Acceleration stops at 50mph - max speed=50mph

[Matrix]

Hi Firewatcher,

I did not replace the valves. For some strange reason, my English metric keys do not fit with the size of the needle valve (from UK, Zenith).
Even the floaters were of different design and required manipulation. I checked the existing one if it had any hole, or any petrol inside, it was ok so i left the old one inside.
I cleaned thoroughly the rear carb that had leaking issues and removed black residue from petrol.
I used throttle cleaner to all parts of the carb (except diaphragm): metal housings, piston, needles and valves and made it shine internally.
After noticing the leak of the front carb i regretted that i did not open the housing of the floater unit and clean it. I cleaned only the upper part of the carb.
Until that moment the front carb had no leak. I hope that it was sporadic, or else i will need to remove again the carbs - though this time will be much easier i know that to dismantle first.
I am stoically waiting for the next Sunday to see "what type of fish i got with my fishing pole" as we say here.

[Firewatcher]

Best way to learn, your own knowledge is essential if you intend to keep it longer term. With respect, many Mechanics these days are more used to modern vehicles, which are very different in terms of fuel and ignition systems. These beauties are a dream to work on by comparison and were built to last. It doesn't have to cost you a fortune, and as you have perhaps found, buying a full service kit is expensive and often unnecessary.
Pete

[johnnydog]

The float valves need a 5/16 BS 1/4 thin walled socket to undo them.

[Matrix]

update: I did a good test today I covered a distance of ~19 miles.

The issue is eliminated, I got past 50mph and reached 70mph without any leak. Seems that the cleaning of the carb and especially the needle valve as you all suggested - thank you for your directions - and the correction of the piston movement did the trick.
BUT i do not think that carbs work perfectly because:
1. I notice very slight smoke, it is quite unnoticeable, but if you look carefully and closely, it can be seen. This is the smoke of the slightly rich mixture (dark color).
The car does not emits any kind of smoke from my experience all these years.
2. reaching 75 mph seemed to be done with a slight engine noise and was done not as fast as i remember. This did not occur in the past.
In the past some engine noise could only be noticed while accelerating past 75mph and considerably more engine noise when accelerating past 85mph.

I remember that when the mechanic adjusted the carbs was BEFORE adding oil in the carbs. So the following theory formed in my mind - i could be wrong:
without the oil the piston moves freely, so it is difficult to have rich mixture while accelerating (which moves piston upwards)
So if one adjusts the carbs with this setup, then when i added oil, which added some hysteresis to the piston movement, this has as an effect to have a richer mixture
than what it should be. Maybe i should readjust the carb to have a weaker mixture to comply with the slower piston movement.
Maybe i am overthinking it, and this theory is not valid because when this happens, happens for a fraction of time.

The problem is that after the addition of the oil last week, i did tests with piston movement with the screwdriver and did not notice increase or decrease of rpm that
indicate the wrong setting of mixture. For sure i have not synchronized carbs - i do not have a flow meter and i did not try the technique using the ear.
Also i have not changed air filter - i have ordered a new one though.
I suppose i just could experimentally turn the screw of mixture setting one half turn counterclockwise (facing upwards) to test if things are improved regarding exhaust emissions.
Is it a good idea to try this ?

[Firewatcher]

Hi Matrix,
Good to hear the positive result. The check for correct mixture is as I described in my earlier post, so if your engine speed remained constant you are there. If you do decide to alter the mixture, you should again check that the air valve (piston) falls freely to make sure the metering needle is still centralised and not touching the jet.
The dynamics will follow assuming the top diaphragms are in good shape and you have no other faults.
The balance isn't usually an issue, but easy to check by listening, I have never used anything else.
Pete

[johnnydog]

It sounds like your mixture settings need some tweaking!
As a basic setting, turn each mixture screw up until the jet contacts the piston (noticeable by slight upwards movement of the piston) then turn them both back down by two complete turns. The slot on the bottom of the mixture screw aids getting the turns equal using a stubby flat bladed screwdriver. You may have to alter the idle speed on each carb equally at this point to get it to tick over ok. The mixture needs to adjusted to give the smoothest idle speed. Listen to the exhaust - if it is irregular then the mixture is weak; a regular hunting suggests it is too rich. A nice smooth idle and exhaust note is more or less right. A road test then should confirm this. Don't forget, the longer the car is ticking over, the hotter the engine can get which can affect the smooth running, so every now and increase the revs for a short while to 'clear' it. When it is more or less right, depress the throttle from inside the car, and the revs should pick up smoothly without stuttering, and then take you foot off, and the revs should fall and settle to tickover speed without faltering. If the idle falls too low, but then increases to your accepted idle, it needs further adjustment.
Without a carb balancer or air flow meter, as long as the throttles are opening simultaneously and equally, they won't be far out.
As a alternative to winding the mixture screw up to get the basic setting, if you take the piston out, and adjust each mixture screw so that the top of the jet is approximately 2mm below the top of the bridge. This will do as a basic starting point.
It's also worth checking that the needle is inserted into the piston correctly - the 'head' of the needle should be positioned flush with the bottom of the piston.
A bit of time and patience to get it right is worth all the effort!!
Sorry if I'm teaching you 'how to suck eggs' so to speak (!) but this is how I do it on mine with good results.

[Matrix]

hi johnnydog,

This is extremely useful info you provided me, and i am saving to use it as a reference. Thank you.
With all this information i think i have a good basis to try it myself!
As soon as i receive my new air filter, as i remove the old one i will try to see what happens.
Also i know what you mean: indeed my exhaust sound is on the heavy note, used to be more silent!

[Matrix]

Hi Matrix,
Good to hear the positive result. The check for correct mixture is as I described in my earlier post, so if your engine speed remained constant you are there. If you do decide to alter the mixture, you should again check that the air valve (piston) falls freely to make sure the metering needle is still centralised and not touching the jet.
The dynamics will follow assuming the top diaphragms are in good shape and you have no other faults.
The balance isn't usually an issue, but easy to check by listening, I have never used anything else.
Pete

thanks for the info Firewatcher , i will check again the piston movement while doing the adjustment.