18 replies to this topic

[sparkymae]

Is it ok to use 20 wt synthetic oil in my HS4 carb? Thought i was buying just 20 wt until i got home and saw it was synthetic. I have a 1990 998cc Mini.  Thanks

[nicklouse]

The recommendation is to use the same oil as that is in your engine.

[hhhh]

Synthetic has the advantage that it is more resistant to temperature induced viscosity changes which is an advantage in the dashpot for more consistent response.

[imack]

Peter Baldwin advised I used atf (automatic transmission fluid) when he set my car up on the rolling road.
I've never used proper SU dash pot oil but I believe it's a very light weight oil, much thinner than engine oil.

[timmy850]

SU brand oil is 20 weight

http://sucarb.co.uk/...e-grade-20.html

[Dusky]

Synthetic has the advantage that it is more resistant to temperature induced viscosity changes which is an advantage in the dashpot for more consistent response.

Not really true. Its still rated eg 20w50 and Will behave like a 20 when cold and like a 50 when hot. Thats why they give it those numbers in the first place.
The OP's oil Will Just behave like a 20 when hot and cold.

Edited by Dusky, 03 January 2018 - 09:11 AM.

[racingbob]

3 in 1 oil works ok on mine so does the su oil

[cal844]

As Nick says su recommend 20/50 engine oil

[Ethel]

A function of the oil is to provide acceleration enrichment by slowing the piston lift - thicker oil = slower piston rise= more enrichment. If you've changed nothing else, when you put your foot down is when you'd know you've got it wrong. You could also see slightly richer running if you slightly increase the throttle opening, but not enough to  generate sufficient pressure differential across the piston to overcome the stiction, increased by thicker oil.

 

BL/Rover did specify engine oil and they set up the carb to match the engine, I'd take their spec as your starting point.

 

While it's true, synthetic or mineral, your oil will have to have met grade stated; synthetics are likely to offer a straighter temperature/viscosity gradient that tails off less outside the grades. If it's a single grade engine oil the viscosity will be @ 100^oC as an approximation of running temperature, your carb dashpot won't get that hot.

[hhhh]

 

Synthetic has the advantage that it is more resistant to temperature induced viscosity changes which is an advantage in the dashpot for more consistent response.

Not really true. Its still rated eg 20w50 and Will behave like a 20 when cold and like a 50 when hot. Thats why they give it those numbers in the first place.
The OP's oil Will Just behave like a 20 when hot and cold.

 

But 20 synthetic behaves more linearly with temperature than 20 conventional which is why it is preferred. It's in "the Bible".

[carbon]

The SAE20 oil as recommended by SU is a lot less viscous than 20W50 engine oil at the temperatures you're likely to get in a SU dashpot.

 

So using the SU recommended oil your carb piston will open quicker than using 20W50.

 

Which may or may not be a good thing depending on the needle profile, how the carb is set up, and what driving characteristics you are hoping to achieve.

[sparkymae]

Thank you all very much. . i see there is a bit of disagreement about the oil type. I will start with 20wt/50wt and then try my 20wt. Thank you all for your input. Wish i was in the UK 

[hhhh]

^{I would tend to go the other way, start with the the thinnest which would be ATF, approximately 8 wt. and do a few throttle punches at various RPM to see if there's any significant hesitation. If there is, go up in viscosity until the transition is smooth. Going higher in viscosity than necessary to give smooth transition will just slow throttle response with no benefit otherwise.}

Edited by hhhh, 04 January 2018 - 02:20 AM.

[carbon]

+1 for above.

 

As alternative to ATF there is also Castrol 10W motorcycle fork oil, available in Halfords in 500ml bottles for about £10. If you need to increase viscosity just blend in some 20W50 engine oil until you get the desired response.

[ACDodd]

Hehehe I always love this dashpot oil question. The answer is there is no one size fits all. Due to wear and tolerances different carbs need different oil to give the right response to the engine. Ita the reason I carry 4 grades when I carry out my optimised tuning sessions. I can categorically say that anyone who has answered this with this particular brand or viscosity suits all is wrong.

Ac