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Mercarb Power Piston

I

ianfs

New member
Hi,

I'm new to the forum and have a challenging question. I own a Sea Ray Seville 1988 with a Mercruiser 4.3L fitted with a Mercarb 2BBL

I've had my old boat a few years but recently stripped the carburettor to re-build it and found the power piston missing. The weird thing is that it looks like it has never been there. The engine runs ok but no much more than about 4000rpm which makes me think that I ought to be there.

I've been trying to find the part but it is not available on it's own. Mercury Marine tell me that Mercarbs were made by Nikki but they don't reply to my e-mail questions.

Has anyone on here come across this before?
 
Yes that's the one....sometimes called a power valve actuator by some techs.
 

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Hi Mik

The Power Valve is situated in the carb base/float chamber! The Power Valve Piston is "or should be" in the Air Horn!
 
Is the power valve in the bowl?

The manual says it should be there.

DId some looking and no parts available like that.

Best bet would be to find a marine repair and see if they have a junk ones with ok internals......maybe someone who participates here may have a spare junk one that cam be scavenged for parts.... I do not have any spares or junk ones......
 
Also I forgot to mention this, someone correct me if I'm mistaken, But I beleive the mercarbs without the power piston were made for smaller engines and the ones with power piston were for v6/v8 engines
 
Thank you all for taking the trouble to reply.....

kghost

Yes the power valve was there when I stripped it, middle picture, the left picture is the Sierra rebuild kit plus a couple of non related parts and the right hand picture is the new valve fitted. My ignorance at this point was priceless because I thought the valve was opened by fuel flow!! :confused:

You mentioned that someone on here might have a scavenged one, that's what I was hoping! :D

MidCarolina

I tried Mike's Carburetors and he said the Rochester piston doesn't fit the Mercarb, that was a real downer as it is only $10 and shipping peanuts!! :(

Interesting about the differences in the carbs for the engines, mine is a V6, but maybe swapped at some stage for the wrong one!
 

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ianfs said:
Thank you all for taking the trouble to reply.....
Interesting about the differences in the carbs for the engines, mine is a V6, but maybe swapped at some stage for the wrong one!
Click to expand...


Other than the wrong prop, maybe that is why you cant reach max rpms.........

Is there a metal tag on the carb? If so reply with the numbers on it.

According to the pdf manuals I have, all 1990-present 4 cyclinder carbed 2 bbl Mercarbs have the power piston and spring shaft.

Looks like either someone has been it it before or it was left out from the factory.
 
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kghost said:
Is there a metal tag on the carb? If so reply with the numbers on it.

According to the pdf manuals I have, all 1990-present 4 cyclinder carbed 2 bbl Mercarbs have the power piston and spring shaft.

Looks like either someone has been it it before or it was left out from the factory.
Click to expand...

No tag, it's stamped on....
Mercarb Model no 3304 9565 dated 7Y03

Are, so they should all have them.
 

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When I picked up my boat it would run but had a bad dead spot at about 1/4 throttle........rebuilt the carb ( mercarb ) and found that the power piston was frozen would not move at all.........got it freed up working right boat runs great...

Like I said I am not sure were I read about power piston deleted in some carbs.......maybe I'm wrong, my memory aint what it once was.......but yours should have it cause without the power piston there is no need to have the power valve there........
 
ianfs said:
Thank you all for taking the trouble to reply.....

kghost

Yes the power valve was there when I stripped it, middle picture, the left picture is the Sierra rebuild kit plus a couple of non related parts and the right hand picture is the new valve fitted. My ignorance at this point was priceless because I thought the valve was opened by fuel flow!! :confused:

You mentioned that someone on here might have a scavenged one, that's what I was hoping! :D

MidCarolina

I tried Mike's Carburetors and he said the Rochester piston doesn't fit the Mercarb, that was a real downer as it is only $10 and shipping peanuts!! :(

Interesting about the differences in the carbs for the engines, mine is a V6, but maybe swapped at some stage for the wrong one!
Click to expand...
Your carb should have the vacuum actuated plunger to operate the bowl installed power valve. If for some reason you cannot locate a replacement, on some carbs, its possible to increase the main jet size a few sizes, and eliminate the power valve. Good luck! Rick
 
It's worth a shot..........but you kinda gotta ask yourself ifn it would work right with bigger jets why was it not engineered with larger jets and no power valve to begin with...........just sayin, but it could work just fine stranger things have happened
 
In my opinion dont go playing carb tuner...........Not a way to go about it.

Go online, buy a replacement carb, Be done with it.

Your time is worth money, so how many hours will you waiste and not needed parts will you buy before you realize nothing worked........

That is what I would do!!
 
ianfs said:
Hmmm.

