I don't think a carb will help....

Also,IMHO I think "just about everyone" are wrong.
I had a 250 ES with a stock 502 that spun a 23P Mirage plus to about 65mph (GPS)
It took allot more ponies to get it to 70+mph

 

jce68 - 1boatnut is right. My boat is a 23' Wellcraft Nova. 502 cu in and 500 hp (according to Chevy). The boat weighs about 4100 lbs. +/-. Spinning a Mirage 25 Plus between 4750 and 4800 rpm. Boat does about 68 to 69 mph on GPS. With a Mirage 25 {not a Plus} I'm getting about 5050 rpm and about 70 to 70.5 on GPS.

"Everyone" told me I'd get mid 70s out of it, before I laid out the hard earned cash. I'm getting educated...one of the 1st things I learned is HP and speed does not not come cheap.

I expect that 1boatnut as well as most of the other contributors on this sight have forgotten more than I know about this stuff. But I think if you go to more carb, you'll need more cam. And regardless of what you do, you're still gonna want more. Hell...I would'nt want it any other way.

 

Another issue with this carb is that when it reaches the 3700-3800 rpm level to open the 4-barrel, it hesitates, almost stalls. If I ease the throttles up, it only slightly hesitates. But if I punch it, it just about stalls the motor and I have to back off. I was told it has to do with vacuum, so we disconnected a hose (not sure which one - I'm not a mechanic) and it seemed to help but still has the symptoms.

 

one way to find out see if you can barrow one and try it.....

 

Another issue with this carb is that when it reaches the 3700-3800 rpm level to open the 4-barrel, it hesitates, almost stalls. If I ease the throttles up, it only slightly hesitates. But if I punch it, it just about stalls the motor and I have to back off. I was told it has to do with vacuum, so we disconnected a hose (not sure which one - I'm not a mechanic) and it seemed to help but still has the symptoms.

There is an adjustment for the secondary air valve spring tension, its hard to explain on here, but if it is set too loose, the secondaries will flap open too soon causing a lean condition, and the "BOG", set too tight, and they will not open soon enough. i am still monkeying with mine, its a pain, but they say when you get it right, its better than a holley. The Quadrajets are 750 CFM, but something about variable???? Should be plenty of carb.

 

Is the carb calibrated for this engine?

By your symptoms I going to guess that this carb came off a lower hp 454. If this is the case you will need different primary jets and rods and different secondary rods.

Make sure fuel psi is correct - min 5psi at WOT and top rpm. If it is and richer jets/rods do not help power it may be the fuel bowl isn't supplying enough fuel - this is a problem with Quadrajets when supplying 400+hp engines. They do make llarger needle and seats to supply more fuel to the bowl however because there is only one small bowl (Holley's and Edelbroc's/Weber have two) 450-500 seems to be the absolute limit!

A hesitation is usally lean while a bog is rich.

Good sources for books on this matter:

HP Books - Big Block Chevy Performance
HP Books - Rochester Carburetors

The first covers some general info on which jets/rods to use and the second explains in full detail how to rebuild/diagnose/modify these carbs.

Everyone remember: a engine might be rated for a certain hp but if your ignition timing and carburetor cailbrations are not 100% correct than in no way are you going to make the same hp! I've seen 4 degrees of spark timing cause over 20hp decrease in performance! Of course carburetor tuning can cause less, the same, or more hp loss depending on how far "out" the calibration is.

Hope this helps.

 

Is the carb calibrated for this engine?

By your symptoms I going to guess that this carb came off a lower hp 454. If this is the case you will need different primary jets and rods and different secondary rods.

Make sure fuel psi is correct - min 5psi at WOT and top rpm. If it is and richer jets/rods do not help power it may be the fuel bowl isn't supplying enough fuel - this is a problem with Quadrajets when supplying 400+hp engines. They do make llarger needle and seats to supply more fuel to the bowl however because there is only one small bowl (Holley's and Edelbroc's/Weber have two) 450-500 seems to be the absolute limit!

A hesitation is usally lean while a bog is rich.

Good sources for books on this matter:

HP Books - Big Block Chevy Performance
HP Books - Rochester Carburetors

The first covers some general info on which jets/rods to use and the second explains in full detail how to rebuild/diagnose/modify these carbs.

Everyone remember: a engine might be rated for a certain hp but if your ignition timing and carburetor cailbrations are not 100% correct than in no way are you going to make the same hp! I've seen 4 degrees of spark timing cause over 20hp decrease in performance! Of course carburetor tuning can cause less, the same, or more hp loss depending on how far "out" the calibration is.

