I need a bit of advice about jetting for the Solex twin choke in the Acadiane.

As I have been slowly working my way through the engine getting it all ship shape, I finally got around to fitting the yellow coil and timing correctly with the gun on full advance to the 7th tooth ahead of the 8 degree point.

It started pinking on the second choke a little while back despite a thorough carb clean so I took a look at the jet sizes.

Firstly I don't appear to have a primary fuel jet, it looks like it's a fixed drilling, and the secondary fuel jet was an 87.5. So I suspect by the time the Acad rolled off the line in '83 tamper-proofing was in vogue.

Now the Haynes book of lies states it should be an 82.5 and the ECAS listing for jets [i]starts [i][ at 95 going up to a 135, with all sorts of other info just to confuse....

For example it states that the 100 jet is 11% increase for 18/24 (although WTF 18/24 is I have no idea)

120 and 125 states it's standard for many 21/24


So colour me a tad confused at this point. Does anyone have an inkling of what 18/24 and 21/24 refer to?

Anyhoo I've Drilled an old Mikuni jet out to 98 size and that has stopped it pinking. I suppose I should really weld a Lambda bung in the exhaust and cast my AFR gauge over the setup. It does look rich at idle as the plug ceramics are bone white but the base ring is sooty black.

Dave,
primary petrol jet position...
It's a little grub screw affair, but the carb body can be tapped out to take a Mikuni jet,
although you have to be careful that the head of the Mikuni jet doesn't foul the n/s float.



primary petrol jet position by slcchassis, on Flickr

21-24 & 18-26 refer to the venturi diameters in mm, with earlier versions of the 26/35 carburettor using the former sizes.

Yours is an 18-26 and I'm afraid that unless you always use full throttle you've altered the wrong jet and also increased its potential flow rate by 25%, as the CSA is proportional to a jet's diameter squared .

Back to the drawing board?
Or just increase the primary jet to a 107, or a 110 if it's got 'blown induction'.
Secondary back to 87.5 for standard induction, maybe try 90 with 'blown'.

I've got plenty of Mikuni jets here in both 107 and 110 sizes.

ken

p.s. Check the heat range of your plugs before using an overly rich mixture to control pinking, B7HS instead of the original B6HS or equivalent.
I use B8HS in our Dyanes as they tend to be worked fairly hard, not because they're trashed, but because they're usually towing something.

Many thanks for the reply Ken - appreciated.

i hadn't realised there was an access hole for the primary venturi jet


The carb will need a rebuild soonest as it's been stood for may years and I doubt the pump diaphragms will last very long now it's in use.

So the std primary fuel jet is a 102, and common upgrade is to a 105 or 107.

At least I won't melt the pistons whilst I await the rebuild kit

hi dave, i replaced the jet that ken has pointed out for a slightly larger one(ecas £7-50)
its best to replace the diaghphram in the accelerator pump
the other "pull off device" on the upper part of the carb also has a diagphram but the whole thing can be completely removed and the 2 holes blocked up
theres an explanation on here somewhere
i cant spell diaghphram)

ayjay
Do you mean diaphragm?

I did pull the plugs today just to check and the porcelain is still bone white.

It was only pinking on the second venturi hence the reason I homed in on the secondary jet.

I think I will weld in a lambda bung and see what's going on, it made tuning the bikes soo much easier.

Dave,
with unleaded fuel, all that guidance from plug manufacturers with photographs
of insulators is no longer applicable.
It belongs to an era when it was possible to get some idea of an engine's condition
and/or state of tune by looking at the exhaust tailpipe for deposits with a light grey
colour.
But since there's no lead in fuel any more...

Also, pinking is directly linked to throttle opening for the simple reason that a partially
open throttle reduces the 'effective' compression ratio.

If you're having to retard the ignition to less than 32* total advance with 95 octane
unleaded and 8.5:1 CR, there's something else wrong.
Or else I've been wasting my time for the last 30 years working on these engines...

ken

Ken I respect your knowledge completely.

Carb rebuild time is approaching rapidly as we have some miles to cover over Christmas, adding it all up comes to around the 700 mark.

On the plus side I did get the headlamps to point earthwards today although it has been interesting looking at the treetops driving through the lanes.