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Borsche911
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Ignition timing advance Vacum dizzy with Carbs
I have just completed the rebuild of a 2.7 S engine with 10.5:1 JE pistons, L cams and Weber Carbs. I have machined the venturis to 34mm, they were originally from my 2.0ltr. Changed the Mains to 130, F3 emulsion tubes and the reccommended air correction jets. Engine starts fine and ran it for 20 minutes at 2000rpm to run in to first oil change. Got quite exited about the first drive and set my shift light to 5000rpm on the tacho, (sunpro aftermarket). However, the engine woulod not pick up past 3000 rpm and hesitated. I had set the timing to 5 degrees BTDC. After checking the MSD 6AL unit, (swapped back to std CDI setup) I noticed that the timing was the same at tick over as it was on full chat. I then kept advancing the timing till the engine responded perfectly and checking with the strobe, it was at 30 degrees BTDC. However, it remains at this setting on tick over which is now at 1500 rpm. Clearly, I have no vacuum pickup from the carbs, the distributor was from the original CIS setup. Am I on the right track trying to find a suitable pick up point to connect the vacuum line to? There does not appear to be any mechanical advance on the Bosch Dizzzy, No 0 231 184 001 JFUD 6. Guidance from a pioneer would be a great help! Regards, Jim
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10-14-2010, 02:27 PM
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Registered
Join Date: Oct 2008
Location: Nash County, NC.
Posts: 8,472
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On a 2.7 CIS the vacuum is a retard, not an advance.
Bruce |
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10-14-2010, 06:03 PM
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Registered
Join Date: May 2003
Posts: 3,346
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Your Dizzy should have a centrifugal advance. You shouldn't need the vacuum line at all. You can get an idea of the health of the centrifugal advance by turning the rotor by hand. It should have a spring holding it at rest and you should be able to advance the rotor by hand about 10-15 degrees against the spring pressure. If you can't do that then the advance is "frozen" and the dizzy needs to be reconditioned.
-Andy
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72 Carrera RS replica, Spec 911 racer |
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10-14-2010, 06:56 PM
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Borsche911
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Vacuum advance and carbs
Guys, thanks so much for your help. The vacuum is indeed a retard not advance function and the rotor was indeed frozen/seized as suggested. I have now stripped and rebuilt the distributor and the engine is now singing like a bird!!
 Best regards, Jim.  
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10-16-2010, 02:36 PM
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beetle
