[WileEcoyote75]

I have a UMA p-lead tachometer that is spiking anytime I transmit on a frequency of more that 121.000. If the engine is off the tach reading will increase anywhere from 500-3500 rpm. The reading will also spike when the engine is running. The tach was just returned from the factory for a recalibration, the indicated was previously reading 300 over actual. I have recently installed a new Garmin GTR200 radio that is working great, and is 2 watts higher in output than the previous radio. I have tried a few different grounding locations, with no change. The tach reading is steady when transmitting on <121.00 or when I disconnect the coax output on the radio. The com antenna is a mounted in the wing tip.

 

[FORANE]

Do not transmit without an antenna connected as it may damage your radio.
Try to rule out the cause with:
sliding another radio in and see if it also does the same.
connect a RG400 to antenna in another location.

I had a similar issue with my EGT when transmitting with my SL40. My issue turned out to be a poorly secured RG400 cable vibrated until internal damage occurred. This led to poor receive. We found and replaced the RG400 but unknown to me at the time was the SL40 was damaged due to transmitting without a good antenna. After connecting the new RG400, the SL40 did better but sprayed RF when transmitting on some frequencies - just as you describe. It took sliding another SL40 in my tray to isolate the EGT problem to a damaged SL40.