Keith

I'm in a jam. Need some experts who've "been there, done that" to offer up some wisdom based on experience. Looking for reliability, and dependability over performance.

I'm used to my old 454 Merc Magnums and Quadrajets. They were bulletproof, ran like a watch with no trouble. The repower project was supposed to yield the same results with 502's, but I'm finding out this setup might not quite get me there.

I bought Merc 502 Gen VI 450HP long blocks. The shop I was doing business with installed Dart intakes and Holley 800 carbs. It's running very rich, and has been rejetted once, I don't know the jet sizes.

Using the Holley website estimator, a 502 engine with 5000 max RPM calls for a 750 carb (actually 726cfm). How reliable is that information?

If the Holley 800 is the right carb to install, what jet sizes do I need?

Griff

The Dart intake and 800 cfm carb will work fine. It definately sounds like a jetting or setup problem.

Jetting should be done on the water or the dyno under load to get it right.

On my hopped up 454mag, I ran a Holley 800 with about 80's in front and 94's in back with no rear powervalve.

When is it rich????? Idle????, Cruise????, WOT????

Keith

It's rich at idle and loading up a bit when I'm in gear for a 6mph zone. If I give the throttles a kick to clear them out, I can come up on plane fine after that. If not, they'll choke and spit when I throttle up. Once cleared, they seem to run okay. Throttle response in the midrange and upper end is good. I haven't gone past 3800 RPM because the engines are new, about 10 hours on them.

I've had 1 suggestion to wait until I get to 20 hours, then assess this situation, explaining that a new engine will tend to use a lot of fuel and run rich until everything is seated properly.

Merc spends a lot in R&D and knows the right combination of parts to build a reliable and conservative package. I asked my shop to build 502's with the same principle, dependable and proven parts found on stock Mercs. Before I knew it, they had purchased and installed these pieces. It appears they've taken a stock 502/450HP block, and put intakes and carbs used on the HP500. If that was a good combination, Merc would have done that way, but they didn't.

Mercruiser stock 502's (Magnums I guess) had dual plane intakes with Quadrajets or Webers. Short of replacing the darts and holleys, I'm just trying to find out if I can make the current combination work, and if so, how.

Scott72

I don't think you are running too much carb. I have a 454 Mag .030 over (461) that has had a lot of work done to it (head work, cam etc...) and am running a 850 Holley, but with a dual plain manifold. I am a tad over carb'd, but it runs pretty well, just a tad flat out of the hole. I'm running 75 front 89 rear jets. You can get away with a little more carb running a dual plain manifold over a single plain. Still, I think with a little more tunning you'll get those 800's to run great.

Griff

Loading up at IDLE has NOTHING to do with the jetting. The idle mixture has its own metering system.
Have you tried to adjust the idle mixture screws????? Make sure you adjust them when the engine is hot. Turn them in till the engine stumbles and then back them out a 1/4-1/2 turn.
IN=leaner idle-----OUT=richer idle

Do you have a choke?? If so make sure its staying closed at idle.

Also a slightly hotter or different spark plug may also help. An MSD ignition is also an option

Keith

Thanks for the replies. I spoke with Tom at Holley today, he suggested similar methodology to troubleshoot the issue. There is no choke on the carb, the plate is wire tied open.

While I was on the phone I asked him about the holley blue marine pumps the shop installed. There's no line attached to the nipple fitting on the pumps, so in the case of a failure . . fuel would be dumped into the bilge. So I'm going to have to run a tube from that fitting to the carb somehow. This is the kind of half-a$$ed stuff I've been dealing with for months now. You'd think a marine performance shop would already know how to deal with these issues.

BadDog

Keith,
I ran a 502 Mag with the stock cam, something like 224/224 etc. Gen VI. I used the Holley 800 off an HP 500 with the larger squirter's etc. and pump cams etc. Used the Dart single plane and Lightning headers. My jets ended up something like Front Port 81 Stbd, 79 and Rear Port 92, Stbd 94. Rear PV valve blocked off. Never did get a really clean idle. I later added the Crane Ignition system and coil and switched to AC Rapid Fire #1. That REALLY helped. The jetting is close to the HP 500. Idle was fine, the plugs just loaded up though. The ultimate cure was a B&M 250 with Nickerson carb...
Tim T.

DanB

Are the carbs new or used? Sounds like an idle circuit problem, easy fix. Is the gas old? Use a vacum gauge to set it up. Make sure butterflys are set right.
Dan

MACDAD260

I had a similiar problem with the new 800 Holley I just bought. I changed from the stock 65 power valve to a 45. I was then able to adjust the idle speed down and turn in the idle mixture screws a 1/4 turn. Now it seems to idle fine. I also have a stock 502 Mag. Maybe it will work for you, too.

Doug

Keith

Griff: We actually already went thru that piece of it, backed them out about a 1/2 turn, probably should try it again next weekend and see if that makes a difference.

DanB: These are new Holley 800's, the entire setup is new. HP450 long blocks, dart intake, and the carbs. Gil Offshore from my old engines are also installed. New fuel too.

MACDAD260: I'm going to live with it for a few weeks and break the engines in, then take it to a shop who knows Holleys. After my last experience, I'm going to want first hand references. Thanks for the input.