Mechanical tacho

[beamdream]

Having recently fitted the correct distributor with mechanical tacho drive to my Series 1, I thought all was well when I fired up the engine and the tacho responded nicley, going smoothly up and down the rev range whilst I blipped the throttle.

I then went for a a drive and at about 3000 rpm the tacho started screeching and pointer bounced wildly around the dial; went back home disconnected the cable from the distributor and connected it to my drill chuck, powering up the drill produced a perfectly smooth rpm reading, again up and down the rev range, no misbehaving.

Reconnected the cable to the distibutor, fired up the engine and tacho again worked smoothly up and down the rev range, now out for another drive, and you guessed it, once again its screeching and needle bounces all over the place from about 3000 revs and up - what am I missing ?? why should a distributor cable drive change its behaviour when the car is in motion ??

 

[Tom H]

My guess is lubrication issue. Both in car and drill driven it works fine for a while. Then at some critical point along the cable or in the tach, the friction causes heat build up and it starts to bind and screech and jump.

Two posibilities - one - the drlll test does not run long enough to re-create the problem. or two, when using the drill, the cable is less kinked and thus the critical point along the cable is less stressed.

Try re-routiing the cable slightly and also run some oil down the cable.

Tom

 

[65beam]

tach

several years ago i had a similar problem with the tach in the red harrington. the needle would sometimes stick. i sent it off to a guy that repairs this stuff. he found that the lube inside the tach had hardened and there was no lubrication which was jamming everything. it looked like grease that had all of the oil drained out and all that was left was the thickner. a good cleaning and lubricating and it still works.

 

[bernd_st]

Check the length of the inner cable protruding into the tach and drive . The lower drive may be clogged with some dirt reducing the free play . Heat may reduce play further...

 

[beamdream]

Thankyou all, without wanting to make the original post too longwinded I neglected to explain that the distributor had been stripped and rebuilt prior to installation, the cable and drive were also lubricated.

On the road, the problem occurs within a block of leaving home yet my in shed trials with drill and engine running it has gone for a good ten minutes without fault.

Tom - I thought of the entry angle for the cable being different with the drill driving it, but it still works ok fixed to the engine and car not moving; I think I might have just convinced myself to pull the instrument itself and strip it.

 

[beamdream]

[ I think I might have just convinced myself to pull the instrument itself and strip it.
[/QUOTE]

Update - small amount of spraylube applied to innards of the instrument seems to have cured the problem