Eliminating vacuum on distributor

ahdoman · 07-09-2009, 04:24 PM

Ref; 77 2.7CIS - I know the vacuum mechanism on the side of the distributor is there for retard. Mine hasn't been hooked up and it looks like there is no reason to since the car runs great. Since this is in my 914 I'd like to eliminate the vacuum mechanism since it gets in the way when I am trying get to the distributor lock down nut and timing. Has anybody ever done this? It looks like the plate the vacuum arm hooks to will need to be anchored in place so that it doesn't move. Any input or advice?

moneymanager · 07-09-2009, 05:10 PM

Most people leave it there for that purpose...to anchor it. Easy enough to make some kind of little bracket to cover the hole and anchor the mechanism though. Post a pic when you finish it.

snbush67 · snbush67's Garage

Just a thought... if you anchor that arm wont you loose your mechanical advance? I don't know if that distributer has a weighted spring mechanism for mechanical advance but my SC distributor does. I had thought about doing the same, eliminating the vacuum canister as I have carbs now, but I wanted to do it in a way that is reversible.

ahdoman · 07:25 PM

How do I determine the correct orientation of the advance plate? The vacuum assembly doesn't work anymore so I don't know if it's in the correct orientation.

snbush67 - That's one of the things I was thinking. The distro is a mechanical advance (springs & weights?) but does that top plate need to move? Does the mechanism rely on the tension from the vacuum assembly to return it to neutral?

JFairman · 07-09-2009, 07:41 PM

just leave it there. and set the timing around it.

old european BMW big six cylinder engines used basically the same skinny bosch distributor, but it the vacuum pot was for advance and pulled the breaker plate in the oposite direction of shaft rotation when vacuum from the throttle body advance port was applied.

usa marketed blue streak points are soo much better than the crappy lowest bidder bosch points it's ridiculous.. much more precisely made and with a stronger spring for higher revs without point float.
i don't know if they are still sold or not. i have the part number somewhere.

i used to use them till i put in a pertronix magnetic igniton trigger pickup for early 70's 911, and a digital 6 MSD.

ahdoman · 08:00 PM

JFairman- If I take the vacuum mechanism off the plate is loose. Are you saying to anchor it in the position relative to where it is located with the vacuum assembly on? Does that top plate not need to rotate for mechanical advance?

JFairman · 07-09-2009, 08:05 PM

if the both srews holding the pot onto the distibutor can still hold it on and it anchors the breaker plate steady in a position that the centrifical advance is working correctly around it, that is what i meant by leave it.

of course spending money on rebuilding them completely usually makes things better.

ahdoman · 07-09-2009, 09:12 PM

OK. Here's a shot down the distributor. As you can see the mechanical advance acts on the center shaft and that's where the advance comes from. So, it makes sense to be able to somehow "lock" the points plate in place since it doesn't need to move for the mechanical advance to work. I'm going to go to a Pertronix so I should get some great accuracy.

On another note; notice the gap between the spring end and the weight post (see red arrow)? This pic is with the dizzy at rest. At full advance the spring never even actuates. All the tension comes from the other spring. Is this correct? I did notice that the last time I set timing for the engine I was already at full advance before 4000 RPM. According to specs the full advance needs to be at 6000 RPM. Does anybody else have a dizzy apart that they could check this with? Dizzy is part # 0231 184 007.

ahdoman · 07-09-2009, 09:14 PM

Sorry...forgot the pic

ahdoman · 07-10-2009, 11:45 AM

Edit for update- I took apart another earlier distributor I had (73.5 / 2.4CIS Part #231169008) and both springs are the same as the one on the left in the pic.

snbush67 · snbush67's Garage

Quote:

Originally Posted by ahdoman

How do I determine the correct orientation of the advance plate? The vacuum assembly doesn't work anymore so I don't know if it's in the correct orientation.

snbush67 - That's one of the things I was thinking. The distro is a mechanical advance (springs & weights?) but does that top plate need to move? Does the mechanism rely on the tension from the vacuum assembly to return it to neutral?

