My situation... rear bank belt skipped about 4 teeth over intake cam sprocket ...fixed then happened again. First time: engine at PM 1-4 & cam cover off, I removed the tensioner bearing, allowing the belt to loosen enough to turn the cam to where the marks lined up. Put all back together, etc. fired it up & ran great (new territory for me so felt good). Did a very short drive, engine up to temp & idling on driveway ...gave it a little gas then started backfiring/not responding again... exact same circumstances as 1st time it happened, and exactly the same cause. OK... a couple of things I should have done, such as replace belt & rebuild tensioner w/new spring ...although belt seemed well tensioned according to notes from Birdman (& others). Assuming I do both these second time around, wondering what else I need to do to properly fix the problem? Cam sprockets were replaced 2 years ago along with belts (very minimal driving since) ...no obvious visible damage, a couple of tiny scratches. Existing belt looks fine also. Tensioner bearing looks & feels fine but I could certainly replace. Drive sprocket bearings: looks like my former mechanic replaced at least the outer but for some reason I don't have that documented. (has zero play in/out, tiny amount in rotation, no obvious noise when running) Any good advice very welcome as always... Image Unavailable, Please Login
sorry to hear. how long ago were the belts done? correct belts? correct sprockets? I see a part of the cam pulley that is not touched in your photo
Most likely the belt was not tight enough. Make sure in your tensioning procedure that you lock the tensioner at the point where it is tightest (most extended).
Did you lock the tensioner? As suggested above, turn the crank by hand and at the point where the spring has pushed the tensioner bearing the furthest towards the belt, lock the tensioner. You were lucky as a lagging exhaust cam would probably wack the exhaust valves and break off their heads.
I was thinking the same thing. I have never read Birdman's procedure. However, once you line up the timing marks ( new belt in place) you should rotate the engine and lock the cam tensioner at its most extended position. Never re-use a belt, especially if it has skipped a few teeth.You will probably be lucky not to have tipped a few valves. Good luck
Thanks for the fast responses... New belts 2 years ago... correct belts & sprockets as far as I know (I trusted my mechanic as much as I could trust anyone... 40 years wrenching/restoring all kinds of Ferraris) I thought it normal that the belt doesn't contact the entire width of the sprocket (?) Also thought I followed the right procedure for tensioning the belt (as mentioned above) ...then I tightened the tensioner bearing at 48 lb/ft (to be precise) Hand turned the motor again to check consistent tension throughout revolutions ...seemed good, could only twist belt about 45 degrees on longest stretch for example. Could easily replace tensioner spring this time in any case. Still not sure if there's any issue with the drive sprocket tho' ...obviously I'm on the learning curve here.
Good luck there are more experienced with belts here than me. But just check everything out. Make sure the sprockets are correct and get new belts and s tensioner spring. And maybe new tensioner beatrings. They look like hill bearings. Better than a new motor.
Hmmm...I must disagree re belt tension. If you lock the cam tensioner at the tensioner's most extended point, you are setting belt tension at the loosest point of the belt's run. Depending on the play/tolerances of the cam pulleys/belt in the specific engine, this can result in the belt being too tight and possibly damaging the belt as the run gets to the tightest point. Follow the procedure in the Ferrari manual. There is no reason to pay any attention to what the tensioner does as far as moving in/out. As you rotate the engine several turns, the spring in the tensioner "automatically" adjusts the belt tension. You don't have to set anything at a specific spot - just let the spring do it's job and, after a couple of revolutions, tighten the tensioner bolt. The cam belt tension is now correct. It is neither too tight nor too loose.
OK! Can't argue with Ferrari's bulletin! I had not seen it but it's their engine! Do it as Ferrari states!
No, the belt doesn't contact the entire width of the pulley as the pulley is about 4mm wider than the belt. I would change the belt as they are cheap and yours may have been a bit damaged (even if it looks perfect.)
Thanks, although the Bulletin refers to 4 valve engine, but would make sense on 2 valve as well right? I'll certainly replace belt & tensioner spring then follow those notes. I'm still puzzled why the problem occurred the first time tho' ...engine ran fine for months since my mech. rebuilt all these items. I was running engine in intervals up to 4 weeks during the winter, & car was stored in garage with temp. sometimes as low as 40F. As mentioned before, tensioner bearing & drive sprocket seem good (but I'm no expert)...
