Honda 50 Tachometer needle going backwards

I have just had my old 1995 Honda 50 outboard replaced with a 2007 carb model Honda 50.
She starts up and appears to run well. (Not been able to take the boat out yet.) At normal idling speed the tachometer only reads around 200 rpm but when the throttle is increased the tacho needle goes backwards to zero. Never had this problem on the old engine where idling speed was around 900 rpm and increased with more throttle.
I have not changed any of the boats gauges that were related to the old engine so could the newer engine be mismatched to the old gauges?
I have noticed a tiny sprinkling of metal dust around the starter motor and upper engine area, could this be related to my problem, ie the pickups under the flywheel?
Any pointers would be welcome.

I feel safe assuring you this issue has nothing to do with the coils/pick ups located under the flywheel.

Beyond that, things become a little more hazy. I KNOW that the fuel injected engines do not register properly on the old style tachs (injected engine will register noticeably low). Exactly why is not clear in my mind. Some might say "digital signal", leaving me wondering why that might make a difference in the calibration one vs. the other?

2007 was a transition year, from the carbs to the fuel injected motors, leaving me wondering if that might have something to do with it
THEN, there's the possibility there's some sort of electrical issue, which is most likely in my mind. Some sort of bad connection - somewhere.....

Just some thoughts. Sorry I can't be of any more help. -Al

Thank you for your comments Alan.

I'm now wondering if the rectifier may be at fault here. When I next get to the boat I will check to see if the charging system is working and look out for bad connections.

I have a much older service manual for info, but I can't find the location of the rectifier unit.

Milgrip said:

Thank you for your comments Alan.

I'm now wondering if the rectifier may be at fault here. When I next get to the boat I will check to see if the charging system is working and look out for bad connections.

I have a much older service manual for info, but I can't find the location of the rectifier unit.

Click to expand...

Doubtful the rectifier itself has anything to do with it either. Location is front and center, right below the starter.

What kind of tacho do you have. Allan's correct on the tacho issue, the last of the carb engines ran the same tacho and setting as the EFI motors. That is setting 1 on the farias

iang6766 said:

What kind of tacho do you have. Allan's correct on the tacho issue, the last of the carb engines ran the same tacho and setting as the EFI motors. That is setting 1 on the farias

Click to expand...

Do you know right where the break to the new style is? Could it be the elec. choke models need the new style tach?

Just to update you and to thank you both for your help. I checked the charging system and all was as it should be. Alan was right about that and the bad connection. The rubberized bullet connectors between the remote lever and the boat gauges were corroded so I replaced them.(We used the wiring loom that came with the 2007 engine) The tacho now increases with throttle but is inaccurate and doesn't show correct engine speed. The gauge rpm reads lower than it should at all throttle openings and the trim gauge also reads lower, for example when engine is raised to the prop out of the water, the gauge shows engine in level position.
Ian, the gauges have black dials and are 1995 original. There are no visible identification marks on the dials except for the Honda name. Are all Honda gauges Farias? The adjustment on the rear of the tacho is covered with a sealant and I must suppose the setting is already on no 1 which I believe is the 4 cyl setting) as the gauges worked perfectly on the old engine.
So does it look like the gauges will need to be replaced?

The tach will need to be upgraded for sure. Sorry, no help on the tilt gauge (new style vs. old). No experience there.

Before I replaced it though, I would want to make sure it's following the arm on the sending unit smoothly for assurance there wasn't something going on there. Not unusual for them to be "sticky" causing inaccurate readings.

Congrats on finding the bad fittings. Good job! -Al

After lots of research I sorted the tachometer problem. My old 1995 Honda 50 engine supplied a 6 pole signal to my original 1995 tacho which was fine, whereas my replacement 2007 Honda 50 engine supplies a 4 pole signal, meaning this engine is mismatched to the tacho and giving false rpm readings.

I bought a new Honda tacho (37250-ZV7-913) it's a perfect match and looks identical to the old tacho and reads true.

The 4 pole signal change over took place around 2006.

See attachment for Tacho outboard specs which give more details.