1982 CB750F...Better Devil

pidjones

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Ok, think about it. The engines spin near 10 k. Do you think they want the cable spinning 40k RPM?
 

Jimbonaut

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pidjones said:
Ok, think about it. The engines spin near 10 k. Do you think they want the cable spinning 40k RPM?
Sorry mate, mind expanding on that?

Feel like I should know what you're talking about but, don't.

4:1? 1:4? Both are options apparently, as i said.
 

adventurco

Nick Ol' Eye
DTT BOTM WINNER
I believe that a 4:1 ratio means for every 4 revolutions of the engine, the tacho cable or whatever gears inside the tacho that register the RPMs would do one revolution. So if you're doing 8k RPM, the cable is spinning 2k times per minute.

In a 1:4 ratio, the cable would be turning 4 times for every 1 revolution on the engine. So at 8k RPM your cable would be spinning 32k times per minute.
 

Jimbonaut

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Ok, thanks for that explanation. Still none the wiser though as to which ratio I need for the bike - some sites say 1:4, others 4:1. Pidjones - I'm thinking about it, just utterly non-plussed.
 

teazer

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Are we sure that any tacho runs faster than engine speed? That makes no sense. I suspect that 1:4 means one tacho cable revolution per engine revolution and 4:1 means 4 engine revolutions per tacho revolution. The same thing.
 

Jimbonaut

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teazer said:
Are we sure that any tacho runs faster than engine speed? That makes no sense. I suspect that 1:4 means one tacho cable revolution per engine revolution and 4:1 means 4 engine revolutions per tacho revolution. The same thing.
Yeah, it's crazy pills time. What's thrown me is that I bought a 1:4 tach from Dime City for another DOHC Honda (they list all their tachs 1:2, 1:4, 1:7 etc etc), and yet I contacted the seller of the Triumph repro tach (in the image a few posts ago) that told me it was a 4:1 ratio, and that 4:1 and 1:4 were not the same thing. He told me the tach would not work on my bike for that reason.

WTF?
 

adventurco

Nick Ol' Eye
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The Jimbonaut said:
Yeah, it's crazy pills time. What's thrown me is that I bought a 1:4 tach from Dime City for another DOHC Honda (they list all their tachs 1:2, 1:4, 1:7 etc etc), and yet I contacted the seller of the Triumph repro tach (in the image a few posts ago) that told me it was a 4:1 ratio, and that 4:1 and 1:4 were not the same thing. He told me the tach would not work on my bike for that reason.

WTF?
I think you need to buy and test them both so we can figure this out once and for all ;D
 

Jimbonaut

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advCo said:
I think you need to buy and test them both so we can figure this out once and for all ;D
That. Then maybe I can finally answer the question pidjones keeps asking me ::)
 

Jimbonaut

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Had to google speedhut - ah to have deep pockets, then lots of things won’t matter.


Sent from my iPhone using DO THE TON
 

Jimbonaut

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Thanks for all your input guys. There seems to be a bit of a conflict surrounding the ratio, someplaces it's listed as 4:1, others as 1:4. What compounded the head*ck - for me anyway - is that I was told by a seller of a 4:1 ratio'd tach that it would not work for me as I needed a 1:4, and the two ratios were not the same thing. Screw it.

The voodoo- hoodoo- what you don't dare do- people.
 

Maritime

Well-Known Member
get an electronic tach and plug the tach hole and be done with ratio's all together.

Edit* I have a cheap china one and it works fine, every so often my engine revs to 1500000 rpm for a second but generally shows the correct revs LOL. There are good quaity for decent prices and all you need is to wrap a lead on the coil or plug wire and your done.
 

Jimbonaut

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Thanks for the suggestion Mike, I'll look into them. Does yours have a digital display? Do all the electronic tachs have digital displays?
 

irk miller

You've been mostly-dead all day.
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Here's the gauge cluster for a stock 1970 Triumph Bonneville. Notice the ratio printed on the face of the gauge. Homeboy that is arguing this info is an idiot.

 

Jimbonaut

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DTT SUPPORTER
Thanks Irk - the dude telling me that 1:4 is not the same thing (and therefore non-compatible) as 4:1 has really thrown the cat amongst the pigeons, yeah. He's the seller of the Emgo Triumph repro tach (which is 4:1), and he's the guy telling me it won't work on my ride (which, I've been told, is both 1:4 and 4:1 - depending on who you ask).

So I guess the million dollar question is - is 4:1 the same thing as 1:4?
 

Maritime

Well-Known Member
The Jimbonaut said:
Thanks for the suggestion Mike, I'll look into them. Does yours have a digital display? Do all the electronic tachs have digital displays?
You can get analog dial with needle that run off spark readings. It just might save you headaches. May make things worse IDK for sure LOL.
 

Maritime

Well-Known Member
There are better ones than this but here is a 5 sec ebay search for one:

https://www.ebay.com/itm/NEW-Style-Universal-Motorcycle-Tachometer-Tacho-Gauge-Speedometer/172870436378?epid=2250903503&hash=item283fe1861a:g:eek:jkAAOSwlddcdobm
 

irk miller

You've been mostly-dead all day.
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When the two CB750's that I currently own and the other that I did own changes from 4:1 to 1:4, I'll let you know. ;)
 

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