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Posted by: ctc911ctc Aug 3 2026, 07:36 AM

All,

When my youngest son was 10, I taught him to roam the neighborhood for lawnmowers and snowblowers. Once a month he would harvest a good one and I taught him to clean carburetors or install a rebuild kit. He would clear $50-100 for each on FBMP.

He graduated to cars and boats - boats can be free if you tow them from the yard.

Here is one that he harvested from a yard in Maine - Center Console with a Mercury Optimax 225 3.0L - which is a 200hp with a few additional parts to allow them to say it is 225. It is an early ECU and though the engine is remarkable when running right this one stalls at idle (ECU sets the idle speed) as well as stumbles a bit through the power curve. Compression in all cylinders is perfect. Low time engine. Just like to lawn mowers.




I do not have the Mercury ECU reader ($$$$) so I have not pulled codes, but I replaced all of the 'consumable' parts and diaphragms, only the reed valves have not been replaced.

Air Pump replaced
Fuel to Air pressure rails are within specs


Any Optimax experts out there?

Next stop - pull it out of the lake and into the Mercury repair shop!

Posted by: ctc911ctc Aug 3 2026, 07:37 AM

Apologies for being so far from the 914 world - But I trust you guys and not the people on the boat forums.......

Posted by: Shivers Aug 3 2026, 08:24 AM

I can give you what I found. Maybe some place to start:

Air and Fuel Pressures -An Optimax relies on precise air (~80 PSI) and fuel (~90 PSI) pressures to atomize fuel correctly.A failing air compressor or a leaking direct-injector/tracker valve will cause a pressure drop, leading to an immediate stall at low RPMs.Connect dual gauges to verify you maintain roughly a 10 PSI difference between fuel and air.
Idle Air Control (IAC) and Throttle -A carbon-fouled or broken IAC valve fails to regulate extra air needed when the throttle plate closes at idle.Inspect vacuum lines for cracks, leaks, or disconnections that ruin the idle vacuum.Ignition and Fuel DeliveryCheck spark plugs for heavy fouling or incorrect gap, and test ignition coils under load.Degraded fuel lines or a clogged fuel filter restrict low-speed delivery.

Posted by: brant Aug 3 2026, 09:25 AM

A lot of difference between opti max and diet

They inject their own air
And air leaks in the intake don’t effect the mixture
The pressure differential between incoming air (injected) and fuel (injected) is critical. Can you measure that difference

Posted by: ctc911ctc Aug 4 2026, 06:44 AM

thank you Brant, the pressure is 90/80 - nuts on spec

Right now I am chasing ignition - buying a spark-gap-visualizer to see if the drop outs are spark related.

More to come!

CTC