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  1. #1

    Default 1991 to 1997 7.4L Engine Swap

    I purchased a 1991 2400ssl Maxum with a 7.4L/bravo one towards the end of last season. Had a handful of good outings last year, and was hoping the same for summer 2019. Winterized it and stored it away for the Michigan winter.

    Got it out of storage this year and it ran great all of June, and then I started having issues in July. Somehow the engine took on water and I did not realize it (my guess is blown head gasket), out at the sandbar starter would only make clunking noise, finally got it started and it was knocking really bad so I think I broke a piston connecting rod. Turned it off, got towed back and brought it home. The engine oil was a milkshake and about 3” high on dipstick, pulled plugs and #8 cylinder poured out water, at this time engine would not turn by hand either (seized/hydrolocked). So I started looking at options, really did not want to pay a shop thousands and thousands to rebuild.

    I found on Craigslist a 7.4 carbed engine out of a 1997 Baja that the owner had just replaced with a 502. I went out and bought it as a replacement. Pulled the old engine out with a help of my neighbor and a steel gantry he welded together for his boat engine swap.

    Pretty much everything is swapping over fine except the fuel pump. The original big block has the hole mounted (bravo style?) mechanical fuel pump that’s mounted next to the raw water pump. The new block does not have this hole in the block, so I need an electric fuel pump. I have found a few online and would like input on which one I should go with, I will attach links. The new engine does have an Edelbrock high rise intake, so not sure if that matters on which fuel pump I choose (may need higher psi?). I realize I need to wire in a oil safety switch to kill fuel pump if engine stalls as well.

    I appreciate any input, and will keep updated through this process.

    https://www.summitracing.com/parts/hly-712-801-1/

    https://www.summitracing.com/parts/crt-p4389

    https://www.summitracing.com/parts/dfp-fa0004/

  2. #2
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    Default Re: 1991 to 1997 7.4L Engine Swap

    Sounds like the original engine was a MarkIV block and the newer one is GenV/VI...

    I think you are on the best path with the electric fuel pump and the switch....hard to say which one will be optimal as the flow ratings are unrealistic. I think as long as you can get 35GPH to the carb, at WOT, the engine will stay happy. I'd say the carter will be adequate unless you plan to run it hard and long....

    I think the only other item would be to cap off the sight tube connection at the carb....there should have been a tygon (yellow) tube from the old mechanical pump up to the carb....you won't need it with an electric pump...

  3. #3

    Default Re: 1991 to 1997 7.4L Engine Swap

    Thanks for the quick reply. I did notice the Carter pump did not have that “sight tube” connection on it, but the Holly pump did. My original engine did have the yellow tube from carb to pump, but new engine’s carb, like you said, had a rubber cap attached to it. I will get one ordered up. I’m sure I will have more questions along the way.

  4. #4

    Default Re: 1991 to 1997 7.4L Engine Swap

    Been a slow road with the engine swap, lots going on, always busy...

    I am almost ready to drop in the replacement engine, but I have a few questions.

    1.) I plumbed in the oil pressure safety switch with a brass T-fitting next to the oem oil pressure sensor. I am a little confused as to where each terminal wire is supposed to go on the new safety switch for the fuel pump. There are P, S, & I terminals on the switch — P=Positive side of fuel pump, S=Ties into starter wire(which wire??), I=ignition switch (??).

    2.) I sanded/cleaned/painted my exhaust manifolds. I’m assuming I should go back and re-sand/file the mating surfaces to bare metal on the manifolds as well as the heads? Or is this unnecessary? I did get new manifold and riser gaskets.

    3.) Each manifold has 8 bolts; 4 short, 4 long, I do not remember which holes the long ones came out of, anyone remember off hand which goes where, or if it even matters?

    4.) How necessary is buying an alignment tool to align the engine once it is in? I have done my research on how it’s done, just want some opinions on the topic.

  5. #5
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    Default Re: 1991 to 1997 7.4L Engine Swap

    assuming the terminal labels are as you indicated, for the oil presser switch, the 'S" wire will go to the starter's solenoid's little 'R' terminal....it supplies current to the pump while cranking.the I terminal should go to the purple ignition wire, probably easiest to connect to the coil's '+' post.

