efi to carb conversion
 

has any of you guys ever done this conversion?i was told that all you need to change is manifold,carb,disturbertor,and fuel pump.does the engine wire harness have to be changed?i have a 1994 502,mpi.it is a vst type system.i have heard so many bad thangs about these motors.i have spent about 1700.00 so far and have had no luck.any ideas?

what have you spent 1700 on ?

I do,,,,, find another mechanic

I have no advice, only a question: Why go from EFI to a carb?

While I've only been into performance boating for a few years, I've been hot rodding for 20, and the advantages of EFI (efficiency, tunabiilty, minimal maintenace, and consistent power delivery regardless of lean\tilt angle) seem to outweigh the added cost and complexity.

yes it can be done its very easy and you dont need to buy a harness you seperate the 2 harnesses at the plug where the efi plug is. cut the plug of and that will feed your coil if need be give me a call i just did this 603 234 6363 Gordon

1700.00 was mostly on the fuel system.injectors cleaned.fuel pump r/r.distribor removed and cleaned,plugs,wires.i know very little about injecton.i have al the parts for the motor to do the conversion,except the wiring.i may not beable to spell,but i can work on a carbed motor.....lol.i have been chasing this problem for awhile and i am sick of it.i think it will be cheaper in the long run.

I went efi to carb on my 302 saves alot of room an makes for a cleaner looking motor. Wires you have to seperate cut off harness, need a 800dp holley , intake, I used my old disurbertor. Mine ran great but did give up a little IT WAS WORTH IT.

If i remember right you will be giving up around 4-5 MPH going to a carb in some cases.

well... like everything else, if the guy doesn't know what he is doing then you might as well just light the cash on fire. efi is easier to diagnose and repair than most other elements on the boat. you just need the correct tool , factory manual and fundemental knowledge that is required. you plug in the scanner, read the results, follow the diagnostic flow chart and fix whats wrong... once. you don't just throw parts at it or screw with it in some haphazard sense . the argument , carb vs efi always seems to presume the carbs always run well and the efi is always broken. this is nonsense. in my environment the carb guys re down far more often than the efi guys and even when they are running correctly, there are always some sort of nickle and dime mixture problems that make the boats run badly. the efi works very very well the vast majority of the time and when it does fail ... which mine has twice in 5 years... it repairs quickly and easily. are carbs simpler ? maybe... if you know about carbs. but for people that know the same amount about efi it is always a mystery why anyone would want a carb at all .

if you have spent 1700 with a mechanic on your efi and it still doesn't run then its the mechanic that's no good. not the efi . and i have $100 that says that if you have the same guy do the carb conversion, your problems are far from over.

carb conversion
 

i plan on doing the swap myself.carbs i understand,efi i dont.i am not sure if efi will make more h/p over a carb.i know the efi will use less gas.and another thing.when and if i decide to get more h/p out of my carb motor,its not going to be that much of a hazzle,no re-mapping the computor,changing to bigger injectors etc.i also think as said,a much cleaner apprance in the compartment.and i know the parts are aLOT cheaper.i know some of you guys think this is foolish,but its all about what i can work on and what i feel comfortble with.thanks for all the input.

The stock 502 carb motor was 1 to 2 miles slower, I had one its 415hp to 390hp. What I did was save up buy a custom carb 1000 to 1500, intake brodex or dart are the best for hp. if you can spend a extra 400 on the intake have DEAN GELLNER set you up with custom carb 700 to 1500 /custom intake 800/ roller rockers 300/ cam springs lifters 1200/ plus labor you should end up with 600 to 650hp, that will last 400 hours plus as long as your bottom is good. this is what I had with some combos 390hp/585hp/595hp/637 hp/647hp/ 670hp all started with stock merc 93 502/ did the bottom blew printed and new heads at 637hp and up. now going to his intake carb going for 700hp or add cam and more like 725 . but Dean will hook you up right. I went from 67 gps to mid 80's.

whats the problem anyway?

Yep, it would help to know that.

And are you sure the problem with the engine is with the EFI?

If it's a stock 502 it seems a qualified Merc mechanic would be able to diagnose and then take care of the problem.

