SU Carb Oil

[morfirst]

I tore down my 4 screw SU carbs, cleaned the components, replaced the needles, gaskets, etc. reassembled and reinstalled. When I put the 20W oil in the dampner cylinder, there seemed to be alot of resistance to needle movement. I decided to experiment with various fluids and came to the conclusion that brake fluid seems to provide the best needle movement, without backfire. Has anyone else used DOT3 brake fluid?

[AxtellZ]

Haven't tried brake fluid, but i imagine it would work- the viscosity seems about right. I would avoid it due to it's paint remover qualities, but as long as you don't drip it should be fine.I use ATF, which is about the same viscosity i think. JA

[beandip]

I have used mistery oil for a long time but finaly found 20w . Fork oil available at motorcycle shop. 20W is recomended by the factory and of all that I have tried it's the best for all around use. Mistery oil allowed the piston to rise too quickly and caused a lean condition that showed as a slight lag, sort of a flat spot when starting from a stop, when in normal trafic conditions. I wouldent use brake fluid because it cannot take the heat and will turn into gum and screw everything up. If in racing , auto trans fluid is ok because you are in the upper rpm ranges all the time anyway , driving on the streets 20w will allow the mixture to richen a split secound longer allowing for smoother starts from lower rpm and improve performance.

[EScanlon]

Although you might be getting the "right" response from the Brake Fluid, don't forget that it WILL absorb moisture from the air and that it won't burn.

So, you are dealing with a fluid that will gain moisture and that if aspirated into the cylinders won't burn. Doesn't sound like something I would want in there.

[AxtellZ]

Yikes, hadn't thought of that Escanlon! Very good point. I would certainly skip it for that reason. For my "refreshed" carbs here in NY the ATF does work well in my 260, but so does the motorcycle fork oil (20W) in my 240, FWIW. JA

[morfirst]

The fluid level mark on the plunger is difficult to read at times. I assume if you overfill, the resistance to needle movement will be greater. Should the cylinder be filled to approximately half of it's capacity?

[kmack]

If you over fill the damper, it will push the fluid out of the top when you replace the damper rod. I did this when putting my engine back in my car a few months ago. Best bet I found was to pour a little fluid in at a time and keep checking the level.

[jcdozier]

You can't "overfill" - any excess oil will overflow inside the "pot", seep down the sides of the piston, and be sucked into the intake along with the air/fuel mixture. (Another reason not to use brake fluid.) You can, however "underfill" and lose some of the dampening effeciency.

OBTW, in addition to Beandip's and EScanlon's lists of bad things about brake fluid, it is also one of the best paint removers known to man. So, for those who like to paint stuff under the hood, there's yet another reason not to use it, (especially if you're like I am and often have the oil running down the sides of the carbs when putting the plunger in after "topping off").

I have used both 20 weight oil and ATF in my SU's, and can't tell the difference. Currently have ATF (I think). 20 weight is sometimes hard to find, but ATF is a great substitute. DON'T use a multi-weight oil!

JIM DOZIER

[Ken]

ATF works well here in the NE for the temperature range that is encountered from May thru November. When I need to fill my SU's to achieve a point between the low mark and the high mark I use a medicine dropper and this does a neat and accurate job.