4.3 Carb v 4.3MPI

[Rick Stephens]

Vapor lock is bloody rare in an IO. Unlimited cooling. Only reason for a vapor lock is poor design or poor maintenance.

 

[Searay205]

Vapor lock occurred every day for 30 years when I used it. Ran boat, shut boat off, drank for hour, tried to start vapor locked. 3.0 liter. Had the phenolic gasket between carb and manifold. In end I either pushed throttle all the way open after I shut engine off and let excess gas evaporate or my favorite was starter fluid (I miss that smell with my MPI) . Then engine fired right up. Again carburetors are PIA but manageable. Even my buddies 5.0 tks stalls randomly when pulling to idle. Been rebuilt blah blah blah

 

[jimmbo]

Vapour lock is having the Fuel vapourize in the Fuel Line prior to the Pump, Diaphragm Pumps cannot move a Gas as Gases are Compressible, while a Liquid isn't. If the Fuel is vaporizing in the Line After the Pump due to Heat, when the Inlet valve in the Carb opens the, fuel bowl is overwhemed by a rush of Hot Fuel, and flooding results.

 

[achris]

.. or my favorite was starter fluid ...

Starting fluid (ether) shouldn't be used on petrol engines, unless you like replacing head gaskets. It was designed for diesel engines, only...

Chris...

 

[Rick Stephens]

Vapor lock occurred every day for 30 years when I used it. Ran boat, shut boat off, drank for hour, tried to start vapor locked. 3.0 liter. Had the phenolic gasket between carb and manifold. In end I either pushed throttle all the way open after I shut engine off and let excess gas evaporate or my favorite was starter fluid (I miss that smell with my MPI) . Then engine fired right up. Again carburetors are PIA but manageable. Even my buddies 5.0 tks stalls randomly when pulling to idle. Been rebuilt blah blah blah

Not to get in an argument over it, but neither your events nor your buddy's 5.0 sound like a vapor lock.

 

[jimmbo]

Starting fluid (ether) shouldn't be used on petrol engines, unless you like replacing head gaskets. It was designed for diesel engines, only...

Chris...

At least down under, where you are, they have an appropriately named brand

 

[achris]

At least down under, where you are, they have an appropriately named brand
View attachment 365197

Only in Australia... ‍♂

 

[achris]

At least down under, where you are, they have an appropriately named brand
View attachment 365197

And if you like that one, you'll love this....

 

[Scott06]

Vapor lock occurred every day for 30 years when I used it. Ran boat, shut boat off, drank for hour, tried to start vapor locked. 3.0 liter. Had the phenolic gasket between carb and manifold. In end I either pushed throttle all the way open after I shut engine off and let excess gas evaporate or my favorite was starter fluid (I miss that smell with my MPI) . Then engine fired right up. Again carburetors are PIA but manageable. Even my buddies 5.0 tks stalls randomly when pulling to idle. Been rebuilt blah blah blah

excess gas is not vapor lock it means the bowl is emptying into the engine and it’s flooded , hence open throttle to let more air in.

Vapor lock is gasoline in gas phase in fuel line so pump can’t pump it so carb doesn’t have excess gas it has no gas

 

[QBhoy]

Scot06 interpreted my mess correctly. I would always prefer the higher horsepower and happy with a carb engine over MPI BUT they don't offer a carb 350 just 5.0 tks or 5.0 mpi, sooooo I would take the 5.0 mpi. Actually the Searay 205 was offered with 3 engines the 4.3 MPI, the 5.0 TKS, the 5.0 MPI. Very rare to see the 5.0 mpi in a 205. The 4.3 MPI option was $3K more than the 5.0 TKS. So no brainer take the 5.0 TKS. The 5.0 mph was only $80 more than the 4.3MPI. My take is the 5.0 mpi wasn't pushed. on the customers because it just to much motor for the boat and borderline dangerous. like 63mph on stock prop and black ice felling. I switched to a high 5 prop to take some speed off it, now its the best powered boat I ever been on, tons of torque and 55 with 1 or 8 people aboard!!!

