Mercruiser 4.3 Carburetor Problems? The 165 New Answer

Finding the right replacement parts or ordering the right conversion kit when you don’t know exactly what you’re converting has never been easier – until now! Holley’s conversion kit selection tool changes everything – just enter your carb batch number here and you’re good to go! And have you ever tried to pinpoint exactly what carburetor you had but had no idea how to do it? This short, informative video should answer any other questions you have. You’ll learn the key to figuring out exactly what carbs you have: the list number. It’s like a “VIN” for your carburetor and should be hand stamped on the front of the choke tower to the right of the breather tube on most typical Holley carburetors. The list number is usually four to six digits and may or may not have a suffix number after it. The date code is displayed directly below. Be careful not to confuse the casting number with the listing number – the casting numbers will be raised (not stamped) and alphanumeric. Invaluable information about the model, CFM, factory jets, power valve size, and bleeds can be discovered simply by knowing your listing number. So spend a few minutes watching this video now and save yourself a lot of time later!

If you were lucky enough as a kid to have an older brother or other adult who was genuinely interested in cars, they seemed to have a tremendous amount of knowledge – carburetors in particular. Think of this story as if your cool uncle invited you into his two-car garage behind his house, showed you a bunch of cool Holley carburetors, and filled your head with all sorts of interesting things you never knew.

Choosing a carburetor for the road is a bit more complicated than just making sure the carburetor will allow more air to flow than the engine needs. A street carburetor has an even tougher job than those for race cars. A good street carb needs to be able to start on a cold winter morning and still withstand a 200 degree Fahrenheit heat exposure in the middle of a tennis shoe-melting summer. A street carburettor must also deliver perfect off-idle manners, deliver good fuel performance, and seamlessly handle near-instantaneous transitions from idle to full load (WOT).

Let’s start with a few definitions that will be useful throughout this tour of Holley Street carburetors. Holley offers several configurations of the classic four-barrel carburetor. The entry level version is the 4160 which is bolstered by the 4150 and both are marked as standard mounting flange versions. These two are very similar, with the main difference being that the 4150 uses a thick metering block on both the primary and secondary, while the 4160 is shorter and uses a thin metering plate on the secondary.

There is also a spread-bore style Holley intended as a direct replacement for the Rochester Quadrajet’s small primary and giant secondary designs. These Holley versions are designated 4165 and 4175. Finally, there is a race-only 4500 configuration with huge throttle bores called the Dominator. There are a few other model numbers that are slight offshoots, including the 2300 series two-barrel configuration.

According to my gray cells, the MerCruiser 4.3L has been around since the 1960’s when it was fitted to a front-wheel drive Oldsmobile. The engine continues in marine form and is incredibly popular in both single and twin installations.

Compared to low-emission petrol outboards of the same power, the 4.3-liter looks downright primitive. But it has a simple technique that’s hard to kill even with minimal maintenance. And more importantly, the MPI version (which allows the use of Mercury’s brilliant SmartCraft system) pumps out 15 percent more power than the old two-barrel Carbie version.

MERCRUISER DESIGN

The 262-cubic-inch (4.3L) MerCruiser MPI is a 90-degree V6 version of the long-life 350 (5.7L) V8 and develops 219.8 propshaft horsepower at 4600 rpm with a 4400 wide open throttle range up to 4800 rpm. This rev range is more akin to a current common rail diesel than a petrol engine and when combined with the well over square bore and stroke dimensions of 101.6 x 88.4mm and hydraulic tappets driven by pushrods from the center camshaft is this low-maintenance and durable motor.

Fresh water cooling reduces condensation on the cylinder walls and increases the engine/drive dry weight by 24kg from 393kg, not bad considering the cylinder heads and block are cast iron – none of that newfangled alloy here!

The MerCruiser 4.3L MPI’s 65-amp, voltage-regulated alternator is driven by a serpentine belt, which also drives the power steering and water-cooling pumps.

