Carburetor size

[Ralph Savarese]

I am currently running a Q jet that I know is supposed to be rated at 780 CFM
I have performer rpm Q jet manifold with a H272-2 Crane cam and 1.6 rockers on the intakes . The Carburetor is shot and wanted to replace with an after markert but want to select the best without a astronomical price What would you guys recommend? I am willing to convert to a square bore manifold if nessesary.
Thanks Ralph

[boldts]

Ralph - I know you wouldn't do this, but have seen it in the past. Whatever you do, don't go down to the local speed shop and buy a car carburator. There is a difference between Marine and Auto carbs. (float bowl vents are directed back into the carb rather than out to the air) Thus, the cost difference in most cases also. Do it right and spend the extra for an approved Marine carb. It could save you or someone else from a possible death from an explosion or engine fire.

[BigGrizzly]

Simple question what size motor do you have? If it is a 350 your too big. Some people as in most over carburette their engines. We have gone over this before so I won't explain but you probably don't need more than a 600 cfm. My old stock 454 merc has a 750 cfm. on it. my Corsican has a 351 ford Cleveland with a bunch if Hp and uses a 600 cfm. my 502 blower motor with 690 hp has a 825 cfm. and it goes pretty well so take it from there. the big carb covers up the mistakes that tuners make. big looses velocity and uses extra fuel. so gake it from there. As for what brand this is up for grabs. Some combinations run better with Eldebroch(carter copy) some with Holley and some with Barry Grant Demons me I prefer Holley and BG, for me they are easier to work on, but that is me! As for horse power if the carb is set up right no brand is better than the other, its just the application.

[Ralph Savarese]

I know there are some differences in marine carbs but nothing substantial that I feel would be a safety issue. There are not that many kinds in marine selections. I could probably make the nessasary mods. myself. So based on what I am seeing a 650 cfm would be fine.
Ralph

[TheFees]

Vacuum or mechanical secondary’s...Holly or Rochester Quadra jet...Car or Marine...New or older carburetor
I have a 1987 all orig 18 classic 350 Magnum with the Quadra jet. When I called Donzi back in 1989 and asked them about installing a Holley, at the time they said, "Use the Quadra jet because the Holley has poor idling characteristics" That is before Holley put out a marine carb, I understand that many years ago they did. The Quadra jet has such small primaries that I go out for a day and burn 9 to 15 gallons of fuel. (I don't drive all day, but lay into it as much as I want) As Donzi put it, the 18 does 64.1 mph with an OMC, I have the Mercruiser which is 3mph faster, and the speed prop gives you another 3, which puts me at 70, which was verified with radar.
Maybe someone can help with this, but marine carbs have a coating on the inside.
I had a 69 Camaro with a 350 in it at the end of 69, the car was 10 months old when I got it, and came with a Quadra jet. I converted to a Holley 850cfm quad, with an Edelbrock high rise, Mallory dual point distributor, and my friends told me I was going to bury it with a 355 rear and a mild cam. Oddly enough it worked just fine, due to the fact that it had vacuum secondaries. So at low revs only the primaries would open up, then as the revs built up, the secondaries would kick in. Mechanical secondary’s would open up completely right away and give you way too much fuel.
I'm not an expert on boat carbs, (I guess you can tell) and the other reviewer in this post appears to have the sizes down really well, I just am curious about how the technicals have changed, especially how they pertain to marine applications. So if anyone has some comments on these particulars, I would be curious to learn. Your input is appreciated. Thanks.

