Lowered pitch on new prop, now engine misses at cruise speed

[Wake it Up! 1]

Last summer, I changed props due to damage on the old one and when I did, I dropped the pitch and went up in diameter. I'm not 100% sure what prop I went up to, but I think it was a ACME 497, I will have to look at next time I go up to where it is stored. Any way, the next time we were out on the water, the engine had developed a miss and it would backfire from time to time at cruise. I tried to duplicate this problem out of the water but it does fine on the fake a lake. It seems to only do it with the boat under way with its own resistance. I haven't changed the cap, rotor, or spark plugs in the last two years, I was going to probably start there, but I was wondering if any of you guys had heard of this before and see what else you think it might be? Any input is greatly appreciated. Thanks guys.

[CenturionSkier 43]

You are now hitting the rev limiter on your engine would be my guess. That is the only way a prop could cause a miss. Depending on what model of engine you have, the rev limiter would be at different points. I would bet you have a MerCruiser. PCM engines have what is called a soft limiter and it just limits the fuel injectors at the set point and the throttle has no more effect on the engine. MerCruiser speeds the timing out to cause the motor to miss, but you should also be getting a beep on your warning horn.

 

I would do the cap, rotor, wires, plugs change. And I bet you are good to go.

[Wake it Up! 1]

I lowered the pitch, but I'm not at WOT when it happens, just about 3000 RPM while towing and about 4000 when going around the lake. I also haven't got a warning horn, I will hook up my diagnostic tool and see if I'm throwing codes next time I run it though.

[truekaotik 458]

It is a 100% your cap and rotor bro!!!! I run a 497 and thats not what it is.... mine did this last fall... It will get worse when it rains or a lot of moister is in the compartment... trust me, I will be surprised if its anything else

Edited March 6, 2012 by truekaotik

[truekaotik 458]

I got the alarming horn for debris in the line from the pump to the engine right by the strainer thingy onto the motor... check there?? Centurionskier can probably explain better...

[truekaotik 458]

Or the prop is on crooked or bent in some way which would chime the sensor also..... I think... LOL but it owuldnt back fire from that...

Edited March 6, 2012 by truekaotik

[Wake it Up! 1]

Thanks, I'm willing to bet it is a Cap/Rotor/or plug gap issue. I have no alarm beeps, I also had the warning horn issue two years ago for a bad block pressure sensor. I also have no unusual vibration as a bent/crooked prop would indicate.

[crew chief 4]

Yes check your cap and rotor. When there is. Load on the motor that's when your motor should start missing.