Re: What Intake for a 502ci Carburated?
 

Isn't 5400 rpm at WOT a little high for a stock 502 Carb. I thought 5000 rpm was it.

Re: What Intake for a 502ci Carburated?
 

I was told that the best performance, also depending on weight and prop, that this engine should run in the low 5,000 rpm at WOT. The 950 that is on the engine was just put on before I purchased the boat and is brand new, so with this carb can I jet it to 800 or 850 and replace the intake after I see how it performs?

ROTAX454, I am definitely interested in your intake. I will let you know... Thanks again guys, MIKE

Re: What Intake for a 502ci Carburated?
 

I know of someone who has a 509cid marine engine with some aftermarket Dart 308cc cast iron unported heads, Crane 741 236*/244* hydraulic roller cam, 9 to1 comp ratio, an Edlebrock Air-Gap dual plane intake manifold with a 950cfm Holley carb on top, and Lightning tubular exhausts. On Tyler Crocket's dyno it made 563hp @about 5700rpm. Of course this is with dry, hot, tubular dyno headers. He ran the engine in a 27' Warlock, and was happy with how it performed.

The IMCO exhausts you have should work---at least they're much better than the stock Merc cast iron design exhausts. A better breathing exhaust system ALWAYS helps the engine breath better by helping the other performance mods work together.

A 950 Holley will work, but I think something like an 800cfm carb would have been a better choice. Perhaps you can swap one out with a friend just to try it and see. This is only a guess, but running a single plane intake manifold and big carb with a stock cam is suspect to me.

How is the boat running out on the water while on plane?
What RPM are you turning at wide open throttle?
At what RPM do you usually (or try to) cruise at?
Does it act sluggish while on plane?
Or is it only during idling that the engine acts real sluggish or like it wants to stall?

Just for future reference, an honest dyno test of an engine removes a lot of the doubt, and guess work of how an engine performs. It will tell you all the vital signs, tells you where the power curve is, etc, and will save you a ton of time, money, and grief before you install the engine in the boat! It is money well spent. Just my .02

Re: What Intake for a 502ci Carburated?
 

The boat is running only around 4500-4700 rpm at WOT and running around 55mph on gps with a 3 blade 25 pitch mirage prop. I tried a 4 blade 2 pitch and was running 4800-4900 and 59 mph, quite a bid better performance all around with the 4 blade, still not running lke it should. I really lose power at 4600 rpm, like I am hitting a wall. All of this is running with drive trimmed and tabs up. Cruses at 3400 rpm at 38 mph or so. Idles at 900 rpm and does not chug all to bad, sometimes will stall going in and out of gear.

Re: What Intake for a 502ci Carburated?
 

I dont know if this help in anyway......I just bought a 502/ 415 hp marine crate engine with the stock dual plane manifold and carb with revolution marine exhaust. Bravo 1 1:50 drive on a 27' Carrera and I turn a 24' 4 blade bravo 1 prop at 5000 rpm at 65 -/+ mph. It gets to 5000 rpm and it is like it hits a wall and stops right there at 5000 rpm. According to Dennis Moore Performance book. Mercruisers dual plane manifold is a well designed manifold and there is really no better design for a stock engine. Mercruiser did there homework on that manifold.

Re: What Intake for a 502ci Carburated?
 

You can't "jet" a carb to a lower number. The number, 950 is cubic feet per minute of air. It has nothing to do with your jets. A stock Mercruiser 502 Mag is set to run at 5000 at WOT. Thats where you should prop to run at

You said that you hit a wall at 4600, but then you get it to 4900 with the Bravo 1 prop. Can I guess from that that the engines runs up to 4600 quickly, and then takes its time gainning those last 300 RPM?

Have you done a plug reading on this engine?
Also, what pitch is the 4 blade you ran. You only typed the number 2.

Re: What Intake for a 502ci Carburated?
 

You are getting way to much slip with the 25 Mirage. You are 23-26% slip. Assuming you meant to type 24P for the 4 blade, that is still 25% slip.
You need to find out why your slip numbers are so high. See if the is a hook in your hull and check that your tach is accurate.

Re: What Intake for a 502ci Carburated?
 

Mike,

I would double/triple check your timing and try a 21 pitch Mirage if you can find one. I know it sounds small but I still say that is an awfully big boat for a 25 mirage with a stock-ish 502. You really need to verify your tach also. The easiest way to do it is with a Snap On timing light with digital tach built in and the long cord. Run the boat to max RPM and compare what the Snap On says compared to the dash tach. There should be an adjustment screw on the back of the tach to adjust it if it is wrong. My old boat had the same gauges as you have and I had to adjust the tach every year. That being said, if the engine is basically stock, your carb is too big.

Bill

Re: What Intake for a 502ci Carburated?
 

theres alot of 502 powered single boats running 62-65 mph with 23 pitch 3 blades,I'd make sure you have enough timing & set idle mixture on carb using a vacuum gauge. The 950 carb IS NOT keeping you from rpm'ing higher. You can't "jet" carb down to a 850,your air fuel mix needs to be around 12.5-1 on a n/a motor and you achieve this by usually jetting carb 1-2 jets richer than out of the box. Verify your throttle opens carb to wot by looking down carb with throttle wo.You sound over propped and your tune probably isn't ideal,Smitty

Re: What Intake for a 502ci Carburated?
 

Do we know what is happening ? I still don't get the 'flooding the intake" and 'velocity' thing.

I don't think a box stock 950cfm Holley is too much cfm for any 502, however, it's possible the calibration is not the best choice. These are typically used on pretty 'hot cammed' engines. I'm not just speaking about jetting when mentioning calibration. Boosters, bleeds, channels, etc,etc are not the same from one carb to another.

I could swear I remember Rumrunner (Doug at BG) mention their 850 flows more (wet) than Holley's 950. Don't totally hold me too that.

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We need to get this boat tuned in all aspects - carb, ignition, prop, etc,etc,etc in a correct fashin. Guessing usually leads to dismal performance unless extrememly lucky. Don't count on it.