[DeTomaso] Intake Reversion
sean mundy
seanmundy at hotmail.com
Tue May 7 15:18:10 EDT 2013
Thanks Will for the input. I'm not using the stock damper. I've
checked for intake leaks by spraying carb cleaner all over the intake.
Nothing detected. I just bought an AEM a/f gauge today. I will get
that hooked up an start the process of dialing in everything.
> Date: Mon, 6 May 2013 18:51:35 -0400
> Subject: Re: [DeTomaso] Intake Reversion
> From: will.kooiman at gmail.com
> To: jeffcobb1 at me.com; seanmundy at hotmail.com; detomaso at poca.com
>
> I agree.
>
> And if you are pinging at 22 degrees, you probably have a problem
> somewhere. For example, if you are sucking oil through a bad intake
> gasket seal, you will ping early.
>
> More likely, you are misreading the timing and vacuum. Stock damper? If
> so, the outer ring has likely shifted.
>
> Hard to diagnose over e-mail.
>
> --
> Will
>
>
>
>
>
> On 5/6/13 1:44 PM, "Jeff Cobb" <jeffcobb1 at me.com> wrote:
>
> >Sorry I sent to early.
> >
> >On May 6, 2013, at 12:37 PM, Jeff Cobb wrote:
> >
> >> Sean,
> >> The problems that I read are that your timing only varies by 6 degrees
> >>and does it at 5,000 not about 3,500 +- and that your dyno man came up
> >>with a different number than you.
> >> I thought this engine should run about 32 deg @ about 4,000+-?
> >> The easiest thing to do is to ping test it.
> >> The last time I did it on a Pantera was in 83 for a customer. I found
> >>it to be so easy. Remove the firewall plate and mark and loosen
> >>distributor, then max warm up the engine and short shift to third, then
> >>max load it from 1,500 to 3,500 while turning the distributor to a
> >>consistent ping then backing off about four degrees. Retest for no ping
> >>then maybe back off another degree for good measure.
> >Then your drive will confirm a diff or not. Then you can retest with the
> >timing light for reference only.
> >> Jeff Cobb
> >> Cell 225-907-4514
> >>
> >>
> >> On May 5, 2013, at 8:21 PM, sean mundy wrote:
> >>
> >>> Jeff,
> >>> I have a stock distributor with Accel brand Optical points eliminator
> >>>installed . Standard coil and no vacuum advance connected to the
> >>>distributor. Here is the results I got this afternoon with my cheap
> >>>vacuum gauge and timing light.
> >>>
> >>> IDLE is 1100rpm
> >>>
> >>> RPM Vacuum Timing
> >>> 1100 8 18
> >>> 1500 10 19
> >>> 2000 13 20
> >>> 2500 15 21.5
> >>> 3000 16 22
> >>> 3500 16 23.5
> >>> 4000 17 23.5
> >>> 5000 17 24
> >>>
> >>>
> >>>
> >>> The timing at 4000 to 5000RPM + plus is basically the same. The
> >>>timing is actually rapidly bouncing between 22-24. Not sure if that is
> >>>my light or not. Dyno Tech told me he set the initial timing at 22 and
> >>>total 31. I'm not seeing those numbers with my light but not sure why.
> >>>He said the engine didn't like a lot of advance and that is why he
> >>>recurved the distributor and disconnected the vacuum advance
> >>> The carb is a standard Holley 750 Vacuum Sec electric choke. The
> >>>power valve is standard 65 and the primary jets have been upped to size
> >>>72 (from 70).
> >>> Appreciate your any input you can provide.
> >>> Thank You!!!
> >>> Sean
> >>>
> >>> Subject: Re: [DeTomaso] Intake Reversion
> >>> From: jeffcobb1 at me.com
> >>> Date: Fri, 3 May 2013 19:42:28 -0500
> >>> CC: jeffcobb1 at me.com
> >>> To: seanmundy at hotmail.com; detomaso at poca.com
> >>>
> >>> Your welcome Sean,
> >>>
> >>> On May 3, 2013, at 3:49 PM, sean mundy <seanmundy at hotmail.com> wrote:
> >>>
> >>> Thanks Jeff for taking the time to respond and provide so much
> >>>detailed information. It always amazes me how helpful everyone is on
> >>>the Forum.
> >>> I enjoy helping and solving problems as most people do.
> >>> The dyno tech guy said that he modified the advance curve on my
> >>>distributor.
> >>> Maybe your dyno man is not well versed in this area?
> >>> He said the motor didn't like too much timing.
> >>> What does that mean?
> >>> Are you using low octane fuel?
> >>> Maybe he is wrong.
> >>> I found engines run the best about 4 deg before ping under max load.
> >>> and he set the initial at 18deg. Not sure how that will factor in
> >>>with your note on advancing the timing.
> >>> Is your timing digital where you can dial in diff deg under diff loads?
> >>> You mentioned a graph of timing/vacuum.
> >>> Yes and I believe that all dyno workers can print you out a plot/graph
> >>>of your engines run and your pulls so you can see what timing degrees
> >>>at what rpm under what load against your stoic " fuel/air" ratio.
> >>> I would take timing and vacuum readings at various RPMs?
> >>> Yes.
> >>> Would you take a look at it and let me know what you think?
> >>> Yes.
> >>> Not sure what you mean by primary venturis but we upped the primary
> >>>jets by one size.
> >>> Replaceable throats.
