Back fire when towing?

[Cubby_Z]

I have a 99 21v with the 350 Mag MPI engine. The engine is new last summer and has about 45 hours on it. When I am pulling an adult on a wakeboard, I feel it is sluggish out of the hole and it backfires frequently. The backfiring isn't right, but how sluggish should it be out of hole? Top end speed and RPM are great...Thoughts?

[JCP]

The backfiring could be secondary ignition. Were the cap,rotor and plug wires replaced with the new engine?

[Cubby_Z]

Yes, new cap, wires, plugs, rotor, and coil with the motor...

[TigeFamily]

The low end problem...could be prop related. Especially if top end is "great". Your prop may not be right size...find something that adds low end without giving up a lot of top end. Lots of options out there. Mine will handle anything down low...and still does 40mph so quite happy. Different model than your tho, so I cannot say what you will need.
Call Nettle Prop out of Austin, TX.....or ACME.

[chpthril]

Backfire is typically ignition related. Ran fine for 45 hours? although those items were new with engine, are they quality marine grade? If those items did not cure, it, try ignition module.

[Thegerman618]

I had the Merc 350 in my older boat, got a backfire right before I sold it would run fine when empty but full or towing it would backfire and would not go over 10-12 mph. It was the fuel pump being the issue, the fuel pressure was fine at idle but when adding power the fuel pressure would plummet, not enough to kill the motor but enough to cause a back fire. I would check the pressure on the rail while under a load.

[dj2]

I just started having this issue with my 2008 MP 5.7, mechanic has me checking fuel first. Just replaced the high pressure pump last year but just started having "vapor lock" symptoms this past 2 months. Then all the sudden yesterday backfiring when towing and then progressively worse over a 5 min period to where it was backfiring randomly with any load and then it stopped running. For fear of causing damage from the backfiring or detonation (which I heard right before shutting it off). Hopefully you and I are just having a fuel pressure issue....

[Thegerman618]

I had the Merc 350 in my older boat, got a backfire right before I sold it would run fine when empty but full or towing it would backfire and would not go over 10-12 mph. It was the fuel pump being the issue, the fuel pressure was fine at idle but when adding power the fuel pressure would plummet, not enough to kill the motor but enough to cause a back fire. I would check the pressure on the rail while under a load.

[dj2]

Well, the bushing in my 1yr old high pressure pump is the culprit of my issues.... low pressure pump is good, high pressure pump sounds like he'll and managed to send brass filings into my fuel rail so now I may have to pull all my injectors and have them cleaned.

OP have you figured anything out yet?

[Cubby_Z]

I haven't figured it out yet, I did replace a fuel pump last summer, but don't know if it was the high pressure or low pressure pump. I have also put in a few new injectors.... Kinda makes me think it could be the other pump...

[Cubby_Z]

So I haven't fully dianosed this yet, probably going to the dealer next week...but a 99 21v should be able to pull up a 185lbs man on a slalom ski, right?

[mdk681]

So I haven't fully dianosed this yet, probably going to the dealer next week...but a 99 21v should be able to pull up a 185lbs man on a slalom ski, right?

Yes, without a doubt. I have the same boat and can get on plane with over 2k lbs of ballast in the boat. Good move in taking it to a dealer. Hang in there, these boats are great when dialed in.


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