Oobyscoot Posted January 12, 2011 Share Posted January 12, 2011 What oil do people recommend for the dashpots of SU carbs?It is the last fluid to be refilled following my spit 1500 engine rebuild, but thinking about this, too thick and surely the engine pick-up would suffer, too thin and potential for stalling.I have done a quick search but not found an answer.Cheers all. Link to comment Share on other sites More sharing options...
Lord Sorbington Posted January 12, 2011 Share Posted January 12, 2011 I always used 3 in 1 oil in my SU carbs which seemed fine. I tend to use light oil in the 2000's Stromberg carbs too.I think engine oil is specified for most Triumph carbs so probably best to start with that. I've tried various oils in the 2000's carbs and found it to make a minimal difference. Mental note, must check to see if they have any oil in them at the moment................ Link to comment Share on other sites More sharing options...
Oobyscoot Posted January 12, 2011 Author Share Posted January 12, 2011 Cheers for the quick response.In the past I have always gone with 10/40 engine oil, but just wondering what others like to use......on my shelf I have :General purpose household oil10/14 fully synth bike engine oil20/50 semi synth engine oil15/40 mineral engine oil5/50 semi synth engine oil80/90 hypoid gear oil10w motorcycle fork oil15w motorcycle fork oil20w motorcycle fork oilcitroen LHM fluidhydraulic jack oil2-stroke engine oilpenetrating oiland about 300 litres of used veg oil. (technically this is next to the shelves, but close enough)Which would you go for from that lot? Link to comment Share on other sites More sharing options...
Velocita Rosso Posted January 12, 2011 Share Posted January 12, 2011 The book specifies 20/50 eg your engine oil Link to comment Share on other sites More sharing options...
Oobyscoot Posted January 12, 2011 Author Share Posted January 12, 2011 20/50 it is then.Many thanks. Link to comment Share on other sites More sharing options...
akuma.no.spitfire Posted January 13, 2011 Share Posted January 13, 2011 I suggest you use SAE20, so the 20W motorcycle fork oil.The "book" specified engine oil when only single grades where available.A multigrade like 20W50 will not work as well; it will work OK when the engine is cold but gives too much enrichment when the engine heats up.The oil in an SU enriches the mixture by slowing down the piston, sort of like an accelerator pump on Weber carbs. Link to comment Share on other sites More sharing options...
ferny Posted January 13, 2011 Share Posted January 13, 2011 The best I've found is ATF. Rather helpful having loads left over after doing a service on the Acclaim. Link to comment Share on other sites More sharing options...
Velocita Rosso Posted January 13, 2011 Share Posted January 13, 2011 ferny wrote:The best I've found is ATF. Rather helpful having loads left over after doing a service on the Acclaim.You service an Acclaim....? Link to comment Share on other sites More sharing options...
thebrookster Posted January 13, 2011 Share Posted January 13, 2011 michael_charlton wrote:You service an Acclaim....?He acclaims to a service!!I'll find my coat,Cheers,Phil Link to comment Share on other sites More sharing options...
Oobyscoot Posted January 13, 2011 Author Share Posted January 13, 2011 2099 wrote:I suggest you use SAE20, so the 20W motorcycle fork oil.The "book" specified engine oil when only single grades where available.A multigrade like 20W50 will not work as well; it will work OK when the engine is cold but gives too much enrichment when the engine heats up.The oil in an SU enriches the mixture by slowing down the piston, sort of like an accelerator pump on Weber carbs.the fork oil seems to work well. Eventually (see my distributor post).Thanks. Link to comment Share on other sites More sharing options...
Nick Jackson (2) Posted January 13, 2011 Share Posted January 13, 2011 Quote:I suggest you use SAE20, so the 20W motorcycle fork oil.The "book" specified engine oil when only single grades where available.A multigrade like 20W50 will not work as well; it will work OK when the engine is cold but gives too much enrichment when the engine heats up.I'm with Akuma. When I asked someone at Burlens in Salisbury they specifically said not to use 20/50 multigrade. Only plain SAE20. Link to comment Share on other sites More sharing options...
