Vacumm Advance (or lack of it)

garage dweller · 04-11-2005, 07:07 AM

Todays odd one...
Been fiddling with my distrubtor as part of an ongoing list of "issues" and was surprised to find no diaphragm housing or inlet tube for Vaccum Advance!!!
Distributor is German/assuming bosch...any thoughts or experience.
Webbers appear to have Vaccum tap points plugged so I'm all deeply confused.
Any thoughts???

RoninLB · 04-11-2005, 07:21 AM

Quote:

Originally posted by garage dweller

I'm all deeply confused.

Any thoughts???

yep. I'm confused also.

set up a signature.

I run without vac advance.

Early_S_Man · 04-12-2005, 12:18 AM

What year and model car???

911 engines prior to 1972 models only used centrifugal advance! No Weber-equipped engine ever came with a vacuum module on the distributor.

garage dweller · 04-12-2005, 02:13 AM

The car was originally a us Spec 911T with Solex Carbs and all the emmissions control system. Originally shipped with the Marelli distributors. These have been removed at some point in the past and a set of Webber IDA's put on and the dis. replaced....
hence the loads of questions and lotso of confusion.

RoninLB · 04-13-2005, 12:34 AM

Hello Neil.

vacuum advance advances the ignition at cruise where more timing is acceptable and leads to more efficent combustion. As far as I know all 911 vacuum distributor pots that are pre SC are vacuum Retard at idle and mid rpms, as per my stock '77 dizzy with the vacuum canister hooked up.

I was unable to find the dizzy ignition curve for the carb '70T. I was able to find the '72-'73 911T curves only. My issue is that your current mid range curve may not be optimized.

If it was my 911 I would seek out two things. One is the factory suggested curve, and two being the curve of what you currently have installed. On the side of the distributor is a Bosch part number consisting of ten digets.

Now lets say that you have the right curve installed. I would use a timing light to confirm the operation of this curve at 2k, 3k, 4k, 5k rpms. My issue is that an installed mid range curve that is below desired levels lowers acceleration. I would also seek to have this part of the curve at the upper limits of factory spec's.

Tuning the Webers can be efficently done with the correct curve. I take this seriously because there is the possibility that you can improve your mid-range acceleration.

garage dweller · 04-13-2005, 01:25 AM

Hi Ronin,
All of this has come up because I am getting intermittent blow back up the velocity stack of the webbers on (mainly on 4&6) @1500 to 1800 RPM only. Sometimes feels a bit juddery if I accellerate smoothly & quickly from 2000 to 4000RPM fits with your mid range theory...this has lead me on to the entire thing of fuels and tuning. this is a bit new to me but I learning fast. Currently looking and reading before turning screws etc. Got original workshop manuals but the stock curinfo on this engine is for the marelli with V advance at idle. Will go wading through the oil/filth to find part number and keep you billed.
Regards
Neil

RoninLB · 04-13-2005, 01:52 AM

Quote:

Originally posted by garage dweller

getting intermittent blow back up the velocity stack of the webbers on (mainly on 4&6) @1500 to 1800 RPM only

sounds like it's lean on fuel in 4&6.

I would try a temporary quick fix. It seems that maximizing your acceleration thru ignition curve tuning can wait.

How about if you measure the number of turns each idle mix screw is set to. Write it all down so you can return to this setting. The number of turns should be close to equal. You may find that 4&6 are lower than the others? The "ear" tuning can sometimes be difficult. A 1/4 turn of the idle mix screw is a noticable difference in operation of that cylinder. Turning the screw outward richens the mix. Hopefully this will be good enough for now. If not maybe the 4&6 idle jets are dirty. Idle jets are kinka misnamed as they continue to feed thru about 3k rpms. Just unscrew completely and remove if you have new rubber 0-rings for the idle mix screws available. Use grease on the rubber for install. You can do the full range of dialing in later with the air flow meters, etc. I find it more convient to use 2 meters to balance each carb in sync.

You do have the popular Weber books ?

garage dweller · 04-19-2005, 06:57 AM

Hi Ronin,
Sorry for the delay in getting back to you works been a nightmare...
Well tried the adjustment you mentioned-seems to have made an improovement. Still doing it just less so. Might try cleaning everyhting again and resetting but I still think I need to check the ignition settings and ballance the airflow.
Got the serial number for the dizzy-definitly bosch(funny rivet logo).
Numbers are (serial) 600 951 132 0 and the type/batch number is 6 RDFL. any of this seem familiar.
regards
neil

RoninLB · 04-20-2005, 06:54 AM

I was unable to track down your dizzy part number. Check the curve with a timing light every 500 rpms.

all I could find on part numbers are
http://www.pelicanparts.com/catalog/shopcart/911/Early_911/por_911E_igniti_pg1.htm

"Well tried the adjustment you mentioned-seems to have made an improovement. "
---- maybe the Webers need a full adjustment and a confirm that the throttle shafts are tight enough and tested not to be leaking air.