Bogs down for 1/2 hour

[Grant Brown]

70 Merc 3 Cyl serial # 0A739874<br />The last couple of times I've used my boat, it starts up fine and runs a bit rough at first. After I idle out and try to open it up, it won't rev out. This boat usually just takes right off and is up on plane in seconds. Now it takes minutes. Max speed is usually about 35 mph now it's only 18 mph. I can feel it "kicking" slightly like it wants to go but it doesn't. All of this lasts for about half an hour, and then it kicks in and runs great. It also seems to be using way more fuel than usual too. The first time this happened it just stopped on it's own and the last time it stopped after I shut the motor off for 10 min while fishing. I fired it up again and it blew a bunch of blue smoke for few seconds and then was fine all day. The boat had been in storage for 6 years before I got it so the carbs were pro-rebuilt this season, with new plugs, fuel lines and tanks. <br /><br />Thanks<br />Grant

 

[Chris1956]

Re: Bogs down for 1/2 hour

Grant, That problem is hard to diagnose. I will guess that you may be having partially fouled plugs. Also the fuel pump diagram may have a small hole allowing the bottom cylinder to foul out. I would check the compression and see if it is nearly the same for all cylinders. If so, put a timing light on each cylinder and prove spark is consistant and strong. If so, it is time to investigate the fuel system as I described above.<br />Let us know if you have success

 

[Grant Brown]

Re: Bogs down for 1/2 hour

Compression was exactly the same on all 3 cylinders, 125 if I remember correctly. I'm wondering if maybe there's something wrong with the oil injection because of the oil smoke after I restarted it.

 

[raskal]

Re: Bogs down for 1/2 hour

I had a very similar prob with my '85 50HP. Every morning it would bog and take 20-30min before it would run at full throttle.<br /><br />Finally gave up and took it to my mechanic and he stated that cyls 2 & 3 were not firing during this time. He traced the problem back to the spark plug switchbox. He replaced it and it fires up and runs fine anytime now.<br /><br />Check for all cyls firing during this time.

 

[Grant Brown]

Re: Bogs down for 1/2 hour

I don't have an inductive timing light and I HATE trying to hold the plug near the block without getting Zapped. Does anyone have a technique for testing spark that I'm not thinking of?<br /><br />Thanks,<br />Grant

 

[Grant Brown]

Re: Bogs down for 1/2 hour

I don't have an inductive timing light and I HATE trying to hold the plug near the block without getting Zapped. Does anyone have a technique for testing spark that I'm not thinking of?<br /><br />Thanks,<br />Grant

 

[Barnacle_Bill]

Re: Bogs down for 1/2 hour

Spark plug tester from your local automotive store.

 

[Grant Brown]

Re: Bogs down for 1/2 hour

The boat finally quit totally the other night. It did the usual low rpm/bogging thing until I shut it off to do some fishing. then it ran great the rest of the day. When I stopped to pull my crab traps at the end of the day, it started and then died twice and then nothing. Sounds like no spark cause it turns over fine and it gets fuel. I haven't had a chance to check spark but is there anything that comes to mind right away? <br /><br />Sure am glad I have my 4.5 kicker - started second pull and got me to shore...