5.7 Mercruiser with Rochester will not go over 4000 rpms

[dan2744]

Hi guys, I'm having an issue where I can't get my boat to go over 4000 rpms. I installed a new engine everything is fine but it gets to 4000 rpms and stays there. I adjusted secondarys to open easier but still the same problem. For the heck of it, I took it to the lake and I shoved a screw driver down the secondarys to force it wide open and sure enough it immediately went to 4600 rpms. With engine off and throttle pinned all the way down it looks fine. I was wondering if maybe under load it's not opening all the way or could a clogged flame arrestor cause this problem?

 

[wellcraft-classic210]

Are you sure the primaries are going all the way open when the throttle lever is down ?

The secondaries are pulled open by vacuum and linkage. As you are aware they should fully open at wot assuming your primaries do the same.

I would suggest
--using a vac tool to pull them open and ensure the linkage is allowing full travel ( Not bent, binding etc ) and also for a leaky diaphragm.

I suppose a manifold vacuum leak is a possibility


The experts may have more to add

 

[dan2744]

Yea they are opening really well, when it's full throttle are the flaps supposed to be at 90 degrees? My bottom primary looks to be at about 75 or 80 when engine is off and boat is full forward. I just put the motor in, I remember having to turn the barrel nut to get everything on. Would a few turns be enough to cause this? Thanks for the response, I really appreciate it.

 

[wellcraft-classic210]

Could be I saw a similar case a few years ago.

Suggest you make sure the primaries fully open and try it.

 

[dan2744]

It's the only thing I can think of. Thanks for responding

 

[alldodge]

For the secondaries to open on a Q jet the choke must be full open. If there still not opening, the spring may need to be adjusted for your motor. This guy tells you how to adjust it

https://ericpetersautos.com/forum/showthread.php?15016-Quadrajet-secondary-air-valve-adjustment

 

[dan2744]

I've tried adjusting the spring, mine is really easy, the air horn doesn't even have to come off. I know that when I stuck a screw driver in it, it stumbled for a second and then took off like a rocket. I'm gonna have to take it out on the water some more. My choke pulloff is operating correctly as well. I even tried with the pulloff disconnected. Maybe under load it's not opening as wide as it should or aur way is blocked? When screw driver was in I had the flame arrestor off, Do those clog? I appreciate the input guys, I'll figure it out eventually.

 

[alldodge]

Arrestors do clog but if you can see thru them your good. Can be cleaned with about anything

 

[dan2744]

I can't see through it at all. Not even close. If this ends up being the problem, I'm gonna be really disappointed in myself.

 

[alldodge]

Also assuming you have the regular finned arrestor. I clean them with diesel and then soap and water, the good thing your not going to hurt it with about any kind of cleaner you use

 

[dan2744]

I noticed what looks like a small amount of liquid on the back side of the carb. Coming from the plenum gasket that the carb sits on between carb and intake. Will fuel come out of there if its leaking there and I'm think a vacuum leak there would cause my issue?

 

[alldodge]

Don't understand a plenum and a carb, a plenum (my thinking) is a housing which sites on top the intake on a fuel injected motor. Now if your seeing liquid coming between the arrestor and carb, then this should not happen

 

[dan2744]

Well, no vaccuum leak, maybe I'll just carry a big screwdriver with me every time I go out.

 

[alldodge]

Either the spring tension is not correct, the compression is low (not drawing enough air), or the carb is not adjusted correctly. The Q jet can be adjusted for many motors, and this is why many like it, but can always go Holley and get rid of vacuum secondaries

 

[dan2744]

I have a double pumper Holley, I've thought about switching them out. When I start the boat and work the carb by hand, I can hit the secondary linkage and it pulls the top secondary damper right down. My compression is 150's for all 8 cylinders.

 

[alldodge]

THe good and bad with the Q jet
Good, easy to adjust and can be placed on a V6 to a BBC V8 with only minor adjustments. The vacuum secondaries allow for this because the more air is drawn by the motor, the more the secondaries open. The varying opening allows for smooth air to fuel mix.

The bad, its easy to mis adjust and in doing so, the fuel mix is damaged to to lean to to rich

Holley, easy adjust by using different jets in both primary and secondaries. If adjusted correctly there is no need to calibrated due to spring changes over time.

The bad, it needs to use a air fuel ratio meter to get it my accurate unless this is a standard build with a preset carb design. Installing for example a 800 cfm carb on a SBC where a 600 or no more then a 650 is needed, causes stumble/bog and excess fuel used at high rpm

 

[wellcraft-classic210]

Regarding your statement

My bottom primary looks to be at about 75 or 80 when engine is off and boat is full forward.


I really do think this can cause the issue -- I saw it happen a few years back --

The primaries need to fully open to get the secondaries to do the same ( Assuming all else is good )

Suggest you check travel with the cable disconnected to see if more travel is available & adjust the cable to obtain that,

 

[dan2744]

I cleaned arrestor and monkeyed with throttle cable and it hit 4200 rpms, so I think my problem may be adjustment of throttle. It's amazing the difference it makes with just a couple turns of the barrel but. Thanks for your input guys, it's always a pleasure.