A Couple of Questions before 5.7L to 350Mag

[John_S]

I will going to get the boat out of storage, this weekend. One of the projects planned, before the boat hits the water at the end of May, is converting it from a 5.7L to a 350Mag. I have the cast-iron highrise and a 750 Weber that came of an early 90's Mag. I rebuilt the carb, and added an electric choke from a trashed Edelbrock 1409. Only nervy part was drilling the hole in the base for the vacuum pull-off. <br /><br />

<br /><br />As I waa going through the final list of misc parts, I noticed a shim plate that is used on all 350 Mags with webers. It is not one of those square to spreadbore or carb leveling adapters. It appears to divide the plenium half, into quarters. Kinda like a four hole spacer that fits down into the intake. It is item 21 at this web page Does anyone know its real purpose? I can't imagine that merc would have added it, without a good reason. Kinda pricy, anyone have one laying around th shop?<br /><br />Another mystery to me, is the ears on the Mercruiser webers, that are not found on the equivelent Edelbrock marine carbs. They are item above item 5 (not numbered)in this web page Any thoughts on their purpose?

 

[Bondo]

Re: A Couple of Questions before 5.7L to 350Mag

I can see the little ears on Your picture,....<br />They do 2 things,....<br />1 is they retain the Needles,+ Springs in the Jets,.....<br />And,... They somehow control the Air/Vacuum signal to the carb........<br /><br />The Plenum divider Plate,.. I'm Not sure,....<br />My 4.3l Doesn't use one,...<br />But,... I gotta believe that they also control the Vacuum signals to the Carb for low speed operations......<br /><br />I Don't Know what the ramifications will be for Not installing one.......<br /><br />Using the 750cfm rather than the 625/650cfm carb is going to be borderline Over-Carbed.....<br />Maybe that's Why my smaller carb doesn't use it........

 

[Dunaruna]

Re: A Couple of Questions before 5.7L to 350Mag

What happenned to - 'any grease is better........"?

 

[Bondo]

Re: A Couple of Questions before 5.7L to 350Mag

Ayuh,..... Well,....<br />I've been wandering down to D/C..........It seemed to fit Better...........<br /><br />And,.... Yes,.. I'm having 2nd thoughts about the change........

 

[John_S]

Re: A Couple of Questions before 5.7L to 350Mag

Bondo,<br /><br />Thanks for your input. The divider plate is not used on any of the Q-jet mags or any of the lower profile intakes for the L or LX, from what I can tell. <br /><br />

Using the 750cfm rather than the 625/650cfm carb is going to be borderline Over-Carbed.....<br />

Certainly, if they were mechanical secondaries. The LX and 350 Mag carbed mercruisers, all appear to use 750 cfm carbs, either weber or q-jet. At only 5000rpms, the secondaries are never providing full flow. The primaries on the weber 750 should flow 375cfm, which is still smaller than the 2brl 5.7L, which I believe flows about 450cfm. I went with the mercruiser 750 weber, with the highrise manifold, because I didn't want to change/adjust needles and jets. I think the 600cfm on a 4.3, might be considered just as much over carbed as the 750 on the 5.7, if it was mechanical secondaries. <br /><br />Anyway, leaning toward getting the plate. Anyone know of an aftermarket source of a similiar plate? $35-$40, seems a bit high for the merc part.

 

[Bondo]

Re: A Couple of Questions before 5.7L to 350Mag

I have the Merc./Weber/Edlebrock/Carter Carb on My 4.3LX,.... Merc.'s version of it anyway,.....<br />And, I can tell you that,...<br />It's Over Carbed,... But the 600cfm version is also a Spreadbore body,.... Small Primaries.....<br /><br />Because of this,... I'm going with another copy of this Carb. on the 327SBC Stroker that I'm putting together......<br /><br />The 750cfm version is a Squarebore,....Large Primaries,+ 2ndaries......<br /><br />Considering the fact that a 350cid can only displace 500 or so inches at 5000rpms,.. Even with an OPEN plenum,...... <br />I believe the 750cfm is Too Big......A Smaller Carb will be Crisper in response to Vacuum signals....<br />So,... Even though Merc thought it was the Right Carb,.... I Don't...<br /><br />If you do Any trolling,... You'll soon find out that Merc doesn't Jet them Properly Either......<br />They're WAY to Fat at Idle,+ all thru the low speed circuit.....<br /><br />When I 1st did My Repower,.... <br />Trolling with the breeze would Gag you with Raw Gas Fumes,+ Blackened the Transom,.....That was After a Carb Rebuild.......<br />I Tried to get different Needles or Jets from Merc,... No Go,... <br />Only the pieces They Used are available,+ Of Course,.. Co$t an Arm,+ a Leg.....<br />But,.....<br />I finally found a Carter Strip Kit,.... part # 10-201.....cost me about $40.,.... <br />Nowadays I see a Street/Strip Kit available from Edlebrock,......<br />These are FULL of Needles, Jets, Springs,... Everything for Tuning the Fuel Curve,.....<br />1 of these kits,+ This online Carb Manual,.........................................<br />Will make a Merc/Weber/Edlebrock/Carter Clone Carb,.............. <br />Run like the Wind,.. while Purring like a Kitten.......

