Adding a Tach By ellpee (Read 641 times)

[ellpee]

I bought an aftermarket tach and hooked it up, and part of the setup instructions were to program it for x beats per rotation, or rotations per beat. I tried to reason my way through that, but am still not sure I got it right. On any given cylinder, there should be one intake/compression stroke and one ignition/exhaust stroke, as I recall, which equates to two full rotations. Have I got that right? If so, then on a one-cylinder engine, the tach should be set to show 2 RPM for every one impulse on the plug wire. Si? No?

[steve]

I installed the Trail Tech Vapor on my Viking. They told me set it for 2. That works for me. You usually can tell by listening if the tach is reading some multiple of what's real. Mine read about 2.4k when I knew that was to high. Set it for 2 per tech suport.

[alleyoop]

One guy on another forum pmed me and said he has set it to 2 and at idle it was reading little over 3K and I told him that is wrong and to try setting it to one and that brought the reading down to around 1500 at idle.

So who knows
Alleyoop

[ellpee]

I'm still playing with it; just by ear, I'd guess my idle is around 1200-1300, and I'm trying to find a setting that shows that while also showing a reasonable reading at highway speed. In all honesty I don't care except at the low end when I'm trying to tweak mixture or something, but in a perfect world I'd like to have it set right.

[steve]

Have the same set-up issue with my speedo. Read all kinds of crazy numbers and then quit at abouit 20mph. Mine came with digital and when I looked, I found it had 4 mags in the roter. Computer wasn't set up for that or fast enough to read it. I had to get a mag and mount it on the rim on the other side of the wheel. Very pleased with the useful info from the Trail-Tech.

[ellpee]

Yeah, my deal is that I can get a plausible low-end number, but then at speed it only shows in the 3000's, which seems unlikely. As I posted, all I was really after is does the RPM go up (or down) when I fiddle with the mixture or idle, but I'd be happier if it worked right all the way from idle to WOT.

[steve]

Not an expert by any measure, but this should be linear. It would appear that either the sensor wire is picking up noise, or is a little loose. My tach input is pigtailed to the hot input to the coil. I see where some work by wrapping that wire around the coil output. If that methold is used, you can try to experiment with the number/location of wraps.

[ellpee]

I have the wire on mine wrapped around the spark plug wire, just short of the plug itself, maybe 6 tight wraps, and electric-taped in place. Anybody think that;s not the way to go?

[steve]

My manual lists that as an option and sugests adjusting the number of wraps but I don't remember how many. They do list that as a trouble shooting step but they list a max number of wraps as less than 10 if I remember correctly. Tossed the install sheet. I think they listed between four an 10 wraps. The input to my coil is about 20v and the vapor can handle that easily. There is no queswork with hard wire, so that's the method I used. I am not recommending that method for yours because 20 volts might fry your input. Maybe your tech support can advise you, but it sure sounds like you have it set correctly if it's accurate at low rpm. I would bet it's a pick up issue. I'm thinking it might be a pulse width issue and would start with less wraps.

[steve]

One last thought. My manual says that some engines change the pulses per revolution over a ccertain RPM and allows me to enter that rpm and set a second PPR. I've not been doing this very long, but I've not heard of that being done on any of our machines. It might be, though, or an aftermarket CDI might do that. Sounds like a major PIA to figure out what RPM that change takes place, and what the new PPR might be. I'm like you, though. Use it for carb work and identifying my launch RPM. Would be nice to know if you're in your engines best power band at cruise speeds. And that's everything I think I know about that subject.

[ellpee]

Thanks, your posts have been helpful. The Magnum will be going into hibernation later this week, not to see the road again until next May, but I'll no doubt be experimenting more next summer. Between now and then it's me and my Roketa, terrorizing the roads of Tucson.