Diagnostic tricks

Hi Folks,

I have an unmarked motor, identified through its serial number as a Mercury Mariner 4/5 sn #09852794 mod #1004211rb year 1995. It used to be on the back of a safety boat at a club many seasons ago.

The motor says 'made in USA', and has a Keihin carb.

It was starting well, but not holding full power for more than 5 minutes, so it went to the back of my garage for a while (OK - 2 years). I've got round to setting up a test stand, and now - the motor runs very erratically, though always willing to start.

I've tried cleaning the carb and filters (as have others), and it all SEEMS ok. I have a fairly weak looking spark, but at least there is one.

Thoughts turn to the ignition, and more fundamentally (based on motor bike experience), crankcase pressure.

Does anyone have any tricks to diagnose...

Fuel pump
EHT and ignition performance
Crankcase pressure

... that relate to this motor?

And also, does anyone have any experience with this type to say '15 years old - well that'll be XYZ on that motor...'

Thanks very much

Jeff Adams

Jeff, that motor is a ton of contradictions

The serial is one normally assigned to Mercs plant in Belgium. The carb tells me this is one of the early models that Tohatsu actually built (joint venture with Merc at a plant in Japan).

Anyhow, the fuel pump on this (integral to the carb) has two (2) diaphrams - an inner and outer. A slight perforation in either could give you a "crappy running motor".

It's under 25 bucks for the pair, so wouldn't hurt to try that since the carb has been "done" many times.

In most small outboards it's fairly easy to see a crankcase leak if it's occuring at either the crank seals or where the block mates up. Because they run mixed gas and the motor is enclosed in the cowls, if anything is escaping (blowing out of) the block past the seals you end up with a very oily residue all over the motor and lower cowl when the gas evaporates and leaves the 2 stroke oil behind.

So that's something to look for as a clue.

You might try posting up a question in the Tohatsu/Nissan section also. Tohatsu_guru and Pvanv are both Tohatsu tech's and may have some insight as to the "quirks" of these little single cylinder motors, which despite what the decals say, are "not Mercs"...

Thanks - this is getting interesting

There is evidence of a fairly dry 'sludge' in the base of the cowl. I wonder???

Anyway - thanks for the hint for posting elsewhere - I'll give it a go

Thanks again

Jeff