4 barrel Quadrajet, RPMs, and 5.0 vs 5.7, electric

[cptbraine40]

Good morning........this is probably a stupid question but I am going to ask it anyway. Here is my information:

Mercruiser 5.7 with roller cams, Part: 807447R5 Serial Number: 92019002
Alpha One Gen 2 Outdrive
Quadrajet 4 Rochester, Number: 17080565

I am wondering when the additional 2 barrels kick in? At what RPM. I am just curious to know so when I am on longer cruises I can keep my RPM below that level just to save gas.

Also, when I bought this boat it had a 5.0 in it. The previous owner did not winterize and put a new/reman 5.7 in it from Quicksilver Remanufacturing. I saw the invoice and the shop used all the parts they could from the 5.0 to placed on the 5.7. Is this carb (that was on the 5.0) adequate for the 5.7 or do I need to look at getting another quadrajet that is made for a 5.7?

Again, I know these are probably stupid questions but I am all about knowledge and information.

THanks for your assistance and feedback........Brian

 

[GA_Boater]

Quadrajet secondary operation are both mechanically and vacuum operated. The secondary throttle plates are opened by the linkage and vacuum controls the opening of tapered secondary fuel metering rods to increase fuel flow.

What does all that mean? It means there is no set RPM to go by to save gas. Adding a fuel flow system would allow you to select an acceptable cruise speed and fuel usage.

Before anyone else says it - Hoist the sails or break out the oars to decrease fuel use. Most boats use a lot of gas and that's the way they are.

 

[cptbraine40]

Oh yes, boats do eat up the gas, I was really not that concerned about it, just trying to expand my knowledge some. What do you think about the carb from 5.0 to 5.7? I really appreciate your input and feedback

 

[Scott Danforth]

the same Q-jet is used from 4.3's to 7.4's..... its how its tuned that makes it engine specific (rods, linkages, settings)

and yes, if you want to save fuel in a boat, get oars or a sail.

boat motors burn 0.4# of fuel per HP per hr.

a US gallon of gas weighs 6.82#

so your burn rate is directly tied to the power commanded. the power commanded is directly tied to your right hand, the wind, the hull dynamics, the hull cleanliness, the maintenance on the motor and drive, the pitch of the prop, the angle of the drive, the directly of the current and the weight you are displacing.

As GA stated, get a fuel flow meter as your best fuel sipping RPM will vary

If you want to expand your knowledge on Q-jets, plenty of reputable Q-jet books written over the last 50 years.

 

[alldodge]

Oh yes, boats do eat up the gas, I was really not that concerned about it, just trying to expand my knowledge some. What do you think about the carb from 5.0 to 5.7? I really appreciate your input and feedback

A Qjet from a 5.0 will work fine on a 5.7 if the jetting is correct. The Qjet uses vacuum to open the secondaries based on the amount of air being drawn in. A 5.7 will draw more air then a 5.0 so in theory this will allow more fuel in as air flow increases.

Start by finding out what jet size is in the carb now, and also which primary and secondary rod sizes

 

[cptbraine40]

Great...thank you everyone for all of your input and feedback, I really do appreciate it so much. I have no idea what the jetting is on the carb. I just look up the numbers and everything online states for a 5.0. I just want to make sure I am getting maximum performance from this awesome new 5.7 and don't want to hold it back because of a carb.

 

[GA_Boater]

Well Scott and All Dodge said what I was about to say. Jets and fuel rods may be different. See if you can find parts diagrams for the carbs on both motors and compare part numbers.

Also, if it's running good and the spark plugs look good, then leave it alone. QJets can be hard to work on depending on experience.

 

[Rick Stephens]

I had someone watch my throttle linkage as I opened the throttle and tell me when the secondaries opened on my engine. Gave me a rough idea of how to keep on the primaries. Pretty easy on my boat to hear when the secondaries open. I think I have it down where I cruise with primaries almost all the way open but no secondary flow at all. Never used a flow meter. I can tell you it gets spendy when decide to run the speeds up. Doesn't take much at all to double fuel usage.

 

[Lou C]

You might look up Cliffs high performance Quadrajets website and ask this question on their forums. On the Qjet the secondary throttle blades are mechanically operated but they really don’t come into action till the air doors open (vacuum operated). This you could not see unless the flame arrestor was off, the air doors are right behind the choke plate. However the Qjet howl when they open is unmistakable!

 

[Scott Danforth]

definitely make sure the tuning is done on the carb

90% of the time you can hear the air whistle when the secondaries start to open (Rick mentioned this above). so crack the throttle, and when it starts to whistle, back off a bit.

or realize that 90% of the people that go boating dont put more than 20 hours on a boat a year, and even running at WOT burning all the fuel you can is about $150 per year more per year over getting the best economy you can. go boating, have fun and dont worry about economy.

 

[achris]

'Cheap', fuel economy' and 'boating'... 3 words/phrases that can never be used together.

 

[Lou C]

Everything is relative but I found the Quadrajet on the 4.3 V6 to be surprisingly economical, probably due to the small primaries. If you cruise at approx. 3600 rpm, you will not get into the huge secondaries.

 

[Scott06]

Start by reading your spark plugs do it after a 5-10 min part throttle cruise, cut it off, pull a few plugs and see what they look like. Do the same at WOT. That will tell you what your jetting is like. Google spark plug reading to get an idea what to look for. If they aren’t too lean looking likely the 5.0 carb will work just fine

 

[NHGuy]

I have had a quadrajet apart and rebuilt it. The carb will work just fine.


Edit: the primary jets and secondary rods are the same for both engines if they are both the MCM style, which you should have.

If you want to avoid unnecessary fuel usage just get it on plane, trim for speed and stay there.