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Last edited by 2FORCEFULL; 08-13-2011 at 10:26 PM.
Its a 800dp elec.choke. I have one on my suburban currently. Those jets sound about right. I think I went to a 73 or 4. I figured my 305 Merc motta in the burb could use a little less fuel. The cruise fuel ratio on my 305 is about 14 to 1. WOT is 13.4. I know this is apple and oranges but thats what it is. I would put a couple jet sizes in the Pri. just for grins anyway for your 502. Just because the plugs are white. Check a NEW set of plugs after WOT and an emediate shut down. Look for the smoke ring around the porcelin about a 1/16-3/32" down. The porcelin can still be white with this ring. It looks almost like a thin grey O-ring was rolled down it. And check fuel press. at WOT
The stock HP500 800cfm DP came with the following jetting arrangement:
Star Prim 75 Star Sec 93
Port Prim 81 Port Sec 89
You could have something in the carb blocking fuel (not uncommon today with the ethanol fuels...you should have seen the last 3 carbs I just rebuilt....yuk !) or bad fuel or your ignition has issues or wrong timing.
These are 8:75:1 motors with 222,230 at .050" cams with 110LSA. Not a detonation prone motor by any stretch.
Disconnect the fuel tank/s and run the boat with new (I'd use 87) gas from a portable gas tank and see what you get. If it runs better, put the gas from your boat in your car.
That is what Mercury lists...doubted way back too but over time took a couple of virgin carbs apart and they had those in it. So, that's what is supposed to be in there.
BTW: Not uncommon to find staggered jetting in OEM performance builds. The OEM has the resources and the willingness to go thru the type of testing that will lead to this.
Look up jets for any HP500 in a Merc site or at your dealer and you'll see. They'll list them in orifice size but tell you prim/sec and which side port/starboard.
Are you thinking about an 850 ? Why ? The factory 800 is perfect/already set-up for your motor.
CFM is correct, that is the stock jetting, but he did leave out the 6.5 PV on the primary side only. Its called stagger jetting, and its done due to fuel distribution in the stock GM dual plane manifold. GM and Mercury spend alot of hours on jetting that engine. So if you feel inclined to bypass all their dyno time on one of the most well thought out engine packages built, go for it.
CFM is also right in that if it is pinging, its either running hot, running lean, or you have timing issues. That engine will on 87 octane all day with no problems.
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Merc has sent out many bulletins about fuel lines collapsing. The alc. content is killing normal rubber hose. CFM might correct me but wasnt Merc sayn to use the blue liner stuff for unusually high alc. content. HP 5 hundo is a single plane.
When something goes wrong, I personally do the same thing you seem to be doing. I get all caught up in all the possible answers and if I don't slow my thought process and concentrate on one thing at a time, I end up spending more time and money where it never needed to be spent. In most cases, the solution was way more simple than I made it out to be. Take my words for whatever value you wish. Slow down a bit.
It ran just fine on 91, all the way down to a quarter tank. You topped the tank with 87 and shortly thereafter it's pinging.
No other changes?
What was the engine water temp?
Given the circumstances, the first 3 easy things I'd do is:
Verify fuel quality, verify the cooling system (impeller), and if you have a timing light, verify the timing.
Bang for the buck the AFR305 are an excellent head for a 502 boat engine. No problem making 575 or so with a mild cam and keeping silent choice option.
3HP is an A$$ Whooping!!! JW