Diaphrams

[Andrew S]

Would I be correct in thinking that if I have replaced a split carb diaphragm that the new pair I have fitted will change the mixture as they will float the damper at a different level slightly? I appreciate there is a spring, but the car is now jerking under load at 1500 revs, just at that awkward around town speed. If I put my foot down quickly, its worse, but if I tickle the throttle, she's OK.

Fine at all other revs

Cheers
Andrew

[V Mad]

IME some diaphragms are too thick and impede the free movement of the air valves (pistons). If you take out the dampers, then lift and drop the valve in each carb they should both drop quicky with a clunk, at the same speed.

I ended up refitting some good old ones and binned the stiff new ones. Obviously not on my car I hasten to add!

[Andrew S]

I had another pair and fitted them and they were paper thin and when I took them out to inspect, the diaphram wouldn't even seat back into the carb body and it's as if it was too big and expanded. So, I had another pair new and slightly stiffer surely also, if they are too thin, they can allow the damper to move sideways and catch, especially when there is a spring pushing

[V Mad]

The diaphragms are only there to provide an airtight seal, not to hold the air valves on centre.

[Stagsongas]

Would I be correct in thinking that if I have replaced a split carb diaphragm that the new pair I have fitted will change the mixture as they will float the damper at a different level slightly? I appreciate there is a spring, but the car is now jerking under load at 1500 revs, just at that awkward around town speed. If I put my foot down quickly, its worse, but if I tickle the throttle, she's OK.

Fine at all other revs

Cheers
Andrew

Andrew,

I did write about this issue some time ago, but a here's a short recap;

The thinner the diaphragms are, the less they resist the rising of the carb. pistons. The thicker they are, the more (as you'd expect) they resist the rising of the pistons. As the pistons rise, the tapered fuel metering needles rise as they are attached to the pistons (I know you know this, but it's for others in the future too).

If thinner or thicker diaphragms have been fitted to the carbs they therefore have to potential to change their fuelling characteristics, especially during acceleration, in which case they may need re-tuning. *

I uncovered this effect in my own car when investigating the cause of one pistion rising easily and readily, but the other being much more reluctant to rise. The cause was found to be one thick diaphragm fitted to one carb and one thin one in the other, which clearly illustrates and proves the effect that diaphragm stiffness can have.

To sum up, as long as both diaphragms are the same in thickness (stiffness) as the originals, not much (if any) re-tuning will be required, but it they are markedly different to the originals, re-tuning may be required. In all cases it is best to have both diaphragms being of the same thickness.

This also begs a question - If we require better performance (richer mix) in acceleration, are we better off with thinner diaphragms which allow faster rising of the piston/needle? The reverse may also be true, thicker diaphragms may make for slightly better fuel economy or at least require the carbs to be re-tuned to give a richer mix during acceleration.

The same goes, for that matter, to the viscosity of the oil put into the top damper, the thicker (more viscous) it is, the slower the pistons will rise. Even if the new diaphragms are of identical thickness to the originals, but oil of a different viscosity were put into the dampers, that could make the difference you are experiencing during accleration - that might be a place to look.

Whatever, we can rest assured that the apparent benefits of thinner dia.s (OK, I'm tired of typing that word now) must have drawbacks too - they may not last as long! Same goes for thinner oil in the dampers.

*The springs are hopefully the same as before, so they can be ruled out as a constant.


Regards

Steve

[V Mad]

You cannot realistically compensate ie retune if the diaphragms impede the free movement of the air valves.
If the air valves do not return quickly to rest, with a nice thunk when lifted and dropped then bin the diphragms and get some proper ones.