Sure you're doing KPH?
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| Cause for incorrect RPM https://www.ausmini.com/forums/viewtopic.php?f=2&t=88980 |
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| Author: | iamstib [ Thu Mar 26, 2015 8:02 pm ] |
| Post subject: | Cause for incorrect RPM |
Hey guys was wondering what could be the cause for incorrect RPM? Im assuming its incorrect because im doing about 75kph and in 4 and im on 4500rpm (1330 with twin su's and i believe a 544/643 cam) Obviously the gauge could be faulty but what else? i wanna see if i can fix it up. |
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| Author: | Mick [ Thu Mar 26, 2015 8:06 pm ] |
| Post subject: | Re: Cause for incorrect RPM |
If it isn't jumping around, then it can be gauge calibration (Adjustable via an adjuster on the back usually and a known good reference), or it may be set to the wrong number of cylinders if it modern enough to be configured for different engine types via dip switches. A setting for 6 or 8 cylinders will give a slower RPM than it should be. |
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| Author: | Harley [ Thu Mar 26, 2015 8:42 pm ] |
| Post subject: | Re: Cause for incorrect RPM |
Sure you're doing KPH?
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| Author: | MiniVLT [ Thu Mar 26, 2015 11:14 pm ] |
| Post subject: | Re: Cause for incorrect RPM |
Are we talking std instruments or after market here? Do you know what your diff ratio is and tyre size? You may well be doing 4500 RPM but your indicated road speed will depend on the above + speedo drive gear combo! Cheers Shane |
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| Author: | peterb [ Fri Mar 27, 2015 6:26 am ] |
| Post subject: | Re: Cause for incorrect RPM |
You should just find someone with, or go and get one, an elcheapo tune up kit. Most have a tacho set up to compare with yours, you don't need to be driving it, just bring revs up slowly and compare readouts. If its right then you'll be needing a diff. 
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| Author: | Thorlek [ Fri Mar 27, 2015 8:26 am ] |
| Post subject: | Re: Cause for incorrect RPM |
ebay is good if you just want something cheap to check it against. you can pick up really basic digital tacho's for a few bucks, tho you might have to wait a few weeks for it to arrive from HK/China |
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| Author: | 9YaTaH [ Fri Mar 27, 2015 9:17 am ] |
| Post subject: | Re: Cause for incorrect RPM |
Instrument correctly wired up? |
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| Author: | iamstib [ Fri Mar 27, 2015 6:08 pm ] |
| Post subject: | Re: Cause for incorrect RPM |
Well its been in for 15 years.. So id assume so? So worst case its the diff? How much is it to replace one of them roughly? |
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| Author: | 9YaTaH [ Fri Mar 27, 2015 8:38 pm ] |
| Post subject: | Re: Cause for incorrect RPM |
iamstib wrote: Well its been in for 15 years.. So id assume so? So worst case its the diff? How much is it to replace one of them roughly? Worst case unlikely...leave your gearbox alone but get someone to check the calibration of your Tacho and its wiring...or borrow another and temporarily try that... |
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| Author: | drmini in aust [ Fri Mar 27, 2015 9:54 pm ] |
| Post subject: | Re: Cause for incorrect RPM |
What speedo is it? I have a 200KMH Minispares Cooper S one in mine, it is reading slower and s l o w e r with age. Only done 20K Km or so, IMO these aftermarket pretend Smiths speedos are rubbish. If yours is MPH, not KMH though it's reading about right. As for the gearbox's speedo drive gears, in an Oz built Mini they are all the same. They changed the speedo head for different diff ratios instead. |
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| Author: | iamstib [ Sat Mar 28, 2015 11:27 am ] |
| Post subject: | Re: Cause for incorrect RPM |
I dont think its the speedo.. Speedos a touch off but compared to the gps speed app on my phone its close.. Its gotta be the tacho cuz it changes when hot weather too.. Today i was in 4th doing 35mph and it was at 5k.. Maybe time to get it rebuilt |
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| Author: | drmini in aust [ Sat Mar 28, 2015 11:41 am ] |
| Post subject: | Re: Cause for incorrect RPM |
If it is an old Smiths one yes they change with weather etc. I had one in Barney, it looked good but the readings were rubbish. New Smiths ones may be better, I dunno. I'd buy something else. Years ago I put a Veglia Borletti (ex Fiat 124BC) tacho in mine, it's well damped and reads correct. |
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| Author: | Mick [ Sat Mar 28, 2015 12:11 pm ] |
| Post subject: | Re: Cause for incorrect RPM |
New ones are pretty good being stepper motor driven. My old Smiths drifts by about 800 rpm between a hot day and a cold one 
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| Author: | 9YaTaH [ Sat Mar 28, 2015 1:21 pm ] |
| Post subject: | Re: Cause for incorrect RPM |
drmini in aust wrote: What speedo is it? I have a 200KMH Minispares Cooper S one in mine, it is reading slower and s l o w e r with age. Only done 20K Km or so, IMO these aftermarket pretend Smiths speedos are rubbish. Wear bigger boots or get a big moon go pedal Doc...that'll stop it going slower 
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| Author: | drmini in aust [ Sat Mar 28, 2015 1:34 pm ] |
| Post subject: | Re: Cause for incorrect RPM |
9YaTaH wrote: drmini in aust wrote: What speedo is it? I have a 200KMH Minispares Cooper S one in mine, it is reading slower and s l o w e r with age. Only done 20K Km or so, IMO these aftermarket pretend Smiths speedos are rubbish. Wear bigger boots or get a big moon go pedal Doc...that'll stop it going slower It's getting a Moke 140KMH (800 turns/km) one. I should be able to wind that off the clock in 3rd gear.. on a track, of course!  [edit] I pulled the 200KMH speedo out today and I think found the problem... The threaded bit of the diecast housing where the cable nut screws on has broken right off. The bronze bush inside has moved outwards, probably bringing the magnet bar with it. Less magnetism into needle drum = speed reads low. Moke speedo is mounted now instead.
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