Oil for SU Carburetor Choke Piston

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Kam

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Yep, My 70 year old SU H2 Carburetors have an oil reservoir (complete with their own little dipstick of a sort) which need a few oz of oil in each.
Original Manufacturer SPEC is straight-20 weight, which a lot of people claim is important to ensure the proper viscosity for piston operation under normal operating temperatures. The way it works is kind of like a shock absorber, where the oil bath creates a film to both lubricate, and to slow down the motion of a spring loaded piston, which is pulled up by the engine vacuum. The way this is set up, is so the piston is normally down, and has a slight upward force from the spring, and as the engine's demand for air increases, the piston rises in order to permit more airflow, thus automatically adjusting the Fuel/Air mixture

Anyway, long story short, I need a yellow-metals safe oil roughly equivalent to SAE20w or equivalent viscosity to SAE20w at as wide of a temperature range as I can get (20w20?) Which is yellow-metals safe... This is not a high speed or high wear item, so additives or a lack thereof are probably un-needed.

Ideas?
 
The oil is for the lift chamber damper, not the choke. Most use either 20w-50 or 90w gear oil. Run one, then the other, and see what worked best then stick with it.
 
We used ATF to good effect. I don't know if you want F or Dexron, IIRC F is a bit thicker than GM by a couple cSt - both are in the 20grade range.
 
Originally Posted By: hansj3
Look up 20 at fork oil. Lucus makes a full syn fork oil in a 20 wt


Fork oil was going to be my suggestion. Brand is probably unimportant. I would just go to Cycle Gear and get whatever 20wt fork oil that had in stock.
 
I always used ATF as mentioned in the Volvo manuals of the time. There's no accelerator pump on those "constant velocity" carbs, the oil serves to control how rapidly the internal piston moves the tapered mixture needle up and down with throttle plate caused vacuum changes. Too rapid a lift can cause lean stumble, too slow results in not enough extra air being admitted. Prior to the emissions clampdown the SUs were as good as the "American style" downdrafts of the day, but American companies moved on to better designs while SU did not. The Jags of the time used SUs with a bigger bore size.
Stromberg built similar carbs but with a rubber diaphragm instead of a piston, and Bing still builds similar carbs, which BMW bikes and some aircraft use.
Volvo went from SU through most of the 60s to Stromberg and then back to SUs with some emissions tweeking before they went to FI for all their products. Neither the Strombergs nor the later SUs were that good, a common improvement was a new manifold and a big two barrel Holly.
 
Last edited:
Originally Posted By: HerrStig
I always used ATF as mentioned in the Volvo manuals of the time. There's no accelerator pump on those "constant velocity" carbs, the oil serves to control how rapidly the internal piston moves the tapered mixture needle up and down with throttle plate caused vacuum changes. Too rapid a lift can cause lean stumble, too slow results in not enough extra air being admitted. Prior to the emissions clampdown the SUs were as good as the "American style" downdrafts of the day, but American companies moved on to better designs while SU did not. The Jags of the time used SUs with a bigger bore size.
Stromberg built similar carbs but with a rubber diaphragm instead of a piston, and Bing still builds similar carbs, which BMW bikes and some aircraft use.
Volvo went from SU through most of the 60s to Stromberg and then back to SUs with some emissions tweeking before they went to FI for all their products. Neither the Strombergs nor the later SUs were that good, a common improvement was a new manifold and a big two barrel Holly.


I Agree

(except the bit about two barrel downdrafts necessarily being an improvment)

Realisticly, the same 15w-40 you use in the oil pan will be fine to get you started.

Oil in the Dash pots is really fine tunning.
 
I had triples a couple of times on my holden 6's.

Holden ran the Stromberg cds dry, they didn't want the accelerator pump effect that the dashpot provided.

I tried dry, and the all the way from ATF to straight STP, and everything in between.

It ran better with oil, had better snap throttle for overtaking on the heavier oils (like 25W50/60)...but ATF/20W50 mix (probably top end of 20, low end of 30).

Just get some supertech 5W20, some ATF, and Supertech 5W30, and see which one drives better.
 
