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  #31 (permalink)  
Old 06-26-2011, 08:35 AM
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What would you recommend for a carb? What jet size and metering block would you recommend for my current carb?

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  #32 (permalink)  
Old 06-26-2011, 02:17 PM
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Quote:
Originally Posted by crussell85
What would you recommend for a carb? What jet size and metering block would you recommend for my current carb?
First you need to see if the jetting is where it needs to be or not, to do that requires you to look at the plugs after a hard run to see what they can tell you. What you don't want to do is get into any detonation under high load conditions or WOT runs, so be sure the timing is dialed in first, then read the plugs for how the air/fuel mixture looks.

Another way to go about this would be to replace the primary jets w/a couple sizes larger jets and retest it. If you see an improvement you can be fairly sure the carb is lean for the application. Just increasing the primary side isn't the cure- it's just to see if that's the direction the engine wants. The crossover info for jets to plates is HERE- just be aware that there are also idle restrictions involved when the plates are changed, and you need to know what you have now to know what to change to next.

The power valve may need to be changed to allow enrichment at a higher vacuum than it is set to now. You'd want to know what the idle vacuum was, then what the vacuum is under max load conditions. You want a PV that will be closed at idle but open at max load.

Ultimately, using a 4150-type carb will be the way to go, IMO- so there'd be secondary jets that can be easily changed instead of the secondary plates of the 4160-type carbs. Then I'd want to give it a little more capacity, nothing over 750 cfm should be needed. Any of the vac. sec. carbs that are 4150 or converted to 4150, or either the 4777, 4778 or 4779 DP's are easy to find and don't cost that much.
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Old 06-26-2011, 04:21 PM
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So what you guys are saying is that there should be a 750 cfm double pumper sitting on top of this e engine? The research I did and cf m calculators that I used told me that this carn would be enough for the engine. Could someone suggest a carb for me? Make, model,cfm,etc?
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Old 06-26-2011, 06:45 PM
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http://www.summitracing.com/expertad...=CFMCalculator

5000 max rpm with 383 cubic inches according to this my carb should meet the requirements. Carbs are a new thing to me to please bare with me guys, i'm not trying to be a pita
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Old 06-26-2011, 08:42 PM
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Your carb is fine, it's a boat guys the automotive rules dont really apply. About the issue you are having...sounds like distributor issues...maybe your mechanical advance is worn out or stuck which is not uncommon on a marine engine.

Shouldn't be any vacuum advance on your engine either, is there?
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  #36 (permalink)  
Old 06-27-2011, 12:51 AM
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Spreadbore carbs work great on boats because the small primaries allow the boat to carb well out of the hole, thats why they use Quadrajets. You don't carb or cam for rpms the engine can't handle in a boat, carbing the engine for 6000 rpm is a waste as the engine won't last long at that rpm...most boat V8's peak out at around 5000 rpm for durability.

If you cam the engine for peak power above 5000 rpm it will be a dog out of the hole and likely won't even be able to pull a skier, seen it a million times...guys build hot engines for their boat and its worse than before the mods until they throw the stock carb dual plane intake and cam back on it.

If you want to run that LT4 cam you will have to go down in pitch so it can get out of the hole and then turn the nuts out of it to gain back the speed...say goodbye to any economy but it "might" accelerate better with a light load and a boat with a pad...but not a vee with an outdrive. I'm not even going to mention that a long duration cam doesn't work with a water cooled manifold with high backpressure, you will need open headers for that kind of cam to work in a boat...not to mention that drive won't handle it, they aren't designed for high rpm so its a moot point. Your going to have to be careful with it as is with the increased cubes!

A vee hull boat engine load is like climbing a steep hill that only gets steeper exponentially as speed rises (unless you have a pad), it needs torque to perform. You upped the cubes which is the best you can do, changing the rpm characteristics of the engine is a waste of time unless you run a different style of hull and run a surfacing prop with a very light load.

Be careful applying automotive dynamics to watercraft, they are completely different animals.

I built an engine with a very similar cam to the LT4 (actually a Cam Dynamics 290) for a guy with an inboard boat very similar to yours back in the 80's, Holley 750 DP, very nice 350 putting out about 400HP on the dyno compared to the 225Hp it used to be. We ended up dropping down to a 15 pitch prop just so it could get on plane with a load or a skier (but not both!) and he lost 5mph of the top and about halved his mileage. It was OK with a light fuel load and once it came onto the cam it actually ran not bad from about 3500-5000...but all around performance sucked and he eventually took everything off the engine and put the stock stuff back on...which dropped his 0-40mph times by about 3 seconds compared to the performance modified engine.

Waste of money and totally wrong type of engineering for the application, I tell guys who want to do this type of thing "Think of a tractor pulling giant boots out of the mud, because thats what a boat is...a giant boot in the mud".

Good luck with your project.
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Old 06-27-2011, 02:01 AM
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You didnít say where your backfire was coming from. Exhaust or through the carb. Are you running roller rockers? Lash? Do you have a fuel pressure gauge? What type of pump are you running? Could be a lean pop. I had a pump that couldnít keep up once at WOT it would drop to about 3 psi. It leaned out and bad things happened. (Piston kissed a valve) If it's an Electric pump you have to remember those are pushers not suckers.

