Pierburg 2E3 Issue: Stuck on High Revs

[CJH]

secondary

Could be - but that could equally be a shadow or just the permissible gap IMO. Hard to tell I think.

[CJH]

maybe I-m talking rubbish, vac for 2nd barrel comes from above venturis so would be low at idle.

That's not right, surely. Above the venturis won't there be atmospheric pressure (or nearly) thanks to the feed from the air filter? Surely the 'suck' comes from the inlet manifold below the venturi valves.

[itchyfeet]

when the primary is open vacuum above and below are similar.

[itchyfeet]

that amount of fuel pouring into the secondary can't be right so hows this for an idea

two faults
excess fuel leaking from float chamber because perhaps needle valve not right or gasket leaking

when you restart pump runs slow fuel does not overflow

also may be air leak or seconday is adjusted too far open

[CJH]

I'm going to have to study my carb this evening to see where those vacuum holes go.

There will always be a pressure drop through the carb though because the suck comes from the manifold below, and the top of the carb is nearer atmospheric pressure (apart from the restrictions through the air filter). Even with the pancake removed the engine should still run normally, and in that case the top is definitely at atmospheric pressure. So even if some of the primary vacuum gets round into the top of the second barrel it can't be as much as the vacuum from below the secondary valve. And the design of the secondary diffuser jetting should surely be such that this vacuum leaking round from the primary barrel isn't enough to draw fuel through. So I stand by my theory that something (like a blockage) is affecting the balance of that secondary diffuser circuit.

Either that or a faulty float valve of course....

*..and after seeing Itchy's post above, it could be a combination of both faults, I agree. It would be good if the OP could comment on how far open the secondary flap is when it's in this 'high revving' state.

[itchyfeet]

you need fuel and air for high revs so either the secondary is open or there is an airleak elsewhere

[CJH]

True, but there are holes in both valve flaps. And I wonder what is the purpose of the setup that allows the secondary valve to crack open under vacuum (that 0.4mm gap in the U-bracket that Kev referred to). It's not a lot of air I agree, but maybe it's the reason the engine isn't running away completely - maybe the revs are being partially limited by the available air.

[itchyfeet]

If you were to clamp the fuel hose to the pump cutting off supply with a pipe clamp you would see if or when revs dropped

[CJH]

If you were to clamp the fuel hose to the pump cutting off supply with a pipe clamp you would see if or when revs dropped

Yes - if the float valve isn't working then blocking the pump should stop the over-revving quite quickly, while the reserve in the float chamber should keep the engine idling for some time. I was surprised how long my engine kept idling when I killed the power to my electric pump - I don't recall exactly, maybe 30s or so.

[itchyfeet]

I had a carb once heavily overfueling on idle because the main gasket was shot it didn't rev though because air waslimited

still doesn't explain why so much air is getting in air leak or 2nd choke open imo

[itchyfeet]

im going to have to study my carb this evening to see where those vacuum holes go.]

you can see from my pics above^^^

[CJH]

It shouldn't take much to investigate which, if either, theory is correct. If I had my van at home I'd be tempted to put my finger over the top of the air corrector jet for the secondary diffuser to see if I could replicate the problem. But I don't - my van's having an over-nighter at the local garage trying to get through it's MOT

[itchyfeet]

you b****** you knew I would not be able to resist

if you stick you fingers over correction jets nothing happens, block vent with wire nothing happens

block vent and secondary correction nothing happens

block idle jet slight increase

take vac pipe of second choke actuator and suck while throttling, it opens and stays open while you suck and revs stay high

so if the mech is stuck or the vac has a one way blockage it stays open

still too much fuel, not seeing that, you should only see liquid fuel from the acc pump.

[CJH]

you b****** you knew I would not be able to resist

if you stick you fingers over correction jets nothing happens, block vent with wire nothing happens

Haha - good man.

Just to be clear, when you blocked the corrector jet, did you then increase the rpm by operating the throttle?

[itchyfeet]

No
pointless test then