I would guess that since the arm is connected to the vacuum pod which controls a retard and it is connected to the plate then if you lock it in place the plate wont move and then you wont get any advance as well.

I think that all the pod was supposed to do was provide a rich condition during cold starts to avoid backfireing.

I think that the springs are enough to return it to neutral and the weights advance.

dipso · 07:39 AM

Quote:

Originally Posted by ahdoman

Ref; 77 2.7CIS - I know the vacuum mechanism on the side of the distributor is there for retard. Mine hasn't been hooked up and it looks like there is no reason to since the car runs great. Since this is in my 914 I'd like to eliminate the vacuum mechanism since it gets in the way when I am trying get to the distributor lock down nut and timing. Has anybody ever done this? It looks like the plate the vacuum arm hooks to will need to be anchored in place so that it doesn't move. Any input or advice?

I just locked mine down yesterday. I just put on carbs and the PMO directions say to lock it down by using the little tab near the points. I looked around and only saw one. If you move the plate with a screwdriver you will see that 1 tab doesn't move with it, it is attached to the distributor body. If you see that little pointy tab about 6:35 (near your thumb) , I just put a flathead screwdriver on that and tapped it with a hammer. Presto-Lockedo.

Doesn't move at all now, don't know if it should but I did it anyways, hope I didn't make a mistake. I can't seem to get the carbs running right yet so I don't know for sure.

It runs real good under no load, all the way to 8000 rpm. Under load it bogs under full throttle.
I don't think that is timing related.

I would like to remove the vacuum pod as well and now maybe I will. Sure would look cleaner.

SC-targa · 03-22-2010, 04:18 AM

Are your sure that's not a vacuum advance, not retard? If it's a vacuum advance, you'll lose fuel economy and drivability withouit it.

Regards,

Jerry

dipso · 03-22-2010, 07:02 AM

Yes, on mine it was a vacuum retard, not advance.

Gunter · Gunter's Garage

The correct way to lock the plate is to drill and tap a small hole on the side of the distributor at the same height as the retard plate. The weights and springs are the mechanical advance moving the upper part of the shaft. Make sure that the weights/springs and upper shaft-have move freely. Clean and lube. Install the vac pod so you have a neutral position for the plate. Then select a position on the distributor body for the intended locking screw so it catches the plate inside. Mark it. Remove the pod and dismantle the top portion (Retard plate) Drill and tap for 1/8 NF or 4 mm) keeping the chips out. Assemble the plate and the pod for position. Install the locking screw and lock the plate. Remove the pod and make up a small cover for the hole using the 2 screws from the pod.

Set the ignition according to the new dynamics by trial and error.

dipso · 03-22-2010, 10:11 AM

Jesus that sounds complicated. My instructions from PMO just said to press down the tab next to the points, so that's what I did.

Gunter · Gunter's Garage

Locking the advance/retard plate with a locking screw is what Steve Weiner does when modifying distributors to run without vacuum.

Would we argue with Steve@Rennsport??

It makes perfect sense to this old ME (Not Medical Examiner, the other ME) plus it removes the vac pod so one has more room when adjusting timing.

But, do what feels good.

dipso · 03-22-2010, 04:05 PM

Humm, this is interesting. Tomorrow I put on the correct fuel pump so then I can adjust the carbs properly.
Once that is done I can take it for a spin and see if my 'lockdown tab' worked.
I hope so. I would like to remove that pod thing.

dipso · 03-24-2010, 03:59 PM

Just for future searchers.
Bad idea on the tab thing. I was having a stutter at 5000 rpm. Pulled the points and pried the tab back up.
Then gapped points and timed again.
Now no stutter at 5000.

Don't know if they were related but I think so.

brads911sc · brads911sc's Garage

My .02. If this is an original distributor, it probably has worn out parts inside and is probably in need of a rebuild and youll want it recurved for carbs or non-CIS anyway. Spend the $550, send to Barry Hershon in Detroit. He will replace all the wear parts, bushings, clean it, remove the can, lock it down, put a nice little plate over the hole, curve it where you want it and send it back ready for another 100k miles. It was the best $550 i spent (from a pure performance point of view) on my 5k PMO ITB EFI upgrade...