Boats are a different animal true, but they have the same theory; Suck, squeeze, bang, blow. Most marine engines are bored oversize say .31, .61, etc. the extra .1 is for the open cooling system and the extended RPM'S they will see. A Boat engine is constantly under load. Also remember Fuel miliage sucks in a boat. One last tip, Try and stay away from buying gas at the marina. It always has water in it.

Here is my POS. 327 in a boat. Yes you can run car eqt in a boat.



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  #38 (permalink)  
Old 06-27-2011, 07:21 AM
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My uncle has a383 in his boat also, he is running the stock 305 cam with the stock 305 two barrel carb, he is running a 14.25/20 prop. His boat jumps up out of the water and planes out fast, and runs 60mph.
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Old 06-27-2011, 07:38 AM
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Quote:
Originally Posted by crussell85
So what you guys are saying is that there should be a 750 cfm double pumper sitting on top of this e engine? The research I did and cf m calculators that I used told me that this carn would be enough for the engine. Could someone suggest a carb for me? Make, model,cfm,etc?
That's not necessarily what I'm saying- at least not until you can determine whether the jetting you have now is optimum for the application.

I DO believe the carb can stand to be larger, though. The rest of my thoughts on the subject are already out there, including:

Quote:
Ultimately, using a 4150-type carb will be the way to go, IMO- so there'd be secondary jets that can be easily changed instead of the secondary plates of the 4160-type carbs. Then I'd want to give it a little more capacity, nothing over 750 cfm should be needed. Any of the vac. sec. carbs that are 4150 or converted to 4150, or either the 4777, 4778 or 4779 DP's are easy to find and don't cost that much.
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  #40 (permalink)  
Old 06-27-2011, 08:33 AM
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My uncle has a383 in his boat also, he is running the stock 305 cam with the stock 305 two barrel carb, he is running a 14.25/20 prop. His boat jumps up out of the water and planes out fast, and runs 60mph.
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  #41 (permalink)  
Old 06-27-2011, 08:36 PM
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The factory carb was a two barrel holley. I removed the metering block from the two barrel carb and the one from my 4160. The two plates look identical, can I put the metering block from the two barrel holley in place of the metering plate on the 4160 to have jets in the rear of the carb and also block off the power valve?
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Old 06-27-2011, 09:50 PM
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Quote:
Originally Posted by crussell85
The factory carb was a two barrel holley. I removed the metering block from the two barrel carb and the one from my 4160. The two plates look identical, can I put the metering block from the two barrel holley in place of the metering plate on the 4160 to have jets in the rear of the carb and also block off the power valve?
What is the list number of the original Holley 2 bbl? And how was it jetted?

As far as using the 2 bbl block on the secondary side of a 4160, I have never done this. It might work by using a different length fuel tube and screws. I suppose the accelerator pump could be left in place and just ignore it, block off the PV and the idle mixture screws wouldn't be functional. You'd use 4150 secondary metering block and bowl gaskets.

Holley sells a kit to convert the 4160 to a 4150, more HERE.
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Old 06-28-2011, 02:04 AM
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Make your Holley Marine Carb choices here;

http://www.holley.com/types/Marine%2...erformance.asp

I would suggest staying with the 500cfm carb for best all around performance and economy with the original cam and intake but any of the 4 barrels will work if you want to use an aftermarket or stock style dual plane 4bbl manifold (don't use a single plane unless your going to run "over the top" style headers)...don't expect huge gains from going larger since you won't be turning much over 5000 rpm.

Calibration on these carbs are not the same as automotive carbs, I would avoid mixing and matching except for common parts like power valves, jets etc.

The 650 Spreadbore "Quadrajet replacement" is a great carb for chevy marine engines and more than big enough for your needs, very economical even compared to the 500 cfm two barrel.

BTW, you don't have the electric choke connected to the coil do you? Here is the bulletin from Holley regarding this...seen it before!

Quote:
WARNING: Connecting the choke cap to the ignition or ignition coil could result in unacceptable choke operation, poor fuel economy, and possible engine misfiring, since the voltage delivered to the spark plugs will be severely reduced by the drain imposed by the choke cap. Suitable ignition-activated 12-volt sources are most electrical relays, as well as the leads to the accessories, such as windshield wipers. DO NOT connect this wire to the original equipment (O.E.) electric choke source. This may not be a 12V source.
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  #44 (permalink)  
Old 06-28-2011, 08:43 AM
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Can't I use the same float bowl that I'd already on the 4160 carb? And jay, please start your own thread!
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  #45 (permalink)  
Old 06-28-2011, 04:22 PM
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Quote:
Originally Posted by crussell85
Can't I use the same float bowl that I'd already on the 4160 carb? And jay, please start your own thread!
If you have center hung floats, this will work. I figured the 4 bbl carb was side hung. Might want to post the list numbers of the carbs you're referring to, including any mods that might have been made already. Or a photo or two... otherwise I'm left guessing what's up.
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