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This evening I tested the 75 rev counter in the GT housing and whilst it was fine, it is clear the revcounter needs adjusting.

[/url]IMG_4422 by AJSG1969, on Flickr[/IMG]

I was wondering if anyone has done this as I have absolutely no experience on this. I put the needle back on when there was nothng attached but it seems to under read. I am using the original Alfa 75 needle for testing.

Video of the rev counter

This is the image of a Rheostat that might need twiddling

[/url]Rheostat by AJSG1969, on Flickr[/IMG]
 

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There's two general scenarios about how the tacho is misreading:

1. a fixed value (eg: needle shows 1000rpm when engine doing 2000rpm; needle also shows 3000rpm when engine doing 4000rpm; thus, 'out' by 1000rpm): simply need to reattach needle on the spindle in the correct position
2. a proportionate value (eg: by 20%; needle shows 1200rpm when engine doing 1000rpm; needle also showing 6000rpm when engine doing 5000rpm): presumably adjusting the rheostat will fix this

If you need to actually know how fast the engine is running, one way is to use an 'ignition timing light/gun'. Buy/borrow one that shows rpm on a display. They have a 'clamp' that you attach around one of the spark leads, and it detects the electrical pulse/spark.

:)
 

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I have recently adjusted mine in my 1989 75. I was able to partially dismantle the dash so as to get access to the rheostat. Then, while the engine was warm and idling, I set the idle speed to 800rpm. The adjustment is made simple by putting a flat screwdriver in the top of the rheostat (as it is when installed) and turning it.

Mine used to hit the rev-limiter at 5k rpm when it should have been reading 6.6krpm. It now reads about 6.7k when the rev-limiter kicks in but that is close enough for my needs. The adjustment is very fine i.e. very easy to move it too far.
 
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