Tech Forum : vacuum advance

Posted Wednesday, Nov 1st 9:57am [Edit] [Quote] [IMS] [View car]

Member
Post nr. 9

NY, USA
82 528e2i 84 320i/4 80 320i

I have a stroked '81 320i motor with the stock ignition and the L-jet setup from an '84 318i. My problem is that at light loads and low RPM (driving down the road at 35mph or less) there is a steady misfire which basically jerks the car around and beats up the drivetrain. The idle has never been great either.

I think that the problem is too much vacuum advance. IIRC my timing light showed roughly 25 degrees BTDC at idle with the vacuum connected and maybe 5 degrees disconnected. If I retard the overall timing then there is so little advance at WOT that I lose my low RPM torque. At high RPM there doesn't seem to be a problem either way (no detonation).

Does anyone know if there is some sort of valve I can install in the vacuum line to reduce the vacuum at the distributor? Or could the solenoid+relay from the 318i setup be put to some use?

I have another distributor (both are probably old and high mileage) and that one was a bit worse even.

thanks in advance for any info

imaradiostar

Posted Wednesday, Nov 1st 10:18am [Edit] [Quote] [IMS] [View car]

Member
Post nr. 923

Nashville, TN
81 323i, 82 525i, 85 524td, 90 535i

I would suspect that the springs in the dist are old and weak and can't provide sufficient advance at idle. The vac advance is really only for improving mid-throttle mid-rpm fuel economy.

Try bending the mounting points for the springs in the distributor. This should help a bit. If the spring are totally shot you may have to source some newer ones- perhaps from a later car.

jamie

myersport

Posted Wednesday, Nov 1st 1:56pm [Edit] [Quote] [IMS] [View car]

Member
Post nr. 184

east tennessee
'82 320iS. '91 327iC

The shaft bushings are prone to failure as well, causing excess shaft play and leading to surging and misfiring. The telltale here is a jumpy tach when you feel the hesitation or miss. A new/reman distributor is the fix in this case.
--Dennis

Posted Thursday, Nov 2nd 9:56am [Edit] [Quote] [IMS] [View car]

Member
Post nr. 11

NY, USA
82 528e2i 84 320i/4 80 320i

How do you get the distributor apart anyway? I took out a screw and circlip on top but that didn't seem to do it. When I tried to loosen the screws on the outside I busted my screw driver

Jumpy tach has not been a problem. The timing light jumps around some though, I wonder if that is normal.

I'm going to do some more experiments tomorrow.

aussie323i

Posted Saturday, Nov 4th 2:56am [Edit] [Quote] [IMS] [View car]

Member
Post nr. 536

australia
1979 323i

myersport wrote:

yeh- watch for this one indeed! If the shaft in your dizzy has too much play in it there can be SERIOUS repercussions! One thing to remember is that the oil pump also run off the bottom of the dizzy and it's connected to the timing belt too... I heard of an e21 where the shaft in the dizzy had excessive play, causing the bolt that holds the drive cog onto the shaft to shear off... this jammed the intermediate shaft (run from the timing belt) and the oil pump instantly... everyone can probably guess the consequences- the timing belt jumped off and the valves hit the pistons!

This happened to a newly rebuilt motor... a second hand dizzy was used as a replacement, and 6 months later the same thing happened again!

Bottom line, if you use a second hand/ non- reconditioned dizzy as a replacement, check the shaft/ drive cog to make sure there is no movement before installing or you might be sorry.

*edit... just rechecked the post- you have an M10- wouldn't be effected ofcourse!

Posted Sunday, Nov 5th 8:35am [Edit] [Quote] [IMS] [View car]

Member
Post nr. 12

NY, USA
82 528e2i 84 320i/4 80 320i

I see how the vacuum advance mechanism works now, but still not how to get the distributor apart ><

plan A: put an old piece of brake line with a small hole in it in the line leading to the distributor. Leak = less vacuum = less advance?

plan B: drill a hole in the side of the distributor and put a screw in there to limit how far the four-prong doodad can rotate.

imaradiostar

Posted Sunday, Nov 5th 8:43am [Edit] [Quote] [IMS] [View car]

Member
Post nr. 933

Nashville, TN
81 323i, 82 525i, 85 524td, 90 535i

Plan c- get a new/used distributor or fix it correctly.

If you like I'll look and see if we have one at work. What bosch number is it?

jamie

Posted Monday, Nov 6th 7:41am [Edit] [Quote] [IMS] [View car]

Member
Post nr. 14

NY, USA
82 528e2i 84 320i/4 80 320i

Yes that is a good plan C. I will try them in alphabettical order of course

The 320i distributor is "JGFU," the one in the car is from a 318i and I don't know the designation (or what is the difference).