Carburetor size...

[rotor_av8]

Hello I have the 20' 1968 Seabird Cuddy with the 307 SBC and an OMC electric shift.

I looked up my carburetor, a quadrajet, and it is a 17082403. This carb is off of a 350 or a 454 Chevy marine engine from 1982 from what I gather. Oviously a replacement. What do you people think... should I get a smaller carb made for a 5.0L instead of a 5.7L or 7.4L? If it still runs good will this affect my fuel consumption even though I don't notice it in power?

I seem to be finding that this may be a 750 CFM and thing the carb for the 5.0L 307 would be a 600 or 650 CFM. Does this sound correct?

Maybe this carb will be ok because it has vaccuum secondaries and will only open what it can suck? Then again I'm thinking the setup could be all wrong because of the CFM difference.

I'm a huge fan of quadrajets and have rebuilt a few. I'm wondering though, if I get a standard carb no matter what brand to replace does it have to be for marine? Is it an issue with corrosion or for the bouncing from waves sloshing the floats around? I used to offroad and have seen a lot of offroad carbs set up for all the bouncing.

Thanks,
-Rotor

 

[John_S]

Re: Carburetor size...

It has to be a marine carb to prevent gas leaking into engine compartment, and preventing an explosion.

750 Q-jets were used on multiple sizes of engines. The vacuum secondaries will only open to what you need. On your engine, the air door is probably just openning at wot.

Since you don't know the prior heritage, I would just recommend a rebuild and make sure it has the stock rods and jets for your engine.

 

[oops!]

Re: Carburetor size...

is there somthing wrong with the carb that you dont like?

what i mean is ....if it aint broke......

 

[tombstone]

Re: Carburetor size...

A 750 is way too big for that sbc. Would recommend a 600 cfm. Partial to Holley for my taste.

 

[John_S]

Re: Carburetor size...

He would have to go to a new intake manifold for a 600, or put in a spreadbore to squarebore adapter.

Holley does make a marine 650cfm spreadbore w/vacuum secondaries. But, it will not improve performance over the q-jet.

The q-jet primary barrels are smaller and more efficient than the Edelbrock/Weber or Holley 600cfm squarebore primaries. 99% of the time, that 307 will be only running on the primaries.

 

[rotor_av8]

Re: Carburetor size...

Yeah the vaccuum opened air doors or butterflies for the secondaries open at maybe 3/4 throttle. Again I have not run the boat off the trailer but plan to fix the steering Sat (today) and then take it for it's maiden voyage with me as captain.

Still trying to think of a name....Ocean related sort of like Mighty Mouse. Meaning tough but little.

You guys (and possibly girls) on this forum are awesome! It reminds me of my old jeep club where everyone helps.

-Rotor

 

[John_S]

Re: Carburetor size...

Sorry, can't help much with names.

A couple of things about q-jet:
The secondary butterflies do start to open at 3/4 throttle, and fully open at wot. The secondary barrels will not begin to flow much fuel/air though until the air door (big plate at top) opens. That door is spring loaded, and your engine would have to reach enough vacuum to open it. It is unlikey you can get this to open running on the trailer. FWIW, the Edelbrock/Weber works the same way, except the air door is controlled by a counter-weight.

If you decide to rebuild it, let me know. Maybe someone has info on an old 307/omc. I can post the jets/rods/springs that Merc used when they put 750 cfm q-jets on 305s. I didn't find any info on 307s, but my manuals might not have covered back that far, or merc might not of used that particular engine. But, it should be relatively close.

Good luck with the naming.

 

[Maclin]

Re: Carburetor size...

I will echo that the size is ok. The Q-jet cabs can flow up to their rating, but they cover everything from idle up to their max flow, so they could be used on any engine in the 300-450 cubic inch engine. It is a good design in that respect. The secondary air door openiong rate is the key.

 

[MikDee]

Re: Carburetor size...

It's funny but, I once had an 81' Caddy El Dorado, (mock convertible) that had an O.E. computer controlled 4.3V6 Buick designed engine, that I one time removed the Q-jet to replace a TPS, and I found out it was goverened from the factory, as not to let the mechanical secondaries open completely I guess that was their way of dealing with a big carb on a small V6. I just left it that way, and put it back together, figuring they knew what they were doing in the first place. That was one of the best engines I ever had in a car! smooth, dependable, economical, & peppy, as all get out! But, that particular year vehicle, as nice as it was, had no end of electrical issues

 

[rotor_av8]

Re: Carburetor size...

K.I.S.S. Keep It Simple Stupid

Your saying is one of my favorites.

 

[MikDee]

Re: Carburetor size...

Your saying is one of my favorites.

Thanks, I try to keep it in mind, after many yrs of grief,,,lol

Many yrs ago I put a 4bbl manifold, and an unknown good Q-jet on my son's 1975' Chevy Monza 2+2 hatch back, 262cu.in./2bbl. V8 Auto, and that car became fast for what it was! It ran strong, and surprised a lot of people. We had no problems with the swap, and he didn't lose much gas milage if any.