Need some help- Motor bogging at full throttle

[Boat2fast]

Pappy makes sense. Rule out compression problems first.

If my engine pops while throttling down, my first reaction is to richen up the idle screw. Looking at the picture of the engine, there doesn't appear to be an idle mixture screw. This carb has a bleed orifice. The size of the orifice determines the idle and off idle mix. Now assuming there are no air leaks from a crankcase gasket, bearing bleed tube, intake gaskets, or crank seals; the problem is most likely some crap in the idle circuit restricting fuel flow. If all that is fine, then an adjustment can be done by putting in a slightly smaller orifice to richen the mixture. The orifice meters air so a smaller one richens and a larger orifice leans the idle mix. First I would look for air leaks or dirt in the idle circuit. Possibly got a piece of crap into the thing with no cowl. I can get dirt in the face when my boat sits around for a while, first time I take off into the wind.

 

[thill]

OK, I'm back.

Rob, I agree with your instincts. The lean pop started during high RPMs, as also did it at idle. I checked the cap on the gas tank, squeezed the bulb, and then opened the idle screw before heading to the ramp. Still sneezing, but it got me back.

I just went outside to check compression. Bottom hole- 122#, top hole 122#. They were exactly the same, three checks on a row. Compression is good, thankfully!

There is an idle jet on this motor. It's set into the brass sleeve that you can see at the top left side of the carb. I had it set at 3 turns out, which is a little rich. (Book says 1-3/4 - 2-1/4 turns out) Rich enough that she smokes a bit at idle. I did this because I don't know the history of the motor, except that she set awhile. Set like that, she starts easily, and idles forever without stalling, and I'm sure she's getting good lubrication.

The fact that she STOPPED smoking at idle after the popping and stalling was a bit alarming. Then yesterday, when I first started her, and she was sneezing, I opened the idle jet up even more. Still no smoke. But the longer she ran with the hood on, the better she ran. And when I was loading the boat, she was smoking a bit again at idle. That was quite a relief to see.

Maybe some dirt did cause the problem. That would be one explanation that fits. Fortunately, it wasn't fatal!

-TH

 

[thill]

Oh, almost fogot... Here are the pictures of Gary's race boat:







He told me that with the 30 HP Mercury, the boat ran in the 60's, but with the 40 HP, it went into the 80's. The throttle was a squeeze grip, and the operator kneels when driving, so that he can jump away, in case the boat flips.

This guy really knew his stuff regarding boats and motors. I learned a lot in a very few minutes of listening to him. I hope to meet him again.

-TH

 

[MDCrappie]

Tony - the motor on my 16'er is a 1989-25hp Evinrude. I had a devil of a time getting it to run right when I got it. I cleaned carb, sync/link and got it running perfect, or so I thought. Got in the water and wouldn't stay running. Richen it up (had it 3 turns out), but still popping. Cleaned again, same thing. After 2 weeks of searching I realize I was leaking air at times. Wound up being the core plug right there next to the idle mix screw. Put a small blob of permatex on it and been idling/running perfect for almost a year now.

Just had it out this morning. Caught 3 LM BTW.

 

[thill]

Aaahhhh.... Now THAT was a good find! I just checked the schematics, and my carb is pretty much the same as yours, but with a different bowl. I see the core plug that you mention. Will probably just put a little stuff on it and all of the other seals.

Hopefully, that will prevent this from happening again. Thanks for the tip!

Nice work on the LMB! I hope to be able to get out there sometime soon, not just boat-testing.

By the way, what kind of speed are you getting out of your 25?

-TH

 

[MDCrappie]

thill » Wed Mar 12, 2014 8:41 pm[/url]"]

By the way, what kind of speed are you getting out of your 25?

-TH

I varies a lot. Basically I use the boat in 2 places. First place is 95% no wake. The other place is on the Suqy up close to conowingo dam. Depending on whether I'm going up or down river, and how strong the flow is will determine my speed.

I haven't get run it WOT yet with the 13" prop (just got it last fall), but with the 12" I could get up to almost 27 by myself, or I even had it to 24+(not quite 25 on gps) with 2 other guys, one of which is over 300lbs, but this was downstream with a pretty good flow. I do have a rock guard on the motor, which I'm sure slows me down a bit.

When I'm out any other place I use my 20' Crestliner with 75hp. It's a perfect boat for anything except the Upper River

One other thing I did (not sure if it help or not) was put an additional "O" ring on the idle mix screw so that it couldn't vibrate and would seal out any air from leaking between it and the brass nipple.

 

[thill]

Sorry for the late reply- I just saw your post. Thanks for the info. 27 sounds really good. That is what the Lowe ran before I sold it. Mine likes 23 MPH loaded the way it is.

Well, it's been awhile, but all is well with my 25. I followed your suggestion, and sealed the plug on the carb with a little permatex. I also replaced all the primer hose lines and did the fine adjustments. It didn't change the way the motor ran, but was good peace of mind. Thanks for that!

This little motor runs flawlessly. I'm very happy with it. I have not run it with the cowl off anymore, but on some of the other threads posted recently, other guys are boring out intakes and running with the cowl off, and upsizing the main jets to get more HP. From a .067 up to a .070, I believe. Sounds like they are getting some nice increases, but me, I'm happy to leave my good-running engine alone!


-TH

 

[thill]

Came across this thread, and figured I'd post an update.

The motor has run flawlessly every time I've used it. I hadn't started it since December, so I went out and pulled the primer and gave it a tug... She started on the second pull and purred like a kitten for a few seconds before I shut her off. (no water hooked up)

I have a very rich Seafoam mix in her, and that has worked great for me. Zero issues since I started doing that in all my small engines. Hopefully, Lake Anna will break up soon so I can get out. (It's solid ice mid-lake)

We just got another 5" of snow this morning. That makes 17" in the last 10 days or so! :shock: :shock: :shock:

-TH