1996 Mercury 40hp, idles fine in driveway, won't accelerate.

[mwilson32796]

The motor has new fuel line, primer bulb, gas tank and spark plugs. Had carbs rebuilt and fuel pump as well. Been to the shop twice, they can not figure out what is causing the lack of acceleration. It will idle all day ,but under load it will not plane out. Water tested it again, same thing happens, bulb is firm.
Any suggestions??

 

[Jean Narrache]

I am not a mecanic, but i am playing quite a bit with outboard engines..

I see two reasons why it runs fine in driveway but not in water.
First reason, Gaz / Air mixture might be incorrect. In fact it might be just a little bit too lean.
Other reason is Timing. It might be off a little bit, allowing it to idle properly but lack of power when loaded.
Can be both too !

What is your engine a classic 40 HP 4 cylinders ?
If so, the mixture screw in front of the carbs, located at 1:00 o'clock has to be set to ONE TUNR AND ONE QUARTER OUT from lightly seated position for both carbs. This is a starting point. Adjusted like that, the engine will start. Then while idling at about 1000 rpm on neutral, slowly turn in the mixture screw of the top carb. The engin will accelerate a little bit and then begins to run rough, when the engine will start to run rough, turn the screw OUT untiil it begins to run rough again and COUNT HOW MUCH TURN you will have given to the screw. This count is IMPORTANT. Then when in too rich condition turn the screw back HALF WAY of the TURNS YOU JUST COUNT ans set the screw about 1/8 of a turn OUT to enrich the mixture a little bit. This carb should be set. DO the same for the second one.

YOU DON'T WANT TO SET THE MIXTURE AT THE POINT THE ENGINE RUNS AT A FASTER IDLE, THIS IS TOO LEAN AND WILL CAUSE PROBLEMS, CAN MELT PISTONS AND HAVE CATASTROPHIC FAILURE IN WORST CASE, or many other problem related to an engine running too lean. An engine running too lean will also run too hot.

For the timing, i'm totally not an expert !
Can't help really.

 

[mwilson32796]

Jean, thanks for the response. The engine is in fact the 40 hp 4 cylinder. All the timing and carbs have been inspected and adjusted by the local Marine shop and all is according to them is in order.

 

[Texasmark]

They can't test the engine under load usually, even though some tank test with a specially built tank prop or they actually put it on a boat (like yours) and water test it.......very expensive and you can bet seldom if ever done, especially on an engine of that HP rating. You have to put the engine under load to know. As Mr. Wilson pointed out, I'd open the low speed jet(s), like a 1/16" at a time, test, tweak, test....... till you have at least gone a full ? turn. If that's the problem, the engine will take the load of the boat getting on plane and not bog down.

 

[mwilson32796]

Texasmark, thanks for the response. The carbs have been rebuilt and already adjusted by the shop, could they still be out of sync? Is there anyway it could be a ignition problem?

 

[Texasmark]

Let's try what we said about the low speed jets. If that doesn't work we'll go from there. The on water adjustment is the best way to get them set right. Everything else is just guessing.......obviously it's professionally done, but none the less guessing till it's on the water doing it's job. Good luck.

 

[mwilson32796]

Ok back at it again. It's been in two different Mercury Dealers +$700 and still same problem. The loss of power is intermittent. It will leave from the dock at full throttle (5,500 rpm) and run for awhile. I stop to go through bridge at idle speed and then try to get back up on plane, it plows at 3k rpm for a few minutes and then it might take off again. We have replaced the fuel tank, all fuel lines even under the cowling, new primer bulb, fuel filter and complete carb kit rebuild. All oil injection lines have been replaced fuel pump rebuild. Timing has been checked and tested. Spark will jump almost 7/16". The plugs have been changed. What else could be wrong. I have called Mercury Direct and they want me to spend more money at "another authorized dealer".

 

[mwilson32796]

Oh the shop did a water test last two times and said its ready to go.

 

[MH Hawker]

why ask for advice if you wont follow it :spy:

 

[mwilson32796]

MH Hawker, it a time thing. I have not had the time to do work on it myself.