Mercruiser 3.0 only runs at high RPM

[giossoj]

Mercruiser 3.0 will only run at high RPM's
Will not idle and dies out.

Here is what I did:
Tried to start it last week, would not start. Narrowed it down to bad gas, I think. Drained out all gas in tank.
Replaced plugs, cap, rotor, fuel filter, changed oil and oil filter.
Put new gas in Boat, now it starts but will not run at low RPM's and will eventually die after a while at running at high RPM's
When it starts to die, I have to pump the throttle to get it to stay running.

Any suggestions on what could be causing this?

 

[cr2k]

Re: Mercruiser 3.0 only runs at high RPM

Dirty carb, stuck float, bad fuel pump.

 

[Bondo]

Re: Mercruiser 3.0 only runs at high RPM

Ayuh,... Get a Carb Kit,+ Rebuild it...

 

[giossoj]

Re: Mercruiser 3.0 only runs at high RPM

Yea, that was my first suspicion was that the carb needed a rebuild, but thought I would ask on here to see what you guys thought.

Here's the weird part though, I went out there after I did the post here, took off the flame arrester, looked down into the carb and noticed that there was oil inside the carb. I'm not talking burnt or dried oil, but fresh oil like if someone had poured a couple cap fulls into the carb.

How the heck could this happen?

I took a can of carb cleaner, sprayed the inside really good, she fired right up and ran great for about 6 minutes, died, then starting doing the same exact thing again.

Does anyone think I have a bigger problem than just a dirty carb?

BTW, where is a good place to buy a carb rebuild kit?

 

[cedarjunki]

Re: Mercruiser 3.0 only runs at high RPM

too much crankcase pressure, pushing oil out of vent and into carb...

 

[Simoniz]

Re: Mercruiser 3.0 only runs at high RPM

Mercruiser 3.0 will only run at high RPM's
Will not idle and dies out.

Here is what I did:
Tried to start it last week, would not start. Narrowed it down to bad gas, I think. Drained out all gas in tank.
Replaced plugs, cap, rotor, fuel filter, changed oil and oil filter.
Put new gas in Boat, now it starts but will not run at low RPM's and will eventually die after a while at running at high RPM's
When it starts to die, I have to pump the throttle to get it to stay running.

Any suggestions on what could be causing this?

I had a similar problem with my boat (same engine) shortly after acquiring it second hand.

Someone had put the fue lfliter together wrongly (either with the spring on top of the filter element or no spring - I cant remember exactly).Basically though, the filter was not doing any filtering, with the result that the carb was full of crud, fine particles of dirt out of the tank.

After re assembling the filter properly with a new filter and rebuilding the carb, it runs fine.

 

[cedarjunki]

Re: Mercruiser 3.0 only runs at high RPM

I had a similar problem with my boat (same engine) shortly after acquiring it second hand.

Someone had put the fue lfliter together wrongly (either with the spring on top of the filter element or no spring - I cant remember exactly).Basically though, the filter was not doing any filtering, with the result that the carb was full of crud, fine particles of dirt out of the tank.

After re assembling the filter properly with a new filter and rebuilding the carb, it runs fine.

similar maybe, but did you notice his second post? describing oil not dirt.

Here's the weird part though, I went out there after I did the post here, took off the flame arrester, looked down into the carb and noticed that there was oil inside the carb. I'm not talking burnt or dried oil, but fresh oil like if someone had poured a couple cap fulls into the carb.

I took a can of carb cleaner, sprayed the inside really good, she fired right up and ran great for about 6 minutes, died, then starting doing the same exact thing again.
Does anyone think I have a bigger problem than just a dirty carb?

 

[Simoniz]

Re: Mercruiser 3.0 only runs at high RPM

Well theres only one way oil could be in the carb, and thats from the engine breather.

I can't see what the 2 have to do with each other, you'd need quote a lot of oil going into the carb to prevent the engine running at idle, and even more to make it die after prolonged running at higher rpms.

I suppose if you were to remove the oil breather pipe from the flame arrestor and stick the end into a bottle to collect any oil, then you'd see how much oil the engine was blowing out.

After that, if it were my boat, Id strip the carb and clean it internally and check the fuel filter was OK.

 

[TyeeMan]

Re: Mercruiser 3.0 only runs at high RPM

Get a carb kit at Napa. Take the number off of the carb casting or the aluminum tag that's screwed to the carb and take that to Napa, they'll hook you up. Cheaper than OEM Merc stuff.

 

[cedarjunki]

Re: Mercruiser 3.0 only runs at high RPM

.

I can't see what the 2 have to do with each other, you'd need quote a lot of oil going into the carb to prevent the engine running at idle, and even more to make it die after prolonged running at higher rpms.


After that, if it were my boat, Id strip the carb and clean it internally and check the fuel filter was OK.

bad rings/stuck rings
bad/blown head gasket
bad/worn valves
many ways they can be related.
and with any of these it probably wouldnt idle very well or at all.

but i never disagreed on rebuilding the carb either...

 

[Simoniz]

Re: Mercruiser 3.0 only runs at high RPM

bad rings/stuck rings
bad/blown head gasket
bad/worn valves
many ways they can be related.
and with any of these it probably wouldnt idle very well or at all.

but i never disagreed on rebuilding the carb either...

Im sorry, you are, of course right.

I know you didn't disagree about rebuilding the carb and any of those things could be the cause of the rough idle.

When I posted I was thinking about the spray of carb cleaner that temporarily cured the problem and not about possible mechanical probs.

We do agree though so, Giossoj, rebuild the carb (and check out the fuel filter). Also, how long has the boat been sitting before you tried to start it up last week?

 

[giossoj]

Re: Mercruiser 3.0 only runs at high RPM

OK thanks guys.
Fuel filter is good, I just replaced it. In fact I double checked it when I notticed the oil in the carb to make sure the filter was seated right and it was ok.
I'm going to try rebuilding the carb.
I'm going to try calling Napa like he mentioned above, but just in case no luck...

Do you guys know what the kit part number is for the carb? I'm having a hard time finding the right one. The numbers on the side of my carb are: 807504N and 21 2624.

 

[giossoj]

Re: Mercruiser 3.0 only runs at high RPM

Going to start a new thread on Carb rebuilding for this engine.

 

[cedarjunki]

Re: Mercruiser 3.0 only runs at high RPM

Going to start a new thread on Carb rebuilding for this engine.

have you downloaded the service manual yet?

 

[Bt Doctur]

Re: Mercruiser 3.0 only runs at high RPM

Well ,if the carb dosent work , you can try checking the fuel feed hose(collapsed internally from methanol), a clogged vent hose(Spider/bees nest),
or a sticky anti-syphon valve(rusted from water/clogged with debris)

 

[CharlieB]

Re: Mercruiser 3.0 only runs at high RPM

Excessive oil from the engine breather can foul plugs at an idle.

Pull the dip stick and check for over filling.

Compression test to be sure there isn't something terribly wrong with a cyl. A cyl leak down test would be better but the comp test should give you a clue.