Originally Posted By: expat
Originally Posted By: HerrStig
I always used ATF as mentioned in the Volvo manuals of the time. There's no accelerator pump on those "constant velocity" carbs, the oil serves to control how rapidly the internal piston moves the tapered mixture needle up and down with throttle plate caused vacuum changes. Too rapid a lift can cause lean stumble, too slow results in not enough extra air being admitted. Prior to the emissions clampdown the SUs were as good as the "American style" downdrafts of the day, but American companies moved on to better designs while SU did not. The Jags of the time used SUs with a bigger bore size.
Stromberg built similar carbs but with a rubber diaphragm instead of a piston, and Bing still builds similar carbs, which BMW bikes and some aircraft use.
Volvo went from SU through most of the 60s to Stromberg and then back to SUs with some emissions tweeking before they went to FI for all their products. Neither the Strombergs nor the later SUs were that good, a common improvement was a new manifold and a big two barrel Holly.


I Agree

(except the bit about two barrel downdrafts necessarily being an improvment)

Realisticly, the same 15w-40 you use in the oil pan will be fine to get you started.

Oil in the Dash pots is really fine tunning.
IPD, out in Portland made a two barrel kit which sold very well.
 
Originally Posted By: HerrStig
Originally Posted By: expat
Originally Posted By: HerrStig
I always used ATF as mentioned in the Volvo manuals of the time. There's no accelerator pump on those "constant velocity" carbs, the oil serves to control how rapidly the internal piston moves the tapered mixture needle up and down with throttle plate caused vacuum changes. Too rapid a lift can cause lean stumble, too slow results in not enough extra air being admitted. Prior to the emissions clampdown the SUs were as good as the "American style" downdrafts of the day, but American companies moved on to better designs while SU did not. The Jags of the time used SUs with a bigger bore size.
Stromberg built similar carbs but with a rubber diaphragm instead of a piston, and Bing still builds similar carbs, which BMW bikes and some aircraft use.
Volvo went from SU through most of the 60s to Stromberg and then back to SUs with some emissions tweeking before they went to FI for all their products. Neither the Strombergs nor the later SUs were that good, a common improvement was a new manifold and a big two barrel Holly.


I Agree

(except the bit about two barrel downdrafts necessarily being an improvment)

Realisticly, the same 15w-40 you use in the oil pan will be fine to get you started.

Oil in the Dash pots is really fine tunning.
IPD, out in Portland made a two barrel kit which sold very well.


There are lots of downdraft twin choke carb conversions for old British cars. Weber, Holly, Delorto etc. They are fine carburettors, and a type American mechanics understand.
However, none of the kits I have seen for inline 4cyl or 6cyl engines have a decent inlet manifold. Some are downright awful! delivering un-atomized gas to the end cyls
( ie. cyls 1 and 4 on a 4 cyl engine).Even to the extent that I have seem 'fuel wash' (0il dilution) at the end cyls while the middle cylinder piston crowns, have been light tan colour (lean).
Sharp turns to create turbulance in the manifold runners, and a good Hot Spot under the carb mount would do much to help the situation. But these are hardly features of a 'Performance set up'

Downdraft carbs work very well on a V engine or opposed, but will seldom be the best performance option for an inline.
 
Last edited:
Originally Posted By: expat
Originally Posted By: HerrStig
Originally Posted By: expat
Originally Posted By: HerrStig
I always used ATF as mentioned in the Volvo manuals of the time. There's no accelerator pump on those "constant velocity" carbs, the oil serves to control how rapidly the internal piston moves the tapered mixture needle up and down with throttle plate caused vacuum changes. Too rapid a lift can cause lean stumble, too slow results in not enough extra air being admitted. Prior to the emissions clampdown the SUs were as good as the "American style" downdrafts of the day, but American companies moved on to better designs while SU did not. The Jags of the time used SUs with a bigger bore size.
Stromberg built similar carbs but with a rubber diaphragm instead of a piston, and Bing still builds similar carbs, which BMW bikes and some aircraft use.
Volvo went from SU through most of the 60s to Stromberg and then back to SUs with some emissions tweeking before they went to FI for all their products. Neither the Strombergs nor the later SUs were that good, a common improvement was a new manifold and a big two barrel Holly.


