Jack, my criticism of the intake manifold applies to
the stock 1-bbl item, not the aftermarket dual-carb
pieces - that's a whole different ballgame. VW
recognized the problem and went to an intake manifold
and muffler with DUAL preheat pipes in `74 (the last
year carb'ed bug engines were offered in the US). In
Europe, later carb'ed engines came with intake
manifolds that had larger single heatriser pipes
(nearly 3/4" in diameter). That's a better solution
since it doesn't require an odd-ball muffler, but such
manifolds are extremely rare over here.
With dual carbs there is no manifold heat other than
that achieved by transfer from the heads, and because
of the short distance between carb and intake valve
the fuel condensation problem isn't as big an issue,
at least once the engine's been running for a few
minutes. On cold startup you can still expect a little
"sneezing", especially if the carbs lack chokes or
cold-start enrichment circuits. If it's severe enough
to be objectionable you may have a basic
mechanical-synchronization problem and/or a too-lean
idle mixture on the offending carb. Note that with
dual 1-bbls it is NORMAL for #1 and #3 to idle
slightly richer than #2 and #4, so you must find the
ideal compromise setting to keep all four firing -
typically, #2 & #4 don't contribute much at idle until
the motor warms up a bit. The reason is that due to
the firing order and the fact that fuel has more
inertia than air, the first cylinder on each side to
draw tends to get a slightly lean mixture - once air
flow is established through the carb, there's more
fuel available to the second cylinder. As soon as the
RPM comes up above idle the pauses in intake airflow
become short enough that the mixture difference
diminishes and all four start to fire evenly, but if
only one cylinder per side is firing consistently at
low idle speeds nothing is necessarily wrong, it's
just the nature of the beast with dual 1-bbls.
With dual 2-bbls each cylinder effectively has its own
carburetor and is unaffected by what its neighbor is
doing.
--- Jack Dinan <[EMAIL PROTECTED]> wrote:
> Mark: Well now look what you've gone and done! Last
> winter, I was so
> sick of having my 1600DP stall at the first few
> traffic lights,
> presumably because of icing in the manifold, that I
> pulled the
> 34PICT-3, the stock air cleaner, and the stock
> manifold, and
> replaced these with a set of dual 35 PDSITs that I
> bought from a
> Samba seller.
>
> I was sure that the icing was due to a clogged heat
> riser tube
> because I there is probably five years of carbon
> accumulation in it.
> Also, I wanted the extra power that the dual carbs
> would hopefully
> bring.
>
> And now I read your statement that the dual carb
> manifolds have too
> large a cross section to allow smooth transition
> from idle!
>
> I started up the modified engine for the first time
> last week. Sure
> enough, there is a bit of hesitation off idle and a
> bit of backfiring
> in one of the carbs at low throttle plate opening.
>
> Your explanation certainly makes sense. But now I'm
> committed to the
> dual carbs.
>
> My question is this. The Porsche 356 engines were
> similar to the VW
> 1600 and they were dual carbed. How did Porsche
> avoid the trap that
> you describe?
>
> Thanks, jack
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