Vac. advance on electronic ignition

ga.spyder · Post by **ga.spyder** » Fri Dec 14, 2007 8:27 pm

I have noticed a slight stutter or dead spot at above 5000 rpm.Slight but noticeable.Looking under hood Ive discovered my vac. advance doesnt seem to be working. It isnt moving the arm under throttle ,and you can suck on hose and draw air.Would this cause the problem w/ the high rpm lag? I have ordered a new one. Craig

So Cal Mark · Post by **So Cal Mark** » Sat Dec 15, 2007 3:25 am

the vac adv is mostly for improved gas mileage, since it only works at high vacuum/light load. But the leak thru the diaphragm will also cause a slight lean condition.

ga.spyder · Post by **ga.spyder** » Sat Dec 15, 2007 8:29 am

Thanks ,Mark Im going to wait until I get the new one installed and then see if the lag still there. I had someone recently adjust the mixture as the p/o had 'played' with it to get thru emissions. If the vac advance doesnt help it ,Ill look into the mixture or elsewhere. Craig

So Cal Mark · Post by **So Cal Mark** » Sat Dec 15, 2007 11:04 am

I'd make sure to check not only the initial timing but both the mechanical and vacuum advance totals once you get the dist sorted out. Then check the fuel mixture and make sure the AFM flap works thru its' range without sticking. On the FI units, adjusting the air/fuel mixture will affect it throughout the entire rpm range

bradartigue · Post by **bradartigue** » Wed Dec 26, 2007 4:46 pm

The vacuum advance needs to work for your car to have optimum performance during acceleration. When inoperable the car simply drags until the mechanical advance starts working above 2k RPM. So you really want it to work right.

Above 5000 RPM it is unlikely that the vacuum advance is the issue; however, fix one thing at a time. Your FI computer is compensating for the extra air from the vacuum unit and you need to sort that out. Expect to replace the unit, reset the ignition timing, reset the idle speed screw and stop screw. All very easy to do; I have the procedure documented at my web site http://www.artigue.com/fiat

Bosch L-Jetronic really can't be adjusted for mixture. It can be fooled (very slightly) by modifying the spring tension on the AFM flap, but the air bypass on the AFM body has little affect on mixture. With the system working correctly any adjustment on the air bypass is compensating for once the O2 sensor warms up. I would be inclined to think your previous owner changed the spring tension, which will give you a richer mixture at low RPM but too lean at high RPM (because the door is already fully open but the CPU thinks not). If this is the case you can relieve the spring tension one notch at a time - but finding the factory spot might be tough.

In general L-Jet isn't worth tinkering with. When it all works right the Spider 2000 gets good gas mileage and nice performance. If you want to get more change the pistons to slightly higher compression, it helps.