Vibration at low rpms

[jonvo]

My 944 idles smoothly, and it drives in the upper rpm range smoothly, but when I hold the rpms at around 1200-1500 (in gear or in neutral), it has a lot of vibration (or any rpm under a lot of load). It has always been like this. Is it normal for this car? I recently changed the pass. motor mount, spark plugs, dist. cap/rotor, fuel filter and air filter. The driver side mount looks fine. Any ideas on what it cold be?

[John_AZ]

A balance shaft belt off a tooth could cause this at idle. Have the belts and rollers been changed recently? Another cause is the passenger BB roller upside down on the BB roller shaft.

[jonvo]

Does a BB that isn't timed properly get smoother as the rpms go up? i would imagine it getting worse the higher I rev.

The belts were done about 30k miles ago by the PO. I am planning to do mine next week. Just waiting for a water pump,

[flash968]

it can - because it's a "balance" thing, there can be a particular point that shows vibration, when it doesn't elsewhere - not common, but it happens - pretty common though when the lower gear is off 180 degrees, which happens a LOT if the mechanic isn't paying attention and lets that one drop down

[jonvo]

Alright, thanks. I'll double check the marks before I remove the belt next week. It's going to be my first time doing the front of engine service on a 944. Wish me luck!

[flash968]

unfortunately the marks won't tell you about the lower gear, because both gears can be flipped, and what is important is the keyway on the shaft, and you can't see it without removing the big washer on the front

this is exactly why this becomes a problem - the upper shaft stays put, but the lower one will drop down if you let go, and you can still screw up and put the gear on upside down - the marks will all "line up" but the lower shaft will be 180 out

happens more often than you want to hear

[jonvo]

Originally Posted by flash968

unfortunately the marks won't tell you about the lower gear, because both gears can be flipped, and what is important is the keyway on the shaft, and you can't see it without removing the big washer on the front

this is exactly why this becomes a problem - the upper shaft stays put, but the lower one will drop down if you let go, and you can still screw up and put the gear on upside down - the marks will all "line up" but the lower shaft will be 180 out

happens more often than you want to hear

So do both key ways need to be pointing up?

Would the best method be to put the belt onto the lower shaft and get it lined up, then put the belt onto the upper shaft?

[flash968]

for safety, i always consult the manual to see which way they go (i'm getting older and i tend to forget things from time to time)

once you get the gear on correctly, it's then easy, because you just follow the marks

[John_AZ]

Originally Posted by jonvo

So do both key ways need to be pointing up?

Would the best method be to put the belt onto the lower shaft and get it lined up, then put the belt onto the upper shaft?

Go to Clarks garage for detailed procedure on how to put BB on.

The BB rollers.

From the picture...Both key ways are pointing UP. Tiny dark square on top of BB shaft.
The bottom BB roller is put on opposite the top BB roller. Notice the "o" & "u" marks.



GL
John_AZ

[jonvo]

Originally Posted by John_AZ

Go to Clarks garage for detailed procedure on how to put BB on.

The BB rollers.

From the picture...Both key ways are pointing UP. Tiny dark square on top of BB shaft.
The bottom BB roller is put on opposite the top BB roller. Notice the "o" & "u" marks.



GL
John_AZ

I've seen that diagram before, but i didn't notice the dark spots representing the key holes. Thanks