Centrifugal advance - sanity check please
 

'70 911E (MFI)

When blipping and/or revving the engine at rest (no load), the engine wants to 'hang' at higher rpm for a second after releasing the throttle, then drop to idle. I've convinced myself that it's not a linkage/butterfly issue, and that I should be looking for some sticking in the advance mechanism, on the theory that sticking (i.e. too much) advance will increase the idle, until it un-sticks. A timing light supports the theory (rpm vs. advance), but that seems like kind of a chicken and egg thing.

Is it worth pulling the distributor, or am I barking up the wrong tree? Thanks in advance...

John

Remove the cap and twist the rotor clockwise. It should rotate a little, then return. That alone doesn't discount the weights hanging up, but if it feels stiff, I'd look further into it.

Ater removing the breaker plate, you can see the weights and any lack of lube. You could probably do this in the engine, but I'd do this on a work bench where you fully examine it. Little screws have a way of gravitating to the area directly under the crank pulley.

I'd also take a look at the complete throttle linkage from pedal to engine. Worn bushings can prevent the throttle from closing right away.

Sherwood

John,

Don't pull the distributor just yet. Remove the cap and turn the rotor to verify the advance isn't sticking.

deceleration valve
 

i think it may be your deceleration valve, it is in the system to keep the rpm up between shifts. i think it is a vacuum deive and it is item#7 on the parts breakdown

http://www.pelicanparts.com/911/911_Parts/1978-83/1-7-2.JPG

John,

No decel valve on an MFI engine ...

I would suggest that it is probably time to address both worn throttle linkage bushings [pedal box bellcrank, transaxle bellcrank, & MFI #3 stack bellcrank] as well as distributor cleaning & maintenance issues ... unless you are certain you already took care of those issues in the recent past!

For distributor cleaning, disassembly ... I suggest reading the following thread:

http://forums.pelicanparts.com/showthread.php?s=&threadid=5432&highlight=distribu tor+lube

While changing the throttle linkage bellcrank bushings, I suggest a thorough cleaning of the ball-socket-ends on the linkage rods, followed by a limited-quantity application of Permatex 133K Anti-Seize Lubricant.

http://www.permatex.com/products/automotive/b_lubricants/a_lubricants/Permatex_Anti-Seize_Lubricant_a.htm

sorry did not see the mfi
 

time for new glasses i did not see the mfi after.

John,

The required bushings are: 4 ea 901.423.253.00 at pedal box and TB, 2 ea 914.423.211.00 at transaxle.

At each of the three bushings' location shown in the diagrams below, before putting the split cotter key back in ... check for looseness. If more than typical valve clearance ... try another washer, or combination of washers to reduce side play without binding. Ideal goal is zero, or maybe 0.001" play.

http://forums.pelicanparts.com/uploa...1159156157.jpg

http://forums.pelicanparts.com/uploa...1159156193.jpg

Wow! Thanks for the fast response, the great advice, and the links and resources. I haven't been able detect any pedal/linkage-related issues -- it doesn't appear to hang up mechanically before reaching the idle stop(s) -- but it's an old car, so that stuff is worth a look too.

Thanks again,
John

Finding out is easy enough. Remove the long throttle link where it attaches to the crossbar, then see if the crossbar and all the throttle linkages move freely.

Here's a good thread about throttle linkages in general.
http://forums.pelicanparts.com/showthread.php?s=&threadid=227039&highlight=bushin gs