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89 Mercury 70 hp 8mph full throttle

T

Tyler052

New member
Morning y’all,

I’ve been chasing my tail for a while here. I had the motor completely rebuilt last spring and it ran great the first couple months. Periodically maybe 1 out of 5 times last fall the cost would only go 8 mph full throttle on the initial launch off the trailer. Yes I would let the motor warm up, this isn’t a motor still being cold issue. With the throttle fully engaged after a couple minutes it would go to full speed and I would never have the issue again for the rest of the day. I didn’t think anything of it as it was only a couple of times. The boat goes out 2-3 times a week so it doesn’t sit long in between uses. This year it ran great for the first part of the year and now almost every time going out this same problem is occurring. It’s only on the initial launch but now it sometimes takes 10-15 minutes before it kicks up the rpm and then it’s full speed ahead no issues for the rest of the day. I’m stuck. I’ve changed plugs and fuel filters, I’ve cleaned the carbs. There’s no vacuum leaks. The throttle appears to be pulling all the way. With the motor off and full throttle I am unable to open it any further manually on the motor. I’ve also checked to see if the choke is stuck on but if I choke it, it dies down. Any help or ideas would be greatly appreciated.
 
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Run with a timing light .----Find the cylinder that is not firing.----See when it kicks in as the light will start flashing.-----Was a new impeller installed when motor was rebuilt.
 
fastjeff said:
How were the "carbs cleaned?"

If that's the old two carbed triple, I have a fix for you.

Jeff
Click to expand...

3 carbs, and I ran a high concentrate sea foam mixture at 5 minute intervals, then I pulled them off, soaked them ran a wire thru the ports and blew it out, they were super clean and I didn’t get any gunk out, changed the plugs and filter after that
 
racerone said:
Run with a timing light .----Find the cylinder that is not firing.----See when it kicks in as the light will start flashing.-----Was a new impeller installed when motor was rebuilt.
Click to expand...

So you are thinking that a cylinder isn’t firing and to swap a timing light between the 3 plug wires to see if there isn’t one firing? And yes it has a new impeller that thing squirts water 6 feet at idle lol If a cylinder isn’t firing wouldn’t that make it idle rough? It idles perfect and even at full throttle it sounds normal but just the rpms aren’t going up it sounds like the throttle just isn’t open all the way but it is. If I let off it goes back to idle it doesn’t die out.
 
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I had something similar occurring and it turned out to be a bad plug wire. The only thing the shop could think was the rubber around the plug wire would tight when I got warm making a connection. Took hooking it to a computer to find the issue, motor was a 50hp efi.
 
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