Engine barely running below 1500 rpm

R

RJJohnson

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I have a replacement 2008 mercruiser manufactured 5.7 l weber carb that lately when I back down to below 1500 rpm after running on plane the engine runs bad and eventually will stall and not re start for 12 hours unless I take it back over 1500 rpm and runs perfect until I back down again. The carb was rebuilt last year and had new plugs and wires put in last year. No water in the fuel. Could it be a float problem or a circuit problem or even a coil problem ? Whatever it is it only happens below 1500 and only after I run the engine on plane for 10 minutes. But if I take it back over 1500 rpm the problem goes away. If I dont then it will eventually stall and it wont re start unless I let it sit for 12 hours and it will start back up ss soon as I turn the key. Any ideas ?
 
alldodge

alldodge

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Your probably flooding and need to get back into the carb. Might have a piece of junk stuck in the needle seat
 
wellcraft-classic210

wellcraft-classic210

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Yes -- the fuel is delivered through Different passages in the carb above and below 1500 rpm

idle passages under 1500 // vs venturi's over 1500


Any sediment in the bottom of the float bowl can easily block idle passages. Those same idle passages deliver fuel on start up -- so that can be explained also

Fuel mixture is important also -- If the float is not working properly and flooding is occurring -- that can cause a variety of issues as well.


If it was mine I would clean the carb , check carb float levels and ensure adequate fuel filtration is in place & replace the filters if they are in use. ( You should have 1 that looks like an oil filter ) to prevent debris from getting to the carb..
 
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havoc_squad

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When did this issue begin and how long was it when it was last ran before this problem occured?

If it suddenly happened with zero idle issues beforehand on the water 1 month or less ago, then it is likely an obstruction.

If you dewinterized the boat and found it won't idle, your carb is probably gunked up from improperly stabilized or unstabilized Ethanol gas in the idle circuit.

Either way, time for disassembly and inspection. Hopefully mercruiser covers that model in one of their manuals.
 
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RJJohnson

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It happened once during the end of last year but then stopped. It didnt happen the first time I put it in the water this year but happened the 2nd and 3rd time. But it doesnt happen when I head out on the water, only when coming back into the 6mph zone. That's the part that confuses me.
 
nola mike

nola mike

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I'm pretty sure I own the distinction of being the only person in iboats history where the coil was actually the problem. Coil would be more likely to cause problems when hot/high rpms anyways.
 
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havoc_squad

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RJJohnson said:
It happened once during the end of last year but then stopped. It didnt happen the first time I put it in the water this year but happened the 2nd and 3rd time. But it doesnt happen when I head out on the water, only when coming back into the 6mph zone. That's the part that confuses me.
Click to expand...

What I suspect is your pattern of boat usage is masking the problem.

A good way to isolate if carb is flooding in idle rpms is to do a long in gear in water run at lowest idle speed specified in service manual with the motor fully warmed up, for about 5 minutes.

If you see venturies dripping at all in 1000 or less rpms, you got a flooding problem.

If you have a buildup of too much fuel which keeps growing, it will manifest itself during long idle rpm runs in gear.

It will start running rough, to further confirm that, turn off ignition and start it up. If flooded, you'll have to clear flood with advancing throttle.

Since you have a possible flooding issue, put a fuel pressure gauge in line to the carb to rule out fuel pump over pressure problem.
 
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nola mike

nola mike

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Good point above--when it won't start again, open the throttle all the way and crank it. That will clear out any excess fuel if it is flooded.
 
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RJJohnson

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That's exactly what I did the last time it stalled. I gave it full throttle and cranked until the battery started to weaken. It wouldnt me re start until the next day.
 
nola mike

nola mike

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RJJohnson said:
That's exactly what I did the last time it stalled. I gave it full throttle and cranked until the battery started to weaken. It wouldnt me re start until the next day.
Click to expand...

Your plugs might have been soaked at that point
 
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RJJohnson

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It isn't the coil. I switched it with a old one I had and still having problems. Last night I started it up and it wouldnt idle below 1500 rpm at the dock. This is new. engine not even warm. If I let it stall the only way to re start it was to give it full throttle when cranking. But wouldnt idle under 1500 rpm. I did smell gas so I think it is flooding. Besides the float being stuck what could cause the flooding ? Choke ?
 
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havoc_squad

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#1 fuel pressure to carb test. Do not skip this step.

You should bring an assistant who knows how to properly operate the throttle. Your eyes need to be looking at the intake of the carb and the fuel pressure gauge.

Should be very close to 5 PSI at around 700 rpm idle in neutral. No more than 7 PSI, not even 7.5.

Are you monitoring the venturies for fuel dripping, if not you should be during fuel pressure test.

If fuel pressure is good, the carb has to come off and disassembled per service manual. If you don't have one that applies for that carb, post the carb model number.

With good fuel pressure and venturies dripping at 650 rpm idle, I would focus on needle and seat or float height being the primary source of problem.

I strongly recommend the OP to spend time researching the basics of how fuel/air ratio affects engine performance and how carbs work.

If you understand the problem and how the carb functions in detail, you can diagnose it.

Otherwise the next best recommendation is to send it to a professional or replace it of this is too much required.

If one DIY's, they must teach themselves how to diagnose with the publically available info and hopefully using a service manual where required, or pay a lot in re-work or in paying a professional who can.
 
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