Merc 4.3/TB4 loss of high RPM after lengthy WOT.

Krapht

Cadet
Joined
Aug 30, 2010
Messages
8
Hi All. First timer here - though I've been reading these forums now for a few months. Great information - thanks for providing such a place.

Bought a 1988 Sea Ray Seville Bow Rider with Merc 4.3 and Thunderbolt IV. Previous owner told me he didn't have a chance to use it at all this season, and had just changed the motor and gear oil the season prior. Also, put on a new alternator, starter, and a brand new battery. I wish I would have asked when the last full tuneup was done.

The day before heading out onto the bay for our first trip, the wife and I decide to take the boat out with just us, just to make sure everything's on the up and up.

Boat had 3/4 tank of fuel in it when we bought it, so we decide to head to the launch, slap it in, and go. The river that runs through the city is mostly slow/no-wake, but the stretch right at our launch site is not. Boat starts right up, and after checking everything under the hatch, and letting it warm up, we're off.

Boat planes out nicely, and I'm getting about 40 MPH cruising. Very happy with the performance. We quickly approach the next section of "No wake Zone" past some marinas, and the downtown area. Once we get to the next stretch of "open 'er up", I did just that...and went WOT. Chopped through a couple wakes of other boats, got up to just under 50 MPH. As quickly as the excitement fulfilled us, we're back to another no-wake section, so we sit back and enjoy the tool down the river.

We then decide to turn back around and head back to the ramp. Once we reach the drawbridge where the no-wake zone ends, I slowly give the boat throttle, and it just seems like the power's not there. I'm accelerating - just not worth a damn. More throttle, more throttle, and by now I'm WOT on the handle, but only getting 3,000 RPM. Now the wife's saying, "What's wrong?" Not exactly sure, but let's get to a place where we can tie up and check everything out. Still between 3/4 and WOT, and no sign of power anywhere. No chugging, noises, or other abnormalities from the motor - though it surely didn't sound "happy" that "force-feeding" it fuel. Backing off the throttle to where 3,000 RPM "should" be, and she runs smooth as can be.

We tie up at a dock alongside the seawall downtown. Open the hatch - the engine's just purring like a kitten at idle - although a SLIGHT shake going on. I turn the motor off, check the oil - everything's good there. Made sure none of the spark plug wires came off, coil wires, etc. Not finding anything wrong, we close the hatch, start it back up, and make our way back towards the ramp.

We're now approaching the end of the no-wake zone again, the stretch where our ramp is. Upon applying throttle, she responds perfectly - we plane out and quickly approach 40 MPH. Runs like a dream. We decide to spin back around and take one more run through this stretch, just to make sure our mind's weren't playing tricks on us. She ran flawlessly.

That evening, I turn to the internet, upon which I found the iboats forums. Doing some searching around, I found a post regarding a similar problem - in which the person thought that the "high speed" jets in their carb were possibly plugged. Sounds logical to me - since anything below 1/2 throttle or so ran pretty damn smooth - while anything about would not. Figured I didn't know how long that 3/4 tank of fuel I got with the boat had been sitting there. I went out, got some Stabil, and put it in the tank the next morning.

Next morning, we stop at the local gas station and top off the tank before heading to the beach. Boat runs absolutely fine the entire time. In fact, the next 5 or 6 times we were out, boat ran like a dream. I figured by now that this problem was fixed. (Note: hadn't been running the boat at WOT for very long, if at all during these times. No need to be driving the autobon with the little ones on board).

This past weekend, we finally have a babysitter for the entire day - time to do some tubin/beachin/relaxing with some friends. Boat still has 3/4 tank from the last time we filled her up, so we head to our normal launch site, and head off to the marina to meet up with our friends. We pick them up, and head out to the Bay. Tooled around the Bay for awhile, ended up just parking on the beach for some relaxation and swimming. We decide to head back early into the evening. Boat starts right up, and we're off back cruising towards the shipping channel. Off my port side, I see a 30 or so footer heading the same direction as me. I decide to go WOT and get enough ahead of this boat. We're at WOT for about a minute...maybe two, as we round the buoy of the channel, I start backing off to about 3/4 throttle...and I'm losing power....more than I should. Attempt to give more throttle just does NOTHING, and WOT again won't get me past 3k. Now I'm in a busy shipping channel with powerboats, sailboats, and idiots in go-fasts (not saying everyone is - but there are quite a few around this area) that I need to watch out for, while this crap is going on.