[Brian Becker]

Id run a holley or demon as they have better fuel distrbution at WOT Also vacum secondaries Quads work good thats just my preference. 600 or 650 is fine

[BigGrizzly]

Thefees and others, most of what you have heard about the idling and the coating is was BS. The real reason the the over size worked partly was as you said the vaccume secondaries and the power valve in the Holley. As for coatings this was happening in the 1970s to all carbs because of fuels and corrosion. The guys at Donzi that you talked to just didn't know or just didn't want you to change it. The marine carbs have the vent tubes dump into the ventory and have tighter Teflon seals om the throttle shafts. They also are jetted a little fat. Have I ever put car carbs on boats the answer is yes a lot in my early years lucky no mishaps. Remember in the early years there were no CG regs to go by, so know one knew or cared. Now I don't do it because it is easier to buy a marine carb and jet as opposed to modify and jet. The problem is in a closed compartment you don't see it and you don't smell it until too late. I learned from an old real captain is the bilge blower is good but your nose is better so use it. CG regs were used to save you not just to PO you. I always look the engine over before I go and after I come back and check for leaks. Choice is yours, I have been lucky.

[MOP]

Ralph I am curious why you want to dump the Qjet, I have alway liked them they seem to be less of a hassle then many others. Cruise fuel economy is better then others and hammer down the man hole covers supply plenty of fuel to me that is a good combo. I am getting around to reworking one for my stroker as is Gcarter, there some excellent articles on fine tuning them.

Phil

[mphatc]

Randy is correct . . absolutely don't run an automotive carb on any boat . .

IF something was to happen involving a fire, :crossfing let 's all hope not! . your insurance will likely not cover the incident . . the same with fuel pumps . . . . .

marine fuel systems have their own rules . . all with merit . .for the safety of everyone. IMO, not worth saving a few bucks . . .

Mario L.

[Rootsy]

air bleed and orifice sizes, internally, are different between an automobile and marine carburetor due to load curves, etc etc etc...

[LKSD]

I am personally partial to Holley or Barry grant marine carbs with a mechanical secondary. The Demon is a little more tuneable than the Holley but they are very similar. I like the Mechanical secondaries because there are no diaphragms or BS to deal with. The fuel is there when you need it minus the bog. BTW if the weber & q jet stuff is soooo great then why do top builders use the holleys & barry grant stuff if it isn't injected...:) I think if you went with a 650 on your application from BG or Holley and a mechanical secondary you will be happy.. If you need one give me a shout I will try to hook you up with one.. :) jamie / Lakeside

[Walt. H.]

Ralph,
I retired my 20 yr old Rochester Q-jet for the same reason on my 350 chev and replaced it with a Holley Marine Q-jet/Spread bore 650cfm with vacuum secondaries from Jegs and I also installed a 1" phenolic spacer block so that the front bowl inlet fitting would clear the mercruiser thermostat housing.
Plus it gave my stock manifold a lift.
What a freak-en performance improvement it made right out of the box without the need of any jet changes or secondary spring mods, no more flat spots when you hit the throttle from mid-range and the engine just comes to life from the no-wake zone by jumping right up on plane with no hint of lag anymore!
With this change I picked up another 300 rpm's and a few mph via GPS and I was able to step up from a 23" pitch prop to a 25" pitch which in turn gave me another 2 more mph, so my best mph so far has been a flat on 73.0 mph.
I've sent my current prop to Julie @ Throttle Up in Fla, last month and Matt hopefully has worked a little magic for me for this coming boat season by Labbing & adding one more inch of pitch, and also removing the defuser ring and Boy OH Boy does it ever look Good!.
Anyway heres the Jegs part # for the Carb: 510-0-80552 price:$369.99
The Mr Gasket spacer block p/n#720-3406 price: $29.99 this also includes the longer required studs, nuts, washes & gaskets.
http://www.jegs.com/webapp/wcs/stores/servlet/category_10001_10002_12755_-1_10271
This carbs fuel line inlet is in the same location as your stock Rochester so you can even use your same fuel line, but the fitting that came on the Holley had a smaller female thread for the line fitting but I was able to swap the old Rochester fitting over for a direct fit to the Holley. surprise surprise!
Except I did have to shorten the thread length by a 1/4" so that the internal threads wouldn't bottom out into the float bowl.
I also had to fabricate an aluminum extention plate to use the stock throttle cable bracket so that it would clear the rear Holley float bowl, other then that it was a piece of cake to do and the best money i've spent in a long time. :checkered
Of course having Geoo's old flame arrestor on top i'm sure has also brought me some good performance luck! :yes:
I hope this info will be of help, and
Have a Safe & Happy New Yr's Eve & Day!
Walt :party:

[Rootsy]

i donno kids... i know of a certain SS/IA 69 Camaro knocking on the 9.80's... with a 4MV quadrajunk between the valve covers...

