re-jetting a Mercarb

M

MBAKER

Petty Officer 1st Class
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Jun 9, 2008
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Im rebuilding my mercarb and wondering if anyone has messed with jetting any. The issue is a hesitation or bog off idel when pulling a skier, but why Im in there just thinking if I can help it more. I assume from the factory its prob tuned for fuel effeciency and wondering if a little more fuel might actually help it pull better.



Has anybody messed with or have suggestions on tuning/raising float level, or more main jetting to help with transition and midrange power? Or raising the accelerator pump to the top hole?
 
fat fanny

fat fanny

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1,935
Re: re-jetting a Mercarb

Re-build it to the letter changin jets will only help when sealevel has changed your chance of the carb working perfectly are 50/50 at best after a re-build anyway the mercarb is what it is and any improvement would be minimal!
 
mpdive

mpdive

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Re: re-jetting a Mercarb

You may be looking at an accelerator pump issue or the acc pump arm is located in the wrong linkage hole. It is a lean condition. There were driveability issues with some carbs, and some check balls were removed due to these problems. Rejetting may lead to more driveability issues. Have you checked the timing? I personally would not change the jets, but instead locate the underlying problem. Although the craft is set up for optimal fuel usage, it was also calibrated for performance at all ranges.
 
M

MBAKER

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Re: re-jetting a Mercarb

Im not sure I understand why its a 50/50 shot after rebuilding? Are these carbs really that junky?


According the service manual the pump is in the right hole (middle), but it sure acts like it could use a better shot off idle. Thats why i decided to rebuild it anyway. I could easily try moving it up to the next hole too. I prob wont mess with jetting any then, I just thought maybe somebody had played with them a little bit. Seems like off idle and transition needs the most work anyway. If you ease into it its fine, but if you hammer it with a skier it bogs and then picks up. Just crusing its fine.

and yes... timing is good.
 
mpdive

mpdive

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Re: re-jetting a Mercarb

MBAKER said:
Im not sure I understand why its a 50/50 shot after rebuilding? Are these carbs really that junky?


According the service manual the pump is in the right hole (middle), but it sure acts like it could use a better shot off idle. Thats why i decided to rebuild it anyway. I could easily try moving it up to the next hole too. I prob wont mess with jetting any then, I just thought maybe somebody had played with them a little bit. Seems like off idle and transition needs the most work anyway. If you ease into it its fine, but if you hammer it with a skier it bogs and then picks up. Just crusing its fine.

and yes... timing is good.
Click to expand...

Was the accelerator bore check ball there or had it been removed?
 
M

MBAKER

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Re: re-jetting a Mercarb

I havent taken it apart yet, still sitting on the bench.
 
mpdive

mpdive

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Re: re-jetting a Mercarb

MBAKER said:
I havent taken it apart yet, still sitting on the bench.
Click to expand...

If the check ball was removed be sure to reinstall. The removal was part of a bulletin for over rich conditions. It would allow some fuel to return to the fuel bowl. If you have another hole available on the pump arm, it will give you approx.
.5cc's more fuel on initial opening. I know ther was a bulletin somewhere about it, but doubt yours is removed. Like I said earlier, make sure tune up wise everything is in order. You will always have some hesitation on a carb engine due to the change in manifold vacuum.:)
 
achris

achris

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27,468
Re: re-jetting a Mercarb

MBAKER said:
.... I assume from the factory its prob tuned for fuel effeciency and wondering if a little more fuel might actually help it pull better. ...
Click to expand...

You assume wrong... They are jetted RICH from the factory, to try and reduce the possibility of valves burning by fools who stick props on that are too big because they 'don't like their engines running too fast' :facepalm:....

Leave the jets the way they came from the factory and fix the problem properly, don't just try to mask it....

Apart from 'Mercarb", you haven't told us what you're working on....
 
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friendly_jacek

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May 12, 2008
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Re: re-jetting a Mercarb

fat fanny said:
your chance of the carb working perfectly are 50/50 at best after a re-build anyway the mercarb is what it is and any improvement would be minimal!
Click to expand...

Yes, I would like to hear more about it. I just rebuilt one. Am I doomed?
 
90stingray

90stingray

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Re: re-jetting a Mercarb

I have rebuilt a couple Mercarbs... one i was trying to find a surge and backfire. I drilled out jets and swapped to different sizes during troubleshooting... was easy to do. The jets are not going to help a off idle stumble. I would say, like you mentioned, look at float levels... accelerator pump... etc.
 
M

MBAKER

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Re: re-jetting a Mercarb

I got it apart last night, it looks like it might be a float level problem. It has the spring loaded needle but float level was set at about 3/8, not 9/16 like the manual says. Unless there is some revision that I havent seen anyway. The odd thing is this thing had to come from the factory this way, because you could tell from paint it had never been off the boat. So did Merc not set the float level right to start with?
 
M

MBAKER

Petty Officer 1st Class
Joined
Jun 9, 2008
Messages
279
Re: re-jetting a Mercarb

MBAKER said:
I got it apart last night, it looks like it might be a float level problem. It has the spring loaded needle but float level was set at about 3/8, not 9/16 like the manual says. Unless there is some revision that I havent seen anyway. The odd thing is this thing had to come from the factory this way, because you could tell from paint it had never been off the boat. So did Merc not set the float level right to start with?
Click to expand...

I got ahead of myself... I jsut saw the foot note in the manual about 5.0 engines using float height of 11/32 with the spring loaded needle which would be close to the 3/8 I measure real quick last night. So I guess that part was ok....

Thanks for help all, As long as i can figure out how to get this pesky power valve out I should be good.
 
F

friendly_jacek

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May 12, 2008
Messages
48
Re: re-jetting a Mercarb

MBAKER said:
As long as i can figure out how to get this pesky power valve out I should be good.
Click to expand...

The problem I came across with my rebuilt was the needle seat was stuck and I couldn't unscrew it. Then I remembered about Airkroll I had and soaked it for several minutes. Any good penetrating fluid should work.
 
mpdive

mpdive

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Messages
567
Re: re-jetting a Mercarb

MBAKER said:
I got ahead of myself... I jsut saw the foot note in the manual about 5.0 engines using float height of 11/32 with the spring loaded needle which would be close to the 3/8 I measure real quick last night. So I guess that part was ok....

Thanks for help all, As long as i can figure out how to get this pesky power valve out I should be good.
Click to expand...

If your referring to the small bottom valve on the float bowl, use the widest blade screwdriver. You can give it a little hammer tap first to break the seal. I would not attempt to remove the airhorn plunger as they can get stuck and break. There are no replaceable seals in the airhorn casting anyhow.
 
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