mid range stumble and now no idle on 79 70 hp

[kgross]

First let me say I do not have any real knowledge of boat motors so I have a few questions.

First I just purchased a used boat with a 70 hp merc, yesterday ran I think OK, but had a part throttle stumble/miss. Took it out again today, and it will not idle, has a major part throttle stumble/miss, but seemed to run ok at WOT. I am guessing that it is dirty carbs but I am not sure. Any suggestions on what to do to troubleshoot it?

Also on the SN, where do I find it so I can get me a manual?


Thanks

 

[jebeebe]

Re: mid range stumble and now no idle on 79 70 hp

on the motor bracket below the power head and on the freeze plug on the starboard side of the engine near the top

 

[tashasdaddy]

Re: mid range stumble and now no idle on 79 70 hp

i say the carbs, and impeller are the places to start, on and engine you have no history on. read this: http://forums.iboats.com/showthread.php?t=158086

 

[kgross]

Re: mid range stumble and now no idle on 79 70 hp

Still having problems with this motor. I am pretty sure that it has a carb problem. I purchased carb kits, and cleaned both carbs I think well and rebuilt them with new floats etc. I have also rebuilt the fuel pump.

At part throttle the motor runs lean and has a hard time going to WOT. If I feather the choke button while I am trying to accelerate I can get it to run WOT and it seems to run well.

Today I had the boat out to try it out again after rebuilding the fuel pump over the winter, same problems. I did try adjusting the carbs mix while I was in the water, bottom carb mix screw effected the operation of the engine, top carb mix screw did not effect the operation of the engine. I am guessing that something is still plugged in the top carb does anybody have a good schematic or just just the knowledge to tell me which passages could be plugged to cause this?

Thanks

kim

 

[64 Impella]

Re: mid range stumble and now no idle on 79 70 hp

Kim,
After following Tashasdaddy's advice, I would suggest following the timing 'n synch procedure detailed by Clams Canino in a post a ways down in this forum. This worked awesome for me on my 78 80 hp. Also, after doing the linc 'n synch I had one carb needle that didn't affect the way the engine ran, I pulled it out and shot carb cleaner in there with the can's spray tube. That solved it for me and the needle functioned normally after that. Must have had a piece of something in there, as careful as I was to get the darn thing clean. What I've learned here and by messing with my motor is that dying when shifting into gear is pretty normal for older engines that may have weak reeds, it is a balancing act to find an idle speed that is high enough to stay running when shifting into gear but low enough to not trash your lower end. I also learned that a motor will fall on its face when accelerating if the idle setting is too lean, as without any accellerator pumps the carbs need that rich idle mixture until they get off the idle circuit. The linc 'n synch will tell you how to set this, also.
Hope this helps
Tim