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Fabricator909

Spreadbore (Quadrajet) vs Squarebore (Weber) issues

Hey guys,

 Got a 1994 Malibu Response awhile back, some had put a 5.7l truck motor in instead of a true mercruiser motor, it lacked serious holeshot power and top speed. So I snagged a 5.7 Mercruiser that had been rebuilt for $600 off Craigslist. The issue comes down to the carbs, the motor I'm pulling out of the Bu has a Weber Carb, however the wedgeplate and intake manifold are for a spreadbore carb (quadrajet). The mercruiser motor I purchased does have a quadrajet carb, however the front mount bolts go through the carb, vs. flange mount like the Weber. The big issue is, when trying to bolt it to the wedge plate, the front bolts will not go into the intake manifold, because of the angle of the wedge. Do run the weber carb with the wedge, or the quadrajet without? I'm assuming the weber carb and wedge came off of the original motor before replacing it with the truck motor.

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@Fabricator909:  I suggest you go with the Weber carb, you do need the wedge as it levels the carb (float bowls) to the floor of the boat.  Correct, the original Mercruiser Tournament ski engine was equipped with a Weber carb.  It should be marinized (is it painted black) so it comes with the port to connect the fuel pump vent to the carb, and properly calibrated for marine application.  You will be fighting mixture issues with a carb that does not have a wedge.  Also, the Weber is a Carter AFB or Edelbrock in disguise so easy to source parts.

Edited by Woodski

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6 minutes ago, Woodski said:

@Fabricator909:  I suggest you go with the Weber carb, you do need the wedge as it levels the carb (float bowls) to the floor of the boat.  Correct, the original Mercruiser Tournament ski engine was equipped with a Weber carb.  It should be marinized (is it painted black) so it comes with the port to connect the fuel pump vent to the carb, and properly calibrated for marine application.  You will be fighting mixture issues with a carb that does not have a wedge.

Woodski, thanks for the fast reply, so Malibu just ran a square-bore gasket to a spreadbore wedge? The carb is painted black, my boat as an electric fuel pump however, not mechanical. The quadrajet has the vent line from the carb to the fuel pump, the weber does not.

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How about swapping intake manifolds so you don't have the mis-match?

Also, a wedge is really nice on a direct drive.  If I remember right, I had to oblong the bolt holes in the carb to make it work.

 

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31 minutes ago, MadMan said:

How about swapping intake manifolds so you don't have the mis-match?

Also, a wedge is really nice on a direct drive.  If I remember right, I had to oblong the bolt holes in the carb to make it work.

 

The intake manifolds are the same. Just the front mounting holes on the quadrajet are the issue since they go through the carb body.

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20 minutes ago, Fabricator909 said:

The intake manifolds are the same. Just the front mounting holes on the quadrajet are the issue since they go through the carb body.

You could always put some studs in there, that were bent down by the threads, so they would go through the carb body.

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36 minutes ago, MadMan said:

You could always put some studs in there, that were bent down by the threads, so they would go through the carb body.

The carb is too tall, can't find studs long enough.....also haven't found a QJ wedge that would work either....

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@Fabricator909:  Take this with a grain of salt right now, it has been a long time since I popped the carb off.  I think the wedge does the transition and I think there are studs in the wedge and as I type, I recall the GM intake manifold actually has a dual bolt pattern (but I am rambling off memory not looking at an engine).  Edit:  there are studs originally.

Edited by Woodski

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28 minutes ago, Fabricator909 said:

The carb is too tall, can't find studs long enough.....also haven't found a QJ wedge that would work either....

Make your studs out of all-thread.  Shipping on this wedge is kind of crazy:

http://www.ebay.com/itm/Carburetor-Adapter-Plate-Quadrajet-Wedge-Mercruiser-470-/292027241095?hash=item43fe2e1287:g:WtkAAOSw4CFYoU8u&vxp=mtr

 

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17 minutes ago, MadMan said:

I have a wedge. I'll post pics tonight of the differences, I'm considering all thread. There is no transition from the squarebore to spread bore. I talked to SkiDim and they said it wouldn't be surprising if Merc put the Weber on that way...

I guess the question is, which carb is better??

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Either carb would work fine, but I wouldn't use a carb adapter.  If I was wanting to use the Weber I'd buy another intake, small block Chevy intakes, in any config  are cheap and common.

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Here's my '94, that's the original Weber carb with the original wedge and an Eldebrock Performer intake, FWIW.  I did have to bend the studs a bit to get it to fit over them.

 

57335d243537d_2016enginepic3resized.thumb.jpg.c18857cbb95d749b4d372400a504c8fe.jpg

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So why didn't my 90 mercruiser with quadrajet carb have a wedge?

Lot's of older motors don't have wedges and don't have problems. Is it a carb tuning problem when there's no wedge?

