Low Rpm At Wide Open Throttle

[Ski213]

Wanted to pick some brains. Chasing a problem on a '97 Johnson 225 venom. Current issue is that the engine will not exceed 5000 rpms unless I lift it to the point where I'm blowing out the prop. Engine runs smooth idle to 5k just won't get to where it should be. In the past this boat ran just shy of 75mph GPS @ about 5800 rpms with the same prop etc. In 2012 the number two cylinder went bye bye courtesy of a cooling, or lack thereof, issue that I'm told is pretty common with these. Power head was sleeved on number two, honed on the other five, and six new pistons installed. Work was done by a reputable mechanic and machinist. Boat was broken in as instructed (no long wide open runs, a lot of varying the throttle, doubled up on oil). After build this problem developed. It was run for about ten hours then sat for 8 months. I bought the boat knowing all this. So far it's gone like this: i put fuel on top of the old fuel with ethanol treatment and also premixed with oil at 50:1 just to give it a little more double oil. Took it out couldn't get over 5k rpm and sluggish out of the hole, mechanic checked it out. Good fire, good compression, timing right at idle but wot timing not checked, one cylinder not getting much fuel. That carb flushed and then it was running on all six. Had him make sure carbs were opening all the way up. Took it out, same result. I've seen a lot of engines that sat for awhile fixed by just running fuel through them, so that's what I did. Problem then became same overall issue with the addition of when coming out of the hole it surged off the bat then fell on its face (not died just big push then a lot less power). Once it would die as soon as you tried to move. My buddy leaned over to the primer bulb and squeezed as we took off. It was the one time it came up like it should. Even at 55mph it would surge every time he squeezed the primer. Back to the mechanic. Fuel pressure at the pump was low. Replaced vro with a non vro pump. No difference except lower idle rpm because its not running lean anymore, at least at jdle. Checked carb butterflies just cause I was bored one evening and they weren't opening all the way due to a linkage stop screw that had backed out and was hanging the linkage. Fixed that and thought I would be golden. Nope. Same issues except now that its getting fuel and the carbs are functioning properly its become noticeable that if you come up at 3/4 throttle and don't go to wide open the boat actually gets to its top speed faster than if you give it wide open throttle. Just hoping maybe somebody sees this and has had the same problem or someone has some thoughts on it. Sorry for the length but I'd rather give too much info than not enough.

[sparky241]

Are you getting enough air into the carbs? Also have you replaced plugs and wires? They may be breaking down

[Fishing Rhino]

Could be the fuel that sat for eight months.  Adding new fuel and treatment eight months later will not do much for the old fuel if it turned bad during the eight months it sat.

[Glenn]

Change your fuel filter, fuel lines, and bulb.  I'd fathom a bet the ethanol did it's dirty work while sitting there for 8 months.  Your lines have probably deteriorated and clogged your fuel filter.

[livetofish28]

Change your fuel filter, fuel lines, and bulb.  I'd fathom a bet the ethanol did it's dirty work while sitting there for 8 months.  Your lines have probably deteriorated and clogged your fuel filter.

That's probably it. we only run ethanol free for that reason

Tight lines

Andrew

[jhoffman]

I wouldnt rely on that too long, you cant even buy ethenol free here unless your buying it by the quart premixed two stroke gas.

[jhoffman]

id be amazed if thats not a fuel issue. is the tank vent open?

[Ski213]

Thanks everyone for the replies.

Are you getting enough air into the carbs? Also have you replaced plugs and wires? They may be breaking down

Air should be ok. Plugs basically brand new. Wires are fairly new. Spark is good at idle but I suppose there could be an issue under heavy load.

Could be the fuel that sat for eight months.  Adding new fuel and treatment eight months later will not do much for the old fuel if it turned bad during the eight months it sat.

Agreed. I'm not sure if e treatment can un phase gas once the water has settled out. I've run prob 80 gallons through since though. I would think it should be on good gas by now but who knows. Did not ever drain the tank.

Change your fuel filter, fuel lines, and bulb.  I'd fathom a bet the ethanol did it's dirty work while sitting there for 8 months.  Your lines have probably deteriorated and clogged your fuel filter.

I forgot to mention that part. New fuel lines from tank to pump and new primer bulb. Did not replace all the lines from the pump to the carbs though. Need to give them a look. Hate ethanol. Can get e free gas one place close but its a tough place to get a boat into.

id be amazed if thats not a fuel issue. is the tank vent open?

It sure has the feel of a fuel issue. Vent is ok. I'm pretty confident in everything from the tank to the pump. Based on the plug check its getting fuel but maybe I need to run it wide open then shut it down and check before I idle it back in. I don't really trust the plug check anymore with all the fuel additives though. What if I had weak vacuum to the pump? I'm pretty ignorant on mechanical fuel pumps but I would think weak vacuum would equal low pump output. Pump output is 3psi assuming my gauge is correct. Not super weak but not quite as strong as it should be. I don't know if I can check vacuum since its a pulse type vacuum.

