Twin HP 525 RPM differences
 

I have twin HP 525's with 30 hours on them.The starboard engine is down 300- 400 rpm's to the port engine. I have ran Bravo 1's and Lab finished Bravo 1's with the same results. The engine has been checked for computer code faults and compression by a Mercury dealer. Can anyone tell me what is considered to be acceptable in rpm differences with twins that are suppose to be matched.

Re: Twin HP 525 RPM differences
 

I would suspect that about 50-100 rpms might be a maximum difference if everything is OK. I think you should first swap the tach inputs and see if the same motor still reads low. If it does I would look at fuel pump pressure differences between the two motors and possible valve spring issues. The Merc code readers don't really show power issues only sensor outputs and their ranges. Compression test will not show valve spring problems that occur at high rpms. You may need a good troubleshooter to find the problem?
Raylar

Re: Twin HP 525 RPM differences
 

Did your dealer do a cylinder leak down test? If not I suggest doing one. You can have cylinder leakage past the valves and your compression test will not show it. That would make a power loss. If this is the case check your exhaust header for cracks. I have seen a few of these headers crack. They are not holding up to heat cycling very well. 300-400rpm's is not acceptable at all in my book.

Re: Twin HP 525 RPM differences
 

i had same issue with 575's and it was a drive ready to blow.

Re: Twin HP 525 RPM differences
 

I have the same exact problem with 2003 525EFI's going thru #6 Drives with Mercury 5-Blade props. Howver, I believe the problem lies in the props, that one is turning faster than the other. Or the other problem can be that the Drives are not aligning properly even though dash indicator says they are. I'm still working on the problem.

Re: Twin HP 525 RPM differences
 

Have friend that has had same problem since new 3 years ago, not only at wot but alos at cruise he has to have the throttles at different positions. This last weekend we tryied some hydromotive props & both motors were exact same rpm. If you have bravo drives just change props on drives, just be carefull since forward will now be reverse & reverse will be forward. If problem moves to the other motor you will know it is the props.