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Old 09-10-2018, 09:09 PM   #1
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Default Increased gas usage

At the beginning of the season in May I had my 2 bbl carb rebuild on my Maxum 2500scr with mercruiser 5.7. When installing rebuild carb I also replaced spark plugs, spark plug wires and distributor cap rotor. After installing new carb I had to adjust low idle mixture. Boat starts and runs great with the exception of excessive gas usage. Before I was burning about 10 g per h and now I'm over 12. Also the engine dies on random occasions but starts right up. Not sure if thats related to the same issue. I checked the bottom which is clean including my props. Also pulled out spark plugs and it doesn't look like they are fouled. Can it be caused by to rich idle mixture or should I look somewhere else? Thanks in advance.
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Old 09-11-2018, 01:17 AM   #2
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At what rpms does it stall? How many hours have you put on the new spark plugs?

Did you check/set the timing when doing all this work?

My initial thought is the carburetor is to rich but the plugs should show signs of this. Did you use a new gasket under the carburetor? If not you may have a vacuum leak.
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Old 09-11-2018, 02:32 AM   #3
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At what rpms does it stall? How many hours have you put on the new spark plugs?

Did you check/set the timing when doing all this work?

My initial thought is the carburetor is to rich but the plugs should show signs of this. Did you use a new gasket under the carburetor? If not you may have a vacuum leak.
It stalls very randomly. Mainly right after start up but it happened few times when I was on plane. When idling it will just die without any indication pretty much the same way as you'll turn off ignition.

I didn't check the timing. Is it possible that any of that work could affect it?

I believe carburetor came back from the shop with new gasket but I'm not sure since it was a while ago. Is there any way I could check for vacuum leak?
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Old 09-11-2018, 11:24 AM   #4
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Is the engine still cold when it stalls or warmed up? Verify choke setting and operation.

Yes a new distributor cap and rotor can affect ignition timing.

A vacuum gauge can be connected to measure engine vacuum but you can also mist around the base of the carb with carb cleaner and if the rpms increase you have a leak.
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Old 09-11-2018, 10:34 PM   #5
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Is the engine still cold when it stalls or warmed up? Verify choke setting and operation.

Yes a new distributor cap and rotor can affect ignition timing.

A vacuum gauge can be connected to measure engine vacuum but you can also mist around the base of the carb with carb cleaner and if the rpms increase you have a leak.
Engine stalling happens very randomly and it happened few times when engine was warm but most often it happens right after start up. It's just bizarre because because after start up engine runs smooth and I always give a bit throttle to let it idle around 1000 rpm till warmed up a bit and it still does it without any indication like bugging etc.

I'll look into getting timing light to verify.

I'll check for the vacuum leaks around carb with carb cleaner. Do you think that could cause increased gas usage or stalling?
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Old 09-11-2018, 11:47 PM   #6
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Potentially but from you last post I would check that the choke is set correctly and fully opens in a few minutes.
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Old 09-12-2018, 02:56 AM   #7
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Potentially but from you last post I would check that the choke is set correctly and fully opens in a few minutes.
I believe it does. I'll verify.

Did the vacuum test by spraying carb cleaner around carb. Engine didn't react anyhow to it so it looks good.

Can to reach idle mixture cause drastically increased gas usage or it applies only when on idle? Would that show on spark plugs?
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Old 09-12-2018, 03:19 AM   #8
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Idle mixture screw is only for the idle circuit and if too rich should blacken the plugs. However plugs are mainly blackened by too rich above idle.

Did you use a OEM distributor cap? I have read of some folks having issues with bad caps including OEM ones. Although these folks complained about misfire and stalling but donít recall increase fuel consumption.
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Old 09-12-2018, 03:33 AM   #9
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Idle mixture screw is only for the idle circuit and if too rich should blacken the plugs. However plugs are mainly blackened by too rich above idle.

Did you use a OEM distributor cap? I have read of some folks having issues with bad caps including OEM ones. Although these folks complained about misfire and stalling but donít recall increase fuel consumption.
Yes that was an OEM distributor cap.

If I'm running rich is it something which I can adjust on the carb? I'm thinking to install new spark plugs and observe them one more time.
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Old 09-12-2018, 11:26 AM   #10
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Yes that was an OEM distributor cap.

If I'm running rich is it something which I can adjust on the carb? I'm thinking to install new spark plugs and observe them one more time.
The idle circuit has the idle adjustment screws but the main circuit uses metering rods to set fuel mixture. I don't recall the 2bbl design in detail. 4bbl also has an accelerator pump that can be adjusted by changing the diaphragm and spring as well as the pivot point of the arm that controls how far the diaphragm is depressed.
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