The problem below 2000RPM is due to the idle & progression circuits not being right. You may be able to improve it with larger idle & pilot jets, although the largest displacement I've ever achieved satisfactory results with using a 34PICT-3 carb was 1775cc. With the throttle shut the carb won't pass enough air to sustain an idle so you have to crack it open, and that completely disrupts the transition off-idle. At higher speeds you're operating on the main circuit. The carb is designed to run leaner at mid-range cruise (for economy) than at high speed (to keep from frying pistons). A larger displacement engine operates further to the right on the A/F map - the carb only knows how much air is going through it, not how large the engine or the RPM - so the mixture will actually be RICHER than on a stock-size motor. The carb's still too small for the engine to make anywhere near its full potential, but there's no driveability problem.
--- Bob_Kai Wongthanavimok <[EMAIL PROTECTED]> wrote: > Thank you for your help. > > Just a thought though. If the carb cannot keep up > with larger cylinder > volume, shouldn't it have problem during high rev as > well? It only stumbles > during low rev < 2k rpm. > > ----- Original Message ----- > From: "dan moy" <[EMAIL PROTECTED]> > To: <[email protected]> > Sent: Thursday, March 20, 2008 7:52 PM > Subject: Re: [vintagvw] Stumble engine > > > > Sounds like the addition cylinder volume is the > problem. The carb cannot > > keep up with the demands, have you thought about > dual kadrons? You will > > more then likely have to change your exahust if > you > move to a larger carb > > setup. > > --- On Thu 03/20, Bob_Kai Wongthanavimok < > [EMAIL PROTECTED] > > wrote:From: Bob_Kai Wongthanavimok [mailto: > [EMAIL PROTECTED]: > [EMAIL PROTECTED]: Thu, 20 Mar 2008 > 00:59:33 > -0700 (PDT)Subject: > [vintagvw] Stumble engineBy way of introduction, I'm > a > new old listee (been > with the list few times many years back) from > Bangkok, > Thailand. I have a > 71 RHD Super with recently mod as follow:Existing > stuffs:o 1,600 standard > single port AS case engine with 30-Pict3 carb - now > replaced with belowo 009 > dizzy with Pertronix magnetico Bosch blue coil but > recently changed to > Hitachi Red (my mechanic said it is compatible)o > Aircono 831 trannyo Old > standard muffler Newly put in (~3 months):o New > single > 34 Pict 3 Solex (but > I think made by Bocar) on > > dual port (was single port before)o New 76mm CB > performance cranko New > > 88mm Mahle piston set The two should put me to > a > 1,850cc engineo New > > 044 CB Performance heads (I think 40X37mm valves). > On 34Pict3, I have made > > some mod according to Nate from AirCooledTech.com > by > plugging the throttle > > by-pass hole to boost the vacuum so that 34pict > can > work with 009 see > > link: > http://www.aircooledtech.com/34pict3_modification/ > Here is my > > problem, whilst running nicely on high rev, the > engine stumbles on low rev > > (below ~1,800 - 2,000 rpm). Situations like this > normally occur:a) > > cruising along the road, turn in to cross street, > shift down to 2nd, push > > gas pedal while releasing the clutch, stumble > almost > the engine is going > > to stop.b) cruising along, slow down, shift down > to > 2nd, same to a)c) > > cruising, stop on standstill traffic on neutral, > the > engine stumbly idle. > > However, after a while, the engine came back to > (almost) normal with > > slight hiccup every minute or so. The symptom is > a! > > s > > if the gas does not come through sufficiently. On > a) & b), I have to > > push gas pedal a bit (to keep rpm up) together the > clutch to hold so not > > to go too fast. Any idea? My mechanic suspects on > 2 > things, (1) bad carb > > (he is kind of not trusting Mexico or Brazil made > carbs), or (2) stand > > muffler cannot cope with such a large engine. > thought that exhaust cannot > > come out fast enough. Oh, BTW, > "sometimes", there were some > > backfire like single firecracker too. Hope it is > not > too long. Appreciate > > everyone comments. Bob/Bkk/71 RHD Yellow Super > > ---------------------------------Never miss a > thing. Make Yahoo > your > homepage._______________________________________________vintagvw > site > [EMAIL PROTECTED]://lists.sjsu.edu/mailman/listinfo/vintagvw > > > > _______________________________________________ > > Join Excite! - http://www.excite.com > > The most personalized portal on the Web! > > > > > -------------------------------------------------------------------------------- > > > > _______________________________________________ > > vintagvw site list > > [email protected] > > http://lists.sjsu.edu/mailman/listinfo/vintagvw > > > > > ____________________________________________________________________________________ > Be a better friend, newshound, and > know-it-all with Yahoo! Mobile. Try it now. > http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ > > _______________________________________________ > vintagvw site list > [email protected] > http://lists.sjsu.edu/mailman/listinfo/vintagvw > ____________________________________________________________________________________ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ _______________________________________________ vintagvw site list [email protected] http://lists.sjsu.edu/mailman/listinfo/vintagvw
