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Author Topic: Weber 34 DATA 4/100 carburettor jetting specs for Australian delivered Beta 2000  (Read 999 times)
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Australia Australia

Posts: 44

1979 Coupe 2000

« on: October 30, 2018, 02:26:19 PM »

My father`s 1979 Beta Coupe 2000 has a Weber 34 DATA 4/100 carburettor fitted which was apparently the standard spec twin choke carburettor for Australian delivered Betas with 2000 engines from 1978/79 onwards. I was having trouble finding the standard jetting specs for this Weber model to compare with what is currently in the carburettor. I eventually tracked down the original Weber spec sheet here (at a cost!). I can provide a pdf of this spec sheet if anyone is interested and would like a copy.

The ex factory Weber 34 DATA 4/100 jetting specs are: primary idle 50, secondary idle 80, primary main 115, secondary main 135.  These specs are the same as those fitted to our 79 Beta Coupe with the exception of the installed secondary main jet which is labelled 110. That seems to be a major discrepancy. I am not sure whether the 110 jet has been modified or drilled out to increase the diameter. My father did have some problems with surging years ago and had someone (an Alfa specialist) in Canberra (Australia) look at the carburettor and make some mods although he can`t recall exactly what was done. It is running on the lean side. We were thinking of trying a larger secondary main jet.

I would be interested to know what other Australian Beta 2000 owners with the Weber 34 DATA 4/100 carburettor are running jetting wise and whether they are happy with their carburettor tuning.

We have had some ongoing issues with hesitation on acceleration, some surging, and lack of power. Some of this may be ignition related. I did just recently check and clean the electrical contacts on the distributor cap which had built up some deposits. That did seem to improve the acceleration smoothness and power delivery to some degree, presumably due to better spark voltage getting through to plugs. Plugs, coil and leads have all been replaced at various stages so are unlikely to be adding to the problem.

We took the car to a local Weber carburettor tuning specialist earlier this year who did all of the usual things: cleaned plugs, reset gap, checked leads and coil. He checked fuel line pressure readings and reported a stable 2.5 psi at idle and under load with the mechanical fuel pump only and 4.0 psi with both mechanical and electric fuel pump running. (Electric pump was fitted and is used for cold start priming only). He cleaned carburettor jets and fitted a new needle and seat (preventative maintenance - the existing one was working ok). Carburettor cleaning was probably not as thorough as others have reported as being necessary so there might still be microscopic crud issues or varnish like deposits affecting jetting. Ignition timing was measured at 14 degrees initial increasing to 36 degrees at 4200rpm (22 degrees mechanical advance). Dyno power output was ok (funny how these intermittent problems never materialise when under diagnostic testing) apart from a slight hesitation between primary and secondary throats.

I am not sure how stable and reliable the nearly 40 year old Bosch distributor ignition pick up and ignition control module is operating. Perhaps the terminal block contacts should be checked and cleaned or sprayed with contact cleaner. I read that suggestion on another thread dealing with suspected ignition problems. I am a bit nervous of doing this in case something breaks (a pin or connection point) rendering the whole system defunct. I gather that the Bosch distributor ignition pick up is virtually impossible to source, likewise the original Bosch ignition module. So we are going to have to explore other distributor trigger mechanism options if and when one or both components fail completely. They may be contributing to the hesitation and intermittent power delivery problems.

My brother`s Fiat 124 spider engine died very recently..he could not start it. He had current at the coil so not an ignition switch problem. He replaced the ignition effect. He replaced the ignition leads and effect. He then tracked down a new Marelli distributor ignition pick up from the US  and found that the old magnetic pick up virtually disintegrated when it was removed. The car started at last with the new pick-up but the ignition timing was way out with the car backfiring and developing no power and had to be reset. It is now going better than had probably been suffering from a progressively deteriorating distributor pick-up and mistimed engine.  Lucky for him you can still get Marelli distributor magnetic pick-ups!

I guess a healthy dose of carburettor cleaner in the fuel tank would also be a good idea to help dissolve any pesky build up of deposits in the carburettor. We do use premium 98 octane Shell V-power fuel in the hope that the claimed cleaning additives in it will keep the fuel system free of gunk but I guess an extra burst of additive now and then might be of benefit.
« Last Edit: November 09, 2018, 03:33:40 PM by Gromit » Logged

Family Italian car fleet: 1979 Beta Coupe 2000, Fiat 124 Spyder (and a 2007 Fiat Punto!)
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