IMO, you should confirm that when you apply that torque to the nut that it actually clamps the tensioner bearing well. For example, if the threads are damaged, or if the threaded portion of the bolt is the wrong length, or if the tensioner bearing bore is galled/hanging-up on male boss that it fits over, the relationship between the torque applied to the nut and the clamping force could be not as expected (or non-existant ). Has to be something like that.
Thanks Steve, will look closer... once the nut is tight though, I assume the the belt should feel consistently tensioned when hand turning the engine?
Yes, it seems counter-intuitive, but it has to do with ensuring that the average belt tension is higher. If the tensioner bearing is clamped at the maximum extension, then the tension on the belt is higher at every other position (so the average belt tension is higher). If the tensioner bearing is clamped at the least extension, the instantaneous tension in the belt at that position (from the spring before clamping) is slightly higher than at maximum extension (from the spring before clamping), but (after clamping) the belt tension at every other position will be lower (so the average belt tension is lower). Another way to think about it is that when the tension bearing mechanism is at maximum extension = the longest belt path length (so stretches a fixed length belt the most) = highest average belt tension.
More importantly, once the nut is tightened, you should not be able to move/slide the tensioner bearing mechanism under forces even much higher than the belt tension.
Makes sense ...will renew the tensioner spring first in any case. Again, neither drive sprocket or tensioner bearing have any 'play', so I doubt these are at fault... Thanks again!
Hate to bring this up, but had you (or your Mechanic) just reinstalled the cam caps before the first "skip" event occurred?
Fair question but no ...cam caps were in place & likely checked/ re-tightened by my mech. 2 years ago, engine running v. well up until first event a few months ago.
To Steve's point about nut torque vs. actual clamping, I noticed the yellow paint indicates the nut is about 120 degrees short of it's previous tightened position. Is the photo of it loose? It might simply be the belt was too loose from locking the tensioner in the least tight position but if not too much work I'd look under the cam caps for wear. One of mine had been stamped on the wrong side so clamped the cam once tightened in place. I turned the cap around so that the stamped number was on the inside and it worked like a charm! Since there was no sign of wear on the cap or the head side I can only guess it was put on backwards before line boring.
I actually didn't dare to mention seized camshaft bearings. BTW; did anyone of you ever hear about lost aluminium plugs on the opposite side of the camshaft? On my first big service I found all three (single distributor car) loose on their way to get lost, what would mean close to zero oil pressure on the camshaft bearings. Removed them, cleaned all things and reinstalled them with green Loctite. They probably get loose over time because of the different heat expansion of camshaft steel and plug aluminium. Worth checking each time when at least the front covers are removed. Regarding the QV-procedure; I would not clamp the tensioner at the max. belt tension because of the weaker camdrive bearing design of the earlier cars. I stick with the early carb car procedure described in the manual and never had any problems, though one belt -can't remember which one- is always slightly tighter than the other. Best from Germany Martin
Yes, tensioner nut loose in photo. Regarding tightening, guess the QV bulletin was for QVs only ...think I'll try again with the original 1 rotation procedure. Waiting for the parts now.
Well, I'm going to throw this wrench into the belt tightening mess. This is what the workshop manual for the 308 QV and the 328 says about belt tightening: Image Unavailable, Please Login Note the procedure does not say anything about tensioner extension. Yet the manual was written after the service bulletin I posted above. This speaks to the fact you generally can not go by the WSMs because they are typically incomplete. The SB I posted above is about replacing lower quality belts, that were installed on early QVs, with higher quality belts. My interpretation of the belt tightening paragraph is that it is the procedure Ferrari recommends for tensioning belts on all 308s. Your mileage may vary. Interpret as you see fit.
That's true, but it doesn't guarantee that the material in the later manual was written (by an engineer) after the SB. The procedure you posted from the 308QV/328 WSM matches what's in the Mondial8/QV WSM (which pre-dated the SB). Very possible IMO that a documentation person wrongly copied the Mondial8/QV text (which was the first manual for the 3.0L 4V engine) when assigned to make the 308QV/328 WSM, and wasn't aware of the SB. They did improve the English somewhat, but the technical details are identical. Just a thought (having seen what can go wrong in the F documentation )...