I appreciate the feedback. Keep it coming! Gonna do some further poking around on it tomorrow.

[Way2slow]

That motor should run 5 to 6 psi fuel pressure. Not that it has anything to do with your problem but was the cooling mod on the block. That requires some drilling and plugging of some passages between the cylinders, doing away with those little red and white plastic baffles and going into the lower part of the exhaust chest and plugging off the original exits holes because mod changed the exit point of the cooling water. The cooling mod is almost a must if you run them long and hard or it will probably pop #2 cylinder again. The exhaust gas temps on #2 are about 50 degrees hotter than they should be and that makes the ring locator pin come out and it's also more subject to melt a piston if that carb gets even a little dirty.

If you didn't do the cooling mod, you should increase the main jet size in #2 carb two number sizes larger than the others. If it's a 62 go to a 64 or if it's a 68, got to a 70. That extra fuel helps keep the piston a little cooler.

Did you build it back stock, was any port work done (I hope not because most people only butcher a block doing it), and did you plug the exhaust reliefs. Are you running the stock heads?

Are you running the stock air box? Did you do any mods to the exhaust?

The reason I'm asking is any of this work requires rejecting the carbs. Just this past summer I helped a friend that was working on a customers boat somebody had tried to modify and the couldn't get it to make power, acting like yours. They did a butchered up job on the block and still had the factory #62 jets in it. It was running so lean, it was too lean to melt down because it could make enough power to build enough heat to melt down. I ended up putting a set of #76 jets in it.

The linc and sinc is critical on that motor so hopefully it's done right. How was the timing set on it, it has to be done above 5,000 rpm and under a load. I would also back it down from 18 degrees to 16 degrees for the junk we have for gasoline now.

If it's a bone stock motor, no mods of anykind, then I would start with a linc and sinc, paying very close attention to getting all the carbs in sinc. I would also be concerned about that 3psi fuel pressure, that's way too low. Shoot, I run a Carter electric 15 psi @ 80 gph and regulate it down to 6.5psi. But that's on a motor putting out just a little more hp than yours.

A 1,000 rpm drop is almost a dead giveaway for one cylinder being out. Sluggish hole shot and 4,800 rpm is a classic dead cylinder. Those motors can have a cylinder out and you would swear it's running on all six.

Not saying I know anything about them but I have messed around with one or two over the years.

[jhoffman]

I experienced what you did on a much much smaller motor. Mine was one of two check valves had fallen apart due to ethanol inside the fuel pump. It would run, romp it open and it would fall on its face and shut off. I chased it all summer. My boat has all new lines and a rebuilt carb because of it.

[TorqueConverter]

Sounds like its running lean.  I'd remove and inspect the fuel filter.  Inspect the fuel filter locking nut too.  My Johnson 150 mysteriously started gushing fuel out of the fuel filter nut because the nut had failed due to a defect.  Had the the nut failure not been so sudden, I'm sure I would have faced fuel delivery problems such as leaning out as the leak increased in severity.

 

If it's not a problem with the fuel filter and lines then It's surely an issue with one or more of the carbs.  Without the ability to identify which carb is having problems, I'd say that it's time for 6 carb rebuilt kits.  A lot of people want to blame the VROs on these engines for every little problem.  I'd strongly recommend rebuilding the carbs before taking it to a mechanic, because they're just going to throw a VRO at it.

[Ski213]

So I used to be on another forum for mx quads, I know or at least used to know a lot more about them than outboards. It's amazing how knowledge slips when you're not dealing with it day in and day out. One thing that always bothered me was when somebody had a problem and asked a question and a bunch of people tried to help and there was no final conclusion that any of us knew of. Just kind of left you wondering what ended up happening. Then I come here as a total newb and do the same thing. So my apologies for that.

I got a lot of great advice on my problem here and it's fixed. Has been for awhile. All advice given here could have very easily been the problem,and in fact several mentioned fuel and one mentioned that the symptoms were a dead giveawway for a dead cylinder. It was a fuel issue, it was a dead cylinder In the hopes that someone else has the exact same unlikely issue and can benefit from this here's what it was.

During the engine rebuild the carbs were either rebuilt or at least disassembled and cleaned. I guess during reassembly the problem arose. There's a clip that holds the needle valve on an assembly that is connected to the float. That clip ended up under, not on the assembly. That effectively pinned the needle valve shut, so the number two carb bowl never got full or even full enough to fire. Whatever fuel was forced past the needle by the fuel pump ended up forcing the needle even more closed. So five minute fix and she's been running pretty good. Still don't have it perfect but I guess you never do.

Thanks again to everybody on here. This is as helpful a group as I have ever been a part of.