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lack of rpm above 3800 mercruiser 502 mag mpi

T

tommy396

New member
I have a problem with rpm above 3800.
When running in neutral the engine can take full throttle and hit rev limit, in gear it will not go higher than 3800.
This winther I had the engine out for overhauling, all bearings, pistion, pistion rings, oilfilter, fuel filter, hamonicdamper*and*gaskets replaced, engine bored to +.64 oversize.
Ignition set at 8*BTDC, idel rpm 650-700, engine runs normal up to 3400 rpm in gear,*with normal fuel consumption.*
The engine starts and runs normal hot and cold, before overhauling the engine was running @ 4600 rpm*taking the boat to 48 mph (gps) now I can take it to 3800 rpm 37 mph (gps).
Have I missed something when assamble the engine ?*sensors, cable adjustment*? is there something in the engine control that can be link to*taking the boat into gear and hold the rpm down*?**
Everything on the engine and outdrive is the same.
*
My boat is a Sea Ray 270 Sundancer.
Engine mercruiser 502 Mag Mpi.*
Outdrive Bravo III.
Prop Signature 24P.
 
Would the qts of oil make a difference if the engine is in gear, in neutral it can take WOT until rev limit (4800 rpm) ?
will check oil level, I belive I have it at the between low and high on my dipstick.
I don't know if it is a seacore engine it is a year 1999 gen. 6 engine block.
 
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Re: power problems under load

Re: Lack of power no rpm over 3000rpm

List of possible causes of low WOT

List of possible causes of low WOT (Wide Open Throttle) rpm.

In no particular order

Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel
2. Propeller pitch or diameter, damaged blades
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
5. Marine growth on hull and outdrive
6. Wrong gear ratio in outdrive
7. Restricted carburetor air intake (clogged flame arrestor)
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test
10. Carburetor defective, or wrong type.
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.
13. Engine Overheating
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully.

Above copied nd pasted from another website Don s is the author.

PS running up the revs in neutral means nothing.
 
tommy396 said:
Would the qts of oil make a difference if the engine is in gear, in neutral it can take WOT until rev limit (4800 rpm) ?
will check oil level, I belive I have it at the between low and high on my dipstick.
I don't know if it is a seacore engine it is a year 1999 gen. 6 engine block.
Click to expand...

Did not know the year of your engine. Most likely this is not your problem being it is a 1999. In a newer engine an extra half quart of oil will lower the rpms by 1000.
Chiefalen's list is where to start.

The only thing I would add is the distributor cap and rotor. This will cause a problem like yours.
 
I believe that the rotor and distributor cap, is from the local autoparts shop, not a marine supplyer could that make a difference in the rpm.
 
Still no luck...if I don't have Scan tool, Diagnostic Code Tool or MerCruiser Special Timing Tool, what can I do to be sure that the timing advance is off doing timing.
 
Had some time this weekend to work on my lack of rpm. issue, Going down the list from Chiefalen , item 14 ignition, is where I found the remaining rpm. Taking the engine to 3600-3800 rpm. under load, and adjusting to 20’-22’ of timing advance, the engine runs great and will go to 4800 rpm. engine
Thanks for all inputs and help.
 
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