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fan belt runoff
#1
Having just started my engine for the first time since rebuilding it I find the fan belt doesn't stay within the edges of the crankshaft pulley. The belt runs onto the edge, back towards the engine. Doesn't run right off, but it's not right.

Its a new belt, I think bought from David at Kirby Wiske about 30 years ago.

And a new pulley bought at the same time.

I've tried fitting the belt the other way, front to back.
Tried with the belt tighter, and less tight.

Any ideas please?
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#2
Is the bottom pulley on square?
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#3
(04-01-2020, 04:42 PM)Duncan Grimmond Wrote: Is the bottom pulley on square?

Yes it is.
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#4
Are the top and bottom pulleys in line with each other?
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#5
Have you tried pushing the top pulls assembly towards the rad.
I am always interested in any information about Rosengart details or current owners.
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#6
I understand that pulleys for flat belts have a slightly domed section rather than being completely flat, or perhaps I should say cylindrical. If correctly set up the belt will run straight without the need of the flanges at the sides of the pulleys. With use the pulleys wear and become flat or indented, and particularly if one is more worn than the other, the blt is unlikely to run straight. With a new bottom pulley, as suggested check if it runs true, but also check if it runs in line with the top pulley by putting a straight edge across the face of the bottom pulley. It may well be possible to improve the alignment by a small movement of the top spindle. You refer to a new belt bought 30 years ago, in which case it is no longer new and may have reduced flexibility which will not help the situation. Not also that the belt should be just tight enough to turn the fan without slipping. Over the years I have encountered many tight belts, and when questioned the owner has said 'the fan belt has to be tight doesn't it?' having previously had a car where the belt drives the dynamo as well as the fan. For the Austin Seven a flat belt with minimal tension is perfectly satisfactory.
Robert Leigh
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#7
Your pulleys will not be on the same axis as each other, if the spindles are not parallel with the pulleys in line then you will get this problem.
My dads old 33 box did this for years he just reset the tension with it running on half of the cam pulley and changed the belts when they eventually shredded themselves.
When the engine came out for a rebuild he made sure all the three castings were in correct alignment and the problem went away.

The most likely culprit is the starting handle casting not being in line with the top of the crankcase allowing the dynamo housing to to tip
If this is the case then the dynamo gear will not be at its optimum mesh with the cam gear.
The only way to check is to remove the dynamo and housing.
The nose piece could be up down or slighty rotated.
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#8
When I finished my restoration of my early type 36 Ruby 15 months or so ago I had the same problem.
The original fan belt I fitted came with the car “new” in its original Ferodo box .....but 30+ years old, this constantly ran off the back of the top pulley which was also original, in the end it disintegrated before the car was eventually back on the road. 
I chose to replace both the belt and the top pulley, with no other alteration to alignment, it all runs fine.
Having not found any discernible differences between the new and original pulley, my guess was the rubber in the original belt had simply perished.

Freudian slip, I did mean bottom pulley.
Apologies.
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#9
Thanks for the helpful ideas everyone. At the moment I think I have fixed it, though time will tell.

Checking alignment of pulleys with a straight edge I found the front faces of the pulleys are aligned, but not the back. The top pulley is wider and could take a wider belt, so alignment depends on where the belt runs within the wider pulley. If the belt was to run towards the front of the top pulley all would be good, but it always seems to run towards the back of the top pulley, and then the belt works its way onto the edge flange of the lower pulley.

Interesting the suggestion of having the belt slacker. So I slackened the adjustment even slacker than I had tried earlier. Yay! The belt now runs towards the front of the top pulley, but not against the edge flange, and the belt rides correctly on the lower pulley.

Thanks again.
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#10
check your bottom pulley

if the nut pulls the thread on the cam all the way through. OR NOT?

at the time you bought your pulley and cam belt. kirby wiske was buying these parts from a much cheaper supplier. alot of tapers on the pulleys were not machined deep enough.

this would push your pulley forward.

i take it if you have  a wider top pulley, it is the steel late ruby type. not an aluminium sports type.

tony
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