That sounds like a plan Rick. Thank you!
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Your welcome ianfs. I would still recommend finding the actuator and installing.....and increasing jet size would probably work but as a last resort. I mentioned the increased jets sizes as a "work around" IRT shelling out maybe $600 for a new carb or maybe $400 for a reman. I know that for me, and many others, if I had to purchase a new carb, my boat would have to sit for a while.
 
Fastatv said:
Your welcome ianfs. I would still recommend finding the actuator and installing.....and increasing jet size would probably work but as a last resort. I mentioned the increased jets sizes as a "work around" IRT shelling out maybe $600 for a new carb or maybe $400 for a reman. I know that for me, and many others, if I had to purchase a new carb, my boat would have to sit for a while.
Click to expand...



Do some online reasearch and you will find the carbs are reasonably priced.

Tuning a carb that has already been tuned by merc enigineering is not a good idea. especially on a simple 4.3 ltr engine........



People come here to help fix there boats.

We who offer advice do not offer "work arounds". Either would a marine service center!!

Work arounds are what end up causing issues and possible injury. Maybe not in this case (injury) but I think you get the point.

Many questions asked here are due to work arounds done by someone along the way......Perfect example is when boat owners decide to rewire stuff and dont use the correct wire colors or guage size.........


No one ever said boating was cheap
 
And y'know, even though re jetting sounds like a plan, it probably wouldn't be an option I would honestly look at, for the reasons already mentioned.

I would have gone for a remanufactured carb a while ago if I could find one in the UK, they seem to be in the US and the shipping is about as much as the carb, hence trying to source the piston first.
 
kghost said:
Do some online reasearch and you will find the carbs are reasonably priced.

Tuning a carb that has already been tuned by merc enigineering is not a good idea. especially on a simple 4.3 ltr engine........



People come here to help fix there boats.

We who offer advice do not offer "work arounds". Either would a marine service center!!

Work arounds are what end up causing issues and possible injury. Maybe not in this case (injury) but I think you get the point.

Many questions asked here are due to work arounds done by someone along the way......Perfect example is when boat owners decide to rewire stuff and dont use the correct wire colors or guage size.........


No one ever said boating was cheap
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Actually, what I recommended is really a fix ( tho if you read my posts thoroughly, you will observe that I recommended finding and installing the actuator ), and has been done on countless carbs. My definition of any of my suggested work arounds would be a safe and effective alternative. It appears that your definition of a work around is to "jury rig". I respect your definition, try to respect mine. As for the wiring example you provided, yes, not smart, and unsafe, but that's not a work around....its my definition of "jury rigging"....LOL. And as for marine service centers, well, you can say what they recommend or wouldn't recommend, but in reality, some actually break the emissions rules ( against the law ) by installing different sized air bleeds for fuel signals and different sized jets in order to remove bogs and increase performance. I know this first hand. Anyway, lets hope the man locates the missing actuator, and gets his issue resolved.
 
ianfs said:
And y'know, even though re jetting sounds like a plan, it probably wouldn't be an option I would honestly look at, for the reasons already mentioned.

I would have gone for a remanufactured carb a while ago if I could find one in the UK, they seem to be in the US and the shipping is about as much as the carb, hence trying to source the piston first.
Click to expand...


Well that makes since........I just assumed you are in the States....Sorry
 
midcarolina said:
When I picked up my boat it would run but had a bad dead spot at about 1/4 throttle........rebuilt the carb ( mercarb ) and found that the power piston was frozen would not move at all.........got it freed up working right boat runs great...

Like I said I am not sure were I read about power piston deleted in some carbs.......maybe I'm wrong, my memory aint what it once was.......but yours should have it cause without the power piston there is no need to have the power valve there........
Click to expand...
Hi, sorry tp revive this post. But if your remember( i know its been 10 years....) how did you go about freeing it up. I have just rebuilt the carb and can not get the idel below 2k rpms. And also have a stuck power valve actuator
 
When my Mercarb equipped engine has that problem, it is always hard deposits in the thin ( bottom) ends of the jets on the venturi block. These hard deposits don't come off in any cleaner I've tried and must be removed mechanically. A very thin wire, or better the correct sized numerically sized drill is what I use. Be careful not t o remove any jet ( brass) material itself just the tan colored crap
,
 
sandkicker said:
When my Mercarb equipped engine has that problem, it is always hard deposits in the thin ( bottom) ends of the jets on the venturi block. These hard deposits don't come off in any cleaner I've tried and must be removed mechanically. A very thin wire, or better the correct sized numerically sized drill is what I use. Be careful not t o remove any jet ( brass) material itself just the tan colored crap
,
Click to expand...
Actually its the jets on the emulsion block
 
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