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09-02-2009, 08:05 PM #1Junior Member
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mercury modified 2.4 fuel presure ??
I am currently breaking in a modified Merc 200 offshore exhaust with raised exhaust ports, D ported pistons, with finger ports exhaust cavity work and 30 cc heads. It has 7 pedal reads Jets are at .092 motor will idle fine at 6 lbs on the electric pump but runs out of fuel at 6,000. Motor likes 7 lbs on top end but stumbles at low rpm going on plane
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09-02-2009, 08:42 PM #2
It's not all about pressure - volume is key. I'm running a similar motor and it is happy at 3.5#. Merc carbs (especially wh series 2.4 carbs) are notorious for not being able to handle much pressure.
T-Rex runs a highly modified 2.0L F1 300hp motor with carbs and he runs 3.5# and spins it over 10 grand.
'95 STV "The Blue Goose"
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09-04-2009, 06:57 PM #3Junior Member
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I tried playing with the pressure for the top end.I found that the motor liked more pressure like at 8 #'s ran stronger. Going to try larger jet size possibly.
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09-04-2009, 08:16 PM #4
that is plenty of pressure........... if your pressure is low you will melt one in a hurry....
do you have egt's or just going by a guess that it is running out of gas... at 6000 rpm's and u lean it out "run out of fuel pressure it would have melted...
be very careful.......93 xr 2001 drag with a 15" drag
92 xb 2002 comp with a 15" drag
R.I.P. Phil "rpm racing" Watch over the rest of us my friend.
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09-05-2009, 05:41 PM #5Junior Member
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The boat runs strong up to 6,000 and seems to bog. Don't have egt.
At 8 lbs runs good but low end sucks.
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09-05-2009, 08:14 PM #6
Fuel pressure on a carb motor has NOTHING to do with the way the engine runs at ANY RPM........... UNLESS it's too high. 8 lbs is too high for any Merc needles that I've ever seen. If the engine is varying in the way it runs with low pressure, then you don't have enough volume for one of several reasons. You could mount a gas can on top of the cowl and gravity feed the engine, and if the lines are big enough to keep the float bowls full at WOT, then it'd run exactly the same as it does with a properly regulated pressure system. Carbs are a DEMAND system. The main jets are fed ONLY from the vacuum through the carb venturis. Raising the fuel pressure has no effect on the flow through the main jets. If excessive pressure overwhelms the needles, then you'll get excess fuel back through the vent system, still with NO effect on the main jets. Of course this overflow would richen the engine, but uncontrolled would have a negative effect.
Get a fuel pump with an external regulater and sufficient volume to supply your carbs at 5 PSI.Membership upgrade options: http://www.screamandfly.com/payments.php
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09-06-2009, 10:18 AM #7Junior Member
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I am running an external pump and regulator. When you stated about fuel flow are you talking about changing the existing 5/16 lines over. And or you talking about larger jets?