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 Loss of power at WOT under load. 
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Joined: Sun May 08, 2011 3:30 pm
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Post Loss of power at WOT under load.
I recently finished rigging a new 1854 uncle j boat with my old 2007 Gatortail. Motor has 206 hours on it and mods were done around 175. It has PVT cam and heads were done by PVT also. It also has 42hsr mikuni with 165 main 22.5 pilot on 2nd from bottom need clip running bps crossover exhaust. Im still using a CA19 powertech prop.

Now for the issues. Boat idles great and gets on plane great but at WOT it loses about 300 rpm. At 3/4 throttle it runs about 28mph at 4150rpms if you open it up it falls on its face and drops to about 3900. Out of the water it would rev to the moon. Ive cleaned the carb twice and moved tank to back and checked all fuel connections. Clear fuel filter is staying full. I ran the boat all weekend basicaly on 3/4 throttle and plugs looked great. Also checked and reset valves. Any info would be appreciated.


Wed Nov 22, 2017 9:05 pm
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Joined: Fri Jan 10, 2014 1:16 pm
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Location: Gladewater TX
Post Re: Loss of power at WOT under load.
Sounds like the main jet need to be dialed in. Most heads cam ex motors run a 165 to 175 mains depending on too many variables to list. U have a 170 laying around to try? Or a buddy with a wideband o2?


Wed Nov 22, 2017 11:36 pm
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Post Re: Loss of power at WOT under load.
I went to the harley shoo yesterday and bought a 167.5 and a 170 but havent had a chance to run it yet.


Thu Nov 23, 2017 8:42 am
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Joined: Wed Nov 24, 2010 5:07 am
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Location: Miramar Fl
Post Re: Loss of power at WOT under load.
Have this same problem with my engine now. Let me know how the larger jets work.. keep us posted
I just assumed I reached the capability of the engine, or that my carb slide was opening too much then it'd fall on its face.


Mon Nov 27, 2017 12:15 pm
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Joined: Wed Jan 14, 2009 7:23 am
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Location: Tarkington, TX
Post Re: Loss of power at WOT under load.
Actually reading.... You gain RPM when you let off the throttle. That tells me you're too fat on the main jet. Drop to a 162.5. Porting, cam, and valve jobs make the most of what jetting to run. Its all about flow and port velocity.

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Bayou Surface Drive
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Tue Nov 28, 2017 8:21 am
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Location: Gladewater TX
Post Re: Loss of power at WOT under load.
Agree with him^ the symptoms sound fat on top. I'm just surprised it can't eat a 165 jet. But every engine is different


Tue Nov 28, 2017 9:42 am
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Post Re: Loss of power at WOT under load.

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Bayou Surface Drive
337-351-0362
Tarkington, TX 77327


Tue Nov 28, 2017 11:27 am
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Post Re: Loss of power at WOT under load.
Tried a 167.5 and rpms improved through the throttle range. Plugs still looked a little lean so I went with a 170. She running good at this point. Also called and talked to Al and he stated the 42 typically likes a bigger jet with his cam and head work.


Wed Nov 29, 2017 6:23 am
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Post Re: Loss of power at WOT under load.
What are you looking at on plugs?
You want to look at a FULL TURN of color on the BASE ring of the plug. Don't look at porcelin

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Bayou Surface Drive
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Wed Nov 29, 2017 2:53 pm
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