[DeTomaso] Intake Reversion

[Jeff Cobb]

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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