No life (a.k.a. DattoMaster)  
Joined:
2008/10/10 22:02
From Melbourne Australia (and likely under the car)
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In all honesty, those carbs are too small for even a stock cammed a15. Sure it'll likely make more power than it would with the factory carb, but if you are going to the expense/time to fit twin SUs, give very strong consideration to 1 1/2" ones, and for any non stock/standard cam/head specs, you'd easily be able to run twin 1 3/4 inch carbs and make more power again.
But to answer your question the oil in the carb dashpots acts like a shocker - it slows (or dampens) the rise of the piston inside the carb. if they oil was too thin, or there was no oil in there at all then when you floor it, the piston rises too fast, the fuel circuit won't have time to catch up with the increased airflow, and leans out and has massive flat spots immediately after you floor it. THis is actually the opposite of what most people think. At first glance, most people think that thin oil, or no oil at all will let the piston rise quicker, so better throttle response.
The oil is to 'slow' the piston rising, so that whilst it is temporarily lower than it will eventually be for the new higher throttle opening/rpm - well that means it creates a higher suction (technically a pressure drop) across the jet, and so richens the mixture. It all 'evens out' as the fuel flow catchs up properly and the piston eventually rises to the correct height.
Describing carb's piston as 'eventually' reaching it's proper height - well eventually is a relative term, and in practice it's still only a split second or so.
So we know we need to slow the piston's rise by 'some amount' to avoid the lean out and keep mixtures 'right' during the transition of the pistons rise just after flooring it. To actually find 'how much' or what type is relatively easy.
Try the thinnest oil (sewing machine oil is a good start) you have access to, and try it. And it may or may not then have a flat spot after you floor it. It there's no flat spot, leave it as is. If there is a flat spot, go thicker in the oil and try again. Just go as thick as needed to remove the flat spot (or a very small change beyond that, to go from 'good' throttle response to 'best possible response for this type of carb.)
Obviously there aren't a million oil viscosities floating around, so if you need something between the thinnest off the shelf budget priced motor oil, then mix half and half and put that in a new container (maybe a $2 refillable sauce/ketchop squeeze bottle) and mark it as 50/50 sewing machine oil and 25w50 motor oil (or whatever the oil you use is).
FRom memory the latest dexron vi is around approximately 5w30, but don't take that figure to the bank, double check it.. It starts thinner than earlier dexrons but is more stable and doesn't thin out more (so at engine operating temps won't thin out appreciably). I bring it up because a lot of older SU equipped cars used it (or some form of transmission fluid). That's not a bad idea, but you can find it is thicker than necessary.
Once you get ideal (or acceptable) throttle response, going any thicker DOES slow the rise of the piston long enough to actually reduce throttle response and higher rpm output (at least until the piston finally rises up to the top). Aside from that, as mentioned, no improvement to throttle response (a slight decrease) and it'll be using far more fuel (*during that transition from piston starting position/height to final height) than needed. NO bull, it'd easily be able to make a difference over 1 litre (near enough to 1 pint for our US friends :) ) per 100km - esp city cycle, and a bit less so highway cycle.
Or the fast version:
try sewing machine oil
if there's flat spots:
try 50/50 sewing machine oil/motor oil or sewing mach oil/trans fluid
If there are still flat spots
try 'straight' trans fluid.
If that still has flat spots, I'd suggest very strongly that in fact the problem is that the needle profile (and possibly hte springs) are not right for the combination, and it's way too lean either in the low/midrange (starting point of the acceleration run after you floor it) or too lean up top, and the thick oil is trying its best but is adding more on top of something way too lean to start with. Or potentially both.
If the car has zero flat spots even with NO oil in the carbs at all - then it's a good sign the needle profile is too rich across much of the range, and even if it leans a little when you floor it, it's starting from a point way way way too rich so that 'leanout' is still not sinking below stoich or anything.
Last but not least - do NOT use diff oil in there (the 80w90 or other similar categories). Firstly diff oil viscosity is set on a different test standard/scale. Believe it or not 85w90 *or whatever the most common diff oil is) actually thins out at 100C to be thinner than 30 weight motor oil. That would 'theoretically' be good as it would enrichen more whilst thicker, when the oil/motor is cold. And thin out a litte when it warms up. Problem is it changes too much to really track close to that theoretical 'ideal'.
The second issue with diff oil is far more serious - and that is the fact that it is actually corrosive. Don't get me wrong it's not going to destroy the carb like if you put it in some massively strong acid or alkaline bathtub, but it will damage it over time. The reason for that 'corrosive' property I think has been discussed here, but basically it sort of 'etches' the specially produced/hardened/treated diff teeth so that it will actually 'retain' the diff oil on their surfaces under the strain/load of the gears in operation.
Posted on: 2011/2/15 6:27
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