I don't agree that, in a single engine boat, you're going to loose 5-6 mph if you go to a carb. And as with most any engine, if it's set up properly, reliability shouldn't be a problem either. I've had conversations with a couple of builders who I know pretty well and who are familiar with both carb and EFI and was told that a carb engine tuned/set up properly will produce as much power as a comparable EFI engine. However, I have no personal experience.

Originally my boat had a carbed 7.4/330 in it and it ran flawlessly. The 7.4 was yanked and replaced with a ZZ502/502. I had Nickerson's marinize the carb that came with the ZZ502, and the engine ran flawlessly for 4 seasons, of often very hard running. Last season the ZZ502 was bored and stroked to a 557. It now has a relatively mild rmbuilder tight lash solid roller and a Nickerson Dominator. I only had the chance to get out on the water with it 3 times last season, but it ran very well. And I high idled it a few times (+,-1000 RPMs) for better than a half hour at a time, and the engine would spring to life when I went for the throttle.

That said, I don't understand EFI/computers either so I too prefer a carb engine. My thought is, if you have EFI and if you have someone close by to service the engine, then EFI is probably the best way to go. It's all a matter of preference, and access to someone qualified to service the engine/system. JM .02

symptons
 

when imrun the boat for awhile and shut it off for about a hour,the motor will start,idle like crap,like it has a cylinder out,instead of a smooth ldle,it seems like i have enough fuel to almost get on plain,and then it starts,motor dies,picks back up and then dies agine,some times it picks back up,sometimes not and then will barely run.lately this will happen when i am idling through a no wake zone.temp is normal,140,oil temp 220-240.i have a set of gages on the motor now.one on the suction side of the mech.pump.one at the vst tank at the inlet.and a high pressure gage on the fuel rail.this should tell me if the fuel system is at fault...i hope.i rebuilt the motor last year and had the same problem but just not as bad.i feel it is fuel related because how the motor sounds.....running out of fuel.i just hate paying someone to work on my stuff when i can do it myself...a carb that is.

Have you cleaned the fuel filter in side the VST yet? Also where do you buy fuel from [name]Also land or water? Also are you shur you have the latest style of the VST? I remember merc had to make up a newr style kit for the VST tank to help remove most of the vapor locking.Also the engine temp should be at 160-175 max not at 140 or under it will run rich and start to faul the spark plus.

fuel pump problem??
 

i ran the boat today and it messed up.this time i had the chance to read the gages.i had no fuel pressure on the fuel rail.i could hit the bleed valve on my gage and get frof and then nothing.the motor is running,well trying,and no high pressure.the pressure on the mech. pump was 10psi.after awhile of cutting on and off the pressure builds and the motor clears out.vapor lock,i know.i allways knew.how do i stopit.new pump?the boat was run 1hour and cutt off for about ahour.any fix?

You could add more vents to the deck of the boat, You could install a bilge blower [fan] and aim it at the vst tank while sucking air from the out side, [new vent] You could open engine hatch a little while at rest, Let engine idle for 3-5 min before shut down, Make sure vst fuel filter is clean. The easyest way to know if the vst tank has been opened up is look at the six bolts and see if the paint looks like it peeled, those bolts are blue locktited on you will need a impacket driver to brake them loose or trust me you will strip the bolt head with out even trying, I know I have done my own boat, I have only owend this boat a year and it is a 1996 and the top of the vst tank has never been off and my filter was dirty, I could not see thur the filter, I cleaned it with brake clean. No need to buy a new one the filter is like $100.00-200.00 crazy money, Just don't put your finger thur it, I can now see thur my filter. The filter will twist off it has a U shaped lock,catch on it under the rubber part, just twist it and it will unlock on it's own. Also if you do strip the screws out it is easer to cut the screws to use a flat blade impact drive to remove the bolts. I used a hack saw and I installed new SS bolts usein new allen bolts way easer. [180 hours on mine] Make sure the vst is up dated,You could call merc customer support with you engine serial number and they should be able to get back to you on what to look for on the vst tank to see if it is up dated or not. Also it should not get over 176 degrees under the engine hatch, [merc specks] Good luck.

I ran the return line that goes to the VST back to the tank instead. No more vapor lock issues and I picked up 2-3 mph.

How did you do that? what parts did you use? Did you go to the gas tank? If so where did you tie in to? Thank you in advance.

return line
 

what did you do w/the return line where it went into the vst tank?plug it off?i understand where the return line goes back to the gas tank,just not what to do at the tank?