I'm struggling to keep a 77 308 GTB from jumping time. It only does it on the rear bank and this time its cost me 4 intake valves (luckily no piston damage). I've followed the guides I've seen on here and the front bank (firewall side) always seems fine but I've had the rear bank come out of time twice now in a few months. The drive sprocket at the bottom doesn't appear to be loose. Is there some other component that could cause this to come out of time or am I just failing? The most recent time it had stayed in-time for a few warm up's and small test drives. I was finally starting to feel alright with it and ran it moderately to 5k and when the engine unloaded from letting off the throttle to shift it jumped. Just puzzled about this, I've done a lot of timing belt jobs on other vehicles without issue. But none of the others have had the crank side of the belt turned by something that wasn't the crank either.. Any help is appreciated, Thanks!
Relatively useless comment on my end: My car is also due for a belt job and I just received the parts in the mail the other day. I was going to tear into it as soon as the weather turns and the car is off the road 'till next year. I've read https://www.birdman308.com/service/timing_belt/timing_belt_procedure.htm and it looks like a relatively easy job to me. Now you've got me nervous! The biggest thing I wonder about based on your post is if you're tensioning the belts enough though. Sounds to me like something that would happen if the belt had a bit too much slack in it.
"Front bank" and "rear bank" can mean different things depending on where one is standing -- much better to say "1-4 bank" and "5-8 bank" to be clear about things. What version 1977 308? Is it a US version with its air injection pump still present (if the trouble is on the 1-4 bank)? Also, are you super-sure that the cam caps are in their correct locations and orientations?
You say "jumping time", I presume that means it is not breaking the belt or shearing teeth? I'd be wondering about the tensioner (full rang of motion, spring strength, tensioner bolt holding), the mounting of the cam sprockets, and the drive sprocket (you've checked that).
Have you fitted a new tensioner or at least taken it apart to check the spring movement. Assume you did the bearing and old also seemed fine ?
Thank you, this would be the 1-4 Bank. There used to be an air injection pump but it was removed before I got to it. The ports are blocked off in the head as well. I believe the cam caps are in their correct position - is there any specific methods to verify that I could use to be certain? It'll all be pulled apart at this juncture
It had sat for some time (8 or more years), got it back to running condition briefly and then had the belts replaced to avoid damage but that didn't quite work out
If the belt is not broken or stripped teeth My "guess" would be belt too loose. If you've done a few belts prior I would say it is probably is the tensioner or tensioner assembly.
There should be a number stamped on each cam cap and on each boss that the cam cam mounts onto - the (same) numbers need to be adjacent to each other (i.e., not only does the cam cap need to be in the right position, it needs to not be turned around 180 deg).
Maybe a little more info. Is it just the intake cam that shifted? Is the vernier retainer bolt installed?
Matt when you change the belts for both front and rear banks did they feel like they have the same tension or is the rear bank feel a little looser?
The teeth on the sprockets and belts all appear intact. It has not broken the belt either of the times, just jumped out of time a handful of teeth
You are locking the tensioners by their bolt after rotating the crank to the best position to tension them? Not relying on the spring to keep them tensioned.
As several have already mentioned I would put the tensioner as suspect #1. Do both cams go out of time (equally or not?) or just one cam? One of my heads had been butchered at some point, cam caps where off a different engine as witnessed by having some of the same cap numbers as the other head, some re-stamped with 1/4" sized numbers and some just had punch marks. The numbers should go #1-20 in order starting by cyl #1 on the exhaust cam. The 5/8 head should have caps #11-20. The numbers are very small (2-3mm), stamped outside the engine along the rim that supports the cam cover and on the cap adjacent to the number stamped outside on the head. Despite having non original caps my cams turned over by hand with the same resistance as the head with original caps.
"You are locking the tensioners by their bolt after rotating the crank to the best position to tension them? Not relying on the spring to keep them tensioned." Frankly, this sounds like the problem to me - tensioner not locked down in the correct position (or not locked down at all)! As noted, if the tensioner is not locked in position, a cam belt could jump between acceleration/deceleration because the spring would compress under the changing load and release tension on the belt for a split second. The spring is only there to initially determine the proper position for the tensioner and then the tensioner is locked down. Per Ferrari, after installing the belt, the engine is rotated manually with the tensioner bolt loose through several complete revolutions and then the tensioner is tightened in place. There are other methods used to determine proper tensioner position but, in any case the tensioner is locked down and the spring has no function at that point.
Another issue with not locking down the tensioner: The tensioner is spring loaded. It has natural frequencies one of which could be getting excited during changing engine speeds. Were that to happen, the tensioner could easily act as though it is not there.
And I will add anyone who relies solely on the spring to tension a belt is a fool. The spring does not create belt tension. The tensioner is locked in place and the spring is out of the equation.
