Jump to content

Tachometer identification

Recommended Posts

Hi all

Back in September I managed to find a TR3A that was in need of some TLC and last week after a very enjoyable winter tinkering in the garage it sailed through the MOT.

So the plan is now to use the car and one by one resolve the remaining issues, a sort of rolling restoration. The first of which is the tick over, the engine runs extremely well, I have rebuilt the carburetors and distributor, checked for air leaks and and all appears OK but I cannot get the idle speed below 1000 rpm even with all the linkage disconnected.

Today I took a step back and went to my neighbour who also has a TR3A (it turns out to be only a couple of weeks older then mine) listening to the two cars ticking over side by side my car sounds slower yet the other cars rev counter shows 800ish whereas mine shows 1050!!

So now to the question, my tachometer has a reference number on the face which is partly obscured by the centre of the needle it reads RN14??/09 can anyone confirm if this is correct? I should add my neighbours tachometer does not have a reference number but looks the same.

As an aside if anyone has any info about 931JKK's past I would be most interested in hearing it.



Link to post
Share on other sites

Hi George ~


Here's a shot of my tacho. After I fitted a 2.2L engine she would not idle below 1,000 rpm which was far too high.

Intricate adjustments to the timing and carburation eventually cured it. But it does take some doing. But it can be

achieved. The TR now lies in state in my garage as I've lost interest in it!




Link to post
Share on other sites

Hi George,

you say you have rebuilt the carbs. What did you do.

Did you replace the spindles for the butterflies. Where the holes in the body OK. Air can get in here and cause chaos.


Did it tick over OK before you started tinkering.?



Link to post
Share on other sites

Some multimeters have a tacho function as well. Try that first. My car's tacho points at 1200 rpm when it is 900 in reality. I can live with that knowledge now.


If that doesn't help (both are too high) try this. Put the tip of your show UNDER the throttle and lift it. If the revs fall, you know that there's something not closing correctly, leaving 'room' for higher revs.



Link to post
Share on other sites


Thanks for the photo I can just make out RN and what looks like a 9 at the end of the ident number under your needle and more importantly I just noticed the ratio is marked on the face which matches mine so it looks like the right instrument.

I will persevere with trying to get a smoother slower idle.

One of the other issues with my car that I plan to rectify next winteris the crank shaft pully appears to be 60°ish out from TDC so setting ignition timing is interesting,


When I first had the car the carburetors were a mess with very worn bodys and would you believe very rich needles (KD) to compensate

I had the body's rebushed by Carburetor Exchange and fitted new spindles, SM needles, jets and butterfly disc's I have also replaced manifold gaskets and checked with carb cleaner.


Its interesting that your tacho reads fast, my initial thought was I had the wrong revcounter but from Toms and your info it appears it is just inaccurate.

I am on good terms with my neighbour (I help service his TR3) so I will test my tacho on his car




Link to post
Share on other sites

Hi George no its not about been oval,the bushes that are supplied are just a straight cut,like a tube cut in the vice with an hack saw so has a square end ,when this is pressed into the carb it has to have a radius filed to match the internal curvature of the barrel of the carb otherwise you have a air route x2 through, even if the butterflies are shut tight.


Link to post
Share on other sites

I bought one of these to confirm lathe/drill/mill spindle speeds and have subsequently checked them with a loaned chronometric Smiths tacho - they agreed.


Simple to use the laser thing - with a TR the problem is where the reflective tape will go on a rotating item - I got a contact on the crank extension from underneath.


Peter W

Link to post
Share on other sites

An olde skool engine analyzer will have a tach option, driven by the coil spark. Many of us will have had one of these around for 40+ years -- our family's old one is in perfect condition and kept with my test gear.


They cost nuthin' used -- as in this example, one of many:




Edited by Don H.
Link to post
Share on other sites
  • 2 weeks later...


An update on my tachometer question

Parky very kindly loaned me his electronic tacometer and a dwell meter/tacho instrument and the results are

mechanical tachometer 1100rpm at idle, and both of Peters guages 750rpm.

So as Menno stated these clocks are not very accurate

Thank you all for your advice on this issue



Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

Please familiarise yourself with our Terms and Conditions. By using this site, you agree to the following: Terms of Use.