....

I stopped reading on page 7. Dunno where all you went from there but here are some thoughts.Correction - I jumped ahead to the end.

A Mercruiser 454Mag or 454/330 would last about 5 times longer than the 244 hours that are on the boat. Wonder why the motor swap..
A 502 crate motor would likely be in the 450hp range.

450 hp would spin your [email protected] prop well past 5500rpm.

No GM crate 502 with stock heads and stock compression is going to make 527/570 thru Merc V6 exhaust elbows.
No stock headed 502 with stock compression is going to make 527/570 with a cam that prevents exhaust reversion at low speeds.

You want a reliable motor - you are going the right direction.
Open up exhaust restrictions.
The cam you selected has a little more exhaust duration than you need (230/112) so you DO need to keep an eye on reversion. Not sure I would have recommended it, but if its not reverting water it will work fine. Install it 4 degrees advanced. Check for reversion (chop the throttle from planing speeds, drop off plane to full idle speed/stop, kill motor immediately - check plugs for evidence of water).
Your intake and carb are perfectly fine for your goals.
Set total timing advance to 34 degrees, let initial fall wherever your distributor and box have it.
Adjust idle mix AFTER timing is set as described.
Take care of your oiling issues, make sure the engine is in balance, adjust motor mounts for proper drive alignment, should be good to go for freshwater.

Prop the motor to rev to 4800 with typical load, but not over 5000 with lightest load. You don't have enough cam to warrant operation past 5000 max. You may see best "lightly loaded" top speed when propped for 4850-4900 just you, a splash of fuel, light chop, with the wind, trimmed for best speed.



If you plan to run in salt, there are 50 more things you need to consider.

Had a good run on the lake this weekend and it's really running good. The new Bravo One 22p prop still gets me in the low/mid 50's at a 4600 rpm range in the noisy mode (70 gallons of fuel and the boss). Few hundred rpm less through the prop. It does miss miss a bit in the low 4K range but not WOT. Does not seem to break up through the prop though. Is there a mandatory rpm the the Captains Call should be used?
When cold the oil pressure is still up in the 60-65 psi range (cold) but only peaks in the 45-50 range when oil is up to temp. I'm reading about 240 degrees at the filter in the now 65 degree lake.
I ended up pulling a quart of oil out before launching. I hate that plastic rod dipstick as I struggle reading oil level with it being so glossy. I'd like to modify it to read better.

In case you missed it.

Do not run your boat with a full load of fuel/people until you get the "miss" fixed.
I run my Corsa closed when coming into and leaving the harbor. Open (through transom) everywhere else.
Your cold oil pressure is acceptable.
Your hot oil pressure is acceptable.
Is the 240 degrees at the filter before the cooler? I hope so. I would like to see it a little lower.

No, the boat did not get run on a stand. The engine shop just set up
all non-marine long block...the boat shop set up all the marine stuff. Initial run (assuming on a hose) did have a vac leak prior to carb rebuild...The boat shop's mechanic said it runs great but I'm sure not at 4K.
Although I now know not to run exhaust through the prop above 2500-3000 rpm...I did it. There was NO 4K miss through the prop...only with the exhaust through transom. Made me think my Corsa diverters were rattling BUT I could feel it kinda holding back. It did not "miss" at WOT either position. When idling around the dock I did notice some through transom exhaust like the diverter was not closed fully. Not positive this is new or I just noticed the little blow by (exits under swim platform)...but yes it was a bit louder. Hope I didn't f it up switching at higher rpm. Embarrassing.
The oil temp is read right at the remote oil filter location...before the 215 degree Improved Racing oil thermostat...which directs the hot oil through the cooler...which is a mercruiser cooler shown to be on mercruiser 502's. No I don't have a post cooler reading.
The fuel in the boat (1/2 left over from last October/ and 1/2 fresh this past weekend) is/was 91 octane no ethanol from a new (last fall) station. Gas was treated last fall with Merc's treatment.
My engine builder had mentioned to me (when I picked up the long block) that he would prefer to log A/F ratios in the boat and adjust carb accordingly. Maybe that's the next step?

Thanks for your time. Dave

Run the fuel that your engine was built to run. This is why you should know and/or post the engine's build sheet.
If the engine is like a stock Mercury 502 Mag then don't run premium gas.
A final tune in the boat is always a good thing.

I agree about knowing the original build...you know I've tried. What I do know is the Chevrolet Performance 502HO Crate motor specs out 92 octane premium. I did find out it has Mahl pistons but don't know if they were what Chevy used (?). Still think 87 octane should be used?

I don't know why Chevy would recommend 92 octane. Crazy 8.75 compression small cam and 30 degrees total timing. You will be just fine running 87. How do you plan on checking AFRs in boat with that exhaust?

not sure... maybe he was talking about on his stand, initially. He says in the boat he would run it pig rich minus a hair. Maybe he was pulling my leg 😐.

Talk to me about 30 degrees total timing for the crate motor specs vs. I believe 34 total per ThunderBolt IV recommendation. Which rules?

There are more people buying crate motors for the street than for marine. So there are more ding dongs by shear #'s on the street than the water. So, they are covering their azz. More unerhood heat, higher coolant temps...etc,etc will add to this too.

I know many people that can knock down a barn by just looking at it. :)

That is 502 HO 461 hp stock crate cam. The ZZ502, Lil more duration, lift and 110 lsa might,be the premium fuel spec'd crate motor. And since GM wasn't putting this motor in chevelles, etc I don't think they were under selling the hp numbers like they were in the 70's. 30° total advance would probably not be noticed by the car show-stoplight grand prix guys buying the majority of these engines. And people spending this much for a 502 for et's would probably spend the extra coin for the extra 50 hp and the alum heads. The weak springs they put in these motors is probably in fear of cam and lifter warranty returns you have already taken care of.

