Locking out carburetor secondaries

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Takoradi

Takoradi

Senior Member
Joined
Oct 7, 2017
Messages
152
Location
USA
Vessel Name
The Janie C
Vessel Make
Uniflite 42
I have twin Crusader 454’s that not surprisingly have lousy mpg. I love the full throated roar and punch they can deliver but not so enamored of the gas bill. I’d rather go farther than faster.

I’m considering replacing the 4 barrels with 2’s but it occurs to me that just locking out the secondaries would do the same thing.

Does anyone have any experience locking
the secondaries closed?

Does it help fuel economy?

Is there a dramatic drop in HP?

Are there any risks to the engines involved?

Tak
 
At what speed are you looking to improve fuel economy? Assuming you have Quadrajets, if you're going slow the secondaries are closed and not doing anything anyway.

Things that could improve low speed fuel economy without tearing into the engine for internal changes are (in increasing order of cost/complexity):

  • Set the idle mixture as lean as you can while still getting a good idle
  • Re-jet the carb to lean out the primaries slightly while ensuring it richens up enough under heavier load
  • Upgraded ignition system with a better timing curve and intake manifold vacuum reference, allowing more timing at idle and low rpm / light load

Done correctly, none of those changes will cost you power. Unfortunately, if you're looking to improve fuel economy up on plane, there's not a lot you can do. At low speeds, the stock ignition timing curves, carb calibration, etc. are not ideal for efficiency (they were built for simplicity and durability, which definitely sacrificed some low speed efficiency). Higher compression with aluminum heads, EFI, better cam profile, etc. would all improve efficiency across the board somewhat, but not enough to justify the cost.

Assuming you have closed circuit cooling, run a 160 or 170 degree thermostat and consider adding oil cooler thermostats as well. Those will be nicer to the engines for prolonged low speed running and may save a little fuel as well.

On my own 454 Mercruisers I've been gradually working through some of those changes. I've got a set of 170 thermostats to put in (currently have 160s) and I'm in the process of figuring out oil cooler thermostats. I've also leaned out the idle a bit (makes a small difference for a few hundred rpm above idle as well) and I've been looking into ignition upgrades (currently I have the Mercruiser Thunderbolt IV setup which is good, but falls very short on low rpm timing).
 
My opinion you will not save any money and might just ruin your engine if you figured out a way to disable the secondary jets in your carbureted engine.

A gasoline engine requires a pretty specific air fuel ratio to run. What a carburetor does is to make that happen by increasing fuel delivery as intake air increases.

If you reduce your engines air fuel ratio especially under the high loading of a marine powerplant you run the risk of detonation which will quickly ruin your engine.

If you want to burn less fuel travel at hull speed.
 
ksanders said:
My opinion you will not save any money and might just ruin your engine if you figured out a way to disable the secondary jets in your carbureted engine.

A gasoline engine requires a pretty specific air fuel ratio to run. What a carburetor does is to make that happen by increasing fuel delivery as intake air increases.

If you reduce your engines air fuel ratio especially under the high loading of a marine powerplant you run the risk of detonation which will quickly ruin your engine.

If you want to burn less fuel travel at hull speed.
Click to expand...

Locking out the secondaries would usually involve preventing the secondary throttle plates from opening. So it wouldn't make the engine run lean, it would just limit maximum power (similar effect to just putting a stop on the throttle lever to prevent opening it fully).
 
I don't think locking out the secondaries will hurt the engine, but as others have noted at slower speeds they aren't open anyway.

Is there an EFI kit for your engine. That would be the best way to improve fuel economy. Modern EFI engines make about 12 hp per gph of fuel, whereas older carbureted ones make 10+ hp per gph. Not all of that difference is EFI of course but a lot of it is.

David
 
rslifkin said:
Locking out the secondaries would usually involve preventing the secondary throttle plates from opening. So it wouldn't make the engine run lean, it would just limit maximum power (similar effect to just putting a stop on the throttle lever to prevent opening it fully).
Click to expand...

Yes, any method to limit air flow would be reasonable as it does not affect fuel/air ratios.

In the scenario you described simply not applying more throttle by definition would achieve the exact same result.
 
DavidM said:
I don't think locking out the secondaries will hurt the engine, but as others have noted at slower speeds they aren't open anyway.

Is there an EFI kit for your engine. That would be the best way to improve fuel economy. Modern EFI engines make about 12 hp per gph of fuel, whereas older carbureted ones make 10+ hp per gph. Not all of that difference is EFI of course but a lot of it is.