> >>> You think going smaller will help?
> >>> No, but paying attention to a better air bleed would help.
> >>> Going bigger seemed to help the problem a little but not cure it.
> >>> You may not have gone enough.
> >>> I pulled a few spark plugs last night and they are mostly sooty/dark.
> >>> That is ok.
> >>> Not horrible but it seems to be running on the rich side.
> >>> Someone on the site recommend putting in an air/fuel ratio sensor,
> >>>great idea.
> >>> JT gave very good info on carb flow function.
> >>>
> >>> Your low speed idle air jet control mixture while cruising through the
> >>> transfer slot. The transfer slot feeds fuel to the engine when the
> >>> throttle blades are not open enough to engage the main jets. On my car
> >>> when cruising on the highway in fifth, the throttle blades are barely
> >>> moved and fuel delivery is through the transfer slot. Using a smaller
> >>> idle air jet will richen the mixture in this RPM zone.
> >>>
> >>> You can chase your tail on this stuff all year long. I also recommend
> >>> that you add an o2 sensor to your header and a gauge to your dash to
> >>> inform yourself of where the engine is lean or rich. I spent a decade
> >>> trying to guess what I learned overnight with an air/fuel gauge.
> >>>
> >>> JT
> >>> Good luck,
> >>>
> >>> Jeff Cobb
> >>>
> >>> Subject: Re: [DeTomaso] Intake Reversion
> >>> From: jeffcobb1 at me.com
> >>> Date: Thu, 2 May 2013 09:00:27 -0500
> >>> CC: jeffcobb1 at me.com
> >>> To: seanmundy at hotmail.com; detomaso at poca.com
> >>>
> >>> Sean,
> >>> The vacuum is too low at that rpm and you are lean surging because the
> >>>carb does not have a strong enough vacuum signal to work.
> >>> You are having reversion because the vacuum/exhaust flow combined with
> >>>that cam does not create enough vacuum and flow to suck the air through
> >>>and past the exhaust valve. On our motocross bikes of the 70's, we used
> >>>reed valves to contain the wild overlaps of intake and exhaust.
> >>> A longer distance from the intake valve to the throttle plate will
> >>>help, so the plate could help but that is just a cover up.
> >>> A vacuum to ignition timing based on rpm graph would show any
> >>>deficiencies in the distributor or carb function through the entire
> >>>range.
> >>>
> >>> Increasing the primary jets will allow more fuel to flow if the air
> >>>bleed is strong enough to help it flow.
> >>> Does this carb have interchangeable primary venturis or have I just
> >>>seen too many Webers? If so, go smaller.
> >>> Advancing the ignition timing at that speed/load could cure your
> >>>problem.
> >>>
> >>> Hope this helps.
> >>>
> >>> Jeff Cobb
> >>> Cell 225-907-4514
> >>>
> >>> Jeff Cobb Auto Works
> >>> 1316 South Acadian Thruway
> >>> Baton Rouge
> >>> Louisiana, 70806
> >>> www.liveoakconcours.org
> >>> Work 225-343-7525
> >>> Fax 225-343-7550
> >>> jeffcobb1 at me.com
> >>> ___________________________________________
> >>> On May 1, 2013, at 5:08 PM, sean mundy wrote:
> >>>
> >>> Hey all, I recently had my new 408 Cleveland tuned and dyno'd at the
> >>>shop. 438/411 hp/trq rearwheel.
> >>> Very happy with the numbers but having some issues with how the motor
> >>>runs.
> >>> After driving it 50 miles I noticed at anywhere between 2-3k RPM
> >>> in any gear there was a surging and hesitation in the engine.
> >>> I went back to the shop and we worked on the carb (750 Holley)
> >>> by increasing the primary jets and adjusting the pump squirters.
> >>> We put it back on the dyno and ran it several times. The tech said he
> >>>couldn't get
> >>> good A/F numbers because the sensor placed in the tailpipe couldn't
> >>>get past the
> >>> baffling in the stock muffler. It pulls very good in the top end but
> >>>he noticed the
> >>> 2-3k RPM surging and hesitation that I mentioned.
> >>> The tech told me that he thought since my vaccuum reading was so low
> >>>(10-12 in)
> >>> at low RPM that I was probably experiencing "reversion".
> >>> He said that the lobe separation overlap on my cam specs were probably
> >>>the cause
> >>> of this and also a contributing factor was my single plane manifold.
> >>>Basically he made it
> >>> sound like I was stuck with what I got.
> >>>
> >>> My cam specs are:
> >>>
> >>> GROSS VALVE LIFT .617 .617
> >>> DURATION @ .006
> >>> TAPPET LIFT 286 300
> >>> VALVE TIMING OPEN CLOSE
> >>> @ .050 INT 10 46
> >>> EXH 60 8
> >>> SPECS FOR CAM INST. @ 108.0 CENTERLINE
> >>> INT EXH
> >>> DURATION @ .050 236.00 248.00
> >>> LOBE LIFT .363 .363
> >>> LOBE SEPARATION 112.0
> >>>
> >>> I've done some reading on the internet about reversion and it seems
> >>>like a real thing.
> >>> I read that installing a carb spacer designed to help eliminate the
> >>>phenomen might work in my case.
> >>> Wondering if anyone else has experience with this and what if any
> >>>solution worked for you.
> >>> Thanks, Sean
> >>> _______________________________________________
> >>>
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> >>>
> >>
> >
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