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Old 04-29-2009, 07:20 PM   #1
Tommy2 OP
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Where to get a tachometer for motorcycle testing?

Need a tach to dial in my 2 cyl Honda CM400.

The closest I could find was at Harbor Freight, but it was only for 3-8cyl engines.
Can't seem to find anything for a 2cyl..

Been to the typical auto parts stores..No luck.

Will check Sears tomorrow..but not hopeful.


Any ideas?
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Old 04-29-2009, 08:18 PM   #2
tedder
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should be able to use a 4cyl and multiply/divide by 2. (can't remember which way). But as long as you can do the math in your head, you are good
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Old 04-29-2009, 09:56 PM   #3
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Briggs and Stratton use to make a tach that worked off engine vibrations.

I found one on fleabay.
http://cgi.ebay.com/Briggs-%26-Strat...QQcmdZViewItem
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Old 04-30-2009, 04:56 AM   #4
Tommy2 OP
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Thats certainly interesting..Hard to imagine how accurate it is though.

As far as using a 4cyl tach..I'm not sure its as simple as the RPMs being exactly doubled. (or maybe it is) not really sure.

I just thought I could go into any auto parts store and buy a tach for a 2cyl, no big deal.. Guess its harder than that.

It was the same thing when I was looking for a vacuum gauge a couple years ago. I went into about 4 stores before anyone even knew what I was talking about.
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Old 04-30-2009, 05:56 AM   #5
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Old 04-30-2009, 07:23 AM   #6
Tommy2 OP
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Well, thats for a 1cyl, and its more than I paid for the bike

Maybe it would just be easiest to buy a tach that came from a bike with the same engine..
I just thought it'd be easy to find a cheap tester tach with motorcycle testing. I guess no one works on motorcycles enough for such a product to exist in mainstream stores.
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Old 04-30-2009, 08:11 AM   #7
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Any electrical pickup tach will work if you're willing to scale the numbers (divide by two, multiply by two, whatever). You usually have to do stuff like this anyway because motorcycles tend to be waste-spark engines and fire twice as often as necessary.

- Mark
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Old 04-30-2009, 08:32 PM   #8
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Are you wanting a diagnostic tool or a tach to mount on the bike?

If you want a tool, harbor freight does make one:

http://www.harborfreight.com/cpi/cta...emnumber=95670

Otherwise google up "universal tachometer" and stuff like this pops up:

http://www.buzzillions.com/dz_426461...el-reviewTitle
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Old 05-01-2009, 05:07 AM   #9
Tommy2 OP
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Quote:
Originally Posted by GreaseMonkey
Are you wanting a diagnostic tool or a tach to mount on the bike?

If you want a tool, harbor freight does make one:

http://www.harborfreight.com/cpi/cta...emnumber=95670

Otherwise google up "universal tachometer" and stuff like this pops up:

http://www.buzzillions.com/dz_426461...el-reviewTitle
This is just for testing. The HF is for 3-8cyl engines. (like I mentioned in the first post)

I may buy it and try..I guess I would set it on the 4cyl setting and divide the RPMs by 2?? Or would it be by 4?? Is the CDI system on a 1980 Honda a wasted spark system?
Im just so surprised buying a tach turned into something so confusing! Used to working on cars where you just go in the store and buy one. 10 seconds later your done.

I looked on ebay for a tach for other twin cyl hondas. Havent found one yet. Just something I could temporarily probe for testing (again, not something mounted on the bike).
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Old 05-01-2009, 06:10 AM   #10
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I think I might have a link bookmarked at home, so I'll check on that. In the meantime I've found the following. They're similar to what I may have book marked at home, but I seem to remember there being a wider selection of tachs to cover a wider range of RPMs, cylinders, cycles and spark patterns.


Tiny Tach

ENM

These appear to be made for mounting, but they're relatively cheap and just wrap around the plug wire, so they could be a temporary solution.
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Old 05-01-2009, 07:50 AM   #11
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Quote:
Originally Posted by Mr. 0
I think I might have a link bookmarked at home, so I'll check on that. In the meantime I've found the following. They're similar to what I may have book marked at home, but I seem to remember there being a wider selection of tachs to cover a wider range of RPMs, cylinders, cycles and spark patterns.


