• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Carburettor oil

Share this page

R

r67cat

Senior Member
Country flag
Offline
I seem to be topping up my oil damper reservoir on a regular basis. I am using 20wt oil for the damper. Carbs are SUs rebuilt with a kit from the usual's aprox 3 years ago. I am retopping/ refilling at every other fueling. Car usually resides in Michigan though now in Florida. Did I miss something in the rebuild or would a heavier weight oil be indicated it the warm weather? Thanks, Rick
 
Regular engine oil is just fine for a street machine.

You know there is no place for that to leak.
 
I also top mine off every other fuel load so its not out of the norm see what the rest of the group does?
 
You do know they don't need to be full. Maybe about a third or half is all.
 
Thanks, Just did a search on dashpot oil and found much discussion on the subject. Thanks Rick
 
I have always used hydraulic jack oil. A tip I got a long time ago by a british car mechanic. It works very well in all temperatures and also helps to seal the rubber.
 
Hi all,

I vary my oil depending on the use.
I use a light 3 in 1 oil if I want quick responce and do not require the smoothness or engine oil if I want smootness but do not require the responce.
In warmer climes a slightly thicker oil is suggested

Oil has to be about 1/2 " from top. If you fill it to the top it will find its own level. If you top it up then using your finger lift the piston to its max it should set the level. Watch out for the excess oil coming out of the air hole in the damper knurled top.

You can tell if they need topping up because the engine starts hunting on idle (especially twins).

hope this helps
 
The manual states 3/8" from the top of the tube. Any more than this will just be lost from vibration and heat expansion.

20-weight oil is the correct type for most situations. Moss sells a handy little bottle with oil specifically designed for SU carb dashpots. The last bottle I bought has lasted me many years for five cars.

You don't want to use too light or two heavy an oil. It will prevent the pistons from moving at the proper rate and can affect your mixture upon acceleration. Or so I've been told.
 
Tr8todd,
"It works very well in all temperatures and also helps to seal the rubber."
Shouldn't the oil stay in the piston tube and only be in contact with the metal damper parts?

As mentioned, the oil doesn't go all the way to the top, I believe that most texts call for filling it to within about 1/2" from the top of the tube. When you put the dashpot back in it will displace that much oil anyway.

I use 20W oil and I carry one of the small bottles of
3-in-1 oil in the boot. The "blue" bottles of 3-in-1 are 20W oil for electric motors.

To avoid spilling oil all over your engine and intake when filling the carbs, take a long screwdriver and wipe its shaft clean. Put the tip of the screwdriver in the carb and pour the oil onto its shaft so the oil runs down its length into the carb. It's easy and clean.
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]"It works very well in all temperatures and also helps to seal the rubber."
Shouldn't the oil stay in the piston tube and only be in contact with the metal damper parts? [/QUOTE]

Correct. It is just a metal tube. There are no holes, no parts inside, nothing. Just a solid metal tube. The damper piston which sticks down into the tube is made of brass and only serves as a valve.
 
In this month's MG Driver magazine, John Twist maintains that 90 weight gear oil works best in the SUs. It surprised me, too, but John has forgotten more about MGs than most of us will ever know.
 
I've heard him say that before. The thing is, gear oil is measured differently than motor oil. I don't know what a 90-weight gear oil would relate to in motor oil terms. I also don't see the point in experimenting for weeks to find the best weight gear oil when the research has already been done with other types of oils!
 
That "rubber parts" reference may be in relation to a ZS. The O-ring at the adjustable needle shoulder? But I'd think gear oil would dissolve it instead of lubricating it.
 
I used a LM-1 data recorder to check AFR -vs- RPM when I was messing around w/ various dashpot oils.

The only big difference I saw was that heavy oils would cause the AFR to spike (lean) after gear changes.

Otherwise, the line was flat for everything I tried (ATF to 30wt)

I use ATF and would like to check out something even lighter.
 
Just to show there are differing theories on this...when I picked my '79 up new, the service manager gave me a quart of auto trans fluid & told me to use it in my ZS carb...I've done that religiously & have only had the carb off for rebuild once since then!!

If you're having to add regular engine oil that frequently, you've got a leakage problem inside the carb
 
Aloha Steve,

You are correct in that motor oil and gear oil viscosity values are measured at different temperatures. Viscosity is an internal property of a fluid that offers resistance to flow. SAE motor oil is rated at 210 degrees Fahrenheit and gear oil is at 150 degrees Fahrenheit. Some comparisons I has seen made are 95 weight gear oil flows like a 40 weight motor oil and 75 weight gear oil compares to 10 weight motor oil. A 90 weight gear oil would be about the same as a 30 weight motor oil. For multi grade products, the first number followed by a "W" is viscosity at 0 degrees F and the second number is viscosity at 210 degrees F.
 
tr8todd said:
I have always used hydraulic jack oil. A tip I got a long time ago by a british car mechanic. It works very well in all temperatures and also helps to seal the rubber.
Click to expand...


Rubber!! What Rubber???---Keoke- /bcforum/images/%%GRAEMLIN_URL%%/confused.gif
 
Well, now here's another question also concerning the dashpot oil. I changed the oil today and was generally checking things, one of which was the dashpot oil level. To my surprise, the piston lifted completely out of the carb ('79, just the one carb)and I don't recall that it did this before.. Is this a bad thing and should I be concerned?
Thanks!
 
Dave, thanks for the numbers regarding the two types of measurements. It makes sense, as some people (including John Twist apparently) run a 30-weight oil. Very few people run a thinner oil than 20W.
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]To my surprise, the piston lifted completely out of the carb ('79, just the one carb)and I don't recall that it did this before[/QUOTE]

I know this is dumb; but, you had the carb opened up & lifted the piston out?
 
Back
Top