Okay, this is what is I have: 2.0l type 4 engine (ex fuel injection) twin Weber 40IDFs Pertronix Ignitor dizzy w/vac advance (bought new for FI setup) Trouble setting up the engine, carbs have been rebuilt and set correctly. When timing up, engine at 900rpm, vac off, set to 7.5 degrees. BTDC. Engine seems a bit flat, but I expect to 'fettle' that anyway. The real issue is when I reattach the vacuum - the timing changes to 28 degrees BTDC! I might expect it to change by a couple of degrees, but not that - any thoughts? Possibly the dizzy faulty, or wrong spec (vac advance) It's driving me mad.
As above, what is your vacuum pipe connected to? TBH, I'm surprised a vac can can pull that much advance.
Its the wrong vacuum signal .. you need the ported one from the carburettor body which comes from above the throttle plate, usually combined by drillings so that it is zero at idle, rises at part throttle and drops to zero at wide open throttle. If your carburettors dont feature this port, you are doomed to leaving the vacuum hose blocked off and effectively having a lacklustre 009 and bad performance Bad luck if so, buy new carburettors with vacuum takeoffs or fit working FI, and sell those to a Beetle owner.
I'll probably get shot for saying this, but bigger pump jets can help overcome the lack of a vacuum take off. :runs for cover before the svda police come and get me:
Spanish IDF’s have a vacuum advance port but I believe some old Italian carbs didn’t have it drilled.
Hey, thanks for all the advice guys - much appreciated! Looks like I'm going to have to see if the carbs have been drilled for the vac advance. That photos really useful, @77 Westy
You are part solving the problem. Pump jets stop the dead spot caused by having an engine that cant accelerate fast enough the instant you open the throttle, coupled with a too big venturi. The vacuum advance also requires matched venturi size, ( otherwise the vacuum signal will be too small to be useful) allowing the engine to run more efficiently at mid throttle, delivering more torque to get the acceleration to continue. Using an 009 may be all you can do because you havent got a 3.6 litre engine to suck enough air. Using a programmable ignition like a 123 (the one where you edit advance curves) may be better than a stock SVDA..
The fing is... Vacuum advance allows a leaner idle jet for better mpg. The extra advance allows the leaner mixture time to burn and as the engine is never under much load it's all good. As soon as your foot is more than s few mm into it, the mains take over. So if you fit a non-svda distributor to your stock VW jetted carb, it won't run well in low load and the transition to mains will be weak. Engines I've jetted for myself with non-stock carbs don't seem to care whether I have vac advance or not. Just sayin.