Tiny Tach

ENM

These appear to be made for mounting, but they're relatively cheap and just wrap around the plug wire, so they could be a temporary solution.
I like the 2nd one. Definitely a viable option. Thanks for the links.


I also posted this over on the Garage Journal. (some smart guys there)
http://garagejournal.com/forum/showthread.php?t=35123

Some interesting responses.. One guy looks to have figured out how to make a 4cyl tach work. I would simply multiply the RPMs by 2.

As long as the tach is off by double, it should be easy to figure out which way to multiply/divide.. I would just hate for it to be off by an odd amount (something where I may not realise it)

Honestly, to set the idle is very easy.. I have it set so the idle screw isn't even touching the throttle. Sounds like its at about 1000rpm. For tuning all I need to do is obtain maximum RPM with the adjuster screw on each carb - so even if a tach is way off, it shouldn't even matter.
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Old 05-01-2009, 07:57 AM   #12
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I've used a snap-on rpm probe on a snap-on multimeter, seems to work great. That said, I've never needed an rpm meter for dirt/dual sports and my street bikes have always had tach's - I just used the snap-on gizmo for fun as we had it in the shop for fixing cages. I really don't think you need a tach for whatever it is you are trying to do:)
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Old 05-01-2009, 10:24 AM   #13
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http://www.tinytach.com/tinytach/index.php . . . could be set up as a test tool or permanently mounted.
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Old 05-01-2009, 11:10 AM   #14
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Quote:
Originally Posted by Tommy2
Honestly, to set the idle is very easy.. I have it set so the idle screw isn't even touching the throttle.
Not sure what this means. You always want to have free play in the throttle cables so that the idle stops are setting the idle, not the cable.

Quote:
For tuning all I need to do is obtain maximum RPM with the adjuster screw on each carb - so even if a tach is way off, it shouldn't even matter.
Mixture tuning can be done by ear - in fact, it is easier than way. Typically you want 1/2 turn or so richer on the air/fuel screw than max RPM. And make sure you understand which way turning the screw makes it richer/leaner. Generally, if the mixture screw is on the engine side of the carb, it is a fuel screw and turning it out richens the mixture (adding fuel). If it is on the airbox side of the carb, it is an air screw and turning it out leans the mixture (adding air).

In addition to mixture tuning, you might want to tackle carb synchronization. Here's the standard way to sync old Honda twins without inlet vacuum taps that have a split cable and separate idle stop screws on both carbs. (Not sure how the CM400 is setup.) It works much better than the "place hands behind muffler" method that Honda recommended:

1. Turn the idle screws equally on both sides to a much higher than normal value (i.e., high enough that it will run on one-cyl in step 3).
2. Disconnect one spark plug lead.
3. Start the bike and back down the idle screw on the running side until it just dies.
4. Repeat 2-3 for the other side.
5. Run it on both cylinders and turn the idle screws equally to a suitable idle.
6. Remove the airbox to gain access to the inlet throats of the carbs. You want to be able to get two fingers on one hand down the throats of both carbs.
7. Adjust the splits on the throttle cable so that as you barely crack the throttle open, both sides come off the idle stops as exactly the same time.
8. Finally, adjust throttle free play.

As to what idle to set, I wouldn't get hung up on any number. You want the slowest idle where the engine runs smoothly and has a nice transition off-idle to the needle. Most engines, especially ones with 25-yo carbs, seem to run a better if their idle is slightly higher than spec and for track use, this is a standard trick to make throttle application out of corners less abrupt.

Bottom line: no tach required.

- Mark
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Old 05-01-2009, 02:23 PM   #15
sherpa2
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I tried the ENM on my goose but it didn't work. Though only about $30, I'm sure it was good quality, at least not some Chinese junk, made in USA. However, it was erratic, and seemed to pick up impulses even when it wasn't connected to the plug wires. I suspected it was a bad one, but the mfg said it was good but for some reason didn't work for my bike and they refunded me the $.
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