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 >You can't gain much in the way of fuel economy by >leaning the jetting - lean-misfire will set in and >your mileage may actually worsen. >The factory combinations are nearly ideal, just be >sure you have the correct air correction jet (and >corresponding main jet) for your application. >With a 70Z or 75Z A/C you should have a 127.5 main >With an 80Z A/C you should have a 130 main >With a 60Z A/C you should have a 125 main. >Generally you would run a smaller A/C in a heavy >vehicle (like a bus) and a smaller one in a light car. >A smaller A/C tends to richen the mixture out at the >top end, making it less risky to use a smaller main >jet. Depending upon what kind of driving you do, you >MAY be able to help MPG by juggling the combination, >but don't depart too radically from the >above-mentioned factory settings. Same goes for the >idle jet - in most cases a g55 is best, but stepping >up to a g60 could actually improve your mileage by >allowing you to reduce the main jet size slightly (if >your driving all consists of putting about town). >You can adjust the volume of the accelerator pump shot >on a 34PICT. Reducing it will improve your MPG at the >expense of driveability - it depends upon how much of >a hesitation on acceleration you can tolerate. Too big >of a "flat spot" could be unsafe some day when you >need decent response to avoid a hazard, so don't go >overboard on this adjustment. >Dual-ports are especially susceptible to hesitation on >acceleration because their intake manifold >cross-section is too large to provide good flow >velocity at low speeds - this puts conditions inside >the manifold closer to "dew point" so that when you >open the throttle and vacuum decreases fuel tends to >condense on the manifold walls. This causes a >momentary lean condition...the fuel does get to the >cylinders eventually, but it's in the form of droplets >which are too large to burn completely. Keeping the >manifold warm reduces this tendency, so the proper >function of the preheat system is important. Compared >to the stock muffler, no aftermarket exhaust system >provides near the preheat flow. A properly-functioning >thermostatically-controlled aircleaner and warmair >pickup system contributes to driveability; with a >single carb and a header I'd consider it mandatory. >Lastly, you will (theoretically) get better mileage >with a stock distributor that has both a centrifugal >and a vacuum-operated timing control system than with >a centrifugal-only distributor like the 0 231 178 009. >The stock distributor can respond to load as well as >RPM, allowing more total spark advance at high >speed/light load (highway cruising) which will improve >mileage. In addition, the stock distributor provides a >little extra advance off-idle compared to the >centrifugal type, which helps to fire a lean mixture - >driveability will be better without having to resort >to dumping excess fuel (the usual "answer" to the >flat-spot problem). > >--- Damon & Gay <[EMAIL PROTECTED]> wrote: > >> Does anyone have any recomendations for carberator >> jet sizes? For example, In the main or any others >> that might help fuel economy > > > > >____________________________________________________________________________________ >Looking for last minute shopping deals? >Find them fast with Yahoo! Search. >http://tools.search.yahoo.com/newsearch/category.php?category=shopping > >_______________________________________________ >vintagvw site list >[email protected] >http://lists.sjsu.edu/mailman/listinfo/vintagvw _______________________________________________ vintagvw site list [email protected] http://lists.sjsu.edu/mailman/listinfo/vintagvw
