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Boat stalls out around 3000 Rpm

Old 06-13-2016, 02:03 AM
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Default Boat stalls out around 3000 Rpm

I have a 24' 91 python, I have a brand new 750 Holley double pumper on it and a Holley series black electric fuel pump with a Holley regulator set and 7 psi. Boat Stumbles to get out on plane and wont go over 3000 rpm, when you do it just starts to fall on its face, the RPM's will drop and backfire out the carb like its to lean. Im 80% sure its not a fuel problem, but when i pump the rear accelerator pump i can get it to turn around 3800 RPM. Im stumped, Been changing jets, metering blocks, power valves, boat has just freshly been put together, it has thunderbolt 4 ignition. I'm starting to lean towards ignition. Im not really sure how marine distributor works or the Thunderbolt system. Im kinda clueless of what to do if any of you guys can point me in the right direction! :P
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Old 06-13-2016, 06:26 AM
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What is the fuel psi showing when the problem occurs?
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Old 06-13-2016, 07:30 AM
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does the carb have a vacuum port that did nty get plugged before installing the carb?
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Old 06-13-2016, 12:39 PM
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All Vacuum ports are plugged, with mechanical secondaries, Fuel pressure holds 7 psi, does not drop at all. Today I went back over with the timing light and i noticed 2 wires by the distributor, A Brownish Black wire with a white strip and a solid black wire, they were not plugged in together. I put them together and at 3000 rpm in neutral on the water hose the motor advanced to 34 degrees, sounds about right to me. I'm putting it back in the water tomorrow to see if those wires where maybe the problem. Ill let you guys know how it goes. I'm not familiar with electric advanced distributors, How crazy would it be to put a HEI on it? Heard that they wont last long, the weights will just rust and get sticky because of all the water, but im not keeping my boat on the lake? What do you guys think?

Last edited by AayJay; 06-13-2016 at 12:46 PM.
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Old 06-13-2016, 01:00 PM
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Look on page 4 of below link for Tbolt IV wiring.
http://www.boatfix.com/merc/Servmanl/16/16F4R2.PDF
Move cursor to lower right of page and you'll see a + and - sign for changing the size of the page.


No reason to change distributors unless you need a timing curve that is different from any of the available modules.
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Old 06-13-2016, 01:45 PM
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Well here is whats kinda weird, in the diagram you linked, im using a wiring harness from and Thunderbolt 4 mounted on exhaust elbow, but the distributor is the Thunderbolt 4 mounted on the distributor its self.... Man im stumped.... According to the diagram the brown/white and black im messing with are for the trim sender. Do you think the stock timing curve in the Thunderbolt 4 is enough for my motor? I have a 498 ci BBC with .700 lift solid roller cam, real radical idle and all top end power. On the dyno it dose not start to make power tell the rpms are around 3500-4000, I cant even get that high without it leaning out and backfiring out the carb. Any advice would be sweet, maybe my motor and cam is just to much for the stock distributor and coil?
Edit: Maybe my distributor is not grounding very well? I have a aluminum intake with a bronze gear for the solid roller cam, maybe a should run my own ground? I think im going to slap a 900 cfm holley on it tomorrow ands see if it makes any difference, but the carb i have on it is brand new?

Last edited by AayJay; 06-13-2016 at 01:53 PM.
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Old 06-13-2016, 02:47 PM
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The curve will be wrong, unless maybe a V6 module is used.

And your carburetor is way wrong. I've used those on 350's and 383's.

My 02.
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Old 06-13-2016, 02:48 PM
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Do I dare ask is your exhaust is wet or dry ?
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Old 06-13-2016, 02:52 PM
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Originally Posted by AayJay
Well here is whats kinda weird, in the diagram you linked, im using a wiring harness from and Thunderbolt 4 mounted on exhaust elbow, but the distributor is the Thunderbolt 4 mounted on the distributor its self.... M
Page 5 has the TBolt IV with module on distributor:

http://www.boatfix.com/merc/Servmanl/16/16F4R2.PDF
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Old 06-13-2016, 03:39 PM
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yeah both the harnesses are the same on 4 and 5, but on 5, the one i have, the distributor grounds to the block and the module grounds to the distributor. Maybe I should run a ground wire to the block myself since my intake is aluminum and the gear in the distributor is bronze? Im going to pull the module off and put dielectric grease on it. The exhaust is wet but what does that have to do with anything? I know the carb is a little small for the motor, but it still shouldn't be falling on its face when the secondaries open. Ill try another carb, 900 cfm should be plenty. I have 80-98 jet set, right now i have 84's in the front and 94's in the back.
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