We continue on towards the mouth of the river (about 1/2 mile), running at 3,000 RPM. Still on plane, just no oomph. Once into the river mouth (no-wake zone), we're back down to slow speeds, and the boat's just purring along.

Once we finally reach our ramp, after dropping our friends off at the marina, I notice that at idle, she's now running REALLY rough. Not coughing, sputtering, or missing - just shaky. I definitely think it's time for a tune up.

But what about this WOT issue? My wife pointed out that this only happens after we've been at WOT for any long duration. Short bursts of WOT (to plane quickly, etc) do not result in this issue.

I thought the carb jets were spot on, but maybe not? I'm thinking of doing a complete tune up anyways, just to get that out of the way (fuel filters, plugs, wires, cap, rotor).

Two things that may offer a clue:

1. Strangely, my fuel gauge reads damn near empty. I think that's an unrelated sending unit problem - we should have about 1/4 tank.

2. When this "bogging" occurs - if a pump the throttle - I'll get a short burst of power. Pumping too fast though doesn't provide a continuous burst like I would expect. There's almost a "sweet speed" of pumping that seems to keep (some) speed going.

Reading posts on here I've got one vote on fuel filters/fuel screen in tank, one vote on coil, and two votes for distributor cap. I still am leaning towards fuel issue, since running below 1/2 throttle when this occurs yields a smooth (not 100% smooth, but smooth) running motor.

BTW: This motor has the 2bbl on it.

sorry for the lengthy post - just wanted to cover the whole story. any ideas on this one guys? Thanks!

Bryan
 

Krapht

Cadet
Joined
Aug 30, 2010
Messages
8
Re: Merc 4.3/TB4 loss of high RPM after lengthy WOT.

Found another thread on here that suggests possibly a stuck intake valve? Would just ONE valve really reduce performance THAT much?

Gonna check and replace fuel filters later this week, as well as probably pump out the fuel tank, since I need to address the fuel sending unit issue as well.

Thanks for all who've read.
 

John_S

Rear Admiral
Joined
Jun 21, 2004
Messages
4,269
Re: Merc 4.3/TB4 loss of high RPM after lengthy WOT.

Welcome to iBoats.

I did read the long thread, and think you are on the right path with a fuel related issue. A fuel issue while not the only thing that can cause intermittant issues, I'd say the probability is higher.

I do also recommend you do now, or no latter than over the season close, a full tune-up:

- Engine oil and Filter
- Drive Gear Oil
- Drive Water Pump
- Change Water Sep Filter (check for water)
- Plugs
- Rotor
- Distributer Cap
- Timing
- Idle Mixture
- Idle Speed
- Choke Operation
- Clean Flame Arrester
- Compression Test
- Vacuum Test


And outside of the water pump, doing on a yearly basis, is recommended.
 

Krapht

Cadet
Joined
Aug 30, 2010
Messages
8
Re: Merc 4.3/TB4 loss of high RPM after lengthy WOT.

Hi John. Thanks for the reply.

I'm leaning toward a fuel problem myself - what sets it off from the rest that I've read (ignition) is the fact that pumping the throttle attempts to bring back speed, only to bog down again.

I have read over JustJason/DonS fuel diagnosing plan. I also planned to do the full tune up as you described at the end of the season - guess the end of the season came a little sooner for me. :(

Also managed to download manual #7 (yep..it's still out there on the interwebs) - going to print that out at the office after work tonight (~200 pages front/back).

It's nice to hear from someone that believes I'm on the right track here. I'll keep this updated so others may learn.
 

6meter

Chief Petty Officer
Joined
May 15, 2010
Messages
525
Re: Merc 4.3/TB4 loss of high RPM after lengthy WOT.

You've mentioned a few causes that it could be. Carb overhaul, fuel sitting 2 years could tend to gum up jets. Highly recommend the tune up, complete. If anything, you'll know those parts are right. Anti-siphon valve wasn't mentioned? Coil, vacuum leak, plugs, wires, fuel filters, water in fuel? Good luck.
 

whitney

Seaman
Joined
Jul 26, 2010
Messages
69
Re: Merc 4.3/TB4 loss of high RPM after lengthy WOT.

