Revs drop pressing accelerator at idle. Herald 1200 Solex B30PSE1

[Morgana]

There is a tendency for the revs to die if the accelerator is pressed when idling. My guess is over-fuelling from the accelerator pump, but there are no adjustments on the Solex B30 PSE1. The car idles well, and the engine revs once the initial hiccough has been overcome, but it does make slow speed manoeuvring tricky (especially on a hill) as it's very easy to stall by pressing the pedal too quickly.

The needle valve is clear. I have attempted to address problems of fuel leaking into the manifold by adding a second shim washer under the valve, so the float hits it earlier which ought to result in an average lower level of fuel in the float chamber. With the air filter off, pressing the accelerator results in a good spritz of fuel from the accelerator pump. Otherwise, the carburettor is as normal.

That comes with a caveat: there has never been a ball bearing in the accelerator pump since owning the car, despite the Solex manual showing one (part 45 in the exploded diagram in the attached PDF). Despite having had the carburettor apart a number of times, I don't understand where it could go in order to be a useful valve blocker.

Any ideas?

b30pse1.pdf

[Pete Lewis]

first thing i would check is the slow running jet clear ??   the one on the side of the body 

Pete

[Tipidave]

Interestingly the depth of fuel in the bowl is critical to the correct fuel supply to the main jet. The ‘head ‘ of fuel determines how much low pressure created by the venturi is need to draw fuel into the main jet. A matter of a mm is sufficient to notice an effect. 

I would suggest that since you have changed the height of fuel in the bowl it is now either flooding or lean during the transition from idle circuit to main jet.
Once established at higher revs it becomes less of an issue as the low pressure draw on the fuel increases.  it may be that you need to make a further adjustment  to the height of fuel in the bowl and or … assuming it is momentarily too rich in transition from idle to main circuit lean the idle a bit. 

 

[Pete Lewis]

fuel level does need to be to spec. on change over   thats what the compensating jet should smooth out 

the small holes in the top face and the small holes in the throat near the throttle plate are very important to be clear .

Pete

 

[Morgana]

Over several hundred miles in the last few days it's been OK - I cleaned the slow running jet and redid the idling and mixture, which may have sorted it. Certainly it was idling more smoothly than it ever has before.

However, after the first trip up to Bristol it wouldn't restart. It turned out there was no fuel in the float chamber, and the needle valve was stuck. No gunk or bits of rubber in the bowl, and clearing the valve by blowing and fiddling with it meant it started on reassembly. This happened half a dozen more times over the next few days until I'd got the repair down to under five minutes, whipping off the carburettor top and clearing the valve. It was fine when running, but would not restart after a stop. I can't work out why sitting with the engine off would cause the valve to stick closed.

Is it just worn out? Do they get worn out? It's marked "1.3", but I've seen 1.5 mentioned elsewhere. I can't find a reference to the normal size in any of the literature I have.

[Pete Lewis]

there are changes to needle valves on early and late B30pse1 carbs probably where the changes in size occurs 

its not overly important 

canley show them all throughout the 1200 range of carb  changes 

Solex Carburettor (B30Z1C5) : Canley Classics

cant get the Burlen site to work since its all changed (might be me )

they do wear and can jam up with little to see why 

Pete

[Morgana]

The Burlen site has changed, indeed. The Canley people have different needle valves for early and late as you say - the Herald is a '66, and the carburettor corresponds with the 'late' version they show there, so the needle valve is 509956 rather than 510744 they show in the 'early' drawing. I've got one on order. The exhaust manifold downpipe gasket has gone again, as well... That's not helping.

[Pete Lewis]

,frequent gasket failures  ....do make sure you have the fixed strap down pipe to gearbox  as this steadies  the pipe 

Pete

[Morgana]

On 01/08/2023 at 08:33, Tipidave said:

Interestingly the depth of fuel in the bowl is critical to the correct fuel supply to the main jet. The ‘head ‘ of fuel determines how much low pressure created by the venturi is need to draw fuel into the main jet. A matter of a mm is sufficient to notice an effect. 

I would suggest that since you have changed the height of fuel in the bowl it is now either flooding or lean during the transition from idle circuit to main jet.
Once established at higher revs it becomes less of an issue as the low pressure draw on the fuel increases.  it may be that you need to make a further adjustment  to the height of fuel in the bowl and or … assuming it is momentarily too rich in transition from idle to main circuit lean the idle a bit. 

 

I can't find a reference to what this height ought to be. The gasket kits have a variety of metal washers in to go under the needle valve, but the manual diagrams only list a 'fibre washer' as going beneath it. I have read on here about the different specific gravity of modern fuels, requiring more spacers since the float sits lower for a given fuel level in the bowl. The spacer height is a total of 3mm at the moment.