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 Posted: 01-10-2020 08:45 pm
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jomac
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What is the correct oil level in stromberg carbs.
I am looking for a reference dimension.
Measuring from the top of the casting opening to the
oil level in the fill tube. The manual says top off oil level. what is the favorite oil of choice? I use my 20w50 engine oil.

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 Posted: 01-10-2020 11:41 pm
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redracer
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we don't use "oil" as the old SU carbs with their massive aluminum piston did. The Strombergs have the rubber diaphragm which lightens the "massive" weight of the original SU piston but which will get "eaten" up by regular oil. Instead, we use ATF which will not harm the "rubber"(in fact will slightly expand the diaphragm if it get on it) and just fill to the top: any overflow will clean up some carbon build up(has been used extensively for ages just for this).
So: use ATF but no regular petroleum based oils.

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 Posted: 01-11-2020 02:03 pm
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pokeyjoe
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Moss Motors sells this stuff. It might be repackaged ATF. I have no idea. Click on the pic for the link.

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 Posted: 01-14-2020 03:44 pm
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Esprit2
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The viscosity of the damper oil is to a Zenith-Stromberg what the accelerator pump setting is to a Dellorto or other 'normal' carb. The viscosity influences how quickly the piston reacts (rises/ falls) to a change in throttle. The viscosity is only right for the task when proper consideration is given to how the carb is set-up (jet size and height, damper spring, needle selection and it's adjustment)... and lastly, the damper oil viscosity.

The oil viscosity specified by different engine manufacturers varies because their carb set-ups vary first, then the damper was tuned to provide the additional rich/ lean condition that was required during throttle transition.

Just randomly changing the oil viscosity to what your buddy uses in his Triumph ignores all of that. Or, if you go down that road, at least pay attention to how the engine's throttle response changes as a result. Don't change the oil viscosity, then blame subsequent changes to throttle response on something else, and go off on another Witch Hunt.

Regards,
Tim Engel

Last edited on 02-10-2020 04:48 pm by Esprit2

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 Posted: 02-08-2020 01:04 pm
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chrisl
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Forgive my ignorance; but what does ATF stand for?

Alcohol, Tobacco and Fireams?

Automatic transmission fluid?

And what would be the viscosity of the damper oil that should be used?

My GT has Dellortos but I have a set of Strombergs that need refurbishment and one day I'll fit just to see what the difference is.

I like the thinking behind a damper in a carburettor.

Last edited on 02-08-2020 01:05 pm by chrisl

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 Posted: 02-08-2020 04:12 pm
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pokeyjoe
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ATF = Automatic Transmission Fluid.

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 Posted: 02-09-2020 01:45 am
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Esprit2
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Jensen-Healey and Lotus both called for 20W-50.

Regards,
Tim Engel

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 Posted: 02-09-2020 02:10 am
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chrisl
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Thanks gents, it will be interesting to see what the difference between the two types of carby on the same car will be

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 Posted: 02-09-2020 03:28 pm
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noomg
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I just use 3in1 oil, seems to work fine and has for 20+ years. I've used it in both my J-H and TR-7, they both have the Zenith 175s. Also I read somewhere that a lighter oil will improve throttle response.

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 Posted: 02-09-2020 09:53 pm
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Tom Bradley
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chrisl wrote:
Thanks gents, it will be interesting to see what the difference between the two types of carby on the same car will be

I hope you post your the results. I tried a comparison between SAE20 and 20W-50 and could not tell any difference. Maybe I am not that perceptive.

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. 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.

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 Posted: 02-10-2020 06:41 pm
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Esprit2
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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.

Regards,
Tim Engel

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 Posted: 02-12-2020 05:27 pm
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Tom Bradley
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Thank you for the good information, Tim. One minor correction:

Esprit2 wrote:
...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.

There is (or should be) a spring loaded retaining screw that keeps the metering needle from dropping out when going fully counter-clockwise. At least there is on mine. If not, the screw may be missing, not fully turned in or the slot in the metering needle assembly is not lined up with it.

Last edited on 02-12-2020 05:27 pm by Tom Bradley

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 Posted: 02-13-2020 05:37 pm
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Esprit2
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The carbs in which I measured both needles dropping out at around 4.5-4.75 turns were on a stock 1979 Lotus Eclat's Federal 907.

Stop screw or no... the point is that the adjustment range is limited by the length of the thread. 3.5 turns out is very lean, and about all you can get. 1.5 turns out is richer, and 'about' where the engine will be more happy. That was with B1DK needles, and the exact setting will vary with the needles that are installed.

*~*~*~*
If you're observant about vintage Lotus cars, you might notice a trend of odd fixes used to meet American standards.

The Europa's nose was too low to meet Federal bumper and headlight height standards. Initially, Lotus simply inverted the lower control arm, which raised the nose. That got it into the country and off the dealer's showroom floor. Then all the private owner had to do was flip the lower control arms back over (all the required parts were 'right there') to lower the nose back to Euro ride height.

Another example is the dual-MOP 110 red dot/ 100 blue dot cam pulley on the later emissions 907. On the dual-MOP pulley, the geometry doesn't permit both 110 and 100 to co-exist. So Lotus simply moved the blue dot to the nearest MOP that did exist... 97 MOP. That's worse, so why would they do that? Because once the car was in the country, all the private owner had to do was remove the intake pulley, flip it over, and rotate it 3 teeth to the desired, more powerful 110 red dot timing.

There's a history of Lotus using such creative assembly to get the cars into North America... then you just re-assemble/ re-adjust the same parts to get the car to what is was really meant to be. There is no published list of such monkey business, and no insider NNWWSNM acknowledgement, but I've been playing with Lotus cars long enough to have had more than one occassion to go, hmmmm.

It wouldn't surprise me at all if Lotus used a metering needle with which the engine could be tuned to run 'normally', and then simply adjusted it extremely lean to get the car into the country. Even if setting it that lean meant leaving out a stop screw. The 1979-80 907 powered cars ran like crap rolling off the boat, but they were emissions legal. Then the private owner only had to re-adjust the Z-S metering screws, flip over the existing dual-MOP intake cam pulley to 110 MOP, and adjust the static ignition timing to 12-14 BTDC in order to convert a polished turd into a diamond. Just a Saturday afternoon.

A corporate car manufacturer or their official dealers can't modify cars out of compliance with Federal specs. But a private owner could/ can modify their cars... just look at all the custom-bult hotrods, low-riders and street-tuner cars that are running around.

But you didn't hear any of that from me.

Regards,
Tim Engel

Last edited on 02-13-2020 06:21 pm by Esprit2

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