4hp Merc bogs on full throttle in gear but not idle???

Make a long story short, got a 4 hp merc. and had to replace the impeller and took the carb. apart and cleaned all jets and other metal parts real good because they were gunky as you could get.. It starts right up and idles as good as you can get.

The problem is when it is in idle I can goose the throttle and it will rev up really good to it's full power without hesitation, but when I put it in forward gear it go's to about mid point picking up speed and power then after that it just kinds stays that way, its like I am not getting full power of the motor when I am in gear. If I keep playing with the throttle back and forth it will start to plane for a second but then lose that power, seems like it is only running on half of what it's potential is. It idles like a champ though and puts along great, but I am definitely not getting full power.?

Any ideas on what it may be? The carb looks brand new after I cleaned it, from what I understand it may be an air intake, fuel intake problem? Any other inputs would really be appreciative.

Oh ya, it is pushing a 1432 lowe jon boat if that helps at all.

Thanks in advance.

did you run it before you replaced the parts and cleaned the carb? if so did it run the same as your explaining? sounds like the boat may be overloaded, or underpowered.

It ran like crap before I cleaned out the carb. I wouldn't idle for very long and just puttered out. Now it idles fine. So you think the motor is over loaded? I thought that it should still be screaming at full thottle even if it can't get a boat on plain, but I am new to outboards so I may be VERY wrong. To me it just seems like it should be doing more then it is. it is only me and the jon boat (probably 125 lbs) and it would get on plane for a second when messing with the throttle but then bog down to running like the throttle was set to midpoint. Hmmm...

I would soak the carb overnight in cleaner and then blow it out with compressed air. If that doesn't solve the problem then make sure its firing on both cylinders. Also make sure the carb is opening up all the way at full throttle.

Already took the carb apart and soaked the crap out of it. Blew everything out with carb cleaner and compressed air. I do not believe it to be the carborator because after I cleaned it, it looked brand new. It is odd that in idle it revs up just fine, but in forward gear it only goes to mid point just fine, then bogs when you go past that..... I guess I will be buying a new spark plug, hose, and fuel filter and see if that does it? I don't really know.

My 4hp merc only has one cylinder, and as far as I know it is firing just fine. I read the service manual and it said that with any prop I should be getting full throttle with boats up to 500 lbs. After that then change the prop to proper sized boat.

My boat only weighs 125 lbs and I weigh 195, so I don't think that I am even close to over loading the motor with any prop that is on there even with the motor weight.

Forgot it was a 1 cylinder.
Revving up in neutral is easy for an outboard so that's not going to tell you anything.
That motor should push your boat just fine. I had a 2.5 tohatsu that pushed my 12' boat loaded down without bogging.
What year is this outboard? Does it have electronic ignition or is it points?
Maybe the thing is worn out or it has bad points (if it has them). Check the compression.
If nobody can figure it out here then check out iboats.

When you cleaned the carb did you take out the high speed jet & needle & seat to make sure they where clean?

what is the condition of the spark plug? Symptoms sound very familiar to what I had with my yamaha.

Thanks for the fast replies, yes, I literally took the whole carb apart, including all the jets, needles etc... and soaked everything except the plastic parts of the carb.
It is a 1989 mercury 4hp. made in japan. I haven't even looked at the spark plug yet. Easy enough to change and cheap. I'll have to check on the ignition, I am not sure about point.

It kinda sounds like what my old 15hp mariner used to do in my little john boat with me and my 400+ lb friend in it... i could run WOT and it wouldn't get on plane, and would just sound overloaded, as soon as he got out that thing would fly. Not sure how well the 4hp should push a boat like yours...

You know, after fully dissasembling the carb and cleaning out the gas tank and adding new fuel with a little sea foam, I forgot to replace the fuel filter. I think that this might be pretty clogged up with the same junk that was in the carb, what do you think, would it be causing the problems I am having? The carb was seriously gunked up. This might be giving me problems with not enough fuel getting to the carb wide open. I also probably need to re-do the idle mix screw.

