Just need some troubleshooting reassurance - 1994 Mercury 90 OB

Taxus812

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Aug 5, 2013
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I am helping my neighbor get his motor running on his 94 tracker with a 1994 Mercury 90 OB. He has been through a very difficult time so funds are a bit limited (which is why we are not just taking it to a shop). My background is Aircraft :) but Im helping him anyway.

We can start his motor and it will run if we keep hit the electric prime on the key (doesn't seem to skip). My first thought was that the carbs are in need of some cleaning\rebuilding. I decided to open the throttle (using the button in the center of the throttle to disengage the transmission) a bit . I can get it to run very smooth at a higher RPM for a few min then it will sputter and stall.

Checking the primer bulb post stall it is still hard so its not running out of fuel in the tank lines.

So I am questioning my first thought and thinking fuel pump.

Reasonable ?

Due to his limited budget I don't want to throw parts at it. (the flip side is we don't have the tools and reference books a good marine mechanic would have)
 
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This could be a few things especially if you are running on muffs. The engine will be happy to run on 2 cylinders when its on muffs. A few freebee things to check first is to remove the fuel line and check inside is clear (liner break down). Drain the carbs and try to catch the fuel (yep theres no room so get inventive). Put the fuel in a glass and look for bubbles of water in the fuel. Check the fuel vent on the tank or leave the cap loose. Start engine then pump fuel bulb when running if it takes more than 1 good pump to go firm then the carbs are not getting filled so check for air leeks.
After that its time to open the wallet and buy a spark tester ($8) and test all the sparks at 3/8. A compression tester would be nice and most auto parts places will lend you one for the day all reading should be with in 10% of each other.
After that I think you are looking at rebuilding the carbs. Kits on line are not that expensive but you will need one per carb.

note: running a motor will one carb that's partially blocked will cause a lean condition in the cylinder. This leads to extra heat which will melt the piston. I guess what im trying to say is that saving a few dollars rebuilding the carbs now can lead to a major repair bill later which is why most people see a carb rebuild as preventive maintenance and money well spent even if its not the problem.
 

Taxus812

Petty Officer 2nd Class
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Aug 5, 2013
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173
Thanks for the response Glenn.


We replaced the Fuel filter and inspected the fuel lines from the fuel pump to the carbs (no issues noted).
The primer bulb is staying firm even running.
I already drained all three carb bowls but never inspected the fuel under glass

I have a compression tester but had no idea what a good range was. It makes sense if all three are about equal im set. I will do that later.
I will need a spark tester (NAPA store ?) I am always wishing I had one anyway.
Its $12 for the fuel pump rebuild kit on eBay. I think I'm just going to do it anyway.
Pulling three carbs is a ***** but I'm thinking the same thing. (its about $25 per carb). He doesn't have a working temp gauge preventative is prudent. Anyone have a way to figure out what the float heights should be set to ?

Terry
 

enginepower

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Jul 5, 2014
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If you want to see if you have a fuel pump problem, pump the pimer bulb constantly when it's having the problem. If problem goes away, then you can suspect the pump, if not, no need to rebuild pump unless you just want to. If it's running better when you hold electric choke, carb is running lean or you have an air leak somewhere.
 
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