Hope this helps.

yeah, What he said!!!! I guess that would explain why when I tightened the $hit outta my secondary spring, the feeling changed, but still had the Hesitation/bog

CFM, any idea where I can get the richer jets/rods, needle/seat????

 

Thanks for the input. The original motor was a 502/415. I replaced with a 502/435. The difference in hp may be that the factory rates the 415 at the prop, while the 435 was rated out of the crate. They may be the same motor. I guess my concern or question is - should I go to a Holly carb or mess with my existing Rochester?

 

I have twin 425hp or so engines running qjets. I have adjusted everything possible on these carbs. One thing I did was adjust the air valve rchevelle referred to in 1/8 turn increments from light to heavy spring tension. Too light caused a bog, too heavy causes the power to come on slower but no real bog. Both the air valve and secondary metering rods lift when the valve opens, so I am not sure if opening too soon is a lean condition or rich. I just know it bogs when it opens too soon. Set it at 1/2 to 5/8" turn from zero tension and it will run perfect. Personnally, I am shopping for 800cfm holleys for one of my over the winter upgrades. 365hp mags came with qjets, 420 mags came with 800cfms. I think that may have been for a reason and I am going to find out.

Dave

 

Yes, that is one of the reasons.

1) 2 fuel bowls to sustain higher hp

2) square bore carbs have 4 throttle bores that are the same size - this creates better fuel distribution than 2 small primaries and 2 large secondaries which becomes more critical as hp goes up

3) easier for Merc to tune for the higher hp motors! Most engine builders will agree that a Quadrajet is a great carb for mild motors ( <300 for sbc and <400 for BBC) but once the wick is turned up over these numbers a square bore Holley will typically be better. There are exceptions of course but this is the norm.

4) GM had already figured out the staggered jetting arrangement needed with the Holleys on the GM Hi Perf Dual Plane Intake. Why staggered jetting? Fuel distribution differences from a high rise dual plane (not all runners are same length, same design, same entry + exit, etc,etc) with much of it's divider 'cut down' , and the big fact of the BBC cylinder heads having 2 different sizes and shape of intake ports! Wew!!!


Summary: All carbs have their advantages and disadvantages. Some are better for mild mannered engines while others are better for radical engines. Just because one doesn't work for a given application does not make it a bad carburetor - it means that a poor choice was made in which carburetor to use.

BTW: where to get Quadrajet parts? Your local Chevy/GM dealer should stock them or get them for you. Also, you could do an Internet search - there are many carb rebuilders/parts suppliers. Type in Quadrajet and you'll find them.

 

Thanks for the input CFM-tech. Sounds like I should go ahead and get a Holly Square Bore instead of trying to rebuild / repair this Rochester. What model Holly would be best for my application and where is the best place to buy one? (502/435, Gill offshore exhaust, everything else is stock).

 

From Mercruiser for a whole lot of money (800cfm Holley Tuned for the 500hp - will have to prob detune for your motor)

I quote: "13549T21 CARBURETOR (BLUE) 13549A15 $1,298.10 $1,076.34 "

Ouch!!!

Or from Summit Racing at www.summitracing.com
Part# HLY-0-9022 800cfm mech sec's manual choke $459.95
Part# HLY-0-80443 850cfm mech sec's electric choke $549.95
Part# HLY-0-80444 850 vac sec's elec choke $$$$?Not listed? May have to call them

BTW:
HLY-#745-224 is a electric choke conversion kit for manual choke Holley carb's
HLY-20-59 secondary quick change spring kit for vac sec's
HLY-20-13 Secondary spring assortment for vac sec's

Of course you will have to tune these for your engine but they are Holley's - ie: easy to tune! If real lazy (like me) get the Holley Quick jet change fuel bowls. It' can't get any easier than this! Just watch for fuel leaks.

BTW: Barry Grant makes a marine series of "Demon" carb's. I don't have part #'s or prices for these.

 

JCE, you're right about the hp. Those are essentially the same motor. I've heard that Merc used a different cam with more lobe separation to help keep reversion at bay, but I do not know this for sure. What is your timing at 3000+ rpm?

 

Cheif Engines, 800cfm double pumper $400. Best I've found.

Dave

 

I know Bob Teague always recommends 830 holley carbs (list 80558 marine for these types of applications. He says the secondaries start opening early and slowly on these carburetors, which helps to eliminate a lean condition that is prevalent just before the secondaries start to open on most carburetors. I think this may be the same carb as the 500hp.