Tachometer wiring

[danfrompgh]

I had this question posted on another site and really got no help,probably should have just started here in the first place!
Where does the tach get its reading from?
I originally had a fluxuation at 1200-1500 rpms when run on the muffs,the tach would kinda float for lack of a better word. When I was finally able to get her wet this past weekend the tach read fine at idol and upper rpms. Although...Now it seems to be reading really low at WOT, 3300-3500 when the WOT range is 4400-4800. Motor sounds as strong as ever. Speed is also down to 44mph(speedo & gps)
Tune up/oils are all fresh 20 hrs ago. Fuel & filters are fresh last week. Only thing changed was a new 14x21 Al to replace the dinged 14x21 Al. I know a manufacturer change can cause a difference but 1000 rpms seems to be alot. Before I start changing parts I need to verify my rpms. Need help tracing the circuit.

 

[Mark42]

Re: Tachometer wiring

Experience tells me to check the last thing that changed. That would be the prop.

Check to see if the new prop is cupped, and the old prop isn't. The difference between a cupped and non-cupped prop can be hundreds of RPM. Enough to lower the engines operating range to the point it can't reach the power band and it can suffer.

If the props are identical, then its on to engine diagnostics to see where the power loss is coming from. Fuel starvation, low compression on one or more cylinders, failing/faulty spark plug/wires or ignition components, etc

 

[fdmsiv]

Re: Tachometer wiring

On my o/b the signal to the tach comes from the rectifier. It shoots out a signal at a ever couple rpm. I would check the rectifier first, then the tach. Make sure the tach is on the proper setting for your engine. Strange things can happen when you start troubleshooting, might start with the DUH! ones first

 

[Silvertip]

Re: Tachometer wiring

Seems to me since the tach was not touched, the fact that it now fluctuates says the tach is not the issue. You changed a prop and that alone can cause a significant RPM loss or increase. You didn't say what the "observed" wide open throttle rpm was before you changed the prop. Observed and what the engine manufacturer recommend are not the same. If the existing prop did not allow the engine to reach its recommended WOT rpm there is either a prop mismatch or the engine is not making full power. Don't start throwing parts at this issue until you are sure what the issue is. Put the other prop back on and do a bench-mark.