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Where is my dash pot oil going?


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Hi Sage Ones,

 

I've noticed the oil in the carb dash pots needs topping up after every trip, say 80 miles. It drops to about half way from the top. I use 3 in 1 oil, as I believe it's close to the original spec. When full there is a normal amount of resistance when trying to lift the carb pistons. I guess it's going into the engine but why?

 

Regards, Pete

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I guess you mean half way from the top of the carb body - not halfway down the piston tube? If so you are probably overfilling it and the excess is being forced down past the piston when the piston rises. The SU book says fill only to 1/2 inch above the top of the hollow piston tube.

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You may be filling them too full, they only need to be filled to the point where there is resistance to lifting the piston from the full down position.

 

http://sucarb.co.uk/technical-h-type-carburetter-diagram

 

Bob.

 

Rob posted while I was typing !

Edited by Lebro
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You might also find that 3-in-1 is a bit on the thick side, so more likely to squeeze down the side of the tube. Could also affect your damper resistance.

 

Moss sell a bottle of SU damper oil specifically for that purpose. Around £8 but should last some time.

 

Kevin

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Isn't the cheapest option just ordinary ATF oil?

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If the dash pot is filled to 'full' then the rising piston (I know it doesn;t rise but has that effect) will simply raise the oil above it.

Is it possible the oil raised thus can escape back into the intake.

 

Roger

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By 'fill up' they mean the dashpot tube not the whole thing.

If you fill the whole thing right up to the screw-on cap , then the oil is in the way when the carb piston (not the dashpot plunger) and dashpot tube tries to lift on acceleration. Something has to give and the only place the excess oil can go is either out of the breather hole, or down the side of the tube and eventually into the inlet manifold. There has to be an air gap above the dashpot tube equal to the distance the piston moves upwards at WOT.

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Is it a 2 stroke TR ?

I just use a few drops of engine oil ! ????????????

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Always loads of discussion about the 'correct' oil to put I the dampers.... i'v used 20/50 engine oil for many many many years, filled to half-inch below the top of the tube..... and never had a problem

..... Andy

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Hi guys,

 

The manual clearly means to fill up the inner chamber not including the upper section where the top nut screws in. Rogers point is interesting, I see what he means. Wonder why it's says to fill up the oil chamber then. I will try the correct oil and see how it goes.

 

The viscosity of the oil must have quite an impact on the carb piston speed and therefore engine speed pick up so I'm thinking thinner is better?

 

Sorry to have stirred this hornets nest............. Again.

 

Regards, Pete

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Hi Pete,

I'm sure I have read something recently that suggests that thicker oil will give you a better pick up as it gives more fuel - something akin to an accelerator pump.

 

Therefore thinner may cause the engine to run too lean for a moment and not accelerate as quickly.

 

However I do accept that I could be completely wrong.

 

Roger

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Hi Sage Ones,

 

I've noticed the oil in the carb dash pots needs topping up after every trip, say 80 miles. It drops to about half way from the top. I use 3 in 1 oil, as I believe it's close to the original spec. When full there is a normal amount of resistance when trying to lift the carb pistons. I guess it's going into the engine but why?

 

Regards, Pete

Correct oil and half way is where it should be.

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It IS the accelerator pump and thicker,oil will cause the piston to to remain down a little longer while fuel enters the intake without a blast of air, It's simple and works well with a minimum of parts. I've always filled the tube and allowed it to seek its own level after pushing what it doesn't need out the breather hole.

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It's a compromise between fast pickup when you floor it (a light oil) but you would usually need to set the carbs on the rich side = poor MPG

or a slower response, but usable with weaker mixtures (heavier oil).

I set my carbs using the "lift the pin" method, backed up with colourtune checks, then choose the oil to give me a good throttle response but without "coughing" or "spitting back".

 

Currently got a straight 20 grade engine oil in.

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Years ago racing Jaguars we used to make up our own dashpot oil as a tuning aid utilising anything from engine oil/gearbox oil/transmission fluid/3in1/EP140/Castrol Flick, depending on temperature and state of tune.

Stuart.

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Years ago racing Jaguars we used to make up our own dashpot oil as a tuning aid utilising anything from engine oil/gearbox oil/transmission fluid/3in1/EP140/Castrol Flick, depending on temperature and state of tune.

Stuart.

 

Stuart (as usual) is about spot on, it all depends on a huge number of factors, like air temperature, the temperature the carbs are actually operating at, what engine characteristics like cam, how free reving it is, and what driving style you are using, there is no one correct oil for all engines all driving styles, all weathers, however the recommended oil for an average car, driven in average conditions, will give reasonable average results!

 

When I were a young lad twas recommended to use a light oil like brake fluid for sporting performance. All I can tell you having dozen of data logged tests running back to back tests using different weight oils is that instant WOT response when you need an instant richer mixture that thicker oils are better, if you want economy and generally squeeze the throttle peddle rather than stamp on it then a lighter oil will work. Likewise even in competition different oils work best, if you are running at high revs like on a circuit and are running with fairly wide open throttle then a lighter oil is likely to work best, on hill climbs where you can be going from WOT to hard braking and straight back to WOT heavier oils give a better response.

 

NB virtually every carb book is wrong about the damper only working when the piston is lifting, its not true it also slows the drop however no where near as much, however when you start using 50 weight oils and above this starts to have some more serious affect and you can dramatically alter the throttle response in a WOT, Lift, WOT sequence because the piston hasn't fully dropped on the Lift.

 

FWIW I use 90w140 synthetic on hot summer days for hill climbing, but you wouldn't want to on the road as the economy is appalling!

 

Alan

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I agree with Alan. Using 90 grade as I do gives me the snap throttle opening response that I like together with a bit of an exhaust crackle going from wot to closed.

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As the damper plunger (part of cap assy) fits and works in the central oil reservoir of the piston assy, having any oil too high or over the top of the reservoir will serve no usefull purpose, and will find its way into the engine.

Also as touched on already, thin oil will lift the piston too fast and unless the engine is fully warmed up will weaken the mixture, and make starting more difficult

Chris

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I am sure I read somewhere many years ago that you only need enough oil so that when you drop the piston in damping resistance starts when the screw cap is about half an inch from the top of the dash pot and that's how I have always done it,also I only use engine oil.

 

Graham

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