Mercruiser 4.3 maxes out at 2200 RPMs at WOT - Please Help.

[acevedor2]

Hey guys - took the boat out for the first time this season today and as the title says, she was not very responsive and I couldn't get past 2200 RPMs at WOT. To put it in perspective, at the end of last season, I had her up to near 50 hauling butt and she ran like a top. Took it out of winterization in May and changed oil/filter, fuel filter, drive lube, and replaced the trim pump. Just got around to taking her out today for the first time this season.

Took it out, water was calm, and when I got past the no-wake zone and tried to open her up, she wouldn't go past about 2200 RPMs and about 10 knots. Even though the throttle seems to go through the full range of motion, from about 3/4 to WOT, there is not change in engine whine, RPM, or speed. While at the max throttle of 2200 RPMs, all gauges seemed to read normal. Temp was about 175-180, oil pressure was 55-60, and volts were 13v. Again - just couldn't get it to open up and run like I know she can.

The boat is a 2008 Bayliner F-18 Flight with 4.3l carburated Mercruiser . When I got it back home today and put it on muffs, it idled fine, but when I put it up to about 1200 rpms, it seemed to surge every now and again to 14-1500 and then back down to 1200. Only I can think of is that the throttle cable is bad/misadjusted. In hindsight, now that I think of it, - even last season, though it did seem to go to full throttle at 4k plus RPMs, there did seem to be a little slop in the throttle lever - meaning I would have to move it a little to get it to change. I assumed that was normal, but now maybe not. Ay thoughts as to whether this could be a cable, etc? Thanks!

 

[nateo]

What rpm is she ideling at? Any backfiring or missing?

 

[Scott Danforth]

see #15 in the adults only sticky http://forums.iboats.com/forum/engi...77-must-have-technical-information-for-diyers

 

[acevedor2]

Engine is idling at abut 600-650 and is smooth. No missing, no backfiring.

Thanks Scott - I did see that, but of everything on that list, the only thing I can think of at this point is the cables (the last thing on the list). Gas was new, engine idles smoothly, temp is good, etc.

Is there a way to check the cables to confirm that is the source of the issue?

 

[nateo]

perhaps move remote to wot. remove nut from bow end of cable throttle post and slide cable end off. See if you can move throttle level on carb father by hand. Might be a good place to start as it will only take a few minutes.

 

[Scott Danforth]

I would suspect either crap in the fuel, a plugged vent line or crap in the anti-siphon valve. did you remove the fuel/water separator and pour its contents into a clear jar to inspect. (do not reuse the old filter)

 

[flipbro]

Put throttle to wide open and look down carb and observe the throttle butterflys are opening all the way..

 

[acevedor2]

I would suspect either crap in the fuel, a plugged vent line or crap in the anti-siphon valve. did you remove the fuel/water separator and pour its contents into a clear jar to inspect. (do not reuse the old filter)

I haven't pulled the separator yet. I did replace it a couple of months ago. I know that doesn't mean anything so I'll check it again. I'll also check to make sure the butterfly's are opening at WOT. If the fuel was bad would it still idle correctly? Because it idles fine.

 

[acevedor2]

Also, is there a way to test WOT on shore? My understanding is that going above 1400RPMs or so on muffs is not recommended. But I would like to test whatever I do to correct it without launching her.

 

[Scott Danforth]

you have to be in gear and under load to do a proper test. running it to WOT in neutral doesnt tell you anything.

also, you may want to check your exhaust shutters. they may have broken, and dropped to the bottom of the Y-pipe.

 

[acevedor2]

Thanks Scott - I understand that I need to be in gear when testing it, but is it safe to push the RPMS that high (assuming can even get there) while on the muffs? I have heard and read that you shouldn't go above 12-1400 RPMs on the muffs. I will check the exhaust shutters as well. Someone also mentioned that I should check the ignition timing. I know how to do that and do have a timing gun, but wouldn't the boat be idling/running like crap if the timing were off, not just failingto reach WOT.

