View single post by Esprit2
 Posted: 02-10-2020 06:41 pm
PM Quote Reply Full Topic


Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 460
Tom Bradley wrote:
I tried a comparison between SAE20 and 20W-50 and could not tell any difference. Maybe I am not that perceptive.The effect of damper oil viscosity is transitory, is only effective when/ as the throttle is opened, and only lasts for seconds. It has no affect during normal cruising. You may notice a difference in throttle response for 1-3 seconds after you step on the throttle, but you won't notice anything during the long, steady state cruise to Gramma's house.

Tom Bradley wrote:
Besides the recommended damper oil, I have noticed that the air valve return spring in the JH is much longer and stiffer than in TR4's or TR6's even though the basic carb is the same. It seems like some sort of balancing act is going on here.
Exactly, every carb set-up is a balancing act. And like a spider web, everything you touch 'here' causes it to tingle over 'there'. The air valve return spring, jet size, jet height, needle, needle height, and float height are all balanced for each engine application.

If all those items combined are the dog, then the choice of damper oil viscosity is the tail. A dog might use it's tail to help tweak his balance a bit, but the tail never wags the dog.

As I mentioned earlier, playing with the damper oil viscosity in a Constant Depression carb (ie, Z-S or SU) is similar to tweeking the accelerator pump setting in a venturi carb (Dellorto, Weber, etc). All the hard parts (the dog) have to be right first. Then tweeking the oil viscosity helps smooth throttle transitions when you step on the gas... but the effect only lasts a couple of seconds.

Tom Bradley wrote:
I wonder if the differences are due to maximizing performance or simply adjusting to whatever oil is most easily available. I read in one of my carb manuals that the springs are specific to the particular carb and should not be swapped out.Tuning is never a matter of convenience, or whatever oil is most easily available. Carb tuning is all those other complex things (the dog), NOT the damper oil (the tail).

Z-S carbs became more popular on 1970s/ early 80s Britcars as emissions standards started becoming more strick. Z-S carbs' better resolution gave them a better chance of meeting emissions standards, while other carbs (including SU) were hard pressed to do so. But emissions standards did not make for optimum performance, and early emissions engines often ran like crap. Not because of the type of carb used, but because of how emissions-lean they were tuned.

If a private owner chooses to ignore the emissions standards and tune for performance, then going to a thinner viscosity (richer) damper oil isn't going to get the job done. At that point you're just wagging the tail.

Small performance gains can be made by simply adjusting the 'emissions' parts... like the jet height or needle heights But serious performance tuning will require replacing springs, jets & needles, and for that you really need to know what you're doing.

Most engine manufacturers didn't even have the skills for that. The carb companies, including Zenith-Stromberg, had traveling teams of engineers who would go to the factory and engage in extensive tuning & testing.
You can't improve upon all of their efforts by grabbing a more conveniently available damper oil.

Even after the J-H 907's Z-S carbs' internal parts were chosen, they are adjusted LEAN. Turning the adjustable needles right (clockwise) is richer. Turn 'em full right until they stop, and that's full rich. Then turn them left (counter-clockwise), counting turns as you go, and the needle will drop out of the air valve piston at about 4.5 - 4.75 turns. If you start with an original factory-set 907 Federal Z-S carb, and turn the needle right, counting turns as you go, until it stops, you'll find they were set to about 3.5 turns left/ lean. That's about as far lean as they can be adjusted and still have minimal thread engagement.

If you tune that existing needle's height for best performance (ignoring emissions), you will probably find that it ends up at around 1.5 turns left (lean) from fully seated right (rich).

That's only optimizing what is already in the carbs as they came from the factory. If you really wish to tune for performance, then you'll need to install different parts... at least different needles & springs. But that's more of a black art, and by comparison, tuning Dellortos or Webers is a piece of cake.

Tim Engel