1974 Merc 9.8 hp

[whatsagm]

Question for the Merc experts. Got this outboard out of a basement at a yard sale. Outboard probably had less than 10 original hrs on it. I rewired everything, new coils, points, condenser, rebuilt fuel pump, and Carburator. This outboard is absolutely the best most reliable thing I have ever owned. The only issue I'm running in to is above 90% throttle seems to bog down and go lean. I have checked everything that I can think of. Is the original design of the fuel pump not enough to keep up? This outboard starts easy and runs perfect until 90,% throttle. Any help would be much appreciated. Thank you

 

[merc850]

If you are using the original fuel tank there might be crud plugging the filter. undo the screws that hold the assembly to the tank and check to see if it's clear. Any Merc was designed to run full throttle for hours.

 

[whatsagm]

If you are using the original fuel tank there might be crud plugging the filter. undo the screws that hold the assembly to the tank and check to see if it's clear. Any Merc was designed to run full throttle for hours.

Tank is clean, I run my 3.9hp off the same tank with zero issues.

 

[racerone]

Hopefully you installed a new water pump impeller.----If equipped with a plastic piece to open the throttle n, it might be broken.----I do not believe it is a fuel pump issue.

 

[whatsagm]

Hopefully you installed a new water pump impeller.----If equipped with a plastic piece to open the throttle n, it might be broken.----I do not believe it is a fuel pump issue.

I installed a complete new pump, housing and plastic pipe. Motor pumps water with no issues

 

[QBhoy]

When you say bog? Just thought I’d mention the notion that most engines like that will have next to no real power gain for the last 10% of throttle given. Especially engines that might be de rated versions of the model above in HP…like a 15hp for example.
Or perhaps you mean it actually drops power when applying the last 10%. That’s different altogether of course.

 

[whatsagm]

When you say bog? Just thought I’d mention the notion that most engines like that will have next to no real power gain for the last 10% of throttle given. Especially engines that might be de rated versions of the model above in HP…like a 15hp for example.
Or perhaps you mean it actually drops power when applying the last 10%. That’s different altogether of course.

It drops power last 10%

 

[QBhoy]

It drops power last 10%

Got you. I’d maybe then thing about the fuel air adjustment then. What have you got her at now ? 1 and 1/2 turns out maybe ?

 

[merc850]

Is the throttle plate opening all the way up, while engine is not running turn the tiller handle all the way and look into the carb; the edge of the plate should be facing you. Another possibility is the ignition timing is over-advanced which will slow it down. The carb has a fixed hi-speed jet no adjustment is needed.

 

[tpenfield]

I have the 7.5 HP version of that motor. I would check the throttle linkage and the timing advance linkage just to make sure all is in order.

IIRC, the throttle linkage has stops (screw/nut combos) up around the flywheel housing. Maybe the high speed stop on the linkage is not set right or the screw is missing.

 

[jimmbo]

In 74, there wasn't Garbage called Alcohol added to Gasoline. I suspect the inside of the Fuel Lines are deteriorating and restricting flow. The last 10% of Throttle can increase fuel demand more than 30+%