Modern fuel vs carburettor main jet size increase?

[chriswgawne]

There has been occasional but consistent mention over the last couple of years by Forum members of the benefit of increasing carb main jet sizes because of the apparent different performance of modern fuels as compared to 50's and 60's fuel.
I have never felt that this would benefit any of our cars ( Aurelias (all but one with 1 or 2 Webers)  through to Fulvias (Solex and Dellorto)  typically) as they seem to run perfectly well with standard settings using high octane 98 fuel.
However, because of a very very slight running irregularity ( nothing like as bad as a misfire)  on our B12 with Solex 40PAAI with standard 135 main jets (which Jacky cant feel either as a driver or passenger by the way), I feel inclined to try 140 jets. If I put a Weber 40 with 140 main jets on the car, the running irregularity disappears.
My question is whether increasing jet sizes is logical because of fuel differences?
Chris
 

[lancialulu]

Can you be more specific about the regime of "poorer" running? Is it all the time off idle through to flat out?

If it is on a light throttle it may be that the transition from the idle circuit to main (progression) needs help??

A number of us running solexes on Fulvias have increase the idle jet by 10 or so points which makes a difference.

I have to say though my B12 (Solex carb) enjoys Shell Vpower but is a bit sniffy about most others.....

[Mikenoangelo]

I can see the logic where ethanol is in the fuel as this is supplying some of the oxygen needed but diminishing the amount per litre of the stuff that actually burns. So more of the carbon and hydrogen components are needed. It is a bit annoying to be forced to purchase oxygen in this way when it is freely available in the atmosphere. 

Mike

[GG]

Chris -
Have you checked the hole sizes in the jets? Our Appia ran nicely but languidly. Finally motivated to check the jet, found that after 50 years, it had gummed up and shrunk the actual opening. When drilled back to its original size, fine running emerged (and another 5 hp or so!). Might this be the problem?

We haven't found a need to change jet sizes, but the fuels likely vary, and perhaps your cars are worked more - around here are largely flat roads. Nothing like a long mountain pass road to test an engine.

Geoff 

[chriswgawne]

Of course this might all be in my imagination (but I don't think it is).
There are some useful comments here from Forum members thank you.
Firstly fuel. Here in NE Italy, Shell V Power is not readily available away from Autostradas - or not in our area anyway - so I have 2 local choices.
Tamoil 100 octane is occasionally available close by and ENI 100 Octane is always available in our local town, Este. The ENI definitely gives smoother running in our Fulvias than Tamoil but I have never noticed any difference on the B12.
Our other Aurelia which are on Webers are fine and to repeat myself, putting a Weber 40 set up for 2.5 litre Aurelia on our 2.3 litre B12 works fine.
The 'running imperfection' is only apparent (to me) running around at a constant 80 kph or so. Slower or faster and while accelerating all is OK.
Trying to apply the choke to simulate more fuel is too coarse a way of improving things.
So I will continue to try to get a pair of 140 and 145 main jets and try those first - I don't really want to drill out the existing jets as they are all that I have..

Just to add to the puzzle, most period Aurelia paperwork and drivers handbooks give 135 as the main jet size for Solex 35 and 40 PAAI but there is a leaflet which says 140!!
Chris

[lancialulu]

This guy has huge amount of Solex jets etc and also seems to be able to make a part.

https://www.ebay.com/itm/SOLEX-CARBURETORS-MAIN-JETS/230353229125?epid=2128502367&hash=item35a21f5545:g:H-oAAOSwYGFUx8EO

[chriswgawne]

Thanks Tim, Looks promising. I was surprised that Classic Carbs in the UK didn't appear to have any correct ones.
I will report back. I do have some 1.4mm drills so if needs be I could modify my 135s but this is not my preferred option just yet.
Chris

[simonandjuliet]

I've bought quite a lot of carb bits from him, he's correct and quick to post stuff out

[lancialulu]

Intrigued, I got my owners handbook out and noticed that after 40PAAI carb #676121 the main jet is increased from 135 to 14 as Chris says.

So I went to look at my B12 40PAAI carb which is #704267 so should have a 140 main jet.

It hasn't! Seems it is fitted with a pair of 135's.

The interesting thing though is that these jets are the same form factor as Fulvia and I just happen to have 145's to try if Chris's experiment shows some improvement.