Interesting Low RPM @ WOT

[jdjr24]

I have a 1992 20' Larson with a 165 HP 4.3 Merc. Last year I had issues with the engine and the timing was out 8 deg. So with the timing in check and a full tune up that inclcuded, plugs, wires, dist, ignition coil, fuel filter, new 4 blade 21 pitch prop. When all that was done I ran the boat and it ran @ 4800-4900 RPM and reached 42-43 MPH and was very responsive to the throttle almost jumpy with alot of torque getting out of the hole shot. The next time out it ran at 4000 RPM and 35 MPH but was running great, no hesitiation, no back fire, engine sounded good. Later on in the year I had a bad alternator had it replaced and also had them check the timing to see if it slipped out however the timing was spot on. (All RPM and MPH were done via GPS)

On to this year. First time out the boat started right up and sounded great. First time out of the hole shot the boat was very responsive just like when I first got it back after the timing was adjusted last year. The boat was very quick out of the hole and planed out at 4800-4900 RPM and 42-43 MPH. The boat was running great and very repsonsove to the throttle as it did last year after the timing and tune up was done. So I drove around the lake a little while and stopped for about a half hour. Boat started right up but I instantly notice a difference in the hole shot and once on plane the boat ran at 4000-4100 RPM and 35-36 MPH (All GPS).

I am at a loss as to why at first the boat runs great and at the RPM/MPH that it should then slips down and runs 600-700 RPM's slow which equates to 6-7 MPH.

Any ideas?

 

[callen369]

Re: Interesting Low RPM @ WOT

Sounds like we have a similar issue with our 5.7. It revs great and sounds perfect in neutral, and even sounds and runs good at cruise, but it is lacking a bit of power and top end. While cruising yesterday @ 30mph, it would occasionally take off and speed up to 35 and have all kinds of power, then after a few minutes it would drop off again. We also had a lot of work done last season, tune up, carb boiled and everything tuned. I have pulled all the plugs, w/no problems noted, checked all wires for security, pulled cap and checked it and rotor, and everything looks good, so we are leaning toward a fuel problem. Water separator was also changed, so I am thinking maybe a fuel pump that is weak, causing it to rev alright in neutral, but under a load it may be starving for fuel.

Unfortunately we are on vacation this week, and do not have the tools or time to troubleshoot. Thinking about trying to find a fuel pump local and see what happens since it is lagging more often now. Will let you know what happens if we change it. Best of luck to you.

 

[Don S]

Re: Interesting Low RPM @ WOT

Here is a list of possible causes of low WOT (Wide Open Throttle) rpm.

In no particular order.

Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel
2. Propeller pitch or diameter, damaged blades
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
5. Marine growth on hull and outdrive
6. Wrong gear ratio in outdrive
7. Restricted carburetor air intake (clogged flame arrestor)
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test
10. Carburetor defective, or wrong type.
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.
13. Engine Overheating
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully.

 

[FreeBeeTony]

Re: Interesting Low RPM @ WOT

Is it possible you are loosing a cylinder when this problem occurs?
I would try removing 1 plug wire at a time and see if you find one that doesn't affect the performance......use an insulated pair of pliers.

Just my $0.02.........

 

[callen369]

Re: Interesting Low RPM @ WOT

Well, dads 5.7 and my 4.3 have the same fuel pump, so while it was raining, I pulled swapped mine for his, and then ran it during a break in the rain. Idle to wide open, cruise, idle, cruise, WOT, repeated various settings multiple times, and had no issues using his pump, so I am ruling that out. So swapped them back of course. He keeps saying it's bad fuel, and the marina agreed, though they have no mechanics on staff. But we filled up at the same stations, in town and on water, several tanks this week, and I haven't had any issues. 89 octane ethanol free on the water.

Guess we will have to run through some of Don S. tests. Good luck.

 

[jdjr24]

Re: Interesting Low RPM @ WOT

Well I have started on the list Don S posted. (Thanks for the list Don S) I have eliminated 1, 2, 4, 5, 6, 12, ,13, 14. I figured I would check the easiest things first. I am going onto the anti siphone valve. Once I remove the anti siphon valve is there a way to check to see if it is bad or should I just replace it?

 

[Don S]

Re: Interesting Low RPM @ WOT

Don't just take things apart and look at them, you may not be able to look at it and see if anything is wrong or not. Do the tests with the gauges and clear hose. Then you will know what to go looking for.

 

[aerobat]

Re: Interesting Low RPM @ WOT

strange strange... i can only imagine your linkage got loose and your carb does not open fully or you picked up some trash with your drive without notice and struggled with the additional drag. never heared of such a sudden loss of power without obvious damage signs to the engine.

