[DeTomaso] Why would an engine idle faster after valve adjustment?

[Jeff Cobb]

It runs faster because all you have done on a net level is to make the engine
—air pump—more efficient!

So your idle problem has been overlooked for a while 
and on that type of motor you most likely will have air leakage past one or all of the 
throttle plates if multi throat carb design.
You need to vacuum test each one of the four runners or place the velocity/flow tester 
above the individual carb intakes to correlate flow.

If not multi carb then check for base plate interference to throttle plate closure and base plate/intake air leaks.

Carb mixed or leaked air is entering the engine!
You have a lean run condition which always causes high idle. 
The smaller the motor the most likely it will happen when no load is present.
Jeff Cobb

On Dec 11, 2016, at 1:30 AM, Pantdino via DeTomaso <detomaso at server.detomasolist.com> wrote:

>   My Fiat Abarth has an Autobianchi A112 A2 1050cc pushrod 4 cylinder
>   engine, 10.5:1 CR, fairly aggressive aftermarket cam per previous
>   owner, who used the car in vintage racing in Germany.
> 
> 
>   I checked the tappet clearances set by the previous owner and found
>   them to be 6-8 thou, which is what a stock Fiat 4 cylinder engine of
>   similar type called for, and the engine idled at about 1000 rpm
> 
> 
>   I found the correct specs for this engine with stock cam, which are 10
>   intake, 12 exhaust
> 
> 
>   But I looked up the specs for CatCams cams for the A112 A2 motor on
>   their website and even their most mild asporta cam calls for 0.35mm
>   (0.014a) alash rampa.  So I adjusted the tappets to 14 thou.
> 
> 
>   Now the car idles at 2000 rpm with the idle screw completely out, even
>   after I downsized the idle jet one size.
> 
> 
>   Compression testing shows [1]195 a 198 - 198 a 190 psi with
>   a cold engine, which is higher than the [2]175-195-190-175 with
>   a warm engine I had gotten with the 6-8 thou clearances.
> 
> 
>   Power is very subjective, but I would say the car runs better now.
> 
> 
>   How can adjusting the valves make a car idle so much faster, and can
>   too much lash cause a problem other than loss of power?
> 
> 
>   Thanks,
> 
> 
> 
>   Jim Oddie
> 
> 
> 
> 
> 
> 
> 
> 
> 
>   Sent from AOL Mobile Mail
> 
> References
> 
>   1. tel:195%20-%20198%20-%20198%20-%20190
>   2. tel:175-195-190-175
> _______________________________________________
> 
> 
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-------------- next part --------------
   It runs faster because all you have done on a net level is to make the
   engine

   air pumpmore efficient!

   So your idle problem has been overlooked for a while
   and on that type of motor you most likely will have air leakage past
   one or all of the
   throttle plates if multi throat carb design.
   You need to vacuum test each one of the four runners or place the
   velocity/flow tester
   above the individual carb intakes to correlate flow.
   If not multi carb then check for base plate interference to throttle
   plate closure and base plate/intake air leaks.
   Carb mixed or leaked air is entering the engine!
   You have a lean run condition which always causes high idle.
   The smaller the motor the most likely it will happen when no load is
   present.
   Jeff Cobb
   On Dec 11, 2016, at 1:30 AM, Pantdino via DeTomaso
   <[1]detomaso at server.detomasolist.com> wrote:

       My Fiat Abarth has an Autobianchi A112 A2 1050cc pushrod 4
     cylinder
       engine, 10.5:1 CR, fairly aggressive aftermarket cam per previous
       owner, who used the car in vintage racing in Germany.
       I checked the tappet clearances set by the previous owner and
     found
       them to be 6-8 thou, which is what a stock Fiat 4 cylinder engine
     of
       similar type called for, and the engine idled at about 1000 rpm
       I found the correct specs for this engine with stock cam, which
     are 10
       intake, 12 exhaust
       But I looked up the specs for CatCams cams for the A112 A2 motor
     on
       their website and even their most mild asporta cam calls for
     0.35mm
       (0.014a) alash rampa.  So I adjusted the tappets to 14 thou.
       Now the car idles at 2000 rpm with the idle screw completely out,
     even
       after I downsized the idle jet one size.
       Compression testing shows [1]195 a 198 - 198 a 190 psi with
       a cold engine, which is higher than the [2]175-195-190-175 with
       a warm engine I had gotten with the 6-8 thou clearances.
       Power is very subjective, but I would say the car runs better now.
       How can adjusting the valves make a car idle so much faster, and
     can
       too much lash cause a problem other than loss of power?
       Thanks,
       Jim Oddie
       Sent from AOL Mobile Mail
     References
       1. tel:195%20-%20198%20-%20198%20-%20190
       2. tel:175-195-190-175
     _______________________________________________
     Detomaso Email List is not managed by POCA
     Posted emails must not exceed 1.5 Megabytes
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     To manage your subscription (change email address, unsubscribe,
     etc.) use the links above.
     Members who post to this list grant license to the list to forward
     any message posted here to all past, current, or future members of
     the list. They also grant the list owner permission to maintain an
     archive or approve the archiving of list messages.

References

   1. mailto:detomaso at server.detomasolist.com
   2. mailto:DeTomaso at server.detomasolist.com