 

[FreeBeeTony]

Re: A Couple of Questions before 5.7L to 350Mag

What else are you replacing besides the intake and carb in your upgrade to a Mag?

 

[John_S]

Re: A Couple of Questions before 5.7L to 350Mag

Flame arrestor, fuel line, throttle bracket, and possibly ignition module and prop. Couldn't find any other differences in part numbers. Outdrive is 1.47 ratio.

 

[FreeBeeTony]

Re: A Couple of Questions before 5.7L to 350Mag

Does it already have roller lifters/cam?

 

[John_S]

Re: A Couple of Questions before 5.7L to 350Mag

According to the '97 Mercruiser brochure, even the 5.7L had roller lifters/cam. Won't know for sure until I remove the intake, though.

 

[John_S]

Re: A Couple of Questions before 5.7L to 350Mag

But the 600cfm version is also a Spreadbore body,.... Small Primaries.....<br />

The primaries are 250cfm to the secondaries 375cfm. While smaller, it is hard to call that a true spreadbore. It requires squarebore gaskets and parts. But anyway, I understand your point. While I don't fish with this boat, I do drive a mile of no-wake zone, to and from lake. I considered a Q-jet, but finding a good one, worthy of rebuild, is getting hard. Also, its reputation of being harder or having too many ways to adjust it, scared me off. The Holley q-jet 650cfm marine, seems nice. I do know it has gaskets below the fuel bowl line, but don't know if leaking is a real issue with them. I didn't have any leaking issues in that area on Holleys, back in the day, on my Nova or Charger. <br /><br />PS: weather doesn't look that great to get the boat, this weekend. rain and cold. Wed, may be a good day to take off from work

 

[FreeBeeTony]

Re: A Couple of Questions before 5.7L to 350Mag

I'm sure a '97 would have the roller lifters/cam...<br /><br />I have the Edelbrock 1409..........Also have a Weber that was recently rebuilt. ALso have the cast iron high rise manifold. We could have worked out a deal..........<br /><br />If you remember I was throwing parts at my 350 Mag last season in troubleshooting my low RPM issue.

 

[Bondo]

Re: A Couple of Questions before 5.7L to 350Mag

If you remember I was throwing parts at my 350 Mag last season in troubleshooting my low RPM issue.

Ayuh,......<br />And I sure Wish you'd Splash that thing,......<br />So we can See if the Issue is Resolved................ <br /><br /><br />Tony,... Is your Weber the 600cfm or the 750cfm,..??...<br />The 600cfm will have Different Sized Primaries,+ 2ndaries..... The 750cfm has 4 Equal holes.....<br /><br />If it's the 600cfm,...... Maybe You,+ I can make a Deal..........

 

[John_S]

Re: A Couple of Questions before 5.7L to 350Mag

..........Also have a Weber that was recently rebuilt. ALso have the cast iron high rise manifold. We could have worked out a deal..........<br />

The original manifold is in nice shape......I might be looking to part with the heads, manifold and carb when all is said and done and the boat is running right next spring.<br /><br />

Sorry, I couldn't wait till Spring. Wanted to rebuild/clean carb, and prep and paint manifold over the winter. <br /><br /><br />How about the divider plate I was asking about? It doesn't look like it will fit a square bore intake. <br /><br /><br />PS: Can't wait to see the 's in your first post after you try out those vortec heads.

 

[FreeBeeTony]

Re: A Couple of Questions before 5.7L to 350Mag

Hey bondo.......<br /><br />I hope to be in the water in 2-3 wks. I'm pretty confident that I will be good to go.<br /><br />I believe my Weber is the 750CFM...primaries look like the same size as the secondaries.<br /><br />Guess you remember my situation JohnS........looks like you found one of my earlier posts..... <br /><br />I'm looking forward to trying out the Vortecs also!

 

[John_S]

Re: A Couple of Questions before 5.7L to 350Mag

BabylonBoater, <br /><br />Did you have a divider like item 21?<br /><br />

<br /><br /><br />If not, I wonder if a previous owner removed it, thinking anything blocking the intake "was a bad thing". Possibly, part or all of your original low speed issue???

 

[lilmandavis]

Re: A Couple of Questions before 5.7L to 350Mag

i dont know where you live but its almost 3.15 for a dang gallon down here. why over carb? goin to change the cam too?? goin to vortec heads i would put a roller system up in that. its probably got one but to get the crispy throttle, the cam needs to be matched to the intake, the intake needs to be matched to the carb, the push rods have to be right, sure you could put anything on something, but to make it right- match it up. i mean thats just me though. that plate wont fit a square bore. the purpose of spreadbore is the vacum signals.. but wide open or nothing right!! thats how i catch fish, trolling at 40mph lol

 

[FreeBeeTony]

Re: A Couple of Questions before 5.7L to 350Mag

No divider here John.........<br /><br />Don't think that was my problem though. I was defintly loosing a cylinder up at ~3200RPM's.

 

[John_S]

Re: A Couple of Questions before 5.7L to 350Mag

No, I don't think it would cause issues at 3200rpms. Thought you meant you were having problems at low rpms.

 

[FreeBeeTony]

Re: A Couple of Questions before 5.7L to 350Mag

No.......at low RPM's it ran fine.<br /><br />Gonna go out an do some painting!!!<br /><br />Gotta get done what I can early.......it's my wife's wedding aniversary!!