Pelican Parts
Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   Pelican Parts Forums > Porsche Forums > Porsche 914 & 914-6 Technical Forum


Reply
 
LinkBack Thread Tools Rate Thread
Author
Thread Post New Thread    Reply
Registered User
 
Join Date: Sep 2003
Location: Los Altos, CA
Posts: 483
Setting vacuum advance/retard

So, we seem to have found the source of our blown pistons and ring lands! Our distributor (off a fresh rebuild I might add), does not have enough vacuum retard. Turns out we were way too far advanced (about 38-40 degrees) under load.
My question is, how do you adjust the vacuum unit? The advance and retard work when you pull a vacuum on them. However, I suspect that the adjustment screw (mine has one with a lock nut)is not adjusted properly. It is almost all the way out. Does anyone know how to get this speced right? I am looking for something like this:

1) Remove the hoses to the vacuum advance/retard and plug hoses.
2) Run engine to 3500rpm and set timing to 27 BTDC
3) connect ONLY the advance hose
4) DO SOMETHING TO THE ENGINE HERE
5) You should see XX BTDC now. If not adjust screw 2 turns clockwise to retard or counterclockwise to advance.
6) Once you have set advance correctly, hook up the retard hose.
7) Disconnect the advance hose
8) You should see XX BTDC now.

etc.

Does anyone know this process?


Thanks for your help,
Chris
Old 09-10-2003, 07:45 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #1 (permalink)
Administrator
 
Dave at Pelican Parts's Avatar
 
Join Date: Dec 1969
Location: Silly-Con Valley
Posts: 14,057
Garage
Send a message via AIM to Dave at Pelican Parts Send a message via Yahoo to Dave at Pelican Parts
Instead of "do something to the engine here" in #4, substitute "pull XXX amount of vacuum on the advance port of the distributor dashpot". Ditto with checking the retard function.

For how much change you should see for the amount of vacuum, check:
http://www.dgi.net/914/tech/distrib.html

As for "turn X number of turns on the screw", substitute, "Mess with the screw until you see a change. Keep messing until things seem to be in range."

--DD
__________________
Pelican Parts 914 Tech Support

A few pics of my car: http://www.pelicanparts.com/gallery/Dave_Darling
Old 09-10-2003, 08:14 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #2 (permalink)
Registered User
 
Join Date: Sep 2003
Location: Los Altos, CA
Posts: 483
Thanks Dave. I am taking the dizzy into the shop this morning as I don't have a vacuum pump/gauge and don't really know how to adjust it. I am hoping they can just adjust it back to spec. Seems like the previous owner messed with it based on how far out the screw is. Do you have a cross-section of the vacuum canister? I would like to understand what the screw is doing. Is it just moving the relative posistion of the diaphragm or is it adjusting a spring or pretension?

Chris
Old 09-10-2003, 08:50 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #3 (permalink)
Administrator
 
Dave at Pelican Parts's Avatar
 
Join Date: Dec 1969
Location: Silly-Con Valley
Posts: 14,057
Garage
Send a message via AIM to Dave at Pelican Parts Send a message via Yahoo to Dave at Pelican Parts
Here ya go:




...But frankly, I'm not sure what it says about the adjustment....

--DD
__________________
Pelican Parts 914 Tech Support

A few pics of my car: http://www.pelicanparts.com/gallery/Dave_Darling
Old 09-10-2003, 10:21 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #4 (permalink)
Registered User
 
Ron Meier's Avatar
 
Join Date: Sep 2001
Location: Crestline, CA
Posts: 937
Make sure your shop has a dizzy machine to verify the settings. The setting for the unit can be found via the distributor number in the tech session here or on 914 club
__________________
"Inventor - Blue Flame 914 Seat Heater"

"Yellow Rusty Cars Are Faster"
_____________________________
'70 2.5 (I'll never finish it - Somewhere over the rainbow.....,)
'73 2.0 (Just Not The Same)
'74 2.0 (Heartless & Lungless)
Old 09-10-2003, 10:38 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #5 (permalink)
Registered User
 
Join Date: Sep 2003
Location: Los Altos, CA
Posts: 483
Great. Thanks Dave!

Chris
Old 09-10-2003, 10:47 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #6 (permalink)
 
Registered User
 
pbanders's Avatar
 
Join Date: Jan 2001
Location: Phoenix, AZ
Posts: 1,113
Garage
Send a message via Yahoo to pbanders
Hmm. The idea behind vacuum retard is to reduce emissions at idle and very light part-load. Once you go to even modest throttle angles, the vacuum retard goes away. If the timing is properly set and the mechanical advance is working correctly, I cannot see how you would be seeing 40 deg of overadvancement under normal part load conditions. Look at the engine speed vs advancement charts from the factory workshop manual for examples of proper operation of the vacuum advance, retard, and mechanical advance mechanisms.

I've posted on this in the past, and I think I referenced diagrams from my Bosch guides. I'll see if I can dig them up.
Old 09-10-2003, 12:23 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)
Registered User
 
pbanders's Avatar
 
Join Date: Jan 2001
Location: Phoenix, AZ
Posts: 1,113
Garage
Send a message via Yahoo to pbanders
I just looked at your post again, and I think I see the issue. You're using a throttle body with both vacuum advance and retard ports. Later models didn't use the advance. Under moderate part-load, there is significant advancement of the timing, as the low-density cylinder charge charge takes longer for the flame front to propagate through it for complete burning. As the cylinder harge density increases as full-load is approached, the vacuum advance becomes inactive and timing is set only by the mechanical advance.

Check your dizzy vs. the factory workshop manuals at a shop which has a dizzy tester. You should be able to replicate the factory curve through adjustment of the advance aneroid. If you still have predetonation, look to other factors: plugs, carbon deposits, piston machining problems, fuel octane, etc.
Old 09-10-2003, 12:29 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #8 (permalink)
Registered User
 
Join Date: Sep 2003
Location: Los Altos, CA
Posts: 483
Thanks Brad. If you can find the adjustment procedures/reference diagrams that would be great. I am going to the shop this afternoon to get this set up. Seems like as long as it holds a vacuum on the retard and advance ports, there are enought adjustment points to get teh proper curve....

Chris
Old 09-10-2003, 12:50 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #9 (permalink)
Reply

Thread Tools
Rate This Thread
Rate This Thread:

 


All times are GMT -8. The time now is 12:22 PM.


 
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Pelican Parts Website -    DMCA Registered Agent Contact Page
 

DTO Garage Plus vBulletin Plugins by Drive Thru Online, Inc.