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1 hour ago, Michigan boarder said:

Here's my '94, that's the original Weber carb with the original wedge and an Eldebrock Performer intake, FWIW.  I did have to bend the studs a bit to get it to fit over them.

 

57335d243537d_2016enginepic3resized.thumb.jpg.c18857cbb95d749b4d372400a504c8fe.jpg

@Michigan boarder: Thanks for the pic, it looks like you have the same wedge that I do.

@MadMan : No need for a carb adapter, the bolt patterns are the same on the Weber and QJ.

@JasonK: Not sure of the angle of the motors on the earlier Malibus, but you can see by Boarders pic, the motor sits at a pretty steep angle. I would think it would be a pain getting the float adjustment right without a wedge, but I'm no expert.

I think I'm going to run the weber with the wedge, and see how it goes. The old truck motor was definitely running rich, which tells me the weber is already tuned for the higher HP Mercruiser engine.

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You probably know that the 1994 was pre-vortec (265hp), does your new engine have vortec heads (1996 and up - 320hp)? 

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@Fabricator909:  Is that the truck manifold you are showing or the Merc one.  That intake appears is matched to the Quad, not the Weber if my eyes are seeing the pretty big differential between the primary and secondary holes.  My original Merc intake manifold GM PN# 14098242 has an open dual plane plenum (the Edelbrock Performer RPM is a direct copy).  Carbs are what I call a dumb device, they simply mix fuel with however much air is passing by, so if rich for one engine, will be rich for another until that engine is out of same cfm range. Could be an issue of intake manifold matching or poor atomization.  Marine carbs are tuned rich.  The Weber is super easy to tune and there are is plenty of areas to extract more performance from that unit.

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1 hour ago, Michigan boarder said:

You probably know that the 1994 was pre-vortec (265hp), does your new engine have vortec heads (1996 and up - 320hp)? 

Non Vortec heads unfortunately, tho the old motor would make a nice candidate for a 383 stroker build:biggrin:

37 minutes ago, Woodski said:

@Fabricator909:  Is that the truck manifold you are showing or the Merc one.  That intake appears is matched to the Quad, not the Weber if my eyes are seeing the pretty big differential between the primary and secondary holes.  My original Merc intake manifold GM PN# 14098242 has an open dual plane plenum (the Edelbrock Performer RPM is a direct copy).  Carbs are what I call a dumb device, they simply mix fuel with however much air is passing by, so if rich for one engine, will be rich for another until that engine is out of same cfm range. Could be an issue of intake manifold matching or poor atomization.  Marine carbs are tuned rich.  The Weber is super easy to tune and there are is plenty of areas to extract more performance from that unit.

Good catch @Woodski, the motor that I pulled does have the 8242 manifold, looks like I'll be swapping manifolds as well for the extra torque. It looks like merc machined the casting on that manifold to make it an open plenum, vs. strictly for the spreadbore.

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@Fabricator909:  If you are swapping intake, I strongly suggest finding a set of post '96 Vortec heads, makes the 265 > 310 hp bump, you are 6 gaskets away at that point.  Excellent ROI & should be able to scavenge some used ones.

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34 minutes ago, Woodski said:

@Fabricator909:  If you are swapping intake, I strongly suggest finding a set of post '96 Vortec heads, makes the 265 > 310 hp bump, you are 6 gaskets away at that point.  Excellent ROI & should be able to scavenge some used ones.

@WOODSKI I just missed a set of Voretc heads for $150...doohhh!! Though a set of Gm Performance heads 12558060, just popped up for $400.  I might turn the other motor into a 383 stroker for this ....

I thought the Vortec Manifold is different?

Edited by Fabricator909
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@Fabricator909:  You will have to watch the bolt pattern in the heads, some are vertical some are perpendicular to surface.  Bummer on missing the heads.

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17 hours ago, Fabricator909 said:

@WOODSKI I just missed a set of Voretc heads for $150...doohhh!! Though a set of Gm Performance heads 12558060, just popped up for $400.  I might turn the other motor into a 383 stroker for this ....

I thought the Vortec Manifold is different?

Vortec heads are a lot of bang for the buck.  And yes, they do require a different intake manifold. 

The 383 stroker is great for lowering the RPM of the power curve.  This give you the ability to use a lot of ballast while keeping the RPM down.

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Alright guys, got the new motor in and fired up, only issue seems to be the carb is actually running out of gas, it will run and idle, rev for a bit then die. It will start right back up once after a second, then do the same thing. It has an electric fuel pump, and obviously is getting fuel, wouldn't think it couldn't keep up with the carb?

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maybe floats out of adjustment??

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1 hour ago, Rednucleus said:

maybe floats out of adjustment??

The carb ran fine on the other engine, never touched it besides pulling it off, and putting it on. I did put on a new fuel/water seperator, there isn't a bleeding procedure is there?

 

Got it, a lose wire on the coil causing the fuel pump to shutoff.

Edited by Fabricator909
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