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Facrory New 5.7 bogs down under load

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rl65028

Contributing Member
Just took first trial run on factory new 5.7 mercruiser on my 22 ft walkaround cuddy. At about
3800 rpm the boat just starts to come up on plane and the engine coughs and bogs down and rolls back to idle. It will not take any more fuel. Cant really get past 3000 rpm. Runs fine on garden hose wont run under load. HELP!
 
3800 RPM to plane???? My 4000 # 23 ft cuddy Deep V with a 260 HP 5.7L gets on a plane with two people on board below 2500 PRM! ( 15 x 21 Prop). Are you sure about your tach? timing?
 
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MikDee said:
Check carefully for crossed spark plug wires first.
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I had one guy tell me its due to wrong prop or gear ratio a 4.3 came out and a 5.7 went in. Gears are 1.84 to one and prop is 15 x 15.25. I understand prop may be now too small and even if I do need a 1.47 to one how can that effect Fuel? Its sounds fuel starved when I open it up. It wont take enough fuel to get on plane.
 
re:" it won't take enough fuel to get on plane".... Unless you have a fuel flow meter in line with the engine, there is no way for you to draw this conclusion correctly.

How many RPMs did it take to get the boat to plane with the old engine and old prop (which I assume you are still using?) Also... did you change the tach? You cannot use a tach set up for a 6 cylinder engine on an 8 cylinder one. There are two flavors of tach. Those set up for a particular number of engine cylinders, and the other type has a selector switch for 4/6/8 cylinders.
 
That's like putting 4:11 gears in a car that had 3:36 gears in it, you seem way underpropped? Are you hitting the rev limiter by any chance?

I'd guess you need a 21" pitch prop at least, maybe even a 23" with that gearing.

Either way, I think there's issues with your engines power.

Next, check your timing, make sure the mechanical spark advance in the distributor (rotor turns some one way, and springs back the other way).
 
MikDee said:
That's like putting 4:11 gears in a car that had 3:36 gears in it, you seem way underpropped? Are you hitting the rev limiter by any chance?

I'd guess you need a 21" pitch prop at least, maybe even a 23" with that gearing.

Either way, I think there's issues with your engines power.

Next, check your timing, make sure the mechanical spark advance in the distributor (rotor turns some one way, and springs back the other way).
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Original prop and tach. Timeing has been checked and set to 8 degrees before TDC at idle. This engine is brand new. Yes I agree it is under proped but I should be able to throttle up get on plane and overspeed in theory I have to throttle back from 3800 it will cough and sputter till I throttle back Again it runs great out of water.
 
Yes my Wellcraft planes out just below 2500 are you sure your float level is correct or are the secondary`s not opening sometimes they don't open soon enough
 
rl65028 said:
Original prop and tach. Timeing has been checked and set to 8 degrees before TDC at idle. This engine is brand new. Yes I agree it is under proped but I should be able to throttle up get on plane and overspeed in theory I have to throttle back from 3800 it will cough and sputter till I throttle back Again it runs great out of water.
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You said... "....a 4.3 came out and a 5.7 went in...." . IFSO, a 4.3L is a 6 cylinder engine, a 5.7 is a V8. Hate to harp on this, but is current tach set up for 8 cylinders?
 
yes could be wrong tach it is original for V6 . But the fact remains the boat will not plane Will not take enough fuel to do so. Can only put around reatively slow.If I open throttle it acts as starved for fuel.
 
On some ignition systems, in order to set base timing, a test point (on some engines just a dangling lead) is grounded. If one forgets to unground this point after setting base timing, the ignition will not advance at all and the engine will not develop full HP. Just because you open the throttle more and the engine won't go any faster does not mean "starved for fuel".
 
sandkicker said:
On some ignition systems, in order to set base timing, a test point (on some engines just a dangling lead) is grounded. If one forgets to unground this point after setting base timing, the ignition will not advance at all and the engine will not develop full HP. Just because you open the throttle more and the engine won't go any faster does not mean "starved for fuel".
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Thanks Capt Bob, Mine is delco ign and the advave is disabled with a positive lead installed temporally. I set it @ 8 BTDC at idle with this procedure. Tomorrow Im going to take it on water and check total advance I thnk I should get 26 total @ 3000 RPM I would bet that it is there. I suspect that for some reason my secondary throttle plate is not opening, even though I am in fact hitting physical stops. Another run on water tomorrow and will post findings If I resolve. Thanks to all for inputs.
 
If your secondary air valve on top opens with vacuum, & lifts the metering rods, then the secondarys are opening, if not, then no, they're not. Is your flame arrestor clean?
 
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re: " secondary throttle plate...."
Current production 5.7L engines with carbs use 2BBL carbs, i.e., single throttle plate.

I recently had to dismantle the carb on my 5.7L engine to chip out hard deposits in one of the idle jets in order to clear it (engine would not run under 1500 rpm). Could be hard deposits in main metering jets in your case. In my case, deposits were rock hard and no amount of carb cleaner touched it.
 
This engine is factory NEW not rebuilt. Carb also. (Edelbrock 1409) Got to looking at it today and I see the choke plate will lock the secondary plate from opening if its not wide open. I adjusted the choke looser a couple lines and will run on water tomorrow. I am almost certain the secondarys are not opening for some reason. Does any one think Im heading down the right road?
 
Yes, this can happen. Better yet, get the engine warmed up then just lock out the choke like you said, you don't need it then.
 
I'm not an expert on the MERC product line, but I did research it pretty well when I had my boat re-engined. I think that the current carb on factory 5.7L is a 2BBL (260 HP). For more HP, MERC does not go to a 4BBL these days, but rather fuel injection.
What is the serial number on that engine? It should start with a "0W" where 0 = the number zero.
 
Engine SN MM507G 4 barrel normal aspirated. Pretty Sure I got it figured out. Boat has a GLM 24950 Fuel filter- water seperator Houseing. The houseing has a plastic standpipe that extends down in to center of spin on GLM 24940 filter. If you look up Sierra equivilant filter it will cite Sierra 18-7845 . IT IS NOT. The plastic standpipe will bottom out tight inside this filter. It will fuel starve with a high demand. Thanks To MikDee Sandkicker turbot660 and Bt Doctur Hope I can help someone else Happy boating to all.
 
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