Advice needed

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    Posted: 24 Jan 2021 at 10:26pm
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Got a 2003 Merc 2 stroke with an issue and I reckon I have found where the problem is but not exactly certain what it is. Motor starts and runs when choked and with half throttle or more. Soon as throttle down and choke off she dies. Process of elimination pointed to carb issue. What I found was a little blue plunger that primes the carb when the choke is pulled right out.


This has a rubber boot over it and this is worn and slack. This I think is letting air into the carb.



I found that if I push the plunger down fully with my finger she idles just fine. So till I confirm this I fixed it with a westie solution 😁



So question is whether it is the rubber boot or is it something in the primer pump itself? If anyone has come across this before or has any advice, much appreciated.





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Post Options Post Options   Likes (0) Likes(0)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2021 at 7:52am
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I am guessing it is a smaller Merc 2003. I have a 2003 (90) but not that system.
I am thinking, since it is attached to your choke mechanism, I am guessing it is a fuel bypass system. Letting more fuel in to the carb. Looks like it maybe on the fuel pump - guessing.
So I would be looking for a blockage in the carb system - blocked idle jet, blocked fuel filter or screen in the carb. Or maybe fuel float issue - float level. I am guessing the pump is working OK as you can run OK at higher revs.
Alan
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Post Options Post Options   Likes (0) Likes(0)   Quote bazza Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2021 at 8:29am
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Not sure if it app[es to all models but I always thought my 90 hp merc had a choke that was operated by pushing the starter key in to assist
cold starts.

However have been told by an o/b mechanic that it is not a choke function as such but is an enriching function that when the key is pushed in it cuts out delivery of fuel to one or two of the cylinders therebyincreasing the fuel supply to the remaining cylinders ... hence the reason the initial start is a tad erratic for a few seconds until normal delivery is resumed.
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Post Options Post Options   Likes (0) Likes(0)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2021 at 9:08am
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Correct Bazza - on that motor. Your push key function activates a solenoid in the fuel system that lets extra fuel in from the pump system.
I am guessing this mechanical choke pluger is basically trying to do the same thing.
Thinking on further I am more inclined to something like a blocked idle jet. A blocked screen or bad float setting would probbaly not allow it to run at higher revs while performing poorly at low revs. So it sounds like it is not getting enough low end fuel rather than being starved for fuel overall.
I am thinking a teardown of the carb, or possibly a small airleak in the system. Which would lean the fuel mix out disproportinately at the low rev end.
Check the carb is firmly bolted in place.
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Post Options Post Options   Likes (0) Likes(0)   Quote Transformations Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2021 at 9:10am
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Sorry guys, forgot to add it was a 25hp pull start. so you pull out the choke to close the butterflies, then pull out a bit further which then depresses the plunger to prime the carb. As in the pic, she idles perfectly with the clamp holding the plunger down and I guess stopping air from entering the carb through the worn boot. Hoping no other issues with the carb as the motor is running fine at revs and under load. 
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Post Options Post Options   Likes (0) Likes(0)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2021 at 9:58am
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I doubt that is the explanation for the issue. Could be wrong, but I doubt the boot is an airtight seal. If there was an airleak I suspect you would have a fuel leak there too when you prime the bulb and depress the pin. I think you are basically masking the problem by pouring more fuel in.
I would look for an airleak at the carb or a blockage of the idle jet. You would have to pull the bowl of the carb at a guess to find the idle jet.
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Post Options Post Options   Likes (0) Likes(0)   Quote MATTOO Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2021 at 4:27pm
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I'd go straight for the jets.
Take them out and clean.
Check the bowl for signs of debri.
And replace the inline filter.
Eyes open for water in bowl.
Just cruising in my now sweetas pimped out Southern 755 HT0!
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Post Options Post Options   Likes (0) Likes(0)   Quote Transformations Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2021 at 4:33pm
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Your right Alan, if air can go in fuel can come out. Thanks MATTOO, will follow that advice and strip it down. Cheers.

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Post Options Post Options   Likes (0) Likes(0)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2021 at 5:38pm
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I have just had a look at my Merc manuals - which cover the 25 HP version too. I can't quite relate their diagrams to your pics.
But the other things they refer to - other than what I have posted is
water in fuel - drain carb bowl and check.
A stuck ball valve in the enrichment system , and faulty fuel pump and blocked filter.
I think you can rule out blocked filters, but always good to check. And faulty fuel pump - runs OK at high revs.
There doesn't seem to be an idle jet - but as I say I can't quite relate the pics to the manual diags. The main jet is in the center of the bowl I think. There is of course an idle mixture adjust screw. If you are going to pull it apart, set the screw fully in (gently)- count the turns, then remove and blow that system clean to, and refit at same turns.
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Post Options Post Options   Likes (0) Likes(0)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2021 at 5:54pm
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One thing the manual suggests is to check for airleaks between the pump/enrichment body and carb. I suggested checking the carb is properly seated - but they suggest with an oil can dribbling oil around the gasket/mating surfaces around the pump system and carb. If the engine improves you have found a leak (and temporarily sealed it).
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Post Options Post Options   Likes (0) Likes(0)   Quote Transformations Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2021 at 10:06pm
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Thanks Alan, I think I'll order a gasket kit before I pull it apart. In the meantime I'll test for leaks as you suggested 👍

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Post Options Post Options   Likes (0) Likes(0)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jan 2021 at 7:58am
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Another way I chase airleaks on intake systems is to spray brake cleaner around the suspect areas - while the engine is running. A leak will suck more hydrocarbons in to the intake and the mixture will richen. But you are probably dumping a lot of fuel in with your 'fix'' and it would work best if the engine was struggling.
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Post Options Post Options   Likes (0) Likes(0)   Quote Transformations Quote  Post ReplyReply Direct Link To This Post Posted: 04 Feb 2021 at 8:06pm
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UPDATE......

So, spoke to the local boatshop and the mechanic seemed to know exactly what the issue was. Asked him if the service parts for the primer pump came with the carb service kit. Nah he says, seperate kit and carb kit around $160 and the primer kit even more than that he says..yikes!

Started looking around and finally sourced from promptparts (Kev & Ians marine). $55.55. Managed to change the diaphram without taking the whole carb off and sadly no change. Did a looksee on Youtube and got a pretty good lesson on stripping the carb down for servicing. Decided to give it a try without a carb kit and reassess once opened up

Holy hell, the amount of gunk that came out and off the various parts. Gaskets, needle and jets looked pretty good so cleaned up and put back together. After a few Hail Marys I fired her up and perfect! Never heard it run so smooth and sweet . So all up $55 for the primer kit and $15 for a can of carb cleaner....can only imagine what it would have cost for the boatshop to do it. Dont begrudge them charging for labour and a fair markup on parts but sometimes they seem to be pulling the rrrrr's out of it.

Thanks to all for advice and I think I only got the courage to tackle it because I knew I could get advice and support if I knacked anything up. Cheers



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Post Options Post Options   Likes (0) Likes(0)   Quote smudge Quote  Post ReplyReply Direct Link To This Post Posted: 04 Feb 2021 at 9:02pm
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Great result Transformations. Look forward to you next Manukau report :)
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