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Stumped: boat misfires on WOT after cruising at half throttle.

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Stumped: boat misfires on WOT after cruising at half throttle.

Old 07-20-2020, 08:40 PM
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Default Stumped: boat misfires on WOT after cruising at half throttle.

Hello all,

Im stumped.

Boat: 1992 Four Winns 235 vista
Motor: OMC king cobra Ford 351 w

I've got this odd issue I cant figure out. Boat runs and idles great. Ive poured lots of love and hundreds of hours of labor into it. When its not running right, I cant sleep at night and it becomes an addiction to get it fixed. Most of the motor is new or upgraded.

If I warm the motor up and then run it at full throttle, it runs fine. If I warm it up and run it at full throttle to plane out and then back off the throttle it runs fine. But if I take the family and small kids out and we ride around at 2k rpm for half an hour, once I try to plane out and give it full throttle, it starts to miss fire at the top end of the throttle and looses power when hitting the trim switch. Does not do this if I run it at full throttle at 4k rpm.

No change in thermostat reading between the two speeds. Alternator is good (new). Ground and cables on trim have been checked and fixed. Fuse in charging relay has been fixed. Batteries are on a charger so always topped off. Isolator between starting and house loads so no power drain on the motor from house loads. New plugs this season. Newer wires. New cap and rotor last season. Solid state ignition. Holley marine electric fuel pump. New fuel filters. New Holley carb. Risers are 2 seasons old. New thermostat last year. Outdrive serviced this spring.

I cant figure out why if I run the boat hard, its fine but if I run the boat slow for a while and then try to plane out I have a power issue/misfire.

any ideas?

Thanks in advance.
It feels like a heat issue but I dont know where its from.
Old 07-21-2020, 04:16 AM
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I would replace the plugs again. Maybe got a bad one
Old 07-21-2020, 05:50 AM
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Coil.
Old 07-25-2020, 02:43 PM
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Was leaning torwards a coil too. Sounded like the issue given the behavior shows up after being warm. Installed a new coil today finally. Boat ran better than it has in years when starting out. Sounded better. Higher RPM.

Picked up a friend and we crused around. Drove at 5mph for a bit in. Mooring zone for 15 min and then went to WOT again to plane out. Misfire on the top end again.

Once I use the trim tabs to help get on plane its totally fine.

Could it be a battery issue?
Old 07-25-2020, 03:59 PM
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Not battery... Battery just cranks it .. engine runs on alternator power once started ..
Old 07-25-2020, 04:47 PM
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Brand new alternator. Odd thing I forgot to mention is the trim makes the motor bog when its warmed up and at the point where the miss fires occur. Independently from the miss on WOT. If I trim up or down slightly after its been running for an hour and Im on plane, it will bog the motor. If I trim up and down when Im just starting the trip and the miss fire is not occuring, the trim will not bog the motor.
Old 07-25-2020, 08:40 PM
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Replace the coil.

oops just saw the issue with trimming the motor and bogging. Check all motor and battery grounds. Clean and try again.
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Old 07-26-2020, 07:28 AM
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Coil polarity backwards?
Old 07-26-2020, 07:24 PM
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Polarity on the coil is correct. Ill be checking the grounds next. I just cant figure out why a bad ground would only show its problems after the motor has been running for an hour and not in the first few minutes of use.
Old 07-27-2020, 05:25 AM
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Are you using a coil with a resistor? Solid state ignition needs full 12 volts and require a matching resistor coil. The voltage dropping ballast resistor/run wire is bypassed.
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Old 07-27-2020, 12:55 PM
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Using a coil without a resistor. Was the coil type that was in there when I got the boat 8 years ago. The solid state ignition was wired in prior to me owning so I have not changed the wiring other than tapping a relay off the positive lead of the coil for the electric fuel pump I put in.

The part where it only acts up after an hour or so still has me stumped.
Old 07-27-2020, 01:38 PM
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It sounds to me like a fuel problem, possibly a carburetor. Does this boat have an anti siphon valve? I would replace it with a straight valve. I have had an anti siphon valve cause bogging down at high rpm and stalling out/back firing after coming off plane due to a restriction.
Old 07-27-2020, 01:49 PM
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Originally Posted by drewm3i View Post
It sounds to me like a fuel problem, possibly a carburetor. Does this boat have an anti siphon valve? I would replace it with a straight valve. I have had an anti siphon valve cause bogging down at high rpm and stalling out/back firing after coming off plane due to a restriction.
Could be a plugged tank vent.

Unresisted coils overheat if run on full 12 volts. Ignition must be run on 12 volts. Is the coil really hot after extended usage?
Old 07-27-2020, 01:50 PM
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No anti siphon valve installed. Its been removed.
Old 07-27-2020, 02:26 PM
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Originally Posted by thejtdude View Post
No anti siphon valve installed. Its been removed.
How about the tank vent like the poster above mentioned? Has the carb ever been rebuilt? All of these items could cause a backfire as you describe above. Why do you think it's a coil/ignition issue?
Old 07-27-2020, 02:37 PM
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I reread your post...are you sure your ignition timing is set properly??? The reason I ask is I had a boat that had a 5.7 Yamaha sterndrive with a 19 pitch prop. I wanted a bit more top end, so I put a 21 pitch prop on and then we would be cruising along with the 21 pitch and all of a sudden we would hear a backfire and the engine would stutter and then rev back up to where we were before.