 

[Alumarine]

It tells you nothing if you can get to WOT while in neutral.
It can reach maximum revs running on 3 cylinders at half throttle while in neutral.

I don't think it's good for the engine and I know I always cringe when I hear it.

 

[alldodge]

Don't run the rpms above 1500 on muffs, you'll have a real good chance of burning out the impeller, even for a short burst

Get someone in the boat with you to look down the carb while in the water, with safety glasses on

 

[acevedor2]

So how does the shop test this? I am fairly certain they don't take each boat out and road test it. Do they have a special way to test WOT without burning up the impeller? I m trying to find out because it Is not a small deal to put the boat in to keep checking different things every time. The nearest launch to me is about 40 min away and is $20 a pop to launch. Again, I have no problem checking and trying the things that have ben recommended, but I'm trying to find someone to know that I likely have found and fixed the problem before I launch her next time. Things so far I have on my list to check for:
1 - Pull fuel separator and check to see what the fuel looks like (replace separator).
2 - Pull air cleaner and check to see if carb butterflies open at WOT.
3 - Check exhaust flappers.
4 - Replace fuel anti-syphon valve.
5 - Check timing.

The only thing I am not sure on is the vent line. Which vent line should I check and where is it located? Thanks again for all of the great help!

 

[carrier82]

If you remove the outdrive, you can stick a water nozzle to the water inlet. A large diameter hose with a good pressure could allow higher rpms?

 

[nateo]

Engine is idling at abut 600-650 and is smooth. No missing, no backfiring.

600-650 rpm, was that on muffs or in water? On muffs is usually about a hundred more then in water. 600 sounds low to me, my 5.0L normally runs 650-700 in water. There's a lot you can check on muffs. Compression test is a great way to check engine health and will be a good start to narrowing things down.

 

[bruceb58]

Anyone remove the spark plug wires at any time? If so, check your firing order.

 

[acevedor2]

600-650 rpm, was that on muffs or in water? On muffs is usually about a hundred more then in water. 600 sounds low to me, my 5.0L normally runs 650-700 in water. There's a lot you can check on muffs. Compression test is a great way to check engine health and will be a good start to narrowing things down.

That was in the water. I haven't checked compression yet, but my experience has been that if there were cylinder issues, she would not running near as smoothly.

No one has removed the spark plugs or wires and again, she is firing fine because she is idling smooth and up to 2200 she is fine - just won't go any further.

 

[acevedor2]

One other thing I remember that I figured I would mention in case it helps the troubleshooting effort - When I first put down the throttle after getting out of the no wake zone, it was not super responsive as I mentioned before, but it got up to about 3K RPMs. After coming down fairly hard off a pretty big wave caused by some ass hat that thought I was not going fast enough and screamed around me, I throttled down. When I throttled back up it would only go to 2200 and stayed that way throughout the rest of the trip. I mention it because although something was wrong from the get go, it did make it to 3k not 2200. Could something have shifted as I came down off the wave. We hit the water pretty good.

Also - one of the first things I will be checking when I get home is to see if the exhaust flappers are still there. I am getting exhaust out the back when on muffs, but that doesn't mean that some of the exhaust might not be blocked if my exhaust flappers have come off. Another thing that leads me to believe this could be the issue possibly is because a year and a half ago I had a bad impeller and the engine over-temped. I never checked the exhaust flappers after that. If they are missing or were burned up for some reason, where would they go? I assume I would have to remove the outdrive to find them/remove them?

 

[bruceb58]

That was in the water. I haven't checked compression yet, but my experience has been that if there were cylinder issues, she would not running near as smoothly.

No one has removed the spark plugs or wires and again, she is firing fine because she is idling smooth and up to 2200 she is fine - just won't go any further.

I mentioned it because I had 2 crossed once. It idled perfectly and sounded perfect up to 2500 RPMS as well. Mine was with a V8 so yours would likely not idle as smooth.