My car has started to misfire over 3,000 RPMs. I thought boost leak, but I pulled it apart, nothing looked torn, reassembled with new 0-ring anyway, and same issue. Even pulling away in 1st with no boost, it'll start breaking up right around 3k. I can boost a bit in 2nd under 3k and it's good until it crosses 3k.

So... this is the timing thread, right? I started looking at timing and have thoroughly confused myself again. Pisses me off figuring out stuff I knew 15 years ago. Anyway:

My initial is 15, with or without hoses connected. Doesn't matter.
At 4,000RPMs it's 25 with both hoses plugged with a golf tee.
So my mechanical seems to only be giving me 10 degrees? Doesn't seem right.

When I apply vacuum via mityvact to the front port, timing goes to about 0 TDC and idle drops quite low. Vacuum to the rear port does nothing, but pressure also drops timing to about 0 TDC.

I've never had my distributor apart but remember old Warren Early_S Man posts about breaking them down and cleaning the centrifugal weights at some interval.

Can you anyone verify that I should be getting way more than 10 degrees advance? If that's the case I need to pull this and dig in.http://forums.pelicanparts.com/uploa...1724185711.jpg

I think 10 deg mechanical advance sounds about right. Hopefully others can confirm this.

First, the '86 and later 930's have a distributor that rotates counter-clockwise, so it is the ABSENCE OF VACUUM that produces advance

http://forums.pelicanparts.com/uploa...1724672142.jpg

Here is the hose connection for '86 to '89. The Red hose (your has been replaced with junk) goes from port V (See drawing) to the thermo switch, and from the thermo switch to the front of the distributor. When the engine oil is cold, the vacuum is cut off, spark is advanced, and the engine runs at high idle (cold idle).

When the engine is warm, the vacuum is restored, the timing is retarded and the engine runs at low idle (warm idle).

Port V is ported vacuum, so as the throttle is opened the vacuum decreases, and you again get spark advance.

http://forums.pelicanparts.com/uploa...1724672223.jpg

Here is the USA timing curve. The loss of vacuum puts in about 16 at the crank, and the target is around 26, so that leaves around 10 for the static timing.

http://forums.pelicanparts.com/uploa...1724672285.jpg

Probably should get the admin to post a sticky with my original post on this.

You’ll be chasing your tail if you don’t start by rebuilding your distributor. The bushings get worn and they develop excess play in multiple directions. The fiber washers disintegrate over time. The little plastic parts inside the advance mechanism disintegrate but not the weights themselves. The electrical coil thingy gets crusty and doesn’t advance properly. Several threads here about how to do it. Or use a professional such woods enterprises or parts klassic. Rebuild kits are available with most but not all the needed bits.

Absence of vacuum producing advance? Don't think so, on my dist (1987), with equal vac on both distributer pots it will advance. So in low throttle situation (manifold high vac) it will advance ignition.

I appreciate the information. I decided to inspect/clean/oil my distributor mechanical advance bits in the search for why a 25* total advance at 4000RPMs returns to 15* at idle with both hoses off.

For better or worse, the insides of my distributor didn't appear to be bad at all. Interestingly, the weights appear to be the same size (?) although I've seen reference to a big one and a small one. I decided to install a slightly lighter spring to one of them from a V8 MSD parts kit I had lying around. I did clean and re-oil, checked for excess play without finding any, and am in the process of reassembly now. Vacuum and pressure to the can does hold, and does move the assembly to some degree. I'll drop it back in and report my findings but I'd be surprised if anything changed, other than my maybe slightly earlier advance curve with the one lighter spring. I can always reverse course if I find that necessary. (I find that the longest part of this process is the R&R of the longneck intercooler, which has several accessories on it that are not as easy to access as the OEM one.)



http://forums.pelicanparts.com/uploa...1724696867.jpg
http://forums.pelicanparts.com/uploa...1724696867.jpg
http://forums.pelicanparts.com/uploa...1724696867.jpg

You are correct 908, but if the hoses are connected as intended (Ports I and V) to the throttle body then one has vacuum at idle and one does not, as shown below:

The electric valve 37 cuts off the vacuum at idle when the engine is cold, and that advances the timing for a faster idle.

Read my long winded post earlier in this thread for how the other vac advance/boost retard functions are implemented.

http://forums.pelicanparts.com/uploa...1724865583.jpg