ATF for the carb dampers. Not engine oil unless you want to break down and have to use carbon tetrachloride to clear them by the side of the road some dark night.
.............I have to say that in 30 odd years of bunging engine oil in both SU's and Strombergs I've never had it cause a breakdown or had to use carbon tet to clean them!
I wouldn't get too hung up on the oil- these carbs will function with any or quite often no oil in them- if they're dry it may fluff a bit when you boot the pedal but that's all, it won't cause the lack of power.
I think you should be doing all the normal checks: spark timing/quality, fuel supply, valve clearances, make sure all these are right and then start looking at the fuel mixture. SU's are pretty basic and easy to set up but everything else needs to be right first.
1968 Triumph Vitesse Mk1 2 litre convertible, Junior Miss rusty has a 1989 998cc Mk2 Metro, Mrs Rusty has a modern common rail diesel thing.
The consequence of a low oil level in the dashpots of my twin H4s seems to be stalling when coming to a stop...piston instability or something? Have had mixed results with ATF, has it got thinner over the years? maybe it's the climate here in Sydney.
Isn't Carbon Tet. a bit dangerous to carry around in the boot for those night time roadside repairs?
I stripped the carbs completely hoping to find some dirt in there but nothing obvious.
Re-assembled but still seemed to be stuttering when the throttle blipped.
Swapped the new Lumention coil, (installed as part of a conversion to electronic ignition last year whilst trying to sort this issue) for the old Lucas unit and guess what.... problem disappears!
I've been messing about with this for over a year now and thats what it was. That bl**dy coil cost me about £50 I seem to remember .
Anyway, its sorted now so its going to get an outing to the Oulton Park show in a week or so.
A couple of years ago I chatted with the people at SU about the "right" oil for their carbs. They recommend (and sell in very small bottles) monograde SAE30 oil for my pair of HD6 carbs. They said that an issue with multigrade is that, when the carb piston needs to react quickly once everything is warmed up & driving at varying speeds, it slows down the piston response more than a monograde does.
I also discovered that the oil is to slow down the raising of the piston, but the fall should be rapid and immediate, something that I was unaware of.
Dave' I don't think the fall of the piston should be, as a rule, 'rapid and immediate' as most SUs would use a spring under the piston of varying tensions to achieve just the opposite?
The spring is compressed as the piston rises so it would help push it back down when the throttle is closed though the changes in air pressure probably have a greater effect. But its real job is control how far (and to a degree how fast) the piston rises in the first place to match the fuel delivered with the air flow.
Seth wrote:The spring is compressed as the piston rises so it would help push it back down when the throttle is closed though the changes in air pressure probably have a greater effect. But its real job is control how far (and to a degree how fast) the piston rises in the first place to match the fuel delivered with the air flow.
Indeed. Due to the fact that SU carbs don't have an accelerator jet, if the piston rose too quickly it'd give rise to hesitation and stuttering - an effect which is easy to replicate on old motorbikes with Amal carbs, if you snap the throttle open too quickly. However, a "lazy" throttle that doesn't close more or less as soon as the accelerator pedal is released is quite unpleasant to drive, and possibly dangerous, so you want the piston coming back down pretty rapidly really, certainly much more quickly than it goes up. The SU system achieves both of these quite effectively I think.
Some people are like Slinkies - they serve no useful purpose, but they still bring a smile to your face when you push them downstairs.
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