Converting 502 to a carb need help
 

I have a 96 27 foot powerquest that had a 502 MPI in it. One of the heads rotted out and I got water in the cylinder so I am rebuilding and going with a simple carb set up. It is a roller cam motor that is bored 30 over running CMI e-top headers with a ZZ502-502 cam. Any recommendations on a distributor. I wanted to stay away from the MSD, I was thinking DUIs sounded like a good deal, do I need any kind of a shift interrupter? Do I need to run a melonized gear or composite for that forged camshaft? I am going with the 800 CFM holly carburetor in a dual plane intake how big of an electric fuel pump should I be running and what size lines? I am also looking to buy a wire harness ,my alternator is on the lower forward right side of the engine. The new cast-iron heads I am buying are a direct replacement for the originals but are said to flow a little bit better and give me a bout 20 more horsepower. Any sound advice is greatly appreciated.

What heads you buying

throw the other original head away. don't sell it. it is about to rot out too. spent just as much time full of salt water as the other one. take a close look at the water passages in the block. it is getting thin all over. you need to flush the tampa bay salt out religiously when boat comes out or gets docked.

3/8 stsinless tube or braided lines. 80-100gph pump, fuel pressure regulator at 8lbs. Use ignition protected marine rated electronics. There is a Lot of good stuff out there. I'm other ppl here will have some good suggestions.

You should have a Bravo drive so no shift interrupter is needed

You can use the DUI distributor.
you could also use the Merc distributor with a Daytona sensors ignition box for more tuning parameters.
you will need a melonized gear for the GM cam.

From the GM ZZ 502 install manual,

"Distributor Gear


[img]file:///page2image16072[/img]
A melonized distributor gear must be used in these 502/502 engine assemblies. The deluxe 502/502 engine assembly comes with a melonized distributor gear. For the base 502/502 engine assembly you must install an HEI distributor, part number 93440806, with melonized gear or install a melonized gear on your distributor. Use part number 10456413 gear for 31/64” diameter distributor shafts or use part number 10493532 gear for 27/64” diameter distributor shafts. "


Jeff

Have you run that cam yet? I think it has a lot of overlap. What exhaust? Might be worth finding a 525EFI takeout or cloning it. I don't think that 110lsa is going to be a good deal.

I like the DUI stuff, no problems. You can run the 889275T pump. Can find them for $100.

If you haven't bought the cam yet, I would go with a Crane 168741 or 525EFI cam over the ZZ502 cam.

Thanks for everybody's responses I'll try to answer all questions
 

I am throwing away both of the old original heads. The new heads are a direct replacement for the o88 original heads they are cast in New Zealand and have extra thickness around the water jackets they come with inconel exhaust valves and a swirl polish on the backside of the valves most of the builders here highly recommend them for the rebuilds because of their durability and good performance they are for marine use. I have run that ZZ502 cam for about a year since the last time it was rebuilt and I had no problems with water in the oil or reversion problems as long as I keep my idol above 850 I think I should be OK and before I shut down I always bump the throttle a little bit there is also another distributor out there that I see a lot I think it's a Pertronics. the trigger style is a hall affect what is the difference between that and a magnetic pick up that the DUY has? Also the guy putting my heads together has been informed that I'm using that ZZ502 cam so if I were to purchase another camp he would have to change the valve springs or something to match the camp it's just a little bit too late so I'm going to go with the 502 I think it'll be OK. Glad I don't need to shift interrupter I have read some posts and people were complaining when they install the new distributor that they got no tax signal do I need a special distributor that has a wire probably gray coming out to run my distributor I could ask that question when I call and purchase one from either summit her performance distributors also one other question I had where would be a good place to buy a new wire harness does anybody have one used and like I said the set up is with my alternator on the lower right-hand side of the engine. thanks again everybody.

Griff have you run one of those in a low compression 502?

168761 might fair a little better. Either way break out the $$$$$ for some long travel lifters!!

Jeff

If you use a DUI dizzy, it has a tach connection.
if you use a Pertronix, you will connect your tach wire
to the neg terminal on the coil. If you use an ignition box, there will be a tach connection on there.

you can also use your merc EFI distributor if you swap out the ignition module for Delco Voyager module.

Jeff

I did this with a 94 454 mag , was able to use the old harness just went thru and removed efi connections . The fuel pump runs with the org. pump wire and I used a dui dist. ..... I recall making up wire for the alt. to battery , mines mounted low on port side. Its been over ten years ago so not fresh but still have the boat

Midwest, did you see any gain and performance when switching to the carb. Hallj. Maybe I will use my original distributor two questions do I need to change out the gear with the melonized gear I looked at it closely I don't see any wear but I was only running that camshaft ZZ502/502 for about a year also what is the part number of the ignition module I should run. I have been reading the form quite a bit and I saw a guy ran a D1965 but it only gave him about half the advance he needed should I just buy a module from an HP 500 . Without the MEFI one running the show have I lost Revlimid her protection. And. Is this something I should be concerned about or not worry about it

I think you should switch the gear just to be safe.
this Delco pn should be correct, D1965A.

it looks like it should give you a nominal 20 degrees advance.
I am not sure if it has a rev limiter.
https://bpi.ebasicpower.com/downloads/BPIESTGMV8instructions.pdf

the HP 500 is a completely different distributor and module. Thunderbolt V.