    The gasket surface for the manifold-to-head connection will get sealed by any decent marine grade exhaust gasket...

    The manifold - to head bolts can be determined by looking at the flange going to the exhaust port. the shorter bolts go into the holes with the thinner flange...its been a while so I can't remember if they are the same or different length. Either way, you should retorque them after they get up to operating temp the first time (after they have cooled back down).

    The alignment tool is necessary - you can buy one or make one if you find the dimensions....

  6. #6

    Default Re: 1991 to 1997 7.4L Engine Swap

    Okay so you’re saying with a gasket (I bought Sierra brand), I won’t need to sand off the paint?

  7. #7
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    Default Re: 1991 to 1997 7.4L Engine Swap

    not at the exhaust ports.....some people report 'better results' (no leaks) where the elbow bolts to the top of the manifold (old style design) if the paint is removed before the gasket is applied.

  8. #8

    Default Re: 1991 to 1997 7.4L Engine Swap

    Yeah I went ahead and used a paint removal disc, and then a long flat metal file to get the manifolds and head ports down to bare metal, they mated up well with new gaskets.

    Hoping to get this thing stabbed in sometime this week or weekend.

  9. #9
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    Default Re: 1991 to 1997 7.4L Engine Swap

    FWIW, I like to dab some paint over the joint where the gasket is, and cover the bolt heads too. The cast iron manifolds seem to get janky pretty fast and a lick of paint can't hurt.
    Practicing the ancient art of ren-ching

  10. #10

    Default Re: 1991 to 1997 7.4L Engine Swap

    So I got the replacement engine installed over the weekend, used the alignment tool to get it aligned, and re-installed drive. Re-installed risers and exhaust with new gaskets, put together the new fuel system/lines/filter, along with the new Carter electric pump with oil pressure safety shutoff switch.

    It was a little tough to get it started initially but it finally fired. Lots of black smoke coming out of the exhaust (smells/looks like unburnt fuel- running rich I’m assuming), and it is idling extremely high, like 2000+ rpm. I can manually play with the choke plate on the carb to lower the rpm, but anything below like 900-1000 rpm it wants to stall out. I’m assuming the carb needs to be tuned, or the carb is damaged/needs rebuilt? I don’t really know much of anything about carburetors. The fuel pump states it’s maximum flow is 4-8psi. It has new plugs, and all wires are in good shape.

    We did put a timing light on it and were able to time it, but only while it’s running like 2000rpm so I don’t think it’s accurate.

    The carburetor has number 8867 stamped on it, which I believe is Edelbrock model? Aftermarket intake manifold is also Edelbrock.

    I’m not really sure where to go from here. Anyone with more experience on carbs/tuning/fuel air mixture/timing/etc??? Thanks in advance.
    1991 Maxum 2400SSL 7.4 Mercruiser | Bravo One

  11. #11
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    Default Re: 1991 to 1997 7.4L Engine Swap

    If you timed it at that rpm, it's way retarded at idle.

    Jeff

  12. #12

    Default Re: 1991 to 1997 7.4L Engine Swap

    Yeah, I figured it is pretty off. What would be the best steps to take at this point?
    1991 Maxum 2400SSL 7.4 Mercruiser | Bravo One

  13. #13
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    Default Re: 1991 to 1997 7.4L Engine Swap

    Crank a bunch more advance into the distributor, then fire it up and idle it down until you get it around 700 rpm. Then retime it. Finally, while it's still at 700, retime it yet again.

    Jeff

  14. #14
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    Default Re: 1991 to 1997 7.4L Engine Swap

    The edelbrock has two adjustment screws at the base. Turn these all the way in then back out 1.5 turns. Make sure any open vacuum ports are capped off. Only the hose from the valve cover should be attached, and in most cases this runs to the spark arrestor and not direct to the carb body.
    You probably have a vacuum leak and someone opened up the idle screws to keep it running, thus the black smoke, rough running and poor performance.
    At 2K rpm you're off the idle circuit and into the main jets.
    Last edited by o2batsea; 10-08-2019 at 08:51 AM.
    Practicing the ancient art of ren-ching

  15. #15

    Default Re: 1991 to 1997 7.4L Engine Swap

    Thanks for the replies. I am going to do a little more tinkering tonight after work. I will make sure there are no vacuum leaks, try to idle it down and then re-time it.

