Stock 225 promax won't go past 6000 rpm

[allisonjr]

As said in my title my promax seems to be running fine but once I hit 6000 rpm's it seems like I'm hitting a wall. Didn't know if anyone else has experienced this before. Any help is always appreciated!

 

[Jr in Jax]

How long since you changed Plugs?

 

[allisonjr]

I haven't changed them in probably two years. I did notice last night one red wire off the voltage regulator on the top right of the motor had worked a bare spot in it. I fixed the bad spot in the wire. It still ran the same this morning

 

[whipper]

two things come to mind try swaping the voltage regulator wires right for left left for right. i dont know why or how but a buddy told me to try this and it worked for some reason its been good for years like that now. Also does it start loosing power at 6000 the more you stay at 6000? that pesky pulse pump has made many a promax not have enough fuel in the vst to deliver more rpm. It basically looses its pumping power after a while and needs a new kit installed to meet the demands of higher rpm running not providing enough fuel to meet the demand. Real cheep to try any way. they are supost be changed every couple of years the sping is very small and looses its bounce. The diaphragms also can develop a small tear. You might check all the connections to the switchbox or maybe re tighten them all? My first guess is always that pusle pump though because its cheep and takes 5min to change.

 

[allisonjr]

Is it the diaphram kit for the fuel pump that I need to purchase to repair the pulse pump? I bought a new fuel line, fuel filter, and a new set of ngk plugs today

 

[whipper]

If you follow the line from your VST it being fed from a little crank pump on the side of your motor with 4 screws. In side there is a couple of diaphragms of thin rubber and a very tinny spring. Take a pic with your phone taking them out so you know what order to put the new package of diaphragms back in. the diaphragm kit cost around 20 bucks i think. http://www.mercurymarineoutboardparts.com/pages/OemParts?aribrand=MRC&arian=mercury#/Mercury_Marine/225X_EFI_PROMAX_0T430254_THRU_0T819605_(RACE_OUTBOARD)/Fuel_System/05-002-803934|~02-430/COMMON901514

 

[whipper]

21-857005A1 DIAPHRAGM KIT this is the number and its just called a Diaphragm kit. $20

 

[whipper]

This is the package I used a couple years ago for mine. Just ignore they way I have my fuel lines its an experiment. haha When you see how flimsy the diaphragms are and week that spring is you can see why these need to be changed when you get a chance every year or two depending on use. Ive changed mine 4 or 5 times now in the past 10 years and the last time tried a little heavier spring i just happened upon. Look for the tiniest of tear or hole. It doesn't take much to not give you full throttle fuel delivery. Usually though its just the spring gets week and there is no tears.

Its tough because at idle there's enough to fill the vst. Then you take off. all is good it seems. then as you increase speed and demand the vst slowly starts to drain as the pump cant keep it full thats when you run out of rpm and now are starved for fuel. Then you back off wonder wtf as you slow the vst refills as you have decreased the demand. Then you try it again and the same thing happens. Early signs of a week spring is higher rpm like yours. As it gets worse it will happen at even lower rpm as an example. It might not be this but for 20 bucks its sure a great place to start with the other alternatives other than fuel.

 

[chad202]

Does it have normal power under 6k or feels sluggish?

 

[allisonjr]

Thanks for all the help! Ok today after replacing the fuel line from the boat to engine, replacing the fuel filter, and new spark plugs there was no change. The engine performs normal all the way up to 6000 rpm and then won't go past it. If you didnt know any better youd swear it fine. Even when it's at that mark it hold it steady. No miss or hesitation. That's what I can't figure out. I was running a fellow owner about 3 weeks ago and first noticed it. He kept pulling away and I was stuck at 76 mph.

 

[chad202]

Put a smaller prop on and see if it does the same.

 

[K-DAWG XB 2003]

I had 260 that wouldn't rev up no matter what prop I put on it. Never did figure it out.........Good luck I hope you get it solved.

 

[allisonjr]

Well I did a compression test last night right at 120 psi across the board. I did notice that the two spark plugs in the center of the block were quite a bit more black than any of the other plugs. I don't know if that indicates a possible coil laying down or what. The plugs are new being ran only once since being replaced.

 

[whipper]

If you've tryed all of the above its acu ,stator or switchbox unfortunately. The tests for those thing if i was you will require some help buy someone with the right test equipment and know how to pin point what one it is. The TSP setting could also be suspect but unlikely if it was running fine before. ACU going bad would be the easiest to check. but you will need to borrow a known good one to swap in to try? some one close to you may have a spare to try to rule that out. If that still is the same Stator or bad switch box testing buy some one qualified or has done it before. When my stator went bad. My local merc shop didnt have a merc racing tech. Just a reg merc tech. He spend 4 days and thought it was my tps out of adjustment when it was my stator. So its not as simple as one would think.

 

[whipper]

ya coil actually you just reminded me i had that before also. Those things break easy!! i had mine all taped up at one point till i got new ones. It did make a big difference. I even fiberglass taped the new ones to keep them togeather. good call!!

 

[xb03fs]

Any update?

 

[allisonjr]

I'm going to call my merc mechanic in the morning because I don't have the dva meter to check the ignition parts. I don't know how to check the coils, switch boxes, or stator without a computer or a meter. Is the tps sensor adjustable on my promax? I'm wondering if when I hit that wood it knocked it off the correct position

 

[chester8420]

when I hit that wood it knocked it off the correct position

You need to be more careful with your wood dude.

 

[ROBERT CROSS]

Did you do the pulse pump re-build?
Mine needed to be done last Spring @ 310hrs.
As Whipper noted, pretty flimsy part, bunch of very small pin holes in ours.

 

[Larry33kc]

Coil. Had it happen to me.