One question. What exactly does the idle (or slow speed mixture) screw do for the carb? Does it only affect the low end performance or does it affect the all around performance? I just screwed it back in to what it before I took it out. Maybe it needs to be re-adjusted for the clean carb now? It has no high speed adjustment as this is fixed inside the carb, but it does have a slow speed adjustment.

you likely will have to tinker with the low speed adjustment while it is running. I would clean the fuel filter, sounds like it is starving for fuel.

Hopefully it will be the fuel filter. You can clean the fuel filter, or do you have to replace it? The motor ran fine up to half throttle, then bogs from there, but when you go from full throttle and hit mid throttle again it surges and planes for a second and then goes back to mid throttle like it is suppose too. Hope I didn't mess the piston up messing around on the lake for 30 minutes trying to get it running right.

You can clean the fuel filter, replace it, or leave it out it don't matter.
Running it like you did won't hurt it. If its anything like JohnnyRudes then turn the low speed needle out 1 turn, start the motor and then turn it out a little more until the motor starts idling correctly.

Incahiker said:

Make a long story short, got a 4 hp merc. and had to replace the impeller and took the carb. apart and cleaned all jets and other metal parts real good because they were gunky as you could get.. It starts right up and idles as good as you can get.

The problem is when it is in idle I can goose the throttle and it will rev up really good to it's full power without hesitation, but when I put it in forward gear it go's to about mid point picking up speed and power then after that it just kinds stays that way, its like I am not getting full power of the motor when I am in gear. If I keep playing with the throttle back and forth it will start to plane for a second but then lose that power, seems like it is only running on half of what it's potential is. It idles like a champ though and puts along great, but I am definitely not getting full power.?

Any ideas on what it may be? The carb looks brand new after I cleaned it, from what I understand it may be an air intake, fuel intake problem? Any other inputs would really be appreciative.

Oh ya, it is pushing a 1432 lowe jon boat if that helps at all.

Thanks in advance.

Click to expand...

I've got a 4HP Merc myself that I haven't gotten started yet. Mine is a 1973 made in Fon du Lac. I'm still waiting on a gas tank that BPS put on backorder 3 weeks ago. Keep this thread updated so I'll know what to look for when I start on mine. =D>

Ok, got home and took of the fuel line, filter, etc... the fuel filter had some stuff in it but it was not clogged. I could easily blow through the filter no problem, no restrictions at all. And just gravity feeding fuel through it was fine and fuel would flow out just fine.

I did pull the spark plug and that sucker was black and oily... that is telling me that I am running rich right? Would it have the same symptoms as described earlier? I reset the idle and the slow speed mixture according to mercury's shop manual and now will need to get to a lake to test them out. I guess I will need to be replacing the spark plug cause it is just black and nasty...

That's odd though, definitely sounded like the symptoms were too lean, but I guess if you are running that rich then it could just flood the carb with fuel and not have enough air, would it pretty much be the same thing and bog down? I did notice my fuel was burned at a pretty good rate considering the amount of time I was at the lake.

Does the slow speed mixture affect all performance areas of the carburator, I thought it was just for idle and would not have an affect at WOT.

Low speed adjustment affects idle & throttle response.Don't run without your fuel filter otherwise you defeated cleaning your carb.

Your float level may be off as well. Have you replaced the plug yet?

Nope, haven't replaced the plug. I am going to purchase one today along with a fuel filter. Glad the filter was in there because there is some big pieces of varnish trapped in it that would have really messed things up if it went further down the line. I also checked the gas vent and everything was working out there. So it seems like there was not a restriction in the motor getting the fuel from the integral tank. The fuel lines had no holes in them either. Hopefully the fuel pump is working properly and it is just either the spark plug or the mixture screw was wayyyyyy... too rich, which I think it was. If not then I will move on to the fuel pump, though when I dissasembled it it looked like it was brand new. All the rubber pieces looked great and not dried out with tears or anything.

After I replace the plug and get out on the lake and fiddle with the slow speed gas / air mixture scew I will report back. I hope I get it going good because soon here in Georgia the white bass will start running, as a matter of fact they already have in west point... if the weather holds then they should be up here near allatoona too. I have a 6' light weight rod with 4 lbs. test line and am ready to have some fun... just need a motor to get me there in my leaky Jon boat, lol...

I got a 25 hp for ya cheap Update your profile so we see were your at to BTW. Lots of ga boys in here.