engine RPM question

goodnuss

Cadet
Joined
Aug 10, 2004
Messages
18
I have twin Merc 454 Rochester Quad Carb engines. Rpm for both engines is fine thru 3900 RPM. Once throttles go to wide open the top RPM is 300 short on the port engine. I had engines rebuilt and the aliement and props are good just being reworked. I am looking for any suggestions on what adjustments should be checked to solve this problem. Appreciate any input.
 

Scaaty

Vice Admiral
Joined
May 31, 2004
Messages
5,180
Re: engine RPM question

Could be igniton if you have points
 

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: engine RPM question

Check the timing and advance. If it isn't advancing as much that would cost you some rpm. Also the setup of the carbs. Jet sizes metering rods, secondary opening distance. You could actually swap carbs from one engine to the other and see if the low rpm changes engines also.<br />Even have a look at the rest of the igniton system. Plug wires, cap, rotor. You may also have a tach that is not reading correctly at higher rpms.
 

goodnuss

Cadet
Joined
Aug 10, 2004
Messages
18
Re: engine RPM question

Appeciate comments. Will be checking out. <br /><br />Carbs were recently overhalled sand main jet size increased to .75from .70. This problem is at WOT.<br /><br />Will be completely checking the advance and timing again.
 

ron7000

Banned
Joined
Jul 10, 2004
Messages
498
Re: engine RPM question

the swapping carbs sounds like a really good idea. Would instantly rule out, or pinpoint, the carb as being the problem and would cost $0.<br /><br />And you also might be able to swap tach signal wires to see if it's a bad tach. Either at the coil (depending on ignition system?) or at the tach if you have easy access to the back of them.
 
Top