[TheFees]

Great pictures of your Camaro. Gives me the DT's for mine. I bought it only 10 months old, it came with the SS package, also had hideaway headlights, 350 300hp, 355 posi rear, and a Hurst Mystery shifter, with a competition plus bottom. The thing that shocked me was when the numbers were looked up on the motor, it was an LT1 and had high compression with 4 bolt mains. At that time the hot motor was the 302 which came in the...what was that the Z..? The 350 was a 2 bolt mains I understand, and what some say may have happened is that they ran out of standard 350's that day, so they started putting in the bottom end LT1 vette blocks in. Anyhow, when I made that conversion the thing turned into a monster. Not to the tune of the one you have. I ran it at Cecil County as a stock car and it turned in the high 14's. I ran against my friend in his 66' Corvette. He beat me. When I installed the 850 Holley, manifold and distributor combination, it increased the performance so much, my friend refused to go out on the road and race me.
What really makes me sick is that my Camaro in 69 only cost me $2400 with only 10,000 miles on it, only 10 months old. It went new for $3600. It had the pots in the hood, it was maroon, with the lighter shade of red stripes on the fenders into the doors. (Factory set up) I had it for several years, and sold it for $1875. I had a shop for 20 years, and had a customer bring one in to have it fixed up, and he paid 5 grand for it, and it needed everything. It barely ran. Makes me long for the 60's again. Now their is Donzi, and I don't ever plan on selling it. Nothing matches catching air in a Donzi, although I must admit a custom worked 69 Camaro catching air doing a wheelstand isn't too shabby, must be a hell of an adrenalin rush too.

[Cuda]

If I'm not mistaken, all Q jets are 750 cfm, big block or small block. They have different metering rods as I understand it. You should be able to find a used Q jet in good shape for very little money. Heck, I've got an extra one, and about to have two extras. I've got an Edelbrock on the 350 in the Minx, and it works well, but I had to have it rebuilt because it sat so long. I would have rebuilt it myself if it was a Holley, but the Edelbrock had about ten thousand pieces to it, whereas a Holley has a handful of parts in a rebuild kit.

[BUIZILLA]

If I'm not mistaken, all Q jets are 750 cfm, big block or small block. from '71 on they were 830...

The 350 was a 2 bolt mains I understand, and what some say may have happened is that they ran out of standard 350's that day, so they started putting in the bottom end LT1 vette blocks in. wrong, the LT1 didn't come out until 1970, no way it found life in a '69 breed...

JH

[Ted Guldemond]

I respectfully disagree with those who favor junking the Q-Jet. It is much more tunable than a Holley. Between the wide varity of 2 and 3 step metering rods, hangers and spring kits, the air fuel ratio is almost infinately adjustable. The accellerator pump has multiple settings. The secondaries have an outer trap door to help seal the closed venturis against vacuum leaks. None of these features are found on a Holley. The Q-jet has gotten a bad name because of a lean bog that usually occurs when those large secondaries open up letting in a lot of air and not much fuel in until the venturi effect starts to produce its vacuum signal. This can be minimized by delaying the opening of the secondaries (also adjustable). The Q-jets limitations include a single small float limiting its use to under 450-500 HP. In a car the primary throttle shafts wear and cause vacuum leaks. This would be rare in a boat since the throttle shaft does not see the movement that a car sees. Don't get me wrong, I run a holley on my 19 footer and it's a great carb. I have Q-jets on my cruiser and they have always worked well. I would go the cheap route first and try rebuiding it before I buy a new carb, manifold and flame arrester. GM and Merc sure have successfully used a lot of them. Sorry to run long.