Mercruiser 470 - tach issue

[blkzoe]

My port engine recently began to misbehave. 170 hp 470 w/ Pertronix ignition. Idle was OK, but when accelerating would cough-sputter then die. I would get instant restart, but no rpms higher than 2000 without issues. I chased down both fuel and electrical faults with no resolution. Then I recalled someone on this forum suggested disconnecting the grey wire (tach) from the coil. Instant karma!

With the grey wire disconnected, I went from a Briggs & Stratton lawn mower with bad gas to the starship Enterprise at warp 7. I actually believe I've never heard the engine run so well.

As I was going to replaces the tachs anyway (cosmetic and age issues), the $ issue is no issue.

But the question remains as to why disconnecting the tach made all the difference in the world. Any thoughts from our resident 470 gurus?

 

[Bondo]

But the question remains as to why disconnecting the tach made all the difference in the world.?

Ayuh,... The ignition is fired through groundin' the (-) terminal on the coil,.....

If the tach has an internal ground, If causes the coil to not fire when the ignition tells it too,...

 

[achris]

Internally tachos have capacitors (and quite a few other things). If one, or more, of those capacitors becomes 'leaky', then at the higher frquencies (higher revs) they begin to pass more and more current to ground. That effectively negates the switching effect of the points, so the points opening no longer produces the sharp 'off' signal the coil needs to collapse the field to produce the spark. A much weaker spark is produced. As the frequency (revs) get higher as the points open the faulty capacitor simulates a 'soft off', the current through the coil reduces slowly, and no spark is produced.

If you'd like to fix the tacho yourself, it probably just a matter of opening it up and replacing all the electrolytic capacitors. They have a lifespan of about 15 years (at best) anyway, and in a boat (large swings in temperature) that life will be shortened. If you are going to replace them, use a higher voltage and temperature rating (but the same capacitance) than what's there now and it will last a lot longer.

I have recently replaced all the capacitors in my computer monitor power supplies, computer speakers, boat depth sounder and my old DVD player. All came back to life better than before. BTW, manufacturers really hate people who fix things. Instead of selling a new item (big dollars), all they sell is a handful of parts (very small dollars). In fact, to that end, many consumer goods suppliers don't even carry spare parts. They WANT/NEED people to throw 'broken' things away and buy new. So you need to know where to find parts for things you want to fix. I also fixed a 4 year old battery charger. New- $250, price for me to open up and replace a bridge rectifier and fuse- $3.75.... Parts from the battery charge manufacturer? Nope, local electronics shop. Google is your friend.

Chris...........