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

Pantdino pantdino at aol.com
Sun Dec 11 02:30:47 EST 2016


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 “sport” cam calls for 0.35mm (0.014”) “lash ramp”.  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 195 – 198 - 198 – 190 psi with a cold engine, which is higher than the 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
-------------- next part --------------
   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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