2008 Mercury 20hp 4 stroke problems

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Petty Officer 3rd Class
Joined
Apr 12, 2009
Messages
85
I have a 2008 Mercury 20hp 4 stroke with electric start, short shaft. I bought it used and put it on a 14' Smokercraft aluminum boat. It has always been a difficult starter when cold, will take 10 seconds to crank and finally fire barely. The motor runs poorly when cold, it coughs, stumbles and sometimes quits it almost sounds like it is running on one cylinder. Once the motor has been brought up to operating temp, it runs perfectly wide open and starts perfectly afterwards. Idle's great when warm but low rpm it seems to vibrate the boat an awful lot though no matter if the engine is cold or not. I have changed the plugs, gapped them, new gas tank, line, ran several tanks of no ethanol fuel through it, no change. I am wondering if the Carb would be causing the problem on this?
Thanks
 
M

Maxz695

Guest
Re: 2008 Mercury 20hp 4 stroke problems

i would try to close the mixture valve screw just a tad and see if the engine smooths out a bit. If after adjusting it it still has a great holeshot then it was running to rich. Please mark the amount of turns in to a closed seat and then back out and do the adjustment from there. You may just have some gummed up varnish in the mixture screw. Test at the original setting first then do as instructed in the beginning of the post here. If you hve no luck the float level may be too low or the seat may be leaking. There should be NO FUEL In the carb throats when priming it with the bulb. If so a cleaning and possible new seats are called for.
 

Action

Petty Officer 3rd Class
Joined
Apr 12, 2009
Messages
85
Re: 2008 Mercury 20hp 4 stroke problems

Great thanks. After your tip about the mixture idle screw did a bit more digging and found a service bulletin from Mercury regarding idle and low rpm poor performance as a result of ethanol in the fuels clogging the carb. I don't recall that screw however being covered up? In any event looks like a good clean out is in order in the spring.

https://docs.google.com/viewer?a=v&...K2a9ws&sig=AHIEtbTeTGXjksEbHtIe17IpEqSaakkDYg
 
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