The 4.3L is available with legs from the aging Alpha to the Bravo 3, but in all honesty the 3’s are overkill for twin installations as they prevent the classic one-up/one-back technique of rotating a hull in its own length. On trailer boats, the cooling water intakes are in the drives, but the 4.3L I tested in a Sea Ray 315 Sundancer had through-hull intakes with screens and seacocks. Just remember to open them before you start the engines!

On standalone installations, access to the oil filter and spark plugs is good, but on the 315 Sundancer, which was 9 feet wide, it definitely required a very flexible wrist and fingers to get to the inner spark plugs. The price you pay for safety in a smaller cruiser!

The maintenance intervals are every 100 hours or annually after the first 20 hours. Since most cruiser owners by the age of two only work about 50 hours a year, I would change the oil and filter every six months and use Quicksilver’s SAE 25W40 proprietary oil for sterndrive engines. Standard 20W50 oils just won’t stand up to the rigors of marine engines. Also, Premium (95) Unleaded should be used as MerCruiser gasoline engines require 87 AKI (Anti-Knock Index in the US) which is 92 RON and Standard Oz Unleaded is 91 octane.

PERFOMANCE

The freshwater-cooled MerCruiser 4.3-liter MPIs on test had Alpha legs instead of Bravo 1 or 2, but these worked well in the 315 swinging 14.5 x 17-inch stainless steel Quicksilver Vengeance props driven by relatively high reduction ratios of 1.62:1 drove and were slightly assisted for heavier loads. The engines started instantly, hot or cold, with no oil smoke, and shifting was reasonably smooth, despite the dog clutch drives. For real power aft the drives had to be left out, but at least with around 1000 rpm on the tach the hull could be rotated in its own length in calm conditions.

The engines got the 4.6t 315 out of the hole quickly when the throttles were pushed hard down from trolling revs, and across the rev range the 4.3-litre engines had low levels of vibration and only became overpowered at Rough approach to full load. As with all tests I run on sterndrive cruisers, the trim tabs were left in neutral and the drives were trimmed as needed.

No propeller ventilation occurred through tight turns at 3000 rpm, and a nice touch was how the engines maintained their revs as the MPI added more fuel to compensate for the increased load.

THE TRADE-A-BOAT JUDGMENT

I have no problem with petrol engines in cruisers provided they are well maintained and the bilge fans are run for a few minutes before starting. I have used this technique (and opened the engine room hatches) with the twin inboard V8s in a Halvorsen 40 I drove on Sydney Harbor for private charter work 30 years ago and have never had a problem. And unlike mechanically injected diesel engines, freshwater-cooled gasoline engines do not suffer from cylinder liner glazing when operated under light loads for extended periods. Perfect for pottery around waterways.

The MerCruiser 4.3L MPI makes economic sense in underused sport cruisers. The exhaust note isn’t nearly as satisfying as a V8, but under load the V6 noise isn’t that bad!

Many thanks to Lifestyle Marine, Toronto, NSW – Telephone (02) 4959 1444 – for providing these well set up engines.

MERCRUISER 4.3L MPI SEA PROBE

Sea Ray 315 Sundancer with twin 220hp MerCruiser 4.3L MPI V6 petrol engines. Average of trips in both directions with SmartCraft instrumentation and Raymarine GPS.

RPM SPEED FUEL CONSUMPTION 610 (trolling) 2.5 kts 3.3 l/h 1000 (fast idle) 4.4 kts 4.9 lt/h 1500 5.8 kts 7.3 lt/h 2000 (offshore troll ) 7.6 kts 12.9 lt/h 2800 (min. glide) 13.2 kts 26.2 lt/h 3000 (offshore cruise) 19.5 kts 28.7 lt/h 4000 (max. cruise) 30.1 kts 41.0 lt/h 5000 (WOT) 35.3 kts 61.6 lt/h

* Sea trial data provided by the author. Fuel consumption is per engine.