 

[alldodge]

Re: Interesting Low RPM @ WOT

I agree with Don.

 

[jdjr24]

Re: Interesting Low RPM @ WOT

So I went out tonight and I happened to notice a slight miss I'll call it. Not a knock or anything just a slight miss. It was only noticeable at low RPM around 1600-1800. It was almost a constant at low RPM's but no knocking or backfiring. At WOT I could't really notice it. So while at idle I opened the engine compartment to see if I could hear anything and found the Alternator belt snapped off. It had cracks in it so obviously it was going bad. My question would be if an alternator belt was slipping, prior to snapping, and then eventually snaps off would it cause my problem of low RPM at WOT? I initially thought that it wouldn't have anything to do with the low RPM's at WOT so I figured I would ask the experts.

Also I was thinking tonight that the only thing that I didn't do in a tune up was replace the points but I am also not sure if my boat even has points. I am going to call the shop tomorrow to see if mine has points in it.

 

[Don S]

Re: Interesting Low RPM @ WOT

Also I was thinking tonight that the only thing that I didn't do in a tune up was replace the points but I am also not sure if my boat even has points. I am going to call the shop tomorrow to see if mine has points in it.

You do not have points.

 

[jdjr24]

Re: Interesting Low RPM @ WOT

So I have some time tonight so I was going to change the Alternator belt. I went to the local hardware store and the replacement belt they said matched my belt was 3/8 in. wide and the old belt is 1/4 in. wide so I called my boat shop and they had a replacement belt but was also .38. I am waiting on the mechanic to call me back to find out if they were switvhed to a diff belt. My replacement belt No. is 57-48120. I also checked via my Engine No. to determine if I had the right replacement No. and it matched. My engine No. is 0D511306. Does anyone know if the .38 belt will be ok or know where I can get a 46 X 1/4 belt. I checked this website and wasn't able to locate the measurements.

 

[jdjr24]

Re: Interesting Low RPM @ WOT

I also had a question about the ingnition sensor. Would this cause my problem if it wasn't working properly. I never replaced it because I could't get one of the screws out. My second question would be is there a way to test the voltage on the ignition sensor to see if its bad. I have a multi meter just not sure how to test it or what the proper voltage would be. Any help is much Appreciated.

 

[jdjr24]

Re: Interesting Low RPM @ WOT

Just an update. I replaced my alternator belt and now that the holidays have passed I was able to get out on the lake last night just to run around a little. While I ran the boat last night I was at 3/4 thottle and could feel the RPMs jump up. The boat kicked into the was its suppose to run. I had 4 adults and 2 kids in the boat and ran perfect. The RPMs were running at 4800 with a decent load and 42 MPH however it was short lived as when I slowed down and started to go again it "slipped" back into low RPM mode but figured I would post as it has never slipped in or out of the RPM issue. I still need to do some test just haven't had time but thought someone may know where to point me to start as a "test this first".

 

[alorio1]

Re: Interesting Low RPM @ WOT

You have done the most likely causes, try these most unlikely causes; tie the automatic choke wide open, check the outdrive oil level and grease the gimbal brg......and reckeck the throttle butterfly and cables to engine/carb.

 

[TheHunter990]

Re: Interesting Low RPM @ WOT

Are the mechanical weights in the distributor (take off cap and rotor) moving freely. It they get stuck and do not allow the timing to advance to 30 degrees +- a couple degrees, you could seriously lack power. You can check this with a dial back timing light, even in your driveway.

 

[alorio1]

Re: Interesting Low RPM @ WOT

Are the mechanical weights in the distributor (take off cap and rotor) moving freely. It they get stuck and do not allow the timing to advance to 30 degrees +- a couple degrees, you could seriously lack power. You can check this with a dial back timing light, even in your driveway.

Good point Hunter, I think I would remove point plate and lubricate them anyway to remove that sticking probability......

 

[Don S]

Re: Interesting Low RPM @ WOT

His distributor does not have weights, advance is all done electronically.

 

[alorio1]

Re: Interesting Low RPM @ WOT

His distributor does not have weights, advance is all done electronically.

Well, we can kick that idea to the curb, thanks for the info Oracle............

 

[TheHunter990]

Re: Interesting Low RPM @ WOT

I also had a question about the ingnition sensor. Would this cause my problem if it wasn't working properly. I never replaced it because I could't get one of the screws out. My second question would be is there a way to test the voltage on the ignition sensor to see if its bad. I have a multi meter just not sure how to test it or what the proper voltage would be. Any help is much Appreciated.

Even though you have electonic ignition, I would still try get my hands on a timing light with dial back. Watch how your engine is or isn't advancing. It really seems to me like a spark control related power loss. Checking it on the lake would be best.