JPB Posted January 13, 2011 Share Posted January 13, 2011 ferny wrote:The best I've found is ATF. Rather helpful having loads left over after doing a service on the Acclaim.Good call, Sir, IMHO. :)I trained (on other BL stuff among other things) many years ago now, and ATF, specifically the Donax variety as specified for the Dolomite auto, was what the old gaffers always told us to use in Stromberg dashpots, I still use it in my Dolomite's (on its original Strombergs) dashpots, it works, simple as that. Link to comment Share on other sites More sharing options...
JohnD Posted January 14, 2011 Share Posted January 14, 2011 As usual, this sort of conversation turns into expert statements by experts, when the answer is to understand the problem and make your own decision. There is no "right answer". Not the one selected by the manufacturers for the mass market, but the one to suit you, your car and your driving style.As akuma said, the dashpot slows the piston. A 'dashpot' is a damper, a shock absorber, that prevents a motion being as quick as it wants to be. In the carb, it slows the rise of the piston, which keeps the throat narrow for longer. This narrow throat speeds the air through it, the pressure at the throat falls and so more fuel is sucked up. Lower pressure, more fuel, the mixture is richened to allow better acceleration.Don't forget that both SU and Stromberg carbs are 'constant depression' devices - they equilibrate to a constant pressure in the throat, but the dashpot allows for a temporary increase in 'depression' or vacuum. Tune the dashot to your style of driving and your wish for economy v. performance, by selecting a thinner oil for less enrichment and more economy, and vice versa. You will do no harm by experimenting, though you can't expect to make a big differenceJOhn Link to comment Share on other sites More sharing options...
Oobyscoot Posted January 15, 2011 Author Share Posted January 15, 2011 ah, I feel I may have started something here, something akin to the many "what engine oil" discussions on the bike forums I frequent.Everyone has an opinion.....I have built up my own opinions of engine oils over the years, but no experience of SU carbs, I have only ever run webbers or Zenith carbs before the spit (or the ford VV carbs, But I try to blank that from my mind). I would love to find a manifold that allowed for a Zenith carb conversion as I love them........Ooops..... Link to comment Share on other sites More sharing options...
piman Posted January 15, 2011 Share Posted January 15, 2011 Hello Oobyscoot, once you have some experience with S.U.'s and understand them, you'll appreciate how good they are. Deceptively simple but very effective, and so easy to work on.Alec Link to comment Share on other sites More sharing options...
Beans Posted January 15, 2011 Share Posted January 15, 2011 Don't think there's much difference between a multi or single grade oil as the carburettors are supposed not to heat up to much.For the rest I'd say stick to John's advice. It is pretty simple to experiment with the oil and work out what viscosity suits you best. Link to comment Share on other sites More sharing options...
Velocita Rosso Posted January 15, 2011 Share Posted January 15, 2011 Exactly,each engine has a different requiremnt. "The book says" 20/50 ,however, for the standard car out of the showroom in its eraToday everyone` tweeks` their car to their own requirement from tootling down the road to track days.So as stated ,its down to personal preference, I personally dont find any problem with 20/50, even though Ive experimented from gun oil upwards on viscosity Link to comment Share on other sites More sharing options...
thebrookster Posted January 15, 2011 Share Posted January 15, 2011 And it has just occured to me that having re-fitted my carbs today after a rebuild that I have done a 10 mile test drive without having put any oil in the dashpots at all, so it can be done!!!Right, now I am off out to oil said dashpots before I try and drive them 80 miles back to Glasgow tomorrow.Cheers,Phil Link to comment Share on other sites More sharing options...
tiggrr1 Posted January 15, 2011 Share Posted January 15, 2011 I know when I have not topped up my daspot oil (and it has run out)because my engine pops,farts and bangs on downhill sections "off throttle" on overrun.My cars way of saying give me some attention you lazy git !!! Link to comment Share on other sites More sharing options...
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