30-Pict lean stumble with fast throttle

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30-Pict lean stumble with fast throttle

Post by Kenyon » Sun May 11, 2014 12:14 pm

Hi all,

I've got a 30-Pict carb, and when I move the throttle pedal rapidly, the engine runs lean and stumbles or hesitates, sometimes even quits if I don't work the throttle again or let off. If I move the throttle slowly, there are no problems; if I work the throttle a bit the accelerator pump adds enough fuel to let the motor spin up quickly.

Ive adjusted the accelerator pump by moving the cotter pin as far back as possible. This improved the stumble, but I can't adjust it any further. Are larger jets available for the accelerator pump?

I can also turn the idle adjustment screw and, if I make the engine idle very rich (~11:1), the stumble also improves. If the engine idles near stoichiometric, the stable is worse.

One thing I noticed, the throttle shaft bushings are definitely not new, as I can spray brakeclean around them and the rpms drop. Not sure how typical this is...

What can I change to add more fuel when applying throttle, or otherwise reduce the lean condition?

Also, could a larger idle jet improve anything, since I can adjust idle mixture within range with the adjustment? Whats the point of changing the idle jet when I can adjust that screw? A stoichionetric idle is just a turn or less from closed for me.

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Re: 30-Pict lean stumble with fast throttle

Post by Clonebug » Sat Jun 28, 2014 3:04 pm

You want to set up for best lean idle and not Stoich.

My engine had the best idle at 12.8:1 AFR and any leaner would cause issues.

If I were you I would adjust the idle mixture to get the fastest idle and then try it.

Even my MS2 FI idles best at 12.8:1 AFR.

You might also want to check what your timing is at at idle.
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Re: 30-Pict lean stumble with fast throttle

Post by andy198712 » Mon Jun 30, 2014 6:44 am

What's your timing doing? What dizzy?

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Re: 30-Pict lean stumble with fast throttle

Post by Marc » Mon Jun 30, 2014 7:51 am

What's the stream from the accelerator pump injection pipe look like? There's more to it than the adjustment (which, by the way, rarely needs to be adjusted far from the factory setting). If the stream's feeble, the nozzle may be partly obstructed and merely need cleaning, the diaphragm or spring may be bad, or the inlet check valve in the bottom of the float bowl could be dirty or sticking. There's a discharge check valve too (a steel ball in the passage to the nozzle) although it seldom causes problems - just be on the lookout for the tiny ball if you invert the carb or stroke the pump with the nozzle removed.
As a rule, the injection pipe should be aimed so that the stream just misses the wall of the venturi - putting it as far outboard and as close to the edge of the opening throttle plate as possible.
You can measure the discharge quantity of the accelerator pump (simplest way IMO is to just remove the carb and hold it over a bowl). Spec is 1.3 to 1.6 cc per full stroke of the throttle arm...but if the stream is robust and starts after only a few degrees of throttle arm movement odds are that it'll be in spec.

As for the worn throttle shaft bore, it can be reamed out and fitted with bushings that'll make it better than new if the carb's worth it (IMO a 30PICT-2 never is). But so long as you can still enrichen the idle mixture enough to compensate for the unmetered air leaking in at idle, you've probably got better things to do with your money.
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As you've surmised, since you can adjust the idle mixture that rich with the existing pilot ("idle") jet there would be no point in going to a larger one. There've been variations on the exact plumbing of the idle circuit but generally the pilot jet is sized to properly supply the transition ports which admit fuel as the throttle plate moves off of the idle position. For a Beetle, one would normally run a g55 in any 30PICT.

A lack of adequate intake manifold heat is a primary cause of hesitation on acceleration...if the preheat pipe is coked up you'll have a huge flat spot until the engine's fully warmed up. 99% of aftermarket exhaust systems don't provide as much preheat flow as the stock muffler. A functional thermostatic aircleaner that directs heated air to the carb in cool weather can also improve driveability, Singleports aren't as sensitive as dualports since they have smaller intake tracts than the latter (keeping flow velocity and vacuum up, further from the "dew-point" of fuel condensation on the manifold walls).

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