2017 Mercury 90 hp 4 stroke popping and won’t go above 4200rpm

[Ace327]

Hey guys needing a little help. I have a new to me 2017 Mercury 90 ho four stroke CT out board on a 22 ft sun tracker pontoon. It runs very rough at idle and won’t go above 4200 rpm. I also get a semi loud popping sound on acceleration and de-acceleration (2500-3500 rpm) and sometimes while cruising. I’ve replaced the fuel with known high quality ethanol free gas, replaced the fuel pump assembly and filters, cleaned the injectors, checked all the fuel lines and replaced the spark plugs. It does run some what better after all this but still doesn’t run well. I’m wondering if someone might know what I should do next or what the problem is? I have videos of the problem but can’t seem to get them to post on the forum. Thanks in advance for the help!

[Fishing Addict]

Don will need your serial number. Has the engine ever performed as it should under your ownership?

[Ace327]

It has never performed the way it should. Bought the boat and motor last year and have been fighting it ever since. Serial number is: 2B422925

[Zone18]

when you say popping do you mean a mechanical pop like metal on metal....or a backfire exhaust type pop?

[EuropeanAM]

Also- a couple more questions:

1. Where are you obtaining your RPM info from ? Smartcraft or an Analog/needle Tachometer?

2. What prop is currently on the engine (exactly)? Any damage to the prop?

3. Does the warning horn sound a test beep at key-up?

4. Any warning horns while running?

[Ace327]

Pop is definitely not metal on metal more back-fire exhaust type. Rpm is coming from analog tach in the dash. Prop is new with no damage(replaced last summer with a black max, picked it up from tracker boat center its the Prop they recommended not sure on pitch it’s a 3 blade though). I get one beep when the key is turned on and no other warnings or beeps when running.

[EuropeanAM]

Has the engine ever run properly at WOT since this propeller was installed?

And if not, DID it run properly PRIOR to the propeller replacement?

Sorry for the questions, but Analog tachs are notoriously inaccurate and we need to make sure we're not spinning the engine higher RPM's than we "think".

[Ace327]

Engine has never ran quite right, it ran worse before we cleaned the fuel system and replaced the fuel pump assembly but that didn't completely solve the issues. I replaced the prop shortly after purchasing the boat, the old prop had seen better days (was worn and had several places in the blades), however it was never ran in the water with the old prop (idled albeit terribly) in the driveway however. No problem with the questions! I really appreciate the help!

[EuropeanAM]

Thanks for that info.

A couple items:

1. Spark plugs. Just put a fresh set in it, properly gapped. Don't assume they look good: Replace them. Nothing will make you feel worse than to chase a problem like this and find out at the end of the long tunnel it was a spark plug.

2. Fuel Injectors. Who cleaned them? Did they provide a flow report for Idle, Mid throttle and WOT parameters? Or at the least, WOT and one of the other parameters?

3. Fuel Pressure. Monitor fuel pressure with a gauge, comparing to make sure it's correlating to the engine RPM and load (and not falling off when you hit the power). Some of the VST's in this era had problems with one of the fuel pump connections that would get "hot" and cause difficulties with fuel supply.

[Ace327]

Ok! Thanks for the input, I'll replace the plugs again next weekend when I'm at the boat. I cleaned the fuel injectors at home, no special tools, just made a jumper to charge the injectors and blew fuel injector cleaner through them with pressure. (I'm beginning to think these could be my problem, not sure, but I've already purchased some replacement injectors.) We did check fuel pressure at idle and it was spot on but have not checked it under load. Once i replace the spark plugs and injectors I'll take a trip down the lake with pressure gauge installed and see what happens. Again, i really appreciate all the help!

[EuropeanAM]

Just be careful what you buy for injectors. There are a lot of Chinese knock-off sets that look identical, but the flow rates aren’t even on the same chart.

[Ace327]

Thanks for the information about the injectors! Just about tried the amazon knock offs! I will order the Mercury ones instead.

[Ace327]

Ok, replaced spark plugs, injectors (genuine mercury ones) checked plug wires, replaced coils and even ran the boat on a different fuel tank to make sure that wasn't the issue. All of these things made zero difference in how the boat ran. At this point I've taken it to a marine repair shop. I'll follow up to let you guys know what the ultimate problem was. Thanks for all the help!

[EuropeanAM]

[billnorman1]

Since it is a 4 stroke, or even if it wasn't, have you done a compression test? An intake valve that is bent, or has a broken or weak spring will cause popping in a 4 stroke. No, I don't own one of these engines.

[Ace327]

I haven't, that's the first thing the shop is going to do.