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Old 09-03-2018, 02:15 PM   #1
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Default Twin Drive RPM Differs

I have a 1998 3000 SCR with MerCruiser 5.0 L and Bravo 2 drives. This is a new boat for me and there is a lot of maintenance needed. My main concern is the power differential between the two motor/drives. WOT for Starboard motor only during sea trial was 3500 RPM at roughly 26 mph. Port was only able to get to 2900 RPM and 22 mph.
Compression test showed all cylinders on starboard engine 180 to 190 psi. Port engine showed 180 to 190 psi for 6 of 8 cylinders. Those cylinders are next to each other on the same bank.
Additionally, the port prop is missing a chunk on one blade and is sliding a little on the shaft.

My question is what is causing the differential in performance and which should go after first to correct, motor or drive??
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Old 09-03-2018, 02:34 PM   #2
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Fix loose prop before testing any further. Running with it loose will damage the shaft/splines and a chunk missing would affect performance.

What were the readings for the other two cylinders on the port side?

If you have low compression on those two, that's going to cost you power and may explain he lower rpm.
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Old 09-03-2018, 02:48 PM   #3
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165-170 psi.
The boat is coming out of the water in a few weeks for the winter. I planned on replacing the aluminum props with stainless and all hardware.
I’m hoping the compression issues are stuck valves, the previous owner did not run the boat much about idle in 8 yrs. I changed all the plugs during the compression check because they were all fouled.
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Old 09-03-2018, 09:17 PM   #4
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I donít think the compression test was performed properly as the normal high is 150 tp 160.

The engine should first be warmed up to operating temp.
Disable ignition.
Remove all spark plugs.
Open throttle to half to three quarters.
Perform compression by cranking 3-6 revolutions.
Take readings.

Neither engine is achieving proper WOT which should be 4400-4800 for these engines. They may not be proped correctly. A damaged prop will impact performance.

I would repeat the compression per what I said.

Either recondition both props and see how they do or use Quicksilvers prop calculator to see what it recommends and get a pair.
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Old 09-03-2018, 10:35 PM   #5
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I had the marina mechanics do the compression test and when the first set of readings were that high we changed gauges and did them again with only a 5 psi difference between gauges.
I agree with the assessment of the prop I’m not sure what props are on there now other than they are 3 blade aluminum. When underway I can cruise about 28-29 mph and about 30-32 WOT. But the starboard side is definitely doing the work and revving higher.
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Old 09-04-2018, 01:20 PM   #6
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I've always had a tiny difference in performance with my 5.0's. Nice thing about twins is the ability to compare and swap parts.
However, to eliminate non engine factors, I started with having the props tuned. Small differences in the props can equal a few hundred RPM difference.

Next was basic tune up items. Plugs, cap/rotor, fluid/filter changes (don't forget the small gas filter inside the carb inlet. Carefully set the timing to spec using the prescribed method to insure base mode is set on ignition module.

With all that done, RPM difference was brought within a few hundred. After reading the plugs, I found the stbd was running a bit leaner than the port. For giggles, I swapped the carbs and ran another test run.
Problem followed the carb!! Rebuilt both (just to be safe) and issue is now gone.

John

Note: Incorrect timing was the main culprit, gunked up carb was just the icing on the cake.
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Old 09-04-2018, 01:51 PM   #7
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Those are great insights and have given me a great foundation of things to get started on.
Much appreciated!!
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