    Is a fuel pressure regulator a good idea for this application? Pump specs state 4-8psi, could having 8psi be too much? I think it needs more like 6psi. Just a quick thought, wouldn’t mind spending a few bucks to get an accurate steady flow of fuel if it might help my cause.
    1991 Maxum 2400SSL 7.4 Mercruiser | Bravo One

  16. #16
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    Default Re: 1991 to 1997 7.4L Engine Swap

    That's a bit high. I use a Holley regulator on mine set at 5 psig.

    Jeff

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    Default Re: 1991 to 1997 7.4L Engine Swap

    Oh and I would set the timing to spec, not do any advance until you get it running and idling like a good kitty. Only turn the diz a little bit if you must until it fires. If your dizzy is off by a tooth it can make you nuts. Basics basics basics. Top dead center on the compression stroke. rotor pointing at plug wire number one, blah blah blah
    Practicing the ancient art of ren-ching

  18. #18

    Default Re: 1991 to 1997 7.4L Engine Swap

    So we (neighbor and I) made a little progress tonight, but not much.

    We installed an in-line fuel pressure guage right before the carb and the entire time it was running it read a steady 4.5-5 psi, so I think we’re good there.

    I took off the carb, cleaned up the mating surface, as well as the block/spacer/adapter(?) under the carb, with brake cleaner. There are two seated rubber gaskets and both looked in fine shape. Only thing we did notice is that there in a small 1/16” hole in the block off plate (?) on the small opening on the intake manifold right next to the carb. This looks intentional like it’s supposed to be there, but I can definitely feel air hissing through it while idling, is this normal, or should that hole be sealed?

    We pulled #1 plug, turned engine by hand, found TDC, and reset the initial distributor position #1 pointing at #1. Ran it at about 1000 rpm and set the timing to 8° btdc.

    The issue is that it is idling so poorly, it’s hard to get a good time on it. The RPM’s fluctuate between 700 (always stall out), all the way up to 2500. I can play with the choke flapper by hand and keep it barely open and keep it at about 1000rpm consistently, but no lower for more than a second or it stalls out. It is still pouring out black smoke through exhaust, and overall just very rough.

    We played with the carb adjustment screws, we were able to turn them all the way in (clockwise) and then back out 1.5 turns, no difference. We turned them all the way in again while running, and kept them there and it did not stall the engine, that’s giving it no fuel right?? I’m assuming I have a carburetor issue here, and thus can’t get it base-timed correctly making it run like crap. Might need someone with more carburetor knowledge to look at it (I hate paying people for services for stuff I can do/learn to do!).

    I appreciate the responses, keep them coming! Thanks in advance.
    1991 Maxum 2400SSL 7.4 Mercruiser | Bravo One

  19. #19
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    Default Re: 1991 to 1997 7.4L Engine Swap

    If you can turn the idle mixture screws all the way in, and the motor keeps running, the carb is flooding like crazy.

    However, you also say it dies if you open the choke! So which is it here? It can not be both!

    Jeff

  20. #20

    Default Re: 1991 to 1997 7.4L Engine Swap

    Sorry, what I meant, is that the engine revs very high (2000+rpm) when I open the choke anything more than 1/8” (in neutral). It stalls out if I close the choke all the way, but holding it open that 1/8” with my hand, it stays right around 1000rpm, but when I try to idle it down/close the choke, trying to lower the idle to 600-750, it stalls out.
    1991 Maxum 2400SSL 7.4 Mercruiser | Bravo One

  21. #21
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    Default Re: 1991 to 1997 7.4L Engine Swap

    Do you have this edelbrock tuning instruction guide?
    https://edelbrock-instructions-v1.s3...ning-guide.pdf

