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496 MAG - one uses more fuel and lower max RPM???

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Old 06-07-2010, 10:30 PM
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Problems like yours are a major reason I invested in a prop shaft dyno. It is very easy to identify what's going on. Find a shop that has this capability and you don't have to guess. If the engine is significantly down on HP then you can get mercury to step up to the plate.

Bob Lloyd
Full Throttle Marine
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Old 06-07-2010, 10:57 PM
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Originally Posted by patyacht View Post
Just went throught the same thing. Props looked great but I had them checked and they were way out. Straight from Mercury. Not unusual from what I hear now. Get the props trued and balanced. Fixed my same issue.
I guess it's time for an in-water prop change. I hate chasing these things .... But thank you for the suggestions.
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Old 06-08-2010, 01:59 PM
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Can you post a picture of the props? This may help possibly. If you have a 300-400 RPM difference I would say at the very least you should have the propellers looked at. Try to post some pics and include pics with the part numbers, please.

Julie
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Old 06-08-2010, 02:11 PM
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Bravo III drives? Every set of Bravo III props I have owned are way off from Merc. I always send them in to be trued and balanced. Also, you may have hit something that has thrown one of the two props off. I have found that with the dual props you can damage one but not feel any vibration as the second prop tends to cancel vibrations out.

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Old 06-08-2010, 02:24 PM
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Bravo III drives? Every set of Bravo III props I have owned are way off from Merc. I always send them in to be trued and balanced. Also, you may have hit something that has thrown one of the two props off. I have found that with the dual props you can damage one but not feel any vibration as the second prop tends to cancel vibrations out.
Yes, Bravo III. I will check them next (before getting into the fuel pump related suggestions).

The props "look" fine. But based on the replies here it seems that this type of performance loss can be caused by the props even if they don't have any damage that is apparent to the (untrained) eye.
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Old 06-08-2010, 02:35 PM
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Swap the props between drives and see if the issue follows the props.
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Old 06-08-2010, 02:51 PM
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Swap the props between drives and see if the issue follows the props.
The beauty of having twins!
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Old 06-19-2010, 11:41 AM
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Swapped props - No Difference. Bummer

Still have the same problem ... over 3000 rpm, the port motor needs more throttle and uses more gas than starboard. Port uses about 4-5 gph more and maxes about 300-400 rpm lower.

Any other suggestions, please???

Compression is good and no fault codes stored.
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Old 06-21-2010, 08:49 AM
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Since I have ruled out the props as the cause, and the engine is using too much fuel and running without any hesitation, I am theorizing it's not a fuel delivery issue (at least for now).

Therefore, I am guessing that spark/timing should be my next area to check. I tested all the wires and put in new plugs, but the problems remains.

Is there a knock sensor or something that could be unplugged that would cause the ECU to retard timing?
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Old 06-24-2010, 01:10 AM
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Wink Don't you wish Gremlins would only show up at Halloween!

If the scan tool shows 100% power when running at the affected rpms, then its not a sensor or guardian issue.

How exactly are you measuring the fuel consumption. If its an inline fuel flow metered system, maybe the metering unit is out of calibration? As for rpms, are you reading dash tachs or from the electronic scan as sometimes tachs are off quite a bit?

This being said, if the one engine is not producing as much power as the other it could be that the power loss at the prop difference is whats making your results.
As Bob from Full Throttle is saying, this is where a propshaft dyno comes in very handy.

Compression alone is not always a good measure of power. There could be power loss coming from other issues that are mechanical and would not show up on a scan tool. You might do a leakdown test also to dtermine if there are some out of range losses in a particular cylinder or two. Another suggestion would be to read the spark timing in the ECM with the scan tool at these speeds and conditions and see if there is a substaintial difference between the two engines at that rpm.

As with all problems or gremlins,there is an answer, its just the finding thats sometimes very difficult.

Best Regards,
Ray @ Raylar
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