MERCRUISER 4.3L MPI SPECIFICATIONS

TYPE V6 petrol engine

NOMINAL HP 220

DISPLACEMENT 4.3 lt

WEIGHT 353 kg

BORE x STROKE 101.6 x 88.4 mm

the next steps

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The MerCruiser 4.3 liter MPI engine is a multipoint fuel injected engine. This 262 cubic inch (4.3 liter) V6 engine delivers 220 horsepower (hp) to the sterndrive at 4,400 to 4,800 rpm at WOT (wide open throttle). With a compression ratio of 9.4:1, it runs on 87 octane petrol. The electrical components are powered by a 65 amp, 917 watt alternator. The engine is mated to the MerCruiser Alpha One sterndrive with drive ratios of 1.47:1, 1.62:1 and 1.81:1. These different ratios are achieved by the gearbox integrated in the Z-drive, which indicates the number of revolutions of the propeller per engine revolution. Gear ratios vary depending on what gear the transmission is in. The engine and rear-wheel drive weigh 865 lbs. when paired together.

MerCruisers are sterndrive engines manufactured by Mercury Marine. A sterndrive is also known as an inboard/outboard drive because it combines the features of both inboard and outboard engines. When MerCruisers were introduced in 1961, they featured the first 100+ horsepower sterndrive. Today, MerCruisers are used on both racing and pleasure boats. The MerCruiser 4.3L MPI (Multiple Point Injection) engine can be used in anything from a pontoon to a cruiser.

Turtle175 said: Looking forward to the 1998 21 foot Bayliner Trophy with a 4.3 Mercruiser with approximately 1,000 hours on it. I don’t know how it’s been maintained but the dealer says it’s going great. I don’t want to spend the money on this boat and then have to overhaul the engine next year. Any thoughts on that many hours on a 4.3? Click to expand…

Well it’s a tough question to answer… I got 19 years and 2367 hours (raw water cooled) out of mine and would have gone longer but I failed to replace the distributors in time and get water in the oil so I did it’s been completely overhauled and it now has over 300 hours on it and it’s still running like a raped monkey…that means it’s all about how well it’s been cared for…and in your case the dealer’s statement is “it runs great” no indication of maintenance…if i were with you i would have another mechanic (not the dealer’s) do a compression test, leak test and check the manifolds…if everything is ok and it’s a fair one Price is go for it…if there are some issues you can use that to lower the price and plan a rebuild later and let it run until it drops…remember that a 4.3 the same che is like a 5.7, only with 2 less cylinders. ..a buddy of mine had a 2002 2052(4.3) (raw water cooled) trophy with over 3300 hours on it…and also think of all the 5.7’s you hear about with over 3000 hours on them too… this is my 2c

* ALWAYS CHECK YOUR OWNER’S MANUAL FOR EXACT ENGINE SPEED RANGES.

* Trademarks

Mercruiser is a registered trademark of Brunswick Corporation

Alpha One is a registered trademark of Brunswick Corporation

Bravo One is a registered trademark of Brunswick Corporation

All trademarks remain the property of their respective owners.

Most boaters know how expensive it is to own or operate a boat. So having a problem with this is definitely a problem, especially if you have high idle which can damage the engine!

Why a boat or outboard has a high idle when idling! High idle is mainly caused by too much air getting into the engine after it has been inducted. This extra air leans the air/fuel ratio but increases the idle between 100 and 500 rpm depending on the amount of air.

If you’re looking to fix your boat’s high idle problem, then here are the steps you need to take and the direction you need to go!

Why does my outboard spin up at idle? What is causing this?

It is very rare for too much fuel to enter an engine’s cylinder to cause a high idle. This usually causes rough idle, but you can learn all about rough idle here if that’s your problem!

No, 9 times out of 10 high idle is caused by too much air getting into the engine’s combustion chamber.

An engine is designed to run at a specific air-fuel ratio.

If we deviate from this ratio, the engine will either run lean or rich. Lean means too little fuel and rich means too much fuel.

This can be a bad thing as the fuel is also used to cool the cylinder.

It is not used as a lubricant as often as in diesel engines. When we talk about 4-strokes. We’ll get to 2-stroke in a minute.

At what RPM should an outboard idle?

So now that you know what causes high idle, you want to know what RPM the outboard should be idling at.