Jeff

Yet another option, let the MEFI system run the ignition. You don't spend a dime, and works way better than a "dumb" distributor.

Jeff

Hallj, yeah I'd really didn't think about that, then I would keep my rev limiter using my computer which was raised to 5400 a couple years ago by Mark at I think Precision in Louisiana. I would just let the carb do the fuel metering and I was going to keep the old wire harness as I was thinking I'll just cut off the wires that I don't need leave the grounds. Correct me if I'm wrong but if I keep my computer and original wire harness and hook up my knock sensor with that computer retard timing to stop detonation or did it used to increase fuel flow. I also think that the idle was controlled by the computer, Will this make it hard to tune the idle with the carburetor?

does anybody know where I can get a melonized distributor gear and how was it removed and the new one installed it looks like there's a roll pan to drive out

See my earlier post for your distributor gear.
the ECU would retard timing to control knock so that will still work fine.
keeping the MEFI will also compensate timing for coolant and air temp so keep those sensors hooked up.
you will also want to keep your map sensor hooked up too.
It won't control the idle with the carb so get rid of the IAC motor.
I don't think you will need the TPS either. Not sure if you will need to keep hooked up to avoid any fault codes.
just keep it Ty Wraped to the harness or use a resistor to simulate it.

maybe BUP will see this and comment.

Jeff

Also, yes the gear is fixed by a roll pin.

Jeff

Thanks Jeff for all the words of wisdom sorry I had to ask again about where to find the gear I remember reading that post but I've read so much lately I didn't remember that it was actually in my post. Now you got me thinking this might not be so easy with some of the other sensors like the idle air control and TPS not hooked up it might affect how it runs the ignition .I will definitely have to do some research on this so if anybody out there has any ideas on the specifics of this kind of modification please dievulge any information.

Not really just got away from spending money and time with the mefi1 system , carb is easier to tune when other parts are upgraded at least for me

The cam you listed is just a stock 500efi cam.
I have run the 168731(very similar) in a 454/525SC which is 7.5 CR static and it worked well. I'd go one size bigger for a performance 502.

The melonized gears are all over ebay. 2 questions. Which dia goes with which distributor. And since the ZZ and HO cams are steel cores, what about the other factory GM cams-distributor gears? Since GM got tired of wiped cam lobes, wouldn't ALL of their cams be steel, therefore all distrib gears hardened?

I've done this with a GM truck ECU and it seemed to work well. Verified timing with the scanner and timing light. No IAC or TPS hooked up. No Codes.

Jeff

Sounds like a plan I just remove my old gear and it Mike's out to 17/64. I'm going to stick with this zz 502 cam since it is probably already installed in the engine and my head builder has already set up the springs for that cam ,maybe I should've done some homework earlier. I will post updates or ask questions as they become relevant thank you everybody for the help.

I have the ZZ502 cam also and have been happy with the DUI ignition, although I never switched out the gear so I’ll look into that. I plan on switching to a 731 cam this winter though just to step away from the reversion issues, and should be similar enough to use the existing valve train.

One note with the DUI ignition is that you need a dedicated minimum 12g power source. I ran a new 12g wire instead of using the EFI harness and the change in performance was stupid different. You will also have to rewire the tether kill switch too to cut the ignition.

Thanks

I have done this as well, DUI dist are nice pieces, I have had great success using the H/P HEI's from summit, I get the vac advance eliminator and use the petronix rev limiter, used the gear that came with the dist on both the std cam and as well when swapped in a 500HP cam... if you go with came change go with a single plan intake

i am a little leary about those from ebay,i buy them from summit.

Summit racing equipment in Ohio. Pretty sure there is s .491 Dia and .500 Diameter. Mine are .491. Push the pin out and measure shaft with caliper. If you don' have caliper, go to someplace like autozone. They have them under counter. Or just buy a cheap set. Mallory, msd, ECT. You will have to make sure you have proper clearance between housing wear plate and gear face. .010 is good. My gear was tight against housing, you have assemble measure with feeler gage then take material off of gear face until it's right . Most gears will be tight against housing wear plate. Just call summit hit 2 for help questions wih parts

midwest can you text me 2818816783 have lots of questions?

If you buy a dual plane get a performer rpm, in recent dyno testing i did it had the least amount of afr difference from hot to rich vs a std performer (#3 was 5 points leaner at wot than #2 w std performer), smitty