Advantages to carburetor:
1) quiet, no fuel pump noise
2) low cost, easy to trouble shoot and fix
Disadvantage of Carburator
1) Will not run as good as an MPI engine, will not start as easy, will not idle as well, will act like a mid 70's car
2) most vapor lock, have to know how to clear a flooded situation have to be somewhat mechanically inclined
Advantage of MPI
1) drives like modern car. turn key and it starts, idles perfects, never floods out, excellent throttle response (no lag), no hesitation, anybody that can start a car can start an MPI engine.
2) Expensive to fix if your not mechanically inclined but they don't fail that often. Especially the non-cat engines. Cat engines fail post o2 sensors but there is a work around...

The 5.7 carb hasn’t got more power than the 5.0 mpi though.

 

[jimmbo]

The 5.7 carb hasn’t got more power than the 5.0 mpi though.

It has considerably more Torque. I'll take the 5.7 carb over a 5.0 liter FI. And by adding a few more CFMs to the Picture, it will also be making more HP

 

[Searay205]

Can we agree the MPI has better drivability than a carburetor? Can we agree the MPI has the potential for more complex and expensive problems.

 

[Rick Stephens]

Can we agree the MPI has better drivability than a carburetor? Can we agree the MPI has the potential for more complex and expensive problems.

Barring issues with one or the other, they are equally drivable.

 

[QBhoy]

To be honest. Apart from a few diz caps and an IAC…mine has been faultless. 700 hours later. Sits for long periods unused. Always starts right up. I think it’s all about how you look after them. As with any. Oh. Had a shift interruption fault. But not the fault of the mpi. Common to carb too.
All I know is that I’ve been on plenty of boats of the same model…one with carb and other with mpi of the same engine. The difference in power and economy is noticeably obvious. Particularly one example stands out. Was the 2400. One had a 5.7 carb and the other the 5.7 mpi. There was an enormous difference in power and performance. Like two different boats.

 

[Scott06]

To be honest. Apart from a few diz caps and an IAC…mine has been faultless. 700 hours later. Sits for long periods unused. Always starts right up. I think it’s all about how you look after them. As with any. Oh. Had a shift interruption fault. But not the fault of the mpi. Common to carb too.
All I know is that I’ve been on plenty of boats of the same model…one with carb and other with mpi of the same engine. The difference in power and economy is noticeably obvious. Particularly one example stands out. Was the 2400. One had a 5.7 carb and the other the 5.7 mpi. There was an enormous difference in power and performance. Like two different boats.

Equivalent year carb vs MPI is not 100% equal comparison. Once they came out with MPI they no longer made 4 bbl version of carbed motors. So comparing a utilitarian Mercarb or TKS 2 bbl version of an engine with MPI of the same engine which has a higher hp rating is not a fair rating of carb vs MPI. They had to drop the 4 bbl version to get people to pay for the MPI engines...

Interestingly enough mercs reman carbed engines are all 4 bbl. 4.3 rated at 225 hp, 5.7 (357 alpha/bravi) at 275 (alpha drive), 325 hp for Bravo. I think these are flywheel hp vs prop but you can see the diff vs 2 bbl version. 325 hp at flyhweel would roughly equal the 300 hp mpi 350 mag....

 

[QBhoy]

Equivalent year carb vs MPI is not 100% equal comparison. Once they came out with MPI they no longer made 4 bbl version of carbed motors. So comparing a utilitarian Mercarb or TKS 2 bbl version of an engine with MPI of the same engine which has a higher hp rating is not a fair rating of carb vs MPI. They had to drop the 4 bbl version to get people to pay for the MPI engines...

Interestingly enough mercs reman carbed engines are all 4 bbl. 4.3 rated at 225 hp, 5.7 (357 alpha/bravi) at 275 (alpha drive), 325 hp for Bravo. I think these are flywheel hp vs prop but you can see the diff vs 2 bbl version. 325 hp at flyhweel would roughly equal the 300 hp mpi 350 mag....

I get what you’ve said there…but this maxum with the carb was a vortec engine with 4 barrel and manifold. Started off as a 250hp model previously. I’ll concede that it was through an alpha 1. The mpi was through a bravo 3.
Anyway. There was a fair difference I thought. The carb and alpha could have done with a bit less pitch on the prop to be fair. But still I’m the Rev range and didn’t get near to the mpi top end either. Was about 5mph difference at least.
I knew another 2300 (same hull but older) with a 5.7 efi. That would be about 260hp. Also didn’t come close. The carb version of the 2400 was quicker than that, to be fair.