Merc 650 bottom carb not sucking as much air as top

S

SEABEE#2

Seaman
Joined
May 18, 2011
Messages
59
I just acquired a merc 650 believed to be from 1969 serial number 2715754. The motor is the 4 cylinder version. The motor has good compression and spark but it runs like crap. I noticed that the top carburetor sucks quite a bit of air in which I believe is normal while the bottom one sucks in very little. What can be the cause of this and how can I fix this? I just took the carbs off and will be doing a rebuild. Any suggestions?
 
GA_Boater

GA_Boater

Honorary Moderator Emeritus
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49,038
Re: Merc 650 bottom carb not sucking as much air as top

Have you checked compression? The sealing of the rings and cylinder wall condition affect the "sucking" ability to draw air and fuel into the motor.
 
S

SEABEE#2

Seaman
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May 18, 2011
Messages
59
Re: Merc 650 bottom carb not sucking as much air as top

Yes compression is even across all cylinders last time I checked. I also checked spark and I have spark.
 
mercurymang

mercurymang

Master Chief Petty Officer
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Re: Merc 650 bottom carb not sucking as much air as top

It could also the the lower seals. If they're shot, you'll be sucking in air/water from the there bypassing your carb.
 
S

SEABEE#2

Seaman
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May 18, 2011
Messages
59
Re: Merc 650 bottom carb not sucking as much air as top

Where are the lower seals located?
 
W

Will Liston

Cadet
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Jul 9, 2013
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Re: Merc 650 bottom carb not sucking as much air as top

Make sure your compression test is done at normal operating temp. If its done wrong it can give you a false sense of security.
 
S

SEABEE#2

Seaman
Joined
May 18, 2011
Messages
59
Re: Merc 650 bottom carb not sucking as much air as top

Any reason the carb wouldnt be sucking air?

Anyone know where the lower seals are located?
 
bgc

bgc

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Jul 13, 2011
Messages
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Re: Merc 650 bottom carb not sucking as much air as top

A bad seal allows the air to be drawn in from below vs the carb. Check for clean spark plugs and or pistons on 4 and 2. I smoked some pistons running with a bad lower seal.
 
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