SU Type carby questions...

[mr_sikma]

Hey, just a few questions etc. about SU tupe carbys. I am running a CD Stromberg 175 carby on my sigma turbo.

*Just wondering what type of oil you use in the top of your SU carby?

I was using a 5-40 motor oil but my mechanic has told me to use auto trans fluid and it seems fine. Alot thinner so i think it runs out quicker.

*How do you know when to fill it up?

Sometimes my car starts to idle bad and after hard acceleration and in neutral it idles bad until you give it a bit of a rev. Thats usually when i put oil in.

*How much do you put in?

Mine has a little hole at the top of it that drains the oil out if there is too much.

Thanks for your help, also feel free to post some other info etc. about these carbys as i know NOTHING about them and neither does alot of people i talk to!

[MiniMan]

hey, ive also had this problem but sorted it out, having run them in my mini ive found normal motor oil to be fine in the dashpot.
I used to run a lot thinner oil in it (sewing machine oil) but it ran like a dog (idle problems and stalling after giving it a bootful)

Normally you fill it up the bottom of the threads, but no higher, yes it will seep out of the hole at the top, but thats normal. just when you fill it up. Stick the plunger in and just pump it a few times to rid of any air

[mr_sikma]

aslong as the oil in my carby is full it doesnt idle bad or stall after thrashing it but as soon as it starts doing that i reckon its time to fill it up a bit. i reckon mine is running better now with the thinner oil although it seems to be using more or i could just be giving it more crap

what happens if u overfill it, aprt from it draining the excess out? does it affect it in any way?

and i will make sure i push the plunger in aswell!

thankyou

[hamish]

There is no 'correct' oil to use in an SU/Stromberg, thinner oils like tranny fluid are what is recommended, but you can use the oil viscosity as a tuning variable. It affects the transient mixtures the carb gives, a thicker oil slows the rate at which the piston rises causing the engine to 'suck' harder on the jet giving a richer mixture, think of it as adding more accelerator pump shot.

Do a google search on SU carburettor, there's quite a bit of information out there on how they work.

Predators & most std motorcycle carbs work on the same operating principle (constant depression) as an SU even if they look completely different.

[Datto-Zed]

Most zed ppl swear by 20wt oil. Currently I'm using sewing machine oil in mine without any problems. I do find they need to be topped up about once a month or so.

[mr_sikma]

if i dont top mine up every couple of days or so otherwise it starts to run a bit rough after i give it a bit and then back off. it only takes like a half a cap full of oil every time but seems torun alot better when it completely full.

[hamish]

How's the condition of the throttle shaft? If its worn & sloppy the air leaks around the shaft can give problems with rough idle. A carby specialist can rebush the body for you.

You shouldn't have to top the oil up every couple of days, is it leaking out?

[mr_sikma]

no its not leaking only if i fill it up as it has the little hole at the top.

i only ntice that after giving it a bit and backing off then the revs drop low until u slow down and rev it back up?!? but thats only after a few days without topping up the oil.

when should u need to top it up?

[hamish]

You really should only have to check them when you do a normal service, ie. every 6 months or so.

You may be overfilling it slightly, so when you drive it, the piston rises up & pushes out any excess oil. The oil (in an SU at least, I'm assuming strommies are the same) doesn't come all the way to the top of the metal tube.

Check the rubber diaphragm for holes or perishing, if that's faulty it could give erratic running as well.

Just re-read your original post, are you topping it up through the tiny breather hole in the plastic cap on the top of the carby?
In case you didn't realise, that cap unscrews, & you pull out the little damper piston to top up the oil.

Have a look at this link, http://www.roversd1.nl/sd1web/carburation.html down the bottom of the page there is an animation showing how thay work.

[mr_sikma]

i am topping it up thru the top part after i unscrew the plunger lid thing

i know that i put a bit too much in but it gets pushed out thru the lil breather thing so i am assuming thats ok.

i will get my mechanic to check it out 4 me amd i will also have a read thru that link...

thanks