That is 502 HO 461 hp stock crate cam. The ZZ502, Lil more duration, lift and 110 lsa might,be the premium fuel spec'd crate motor. And since GM wasn't putting this motor in chevelles, etc I don't think they were under selling the hp numbers like they were in the 70's. 30° total advance would probably not be noticed by the car show-stoplight grand prix guys buying the majority of these engines. And people spending this much for a 502 for et's would probably spend the extra coin for the extra 50 hp and the alum heads. The weak springs they put in these motors is probably in fear of cam and lifter warranty returns. And you have already taken care of that issue.

Big-Block: GM Performance Motor

"Crate engine" can mean many things. 502's from GM came in a variety of flavors.
I still don't know where your 502HO with 527HP/570TQ came from or what it actually started out in life.
A few rebuilds will do that. More detective work on your part seems in order.

Good luck.

You know, I really don't care anymore about the past...beginning to think it was a snow job by this shop that went out of business. The dyno numbers were written on the receipt, no dyno sheet. When we unveiled the cam it was some half assed grind that was through the heat treatment. Back to my unanswered question...is it possible to miss@4k exhausting through the transom but not the prop?
I did hear this boat was local in the 90's and the guy who sold it new said it was running with an blower at some point...that would apply to the original 454.

Yes, if the carb is jetted too lean. An a lean misfire can ruin your day/season in an instant.

Did you take my recomendation last year and ditch that Weber/Edelbrock 750 ? Or , if you kept it, did you tune it on dyno or water ?

SB/ no tune/dyno.. So I need to get it re-jetted (it idles perfect & was just rebuilt)...or buy which other carb?

figure out how to measure the afr's and then rejet as necessary.

Yup.

Out of the Box Edelbrock/Weber 750 is basically calibrated for the low performance 300/330 HP 454.

So if I had more money than time...which model marine carb would be a better match out of the box? Just weighing my options...And wouldn't a lean misfire at 4000 also misfire at 4600?

800-850 holley-quick fuel.

I just ordered an Edelbrock 1480 calibration kit and will give it a go...seems fairly easy to do plus I can have it tomorrow morning at 8am.

That carb is too small for a 502. I have ran an engine lean before and it was expensive. Broke the ring landings on several Pistons. If you have a backfire at those RPMs you have a serious problem. That's why you need A/F readings or your going to ruin another engine.

then make sure it is too rich.

running rich will cause high EGTs just as bad as running to lean.

Edelbrock says it's big enough for 502/461hp...I'm going to see what what's in it and try some adjustments.

Ok, after further review...the BOSS says we should buy the correct cfm carb to be safe �� If that's what it takes. So there was mention of the Holley or quick fuel 800 or 850 DP's. Any other recommendations? Is it the easy way to correct or would it still need to be dialed in?

As long as the "BOSS" is making decisions on this, hope she is doing the wrenching or make the coin to pay for it!

Will still need to be dialed in, but prob easier and quicker than the Edelbrock, depending on your knowledge and skills of either type carb.

...on an Edelbrock RPM Air Gap. And I'd like to keep the brand new K&N arrestor that fit the Edelbrock.

I forgot what exhaust you are using, but I couldn't recommend a wideband monitor enough. Even after you get things tuned in you can keep an eye on it should something go amiss. I have a QF850 and pretty happy with it, but for not much more you can get the Holley Sniper FI. Have you considered that route?

Will consider your common sense recommendation. Haven't considered Sniper EFI at all. Thanks

Wow, EFI conversion for $1k would be sweet if only USCG approved?

Not USCG approved as of now, even though the sum of the parts seem safer than a bucket full of fuel on top of the motor, lol.

This was a good little Q&A with Holley.
​​​​​​Converting Marine Engines to EFI | Holley EFI University | General Studies

I Guess I need to remember my goals here and stay on track...if the boat ran as great as it did except for that 4K misfire (if that's what it really was)...I would be thrilled.

side note/question: when looking at (even) Holley 's site and formula for sizing carbs, consider 502 cubes & max rpm of 5k...the general recommendation is <700 cfm. Why even consider 850 cfm? Sure wish I had made the effort to run it on the stand after re-marinizing the long block, but that was part of my thinking in staying somewhat stockish
and reliable. I would hate to throw on a $700 QF and find out it's something like a 25 year old fuel pump, or an ignition thing. I suppose that would rear its head in other scenarios. If it is in fact a need to calibrate (kit is $65) , which mode would I be trying to richen (power mode or cruise mode)? The misfire occurred by carefully inching up the rpm). I never even tried jamming throttle through that power band...just tapping to increase. I can see myself doing both the recalibrate attempt and ultimately replacing the carb (just sayin'). Once again thanks for everybody's info, I'm learning. Dave

Well, then check out all those items. There is only so much we can do behind a computer screen.

Remember when comparing cars to boats, a certain size might be fine on a car that sees 5000 rpm and or full throttle from traffic light to traffic light or the occasional blast down a back road.It sees that in a transient state. A boat motor may see that for miles, so it may just run at a steady state for a prolonged period of time what a car motor only occasionally sees. With carbs, valvetrain or anything else whats fine on a car motor may be marginal on a boat motor. Whats marginal on a car motor my be disastrous in a boat motor...

Alright already, I'm convinced. So Quick Fuel m-850? Will work well with the Edelbrock RPM AIR GAP?... and I'll place the order today and give you guys the weekend off...looks like I will I need some supporting parts...fuel line kit, and what about a fuel pressure gauge &/or regulator? Thanks

Throttle bracket. But I butchered a merc q-jet-afb one by cutting it off next to the left mount bolt. Works fine as an interim setup.

Where can that be had? Hopefully summit carries them?