[DeTomaso] Intake Reversion

[Jeff Cobb]

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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