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Anyone know how much oil goes into the carb pots? SU HS4 1 1/2


hardhatharry
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2 minutes ago, hardhatharry said:

quite happy to pay to save a great deal of time, messing and mess of trying to fill the carb pots using a tiny funnel and a litre of oil

I use a small syringe, no mess no waste.

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Instead of stating that the oil should be this or that viscosity, let's discuss the purpose of the oil and the damper.     SU and Stromberg carbs lack the 'accelerator pump' mechanism that Webers and similar have.   This makes the mixture richer on acceleration.

Instead, recall that in a constant velocity carb ( SU or S'rg) the damper slows the rise of the piston as the engine speed increases.  This keeps the airflow, over the bridge where the fuel jet and needle live, higher.     By Bernouille, faster flow means lower pressure, so more fuel is sucked out into the air stream, making the mixture richer.

So, if you want maximum performance, use a thicker oil.   For economy, use thinner.

Otherwise there is nothing magic about the oil you use.    Even the "Marvel Mystery Oil" so beloved by our American cousins isn't the answer, unless you know what it's viscosity is!

John

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37 minutes ago, Pete Lewis said:

syringe/funnels  come on   get a pump oil can use it on all the throttles door hinges locks and any thing that moves 

 

Got one but I find the syringe more accurate for this sort of 'precision engineering' 🙄

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  • 2 weeks later...

I have a lovely little lever action oil gun like Jeff's, that was my Dad's. It holds about a cup full off clean engine oil that I use in the dash pots after Uncle Pete b*llocked me for using 3 in 1 (calm down Pete, take a pill!!). I think of Dad everytime I service the car, leaning over the engine of my Hillman Husky, (painted by hand with best blue Dulux house paint) dragging on his lighted fag, like you did. Happy memories.

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On 04/08/2021 at 21:29, JohnD said:

So, if you want maximum performance, use a thicker oil.   For economy, use thinner.

A layman's question. Does it not need more air as well, to produce a bigger bang? (genuinely interested). 

Edited by daverclasper
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5 minutes ago, daverclasper said:

A layman's question. Does it not more air as well, to produce a bigger bang?.

You are only enriching the mixture for rapid acceleration, (boost) once equilibrium is acheived the "normal" fuel ratio is correct(ed) for running.

Pete

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9 minutes ago, daverclasper said:

I understand that, though thought it needed air as well for any fuel/ air, basic power ratio?.

On a modern car, the air/fuel ratio is 14.7 for perfect combustion, but for cruising it will go up to 15 or more. Maximum power is at 12.5, a bit rick. So during acceleration, you want a rich mixture to provide extra power. 

In reality, a carb will probably run richer than optimum as it can't cope with very rapid changes. But SUs are very clever in the way they work, and can be extremely efficient (strombergs effectively the same)

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2 hours ago, daverclasper said:

I understand that, though thought it needed air as well for any fuel/ air, basic power ratio?.

Yes, but the point is that you've just whacked the throttle plate open so you're getting a whole load more air. The dashpot holds the piston down to increase the fuel flow to suit.

Incidentally, Clive's comment about modern cars isn't quite right. Unless it's a GDI it will have a 3-way catalyst for emissions, and they only work at stoichiometric, so there's no leaning off for cruise nor enriching at wide throttle. The transient fuelling is likewise precisely calibrated to hold 14.7:1 throughout the throttle movement.

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This is confusing me ! (Hopefully just an age thing)....if the piston (and jet) are 'down' surely the mixture is weakest. You need the jet 'up' for more fuel. 

The damper oil resists the jet rising so it would be thinner oil for more fuel. Allowing the jet to rise more quickly???

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12 minutes ago, Jeffds1360 said:

This is confusing me ! (Hopefully just an age thing)....if the piston (and jet) are 'down' surely the mixture is weakest.

However, you have ignored the other factor. The speed of the airflow over the jet. This is vastly increased with te butterfly opened up but the piston forced down. So much more fuel is sucked out of the jet. (faster airflow, lower pressure/more vacuum)

The carbs are a VERY clever design!

Your initial thoughts are why people think thinner oils are better, when the reverse is true. 

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9 hours ago, NonMember said:

stoichiometric

God I havent seen that, since I did My First Class Certificate some 50 years ago!. (well not quite true!). May be time to get the Books out again?. We had a Lecturer who led us through "tribology". Which at the time some thought was the study of anchient peoples?.

Pete

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19 hours ago, NonMember said:

Yes, but the point is that you've just whacked the throttle plate open so you're getting a whole load more air. The dashpot holds the piston down to increase the fuel flow to suit.

Incidentally, Clive's comment about modern cars isn't quite right. Unless it's a GDI it will have a 3-way catalyst for emissions, and they only work at stoichiometric, so there's no leaning off for cruise nor enriching at wide throttle. The transient fuelling is likewise precisely calibrated to hold 14.7:1 throughout the throttle movement.

My spitfire runs 15.2 at cruise. I want it to be 12.5 at wot, but it is more than tricky to get it right. Some points I am at 11, but it all happens very fast,  and so not a lot of fuel is wasted.

Gill (skilled button presser when I drive) only has so much patience with tweaking fuelling

I know when Honda were playing with lean burn tech they used very weak mixtures. I am sure I heard 18 or 20 mentioned.

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