I'll vote for coil or points/electronic pickup.

I had a similar problem and spent a few weeks ruling out fuel. It was all ignition heat soak related.

http://forums.iboats.com/showthread.php?t=430118

Could still be fuel but it's cheap to inspect and tune up ignition first. Also if you're mechanically inclined, set up a fuel pressure gauge. This is very telling when diagnosing WOT problems.
 

Krapht

Cadet
Joined
Aug 30, 2010
Messages
8
Re: Merc 4.3/TB4 loss of high RPM after lengthy WOT.

I, too thought coil - but since I can pump the throttle and get that burst of RPM for a brief second leads me to believe fuel.

If the coil wasn't producing high enough voltage, wouldn't pumping (dumping more fuel through the carb) essentially flood (thus increasing the 'bog') the engine, rather than bring it "to life"?

Not trying to challenge you, Whitney. Just trying to play the logic in my head. I plan on purchasing a fuel pressure/vacuum gauge in the next few days, as well as tune up parts.
 

fish-n-fool

Cadet
Joined
May 12, 2010
Messages
14
Re: Merc 4.3/TB4 loss of high RPM after lengthy WOT.

KRAPHT, I would like to know were you found the sevice manual, I have the same 4.3 and haven't been able to find it, Thanks, and good luck, GW.
 

whitney

Seaman
Joined
Jul 26, 2010
Messages
69
Re: Merc 4.3/TB4 loss of high RPM after lengthy WOT.

I, too thought coil - but since I can pump the throttle and get that burst of RPM for a brief second leads me to believe fuel.

If the coil wasn't producing high enough voltage, wouldn't pumping (dumping more fuel through the carb) essentially flood (thus increasing the 'bog') the engine, rather than bring it "to life"?

Not trying to challenge you, Whitney. Just trying to play the logic in my head. I plan on purchasing a fuel pressure/vacuum gauge in the next few days, as well as tune up parts.

No worries :) In fact I had been doing the same thing with the accel pump when my engine wasn't running well. In my case I also the engine did run just a little bit better when pumping, which is why I chased fuel for a long time. I think with the weak spark, somehow the richer mixture fired a little easier? More gas molecules? Less HV needed for the spark to bridge when saturated with more fuel? Not sure.

If you are definitely getting a "burst" of power with the accel pump then I'd have to say you've got a problem in the carb fuel circuits. Probably not fuel delivery related, cause the accel pump is getting fuel. It would have to be the main jet circuits ... which, to me, doesn't really match your symptoms.

Still, your plan is good. Get a fuel pressure gauge, inspect/test ignition system, and replace all the common tuneup parts (plus coil, especially if it's original!!)
 

Krapht

Cadet
Joined
Aug 30, 2010
Messages
8
Re: Merc 4.3/TB4 loss of high RPM after lengthy WOT.

Coil definitely looks original. It's worth a shot. Your explanation makes sense as well.

We've had quite a bit of rain here the past few days (or at least the threat of rain); add that to the fact that I don't have a garage - so I can't work on it as much as I would like.

I took a look quick on my lunch hour for fuel filters on my boat, because I didn't remember seeing them. I knew for a fact I didn't have the water separating filter. Opened the hatch, and I find a 3/8in line running from the (stell) tank directly to the mechanical fuel pump. No filters. According to Manual #7, all fuel filters are optional.

I am aware of the filter at the carb. However, I'm thinking I should probably look into getting a water seperating filter installed ASAP.
 

apollo5646

Cadet
Joined
Aug 16, 2009
Messages
28
Re: Merc 4.3/TB4 loss of high RPM after lengthy WOT.

Hey let me know how you come out, I am having the same problem with my boat same motor and everything. I did run into this last year and I had found that the anti syphon valve was stuck half shut. I replaced and didn't have any issue for a bit. One thing I did notice for me is that I will run fine for awhile but if i hit a good size wave and bounce the boat around a little the problem shows up. Are you seeing the same thing? Lose of power after bouncing around in the chop?
 
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