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Merc 488 3.7l carb swap feedback

C

coachant

New member
Hi everyone,

1st time boat owner and picked up an older boat for about $300 (owner was asking $1000). It has a 1984 Merc 488 (224 cu in/3.7l) 190 HP with a 4bbl. The previous owner claimed he had the engine swapped for a rebuilt motor about 2 years ago, what exactly was "rebuilt" the previous owner did not know but the original 1979 470 is now a 1984 488. I got this boat since everything is pretty straight forward, points, coil, carb, etc and was hoping to stay around $1000 to get everything going.

Anyway, when I got this boat the owner stated it failed him on the water and he would not figure out what was wrong. He got a newer boat so wanted to unload this money pit on me. So here I am... Fixed the spark issue (burnt points and dead coil), fixed the trickle of fuel issue at the carb, water separator and clogged mechanical fuel pump, so now have spark to the plugs and fuel to the carb. It turns over and "starts" when fuel is put directly into the throttle body but it was clear no fuel was going through the carb normally. I bought a Qjet rebuild kit for $25 but there are many issues with the carb after opening it up and it can't be rebuilt (mainly stripped threads and things were epoxied in place). At this point I am an additional $150 in (basic tuneup parts and fixing the issues above) so this boat has not hit the $1000 mark... yet

I looked around on the internet and see many of the 6/8 cyl motors have swapped successfully to a Edelbrock or Holley. On cars I have done the swap many times and it was pretty easy since I also swapped out the intake manifold and bought any aftermarket brackets to support the application. For this 4 banger I could not find any info on the swap. I will likely have to do some throttle linkage and fuel line changes but wanted to know if anyone has already done this and what it took. Any advice would be very helpful especially how the boat performed with the secondaries opened (lag?), linkage tips, fuel line routing, etc.

I believe the stock Qjet 4bbl is 750 CFM which seems like overkill for this engine. I was looking at a Edelbrock 1409 in a 600 CFM or a Holley 4150. The Holley 4BBL comes in a 570, 670, and 770 CFM with vacuum secondaries or a 600 CFM with manual (like the Qjet I have now). I have zero experience with the Merc 470/488 so do not know how much vacuum it will pull and if the vacuum secondaries will even open... or if I should think about a 2bbl. Holley seems to have many other marine carb options. Since I got the boat DOA I have no idea how it ran with the stock 750 CFM Qjet. Any other carb recommendations?

224 cu in, 4700 RPM max (Summit Carb Calculator)
Street Carb CFM :258.94
Racing Carb CFM :335.09

Any insight and help would be highly appreciated. Thanks!

 
Edlebrock makes an adapter for Q-Jet manifolds--just bolt up an Edelbrock 1409 carb and go boating. And it will work just fine.


You're over analyzing a non-problem of vacuum and etc.

Jeff
 
What Jeff said. Plus, the Edelbrock has electric choke which saves you the hassle of fooling around with that business. You might need to detune the carb a little since it will probably have the wrong needles for your engine, but it should run OK out of the box and you can futz with the jetting later. It'll just suck down gas like a pig in slop.
This is going to be the most expensive 300 bucks you ever spent hahahaha

Your $300 boat:

Lurssen’s-Apoise-Luxury-Megayacht-1.jpg

Everybody else's $300 boat:

6a00d83451cb8069e2022ad37e1054200c-800wi.jpg
 
Last edited:
First the Rochester is variable cfm.
There also is a very small tab that limits this cfm if not removed.
All marine oem non high perfromance are actuall 650 CFM.

The 470 handles the rochester 4 bbl very well.

So in making your decision, if a replacement 4bbl is chosen make sure the secondaries are vacuum actuated.

If a holley 390 cfm Marine 4bbl if available that would be a good choice.

Just have to monitor plugs for potential lean condtion.
If you dont plan on running at higher rpm then this would work well.

If you get a generic marine 4 bbl it will most likely be aet up for a V8
 
kghost said:
Oh and try NOT to go above 4200- 4400 rpm. That engine does NOT like high rpm
Click to expand...

So far I've only managed 0 RPM... not planning on seeing what it can do since it is an older engine, just want to fish and drag the kids around on a tube, not at the same time :)

I just ordered the Edelbrock 1409 and adapter, familiar with tuning Edelbrock's from a long time back. Hopefully the linkage won't be too much of an issue.

I thought I was over thinking the CFM, just not very familiar with this engine or the Ford 460. Was a mainly a Chevy guy, played with Dodge, but did not play around with Ford too often.

Thanks for all the feedback! I'll try to post before an afters. The boat needs a little TLC and I'll try to document the process. Hopefully I'll be on the water in a couple weeks!
 
Mercruiser block, aluminum.
Ford head,
Gm point ignition.

You will have to detune that carb.
Way to much for a 4 cyclinder.

the primary may work ok but the secondary jetting and secondary timing may need to adjusted.

I have never seen this swap.

Whats wrong with the Rochester?
 
I was successful in tearing it down but while doing that quickly realized I would not be able to put it back together unless I had a tube of epoxy. Whoever did the previous rebuild for the owner stripped a lot of threads so I can't use the core.
 
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