David
Click to expand...

EFI would help (and there are EFI setups that could be retrofitted on that engine), but a lot of the gains come from better spark control and a better tuned fuel curve on newer engines, plus higher compression, better combustion chambers, etc. A well tuned Quadrajet (they meter pretty well at low speeds compared a lot of other carbs) and a better ignition system will get easily 80% of the EFI gains for probably 50% of the cost on an engine like this.
 
Quadrajets are almost always tuned wrong and one of the settings that's misadjusted 90+% if the time is the secondary spring tension. This is what causes the world famous Quadrajet bog when accelerating quickly.
Basically, when you open the throttles far enough the bottom butterflies open mechanically and when manifold vacuum gets high enough, it pulls the top butterflies open. On the left rear of the airhorn there is a screw that controls the tension on the upper butterflies and there is a small Allen screw that threads in from the bottom that locks it. When properly adjusted, there should be considerable tension on the top butterflies with the throttle at WOT. With the engine/s off, go to WOT and use your finger to open the secondaries. If you can tap on the butterfly and make them open, they're too loose and will open too soon. You should have to actually push fairly hard to open them.
You can look up Quadrajet secondary tuning and find all sorts of videos I'm sure.
With the engine running, when you got WOT, the secondaries should only open after RPM's build and you should have a smooth transition when they do.
As the guy that trained me used to say, you should get a "WhaaaAAA' instead of a "Whoaaa". ��
 
The old Uniflite Gas engine conundrum. So quiet, so smooth, so inexpensive to maintain and so short on range. There is a way to set up the engine that will get you 50% increase in fuel economy but you will never be able to go more than 7.5 knots. It requires leaning out the jets and advancing the timing but it turns the engine into a bomb above 2200 RPMs.

There is no win with a gas uni42 other than to enjoy her the way she is a great, smooth, quiet, go fast boat that never passes a chance to stop at a fuel station.

My dream boat, a Uni42 with 440hp Yanmar’s.
 
I have done what you want on a previous boat by disconnecting the rod between the primary and secondary throttle plates. There was no gain in miles per gallon as measured with Floscans. It pretty much limited the engines to 3200 rpm.
If you still have points buy a whole new electronic distributor instead of the Pertronix points replacement kit.
 
Last edited:
As others have said, locking out the secondaries doesn't improve economy, it just reduces maximum power. The secondaries won't open at low speeds and the primaries on QJs are small, which is intended to improve mixing and economy (i.e. 'spread bore'). I'm not actually a fan of the QJ, I far prefer the Edelbrock carbs for tunability, but the inlet manifold on a 454 is probably bored for the spread bore QJ. The aftermarket EFI systems can help but you may never recover the cost. I'm not a huge fan of leaning out mixtures, I want to make sure my engine keeps running when maneuvering so want the mixture to optimized. If you don't have it already a strong electronic ignition is big plus.

The reality is that the 454 is not an economical engine in any application. Obviously you want the engines to as well tuned as you can but ultimately I seriously doubt you'll find that anything you do will result in a material change in fuel economy. This simply is an old design engine that isn't very efficient.
 
The easiest and cheapest solution is not to push so hard on the throttle(s).
 
OP is running a boat that burns 1.25 gallons a mile at 8 to 8.5 knots. There are ways to improve the fuel mileage but it comes with a compromise.
 
FWIW on 454 fuel economy, with the stock ignition system and carb jetting, my 454s return about 0.55 nmpg at 17 kts (3300 rpm) and somewhere around 1.2 - 1.3 nmpg (maybe a little better, my numbers aren't as good for this speed) at 6.5 - 7 kts (1300 rpm). That's in a 38 foot boat weighing in a bit over 26k with full tanks and a normal load of stuff (not including people weight).
 
"The easiest and cheapest solution is not to push so hard on the throttle(s)."

YES , If a vacuum gauge ( under $20.00 ) is installed you can easily see when the secondaries start to open .

If you are happy with the cruse speed on just the primaries , converting to a good sized 2bbl will give back some of the higher RPM, and not increase the fuel burn at cruise speed.
 
This discussion brings back memories of trying to get better fuel economy in a 60s muscle car. Keeping the secondaries from opening certainly worked in that application, but the difference was similar. Without using the secondaries, there was no full throated roar, no rubber in all 4 gears, no 0 to 60 in under 6 seconds, but 10 to 15 mpg instead of 3 to 5.
Always depended on the weight of the right foot on the pedal.
 
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