It seems the extra load of the bigger prop caused the engine to backfire momentarily. Upon switch back to the 19, the issue went away. I never checked base timing on this boat but I suspect it was off hence the backfire that shouldn't have ever been present. It seems to me you may have a similar issue because it presents itself when you are putting excess load on the motor (trying to plane) and perhaps the prop slips causing backfire?

Check base timing and get it set perfectly. Every boat I've owned has been slightly off before checking and adjusting it...
Old 07-27-2020, 08:01 PM
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I had a 5.7 a few years ago that we replaced the engine on. It kept misfiring and bending valves. After replacing the heads three times we replaced the distributor and that fixed it. We upgraded everything chasing the problem. Did exactly what you are saying, the trim would cause it to do it more often than not.
Old 07-27-2020, 08:15 PM
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If the engine bogs when running the trim up/down, sounds like you have a bit of crappy wiring that creates a volt drop under load.
Old 07-29-2020, 07:31 AM
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The coil is in its factory installed location which is under a cover with 6 screws on it so hard to tell if its hot or not without disassembling. I have not checked this but will on the next long run.

Brand new tuned carburetor. Its a Holley 4160. There are no adjustments on this carb other than the idle mixture and the throttle plate stops. It idles and runs great until it gets to WOT under load trying to plane out. Not sure if the carb is the issue source.

I replaced the fuel tank a few years ago with a new one that I had custom built and installed myself so I can confirm the configuration. There is no anti siphon valve on the fuel line to the fuel pump. This was removed by the previous owner due to similar headaches and I just kept it out to avoid any similar issues as you have all mentioned. There is no anti siphon valve on the vent line. Just a dip or curl in the line at the top by the vent to prevent water entry. The vent its self is on the list to replace but I check it every season and its still clear enough to not cause blockage.

I have been running the same prop for several years now. I actually ran into the same or a similar issue when the boat was new to me. The original owner tried to fix a sick motor by changing the prop. The prop was too big and would cause the motor to just die under load. Figured out what the factory installed prop was and changed it back to that and never had that issue again.

The timing was adjusted and set about 3 seasons ago. I agree it is worth me checking that for sure. I will have to do that. I always hate checking the timing out of fear that I can make decent timing worse by changing something that's not broken haha. But agreed, I should at least check it and start from there. That's on the list for the next chance I have.

I am leaning towards an ignition issue because the key in all of this to me is that this issue ONLY occurs when the boat is very warmed up (not going to say hot because everything is running at spec temps). These issues do not occur in the first 30 minutes of running the boat. They occur after an hour of running the boat. That sounds like something is warming up and failing. I don't know how an anti siphon valve or carburetor or timing issue could not show its issues in the first 30 minutes but would in the hour of run time. That's where I have been stumped. Of course its taken me a season to get to this point. I use to think it was all of the above until i realized it only happens after a long run this season. Started doing short runs a lot this season mixed in with long family runs.

I agree that this feels electrical for sure in that its directly effected by additional electrical loads put on the system like the trim pump. A bad ground would defiantly cause this. When I first encountered this issue a few seasons ago, I traced down all of the wires and grounds to my trim pump. I re grounded the trim pump directly to the block. The trim runs through a relay so the actual voltage is pulled right from the battery loop and not from the same wiring as the ignition. Back to the part that stumps me on this is that the trim causing the motor to bog and miss ONLY happens after the motor is warm after a long run. It does not occur if I warm the boat up for 15 minutes at the mooring, drive a few yards away from the mooring at 5 mph and then floor the throttle to plane out and max out my speed while adjusting the trim to raise up the bow in the waves. It occurs after I've been putting around for an hour with the kid on board, drop them at the dock and try to cruse back to the mooring but when trying to get on plane and adjust the bow with the trim, the issue presents its self.

So new alternator this season. The old one was charging at and over 15.5ish volts. New one is charging right where it needs to be and no more bearing noise. Connections are tight. I even have an extra ground cable installed just in case the original wiring is bad.

As for the idea of replacing the distributor.... I had not thought of that one. Good call. Did some research and learned that an over charged system can break the Mallory magnetic ignition module I have installed. I have known for years that leaving the key on when the motor is not running can also burn some of these up. Its a practice to not do this but there are times now and then when this is accidentally done during frustrated spring start ups or motor stalls in the past which could have also added up. There is a test for determining if this unit is bad which I discovered last night after getting home from the mooring and already ordering the part so Ill conduct that the next chance I have just to see and then replace the part anyways since it appears to be heat related and may not show up on the test. One of the things I read about this part is that it comes with a silicone that you put all over the part to shield it from heat. I can see some old gummy gunk on the existing one that's all dried up and falling off. Maybe that was the death of the currently installed one.

Want to mention that I am greatly appreciative of all of your input. Please keep it coming! I am not one to disagree with anyone's ideas here as pretty much every single thing I have learned about boat mechanics (and I will confidently state I now know a lot) has been from here and contributors like you all. So much appreciated and keep on throwing things at me if you can. Ready to try anything (unless I already did and forgot to mention it which some of the above items list out, oops).
Old 07-29-2020, 02:31 PM
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Backfiring is almost always electrical. Second is a valve not seating properly.
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