I Agree

(except the bit about two barrel downdrafts necessarily being an improvment)

Realisticly, the same 15w-40 you use in the oil pan will be fine to get you started.

Oil in the Dash pots is really fine tunning.
IPD, out in Portland made a two barrel kit which sold very well.


There are lots of downdraft twin choke carb conversions for old British cars. Weber, Holly, Delorto etc. They are fine carburettors, and a type American mechanics understand.
However, none of the kits I have seen for inline 4cyl or 6cyl engines have a decent inlet manifold. Some are downright awful! delivering un-atomized gas to the end cyls
( ie. cyls 1 and 4 on a 4 cyl engine).Even to the extent that I have seem 'fuel wash' (0il dilution) at the end cyls while the middle cylinder piston crowns, have been light tan colour (lean).
Sharp turns to create turbulance in the manifold runners, and a good Hot Spot under the carb mount would do much to help the situation. But these are hardly features of a 'Performance set up'

Downdraft carbs work very well on a V engine or opposed, but will seldom be the best performance option for an inline.
The Chrysler slant 6 ran a downdraft although those sold down under had several variations on the theme. The end runners on the six were quite long and the last US versions took a different heat range plug in the end cylinders.
 
Originally Posted By: expat

There are lots of downdraft twin choke carb conversions for old British cars. Weber, Holly, Delorto etc. They are fine carburettors, and a type American mechanics understand.
However, none of the kits I have seen for inline 4cyl or 6cyl engines have a decent inlet manifold. Some are downright awful! delivering un-atomized gas to the end cyls
( ie. cyls 1 and 4 on a 4 cyl engine).Even to the extent that I have seem 'fuel wash' (0il dilution) at the end cyls while the middle cylinder piston crowns, have been light tan colour (lean).
Sharp turns to create turbulance in the manifold runners, and a good Hot Spot under the carb mount would do much to help the situation. That earlier mentioned heat source though would sure help when it's cold.



Another possible strategy, if the stock intake has to be kept for whatever reason, is to try extrude honing it. A buddy of mine raced SuperV and I helped him tow it to a few races. He had rejetted and done as much as he could do (get away with) under the rules. But the car was bogging on throttle application and a quick examination of the intake and even the plugs looked like too much unatomized fuel was making it through. We took it home and a couple of weeks later he called and said he had had the intake extrude honed and that the extra fuel was now being put to good use. Too good in fact as he got protested more than once and tech inspection finally caught on. It was the first time I had heard of extrude honing and I don't know that much about it other than obviously it is intended to increase flow rates. Possibly though it may help maintain atomization rather than presenting the flow with a zillion tiny ridges before it can get to the intake valve. I haven't heard that much more about it over the years except to know that it is still available
 
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You would be fairly close with a DEXRON-VI ATF. SAE 20 is or was min 5.6 cSt at 100C and DEXRON-VI is max 6.4 cSt at the same temperature, and corrosion is not going to be an issue with that fluid.
 
Originally Posted By: HerrStig
I always used ATF as mentioned in the Volvo manuals of the time. There's no accelerator pump on those "constant velocity" carbs, the oil serves to control how rapidly the internal piston moves the tapered mixture needle up and down with throttle plate caused vacuum changes. Too rapid a lift can cause lean stumble, too slow results in not enough extra air being admitted. Prior to the emissions clampdown the SUs were as good as the "American style" downdrafts of the day, but American companies moved on to better designs while SU did not. The Jags of the time used SUs with a bigger bore size.
Stromberg built similar carbs but with a rubber diaphragm instead of a piston, and Bing still builds similar carbs, which BMW bikes and some aircraft use.
Volvo went from SU through most of the 60s to Stromberg and then back to SUs with some emissions tweeking before they went to FI for all their products. Neither the Strombergs nor the later SUs were that good, a common improvement was a new manifold and a big two barrel Holly.


Volvo (with their infinite wisdom) went back to the SU and Zenith/Stromberg in the late 70's until the early 80's for the Canadian DL models. Oddly enough, these cars were marketed as the "entry level" Volvo's yet required premium fuel as recommended in the owners manual...go figure!
 
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