Low Revs

[Private Nemo]

Hi,
I have twin Mercruiser 5.7, 1985. 4bbl, Thunderbolt IV ignition. Over the winter I install rebuilt heads on both motors. Now one engine maxs out at 4600 rpm and the other maxs out at 4000 rpm. Check carbs , both open all the way. New plugs , caps and rotors. Same drives, same props. Cant figure it out. Open to suggestions.

thanks

Drew

 

[alldodge]

Re: Low Revs

How many hours are on the engines?
Why did you want to rebuild just the heads?

 

[Private Nemo]

Re: Low Revs

Not sure of the hours, gauge was broken when i bought the boat. Just wanted to make the engines run best as possible. They ran great last season , both 4600 rpm. Now only one.

 

[alldodge]

Re: Low Revs

Not sure but an engine as it wears over time some items get weaker than others. The engine when new lets say it puts out 300HP and is strong. Over time the HP may drop some and all things are getting a bit weaker than new. By rebuilding the heads you may have shot yourself in the foot... so to speak, maybe. The heads are no longer weak and what use to pass some preasures around different other items slightly, now blows all pressures around the rings. maybe. I'm not saying this did happen but it could have because you fixed all the issues with the heads, and maybe the piston and rings can't handle all the extras.

 

[glenn property of pam]

Re: Low Revs

back to basics. Plugs in correct order, pull plugs do they look the same. Timing check, throttle plate opens all the way, Check each lead with timing light when running to make sure all plugs are getting power.

 

[Private Nemo]

Re: Low Revs

Check firing , perfect. Check plates, perfect. Check timing, perfect. I will pull plugs tomorrow and c what they look like.

FYI, engine runs perfect except for low rpms.

 

[achris]

Re: Low Revs

Compression test, and leakdown if you have a tester for that....

 

[QC]

Re: Low Revs

A compression check is also in order.

Edit: Sorry Chris, posting at the same time.

 

[achris]

Re: Low Revs

A compression check is also in order.

Edit: Sorry Chris, posting at the same time.

That's cool... happens to me all the time... slow fingers.. :facepalm: