta "restraint" when driving. With any 'hot rod' the temptation to just "stomp-the-ever-lov'in-poop-out-of-it"  can be overwhelming at times! This is where a vacuum controlled secondary carb can be the best choice. Remember to 'walk-into-it' when doing the full acceleration thang. Its very EZ to over-carb with a DP unless you got the gears to get the RPM's up in a hurry. Jus say'in!
    Tweak the air idle screws to get the max amount of vacuum on a gauge.
    6sal6

    Last edited by 6sally6 (5/07/2020 6:24 PM)

    Get busy Liv'in or get busy Die'n....Host of the 2020 Bash at the Beach/The only Bash that got cancelled  )8
     

    5/07/2020 11:48 PM  #8

    Mach173
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    From: Sweden
    Registered: 8/08/2018
    Posts: 186

    Re: Timing Curve and of IDLE Bog holley 750 DP

    Great suggestions everybody. Dizzy is pro billet no vacuum advance at all. TDC is verified and timing set with timing light so the is 100 % correct. With the AT and wide ratio of course the DP with mechanical secondaries seems like a bad choice. I will try to change the springs and see how it affects the curve and so :-) Il be back with the result.
    Thanks for all inputs.

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    5/10/2020 11:24 PM  #9

    Mach173
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    From: Sweden
    Registered: 8/08/2018
    Posts: 186

    Re: Timing Curve and of IDLE Bog holley 750 DP

    Okay changed springs to one heavy silver and one light silver , engine revved better and I didnt get the bog for now. More testing has to be done to confirm that it works 100 %
     

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