Weber/Distributor/Timing formula

Ok- I have a 2.7, "S" (stiock) cam with early exhaust, Weber 40IDAs and Bosch distributor JFUD6 #0231 184 001. The dist. has a vacuum advance attached. I need to know where the vacuum advance hose from the distributor should attach to the intake manifold, or where the correct attachment point should be. There are two spigots (both of which are brazed shut) screwed onto the driver's side intake manifold closing off passagewayws into it's throats. I also need to know the correct timing figures for this configuration. I know some of you can give me precise data Thanx!

That is actually a vacuum retard that only works during base idle for emissions purposes. Just put a plug over the nipple on the retard unit. Or just leave it, it doesn't matter. Set your timing at idle to 5 deg. Before TDC.
On the off chance that you want the vacuum retard to work,(don't know why) you would need to plumb in vacuum ports to opposing cylinders. 1 and 4 are preferable. Then tee them together and connect to the distributor nipple. In reality, you want the retard gone for carbs. Helps reduce the possibility of spitting back through the carbs at idle. Carb fires are a bad thing!

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Tyson Schmidt
72 911 Cabriolet

Thanx, Tyson- So what does advance the timing at higher rpm? Certainly it is not set the same for all speeds.

Mechanical advance.

Alright then what about the distributor itself. Will it have the right advance curve for my engine/carbs? Am I good to go as is?

Any more advise on this subject?

It should be fine. After setting base timing check it at 6000 rpm. (briefly) I believe it should be 30-32 BTDC, I don't have the specs in front of me. But the vacuum unit only worked at idle before anyway, so if the timing was OK before then nothing will need changing.

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Tyson Schmidt
72 911 Cabriolet

Thanks guys for the help- I feel better now!

I agree that no vacuum line is necessary for the dizzy with carbs. The vacuum is for at idle and just off idle.

Now without this vacuum you may feel sluggish off the line until the mechanical advance starts kicking in, also low RPM "pull" will be less.

From what I can remember Bruce Anderson says it is okay to run without vacuum as long as the dizzy is from a sub-1980 911.




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Adrian Pillow
1979 911 SC
1966 VW Microbus
PCA - Peachstate Region

Pillow are you saying that if I did hook up the vacuum line it would be better for my low end acceleration? What else would improve the response at those low revs? What simple fix is there to optimize this setup?

Another thing. Are the numbers I quoted above from my distributor identify it as a sub '80. It came with my '77 engine, but it is new.

Brainiac has an good point that I forgot to take into consideration, retard vs. advance. If it is a vacuum advance then I would recommend hooking up the vacuum lines as Brainiac described on 1 & 4. If the dizzy is a retard vacuum plug all the holes and forget about it.

How can you tell what vacuum you have? Easy, let the car warm up and then remove the vacuum line from the dizzy. If the revs go down a little it is a vacuum advance. If the revs go up then it is a retard vacuum.

From my experience it is a wise purchase to get a dynamic timing light. Then you can dial in the advance you are looking for and rev 'er up! I am thinking the dizzy advance will be "all in" around 4500-5000 RPM. Consult the manuals on what max advance you need, I think between 32-34 degrees. I am not sure if it possible to static time a Porsche accurately?

I am not familiar with Bosch part numbers to be able to identify what dizzy you have, sorry.



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Adrian Pillow
1979 911 SC
1966 VW Microbus
PCA - Peachstate Region

This engine is newly built and the distributor came with it in the package. It is not yet installed in the car. I'm pretty savy about using a timing light and understand what you're saying about fine tune timing. I've done it for years on MC engines.

What I'm hearing here is that- though my distributor is set up for CIS, with a retard funtion to compensate for that system's more regular fuel flow at low rpm, it also has a mechanically weighted advance which is activated (over a curve) to fully advance, maybe 30-34 degrees, at 4500-5000 rpm. I should set my idle (900-1000rpm) to show an advance then of 5 degrees, and since I'm using carbs and have need of more advance at idle than CIS, then I should plug the vacuum spigots on the manifold and distributor.
Of course I'm figuring the idle timing may change depending on where I twist the distibutor to match the optimum advanced setting, which at 6000rpm should be in that 30-34 deg. range

That brings me again to the questions- What is that optimum setting? I have most the books and They don't seem to mention how to set a 2.7 with webers, only the stock CIS setting, which must be different. Also does is the advance curve of this distributor (and they can be quite different) in tandem with these carbs give me the right kind of low end power I need for all around driving?

I guess I'm probibly back to doing like I do with Harleys- Running up and down the highway, adjusting and then running up and down the highway again til it runs best.

Thanx so far, Guys!
Perhaps there is a person out there who has had an Identical setup. It can't be that rare.

I think you will be fine with the 32-34 at 6,000 RPM. The low end will have to take care of itself because you really have no other option, unless you have the dizzy recurved.

The hard part is getting the engine to perform well at low RPM and still be safe at high RPM. The more advance at low RPM makes for a more responsive low end engine, but at high revs the engine is timed too far out producing extra head heat and pinging. Plus the further advance on the timing the lower MPG you will see and you will have a hotter running engine.

I think you are money in the stock range.

Good Luck!