    It's an electric choke 1409/1410, yes?
    Practicing the ancient art of ren-ching

  22. #22
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    Default Re: 1991 to 1997 7.4L Engine Swap

    Quote Originally Posted by lucasmahony View Post
    Sorry, what I meant, is that the engine revs very high (2000+rpm) when I open the choke anything more than 1/8” (in neutral). It stalls out if I close the choke all the way, but holding it open that 1/8” with my hand, it stays right around 1000rpm, but when I try to idle it down/close the choke, trying to lower the idle to 600-750, it stalls out.
    sounds like the throttle plate is open too far to allow the engine to idle.....and have the choke closed that much is inducing the rich condition = all the black smoke....

  23. #23

    Default Re: 1991 to 1997 7.4L Engine Swap

    So sure enough, the throttle cable was adjusted too short compared to old engine, and left the throttle open a tiny bit while in neutral, making the engine rev up. So we adjusted that so that it was closed in neutral, and she purred like a good kitten. We retimed it one last time and it idled smooth at around 750rpm, got up to temp, no more black smoke!

    There are a few minor bugs to work out in the mean time. I don’t think the electric choke is working at all. It’s positive is wired to the yellow/red hot wire on starter solenoid, and then grounded to engine. The choke flapper is not moving at all while running. Thoughts?

    My engine alarm is going off. Oil is at 40psi in neutral at 750ish rpm, and increasing throttle, it does not raise over 60psi. Engine temp got up to about 175°(? Top middle of guage) and didn’t go above that. I did install a new quicksilver water temp sender previously (broke old one upon removal). Other than that, I’m not sure what else would set that alarm off? It’s a constant alarm anytime the engine is running.

    thanks again for the replies!
    1991 Maxum 2400SSL 7.4 Mercruiser | Bravo One

  24. #24

    Default Re: 1991 to 1997 7.4L Engine Swap

    The oil pressure switch could be bad , which is a different sensor than gauge sender.

    Drive lube level monitor would also set it off.

    I think both of these you can take these off and either jumper together or leave disconnected (not sure if they are NC or NO) see if the alarm goes away

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    Default Re: 1991 to 1997 7.4L Engine Swap

    Choke is to be wires to PURPLE (ignition on).

    The choke needs power all the time when engine is running.
    The electric choke is a resistor. With power to it, it gets hot and makes by-metal spring wind/unwind.


    There should be a purple wire for that purpose. Not sure where you got the Yellow with red????
    Yellow with red is only active when CRANKING engine over to start.
    Jack
    Mass.

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    Default Re: 1991 to 1997 7.4L Engine Swap

    Glad you got it sorted. You probably know more about your boat than anyone else at this point ha ha ha!
    Alarm is on a tan/white wire. NOT saying that you should delete the alarm but it's one of those things that gives warning but doesn't tell you anything about what caused it. Like a crying baby, you have to drop everything and figure out what it needs.
    Practicing the ancient art of ren-ching

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    Default Re: 1991 to 1997 7.4L Engine Swap

    On the +12VDC feed for the choke, like Jack said, the PURPLE wire at the coil is the best source.

    On the alarm, there are three SWITCHES running in parallel....if either of them turn ON, the alarm sounds....best way to figure out which one is active is to remove them from the circuit, one by one....they are spliced together in the harness, so there isn't any 'daisy chain' to be concerned about (assuming you are using the factory [91] harness)...and the wire should be TAN with a BLUE stripe.

  28. #28
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    Default Re: 1991 to 1997 7.4L Engine Swap

    yeh well that's why I don't defuse bombs for a living...I have CRS syndrome
    Practicing the ancient art of ren-ching

  29. #29
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    Default Re: 1991 to 1997 7.4L Engine Swap

    ..."I have CRS syndrome"

    What is that?

    Mr. Curious

  30. #30

    Default Re: 1991 to 1997 7.4L Engine Swap

    Quote Originally Posted by fastjeff View Post
    ..."I have CRS syndrome"

    What is that?

    Mr. Curious
    Can't Remeber Sh$t

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