Tooted belt slips on 4,5l - M28/01
 

Hi. I have during this autumn had the engine out of my ´78 928, for an overhault and new cam belt and so on. After having driven the car for tree times with no problems, the 4´th time it did not start. I heard someting similar to a "puff", and the engine would not start. After some time I found out that the toothed belt had slipped over 6 steps on the right hand side on the cam and 4 steps on the left hand side. Now we have retimed the engine, and it starts and runs great.

Video:
Is there any one having any thoughts on what is wrong here, or what could have gone wrong?
Thanks! - Kind Regards Finn Erik

please take a picture of the belt and the cam pulley,

Might you have a round tooth belt and square tooth pulleys.
What was the last thing you fixed

Hi - have both the square belt and pully. The last thing we did was the toothed belt and water pump, with all the other belts, hoses and gaskets on the engine. We pulled the engine out to do this.
https://cimg5.ibsrv.net/gimg/rennlis...d1b45b75f3.jpg

https://cimg3.ibsrv.net/gimg/rennlis...86636c5a07.jpg

So where do you think you've screwed up?

...Yes, That's the question!

Missing the circlip behind the main roller.

Ok, what do you mean is the main roller? The crankshaft?

Sean, do you mean that missing the circlip is making the main roller looked "behind" the belt or rotating towards the block closer than it should as it appears in the picture?

-Hoi

Circlip is #4. Factory part no. N0423641

http://ab060d1e38dfcecd7276-1ee42b6e...com/103-10.gif

The missing circlip will not cause the belt to jump. Anything fell between the crank or cam gears that made the belt slip? Happened to me once, a piece of the timing belt cover broke off and wrapped around the crank gear. Every time the crank wen around the belt slipped a little.

Thanks, that might just be it! There was some demage On one of the covers...

I'll go out on a limb here a bit, and suggest that you had an intake backfire (that whummpf noise) that rotated the crank backwards a little as you cranked to start. You'll want to find the cause of that backfire. Might be over-fueling, for instance.

Thanks. Have to look into this :)

This happened to me twice. Once was because I used a Conti belt which stretched in the first 100 miles or so as they often do. Once was because the plastic bushings that the tensioner arm pivots on were worn.

Thanks, I will check both these tips! 👍🏼