Engine will not go over 3400 RPM

[BILL KERN]

Recently rebuilt Merc Cruiser 350/260HP engine. Engine will not go over 3400 RPM. I have swaped rochester carborators. I have swaped coils. I have swapped igniton interrupters. I have taken out the anti syphon valve on top of tank. With engine running there appears to be fuel coming out of the secondaries at 3400 rpm. Throttle plates are also wide open at this point. Choke is wide open. Also I have swapped ignition modules which is running thunder bolt IV system. Base igniton timing also set at 8 degrees BTDC. Any suggestions?

 

[Boomyal]

Re: Engine will not go over 3400 RPM

what do the plugs look like? How does it sound and feel at 3.4K? How much history do you have with this boat. Could it be a prop issue? a bottom fouling issue?

 

[Coors]

Re: Engine will not go over 3400 RPM

Per the secondaries; are the bottom/actual throttle plates open
Or are you talking about the top air valve plates?
Huge difference.

 

[Boomyal]

Re: Engine will not go over 3400 RPM

Per the secondaries; are the bottom/actual throttle plates open
Or are you talking about the top air valve plates?
Huge difference.

Coors, you are talking about an Edelbrock type 4 bbl carb. Not sure what his combination came with or even if it has the original stuff. Even so, if Edelbrock style with 'air doors', that 350 should be able to get up to max speed on the primaries. It would just take longer.

 

[180shabah]

Re: Engine will not go over 3400 RPM

Also, how old/what is the condition of cap, rotor, plugs and wires?

Have you run it using an outboard tank of fresh, new gas to eliminate that as a possible problem?

 

[SuperNova]

Re: Engine will not go over 3400 RPM

Hey Boom--he stated in his first post "rochester carborators". With an Edelbrock(Carter AFB) you're right, it should go on the primaries, but a Rochester "Quadra-bog" has really teensy primaries.

My guess would be linkage problem on the secondary air doors(the top ones). The bottom butterflies are mechanically controlled and should always open when the throttle is in the wide-open position. The top airdoor is controlled by airflow(actually vacuum, but that is a discussion for another time). However, there is a small link from the choke horn that prevents the secondary airdoor from opening when the choke is engaged. So, the first thing you need to do is make sure the choke is coming all the way off and allowing the airdoor to open and then make sure the secondary butterflies are standing straight up when the throttle is wide open.

If everthing mechanically is alright, then you need to check the hull for any type of resistance(marine growth, etc).
--
Stan

 

[Maclin]

Re: Engine will not go over 3400 RPM

BK,

Is the tachometer correct?
Are you talking about under load out on the water?
Do you open the throttle slowly or stab it to wide open?
If slowly does it have any flat spots?
Does the carb make a really loud bogging type noise at the 3400rpm?
Does it matter where the throttle is at 3400rpm?

 

[FreeBeeTony]

Re: Engine will not go over 3400 RPM

Was anything done to this engine recently? Did it run OK before any work was done?

I had a similar problem where I couldn't get over 3200RPM's and had fuel coming out of the carb when this condition occurred. Below 3200 it ran great.

My problem ended up being that the valves were adjusted too tight.