These may vary depending on the engine and manufacturer.

But in general, most engines idle between 600 and 900 rpm.

When we talk about 2-stroke vs 4-stroke, we’re going to break this down a little more.

Because one will be higher than the other. But both will give you the answer you need for the engine.

If we look at the side of the motor box. Or somewhere on the side of the mount, depending on the brand and manufacturer.

You will find a tag that contains some information.

And one of those lines of information will be your engine’s idle speed.

That way you know what RPM your engine should be idling at.

What happens when your idle is too high

As mentioned, this can be a bad thing.

When the engine idles too high, the internal parts of the engine are subjected to unnecessary stress.

And then go one step further.

What you will most likely find is that you will have trouble shifting the engine. Which will unduly stress the gearbox.

Hard shifting or shifting problems leave a loud grinding noise when shifting. (<—Click here to learn more about these issues!) That grinding noise will be the clutch claw located in your lower unit. The high idle means the lower unit driveshaft is spinning faster than it should be. This spins the gears in the bottom unit faster than they should. when they are moved. This will blow up a transmission case that can range from $3,000 to $8,000 depending on the trim! So, let's fix that high idle! (Here are some signs of incipient gear case failure, catch it early!) 2-stroke Vs. 4-stroke high idle There is a much bigger difference between high idle in a four-stroke than a two-stroke than you might think. In most cases, there are very few carbureted four-strokes on the market today. Most 4-strokes these days have electronic throttle bodies. Which just means that a computer controls the throttle on the engine. This regulates the air entering the engine. The idle speed is therefore electronically controlled and cannot be changed! Now if you have a high idle on a 4 stroke without a carb then the problem will be an air leak found after the throttle but we'll get to that next. When it comes to a two-stroke engine, the most common are carburetor engines. Which have an adjustable idle speed that can be changed. But will also be affected by an air leak somewhere in the system, just like the four-stroke. However, damage from an air leak in a two-stroke is a much greater risk than in a four-stroke. With two-stroke engines, the oil is mixed with the fuel that lubricates the cylinder walls of the engine. So a lean condition is more damaging to a 2-stroke than a 4-stroke. Mercury Outboard, Optimax or Verado High Idle These engines will all suffer from this air leak situation. When it comes to an L6 or inline 6 cylinder Verado, you will often find these places to be leaking. Usually due to a broken clamp or hose. (Here is a list of other Verado issues!) Starting with the throttle body assembly we have what is called an isolator. This clamp or rubber can tear and cause a leak. You can also find them on the hoses or clamps between the compressor and the intake manifold. As well as the hoses between the boost pressure control valve. Now finally the map sensor has a rubber boot on top of the intake manifold. They can also tear or have a broken clip. Air entering the system will cause a high idle problem. Now there are just as many places this can happen on a Mercury Optimax. But this engine is a direct injection outboard. And it uses an air compressor to build up air pressure for the fuel rails. It can leak at any point in the rail, as can the air injectors! Which can be a bit more difficult to diagnose, so check out this video here for more info! How do I lower my idle? Well when it comes to lowering your engine idle. There are usually idle mixture screws on carburetors. As well as idle air screws on the throttle bodies of the engine's throttle body assemblies. These are pretty much only on 2-strokes unless you have a carbureted 4-stroke. And you need to do something called sync and link in the engine to set it. In addition to performing synchronization and linking. You should refer to your engine manufacturer's service manual for the procedure to adjust these bolts. To lower your idle and get it where it needs to be. It is important to refer to the service manual rather than just turning the screw. Because if you just reduce the air or increase the air. You could be covering up another issue that needs addressing! put you in danger! We would like to thank you for checking us out today and hopefully this article has been of some help to you. You can learn a lot more about your boat and its systems by joining our Born Again Boating Academy where we will teach you all about your boat's systems! You can also visit us on our Youtube channel where we create weekly content to update you about your boat! And if you'd like to help us keep bringing you great content, please click the link below to go to Amazon, where we'll get a commission on everything you'll already buy! Click here to Amazon! Here are some other helpful articles you might find useful 🙂