Tacho

modelman · Post by **modelman** » Mon Sep 14, 2015 10:08 am

Gentlemen, I am in need of your expertise once again, upon starting the CB77 it 'raced' up to around 2000 rpm, managed to sort it by carb adjustments, ticks over nicely now, but I notice that even on tick-over the tacho needle sweeps right round & stops at about 6-o'clock, what would that be, about 20,000 rpm??? I don't think so.
Its the one thing I haven't touched (yet) so, at least I know it isn't seized ! Anything I should look out for?
Its has a brand-new cable & the worm-drive is nice & free, & I'm guessing its at the head-end.

PhilD · Post by **PhilD** » Mon Sep 14, 2015 3:28 pm

Hi modelman, glad to hear that you've sorted the high idle problem. The tacho problem sounds like a broken spring inside the meter. Its fairly easy to dismantle the unit to look inside for the cause, there's plenty of information on the forum on how to do this and it won't be a problem for someone with your obvious skills. I recently stripped mine down to sort out a seized tacho and reface the dial, the trickiest bit was getting the needles off without damage but G man describes a good method using two teaspoons after protecting the dial, just be ready to catch the b*****s as they tend to fly a long way.
PhilD

G-Man · Post by **G-Man** » Mon Sep 14, 2015 4:41 pm

Modelman

If you get the unit apart - I have plenty of spares and can either help you with replacement innards or talk you through what to look for.

G

PhilD wrote:Hi modelman, glad to hear that you've sorted the high idle problem. The tacho problem sounds like a broken spring inside the meter. Its fairly easy to dismantle the unit to look inside for the cause, there's plenty of information on the forum on how to do this and it won't be a problem for someone with your obvious skills. I recently stripped mine down to sort out a seized tacho and reface the dial, the trickiest bit was getting the needles off without damage but G man describes a good method using two teaspoons after protecting the dial, just be ready to catch the b*****s as they tend to fly a long way.
PhilD

modelman · Post by **modelman** » Tue Sep 15, 2015 1:32 am

OK gents, thanks for that, it looks like the instrument strip down is coming up as soon as the tail-fin is finished.

Next question, is there any 'pre-load' on the needles upon re-assembly?

I have found in the past that these old eddy-drum units require a small load/tension against the zero stop pin, & have needed several fittings on the locating pin to get the readings right.
I once rebuilt my Bonnie speedo & simply pressed the needle against the pin & it then read 50 when it was doing about 30!!

G-Man · Post by **G-Man** » Tue Sep 15, 2015 1:45 am

Modelman

I have not noticed any need for pre-load. I have tested the units I have fixed against a NOS speedo / tacho and they compare well.

G

modelman wrote:OK gents, thanks for that, it looks like the instrument strip down is coming up as soon as the tail-fin is finished.

Next question, is there any 'pre-load' on the needles upon re-assembly?

I have found in the past that these old eddy-drum units require a small load/tension against the zero stop pin, & have needed several fittings on the locating pin to get the readings right.
I once rebuilt my Bonnie speedo & simply pressed the needle against the pin & it then read 50 when it was doing about 30!!

modelman · Post by **modelman** » Tue Sep 15, 2015 2:05 am

Magic, thank you very much, its great to tackle a new job with a bit of knowledge gleaned from you experts.

G-Man · Post by **G-Man** » Tue Sep 15, 2015 5:54 am

Modelman

We're all just learning all the time. The more we share, the more others share with us....

Today I'm meeting someone from the forum to swap a few early parts. The Forum is a great help all round.

G

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