Need help with Mercruise and Quadrajet carb

I got my first I/O boat this year and the Q-jet carb is giving me a fit. The boat is a Wellcraft Nova, engine is a Mercruiser 350/260 horse. The boat will cruise/putt/putt along all day, but as soon as you open it up to ski, pull tubes, etc. you plane out about 50 yards and the engines quits. I put it in the marina shop, they replaced the fuel pump. Back out last weekend, it did the same thing. The mechanic thinks the carb is running the bowls dry, thinks the float is damaged or not set correctly? I don't know much about the Q-jet, but on the paddles I take it all the way to 5000 rpm and the secondaries flap never opens? The big bowls in the rear are bone dry. Car mechanics tell me the Q-jet will not open by rpm in nuetral, mine will not open even with the prop turning? There is a strange little "can" to the lower left of the carb that is linked to the secondary flap, is this a vacuum advance? Could that be why they are not opening? I think that the 4 barrel never dumps, and when I give it throttle it just runs the carb out of fuel. Help, I don't have $500 a week to throw at the boat mechanic!

Re: Need help with Mercruise and Quadrajet carb

Ayuh,... Welcome Aboard,... Sounds like it needs to be rebuilt, or replaced...

Merc is now using their own version of the Carter AFB, Edlebrock Performer carbs..

Re: Need help with Mercruise and Quadrajet carb

Joe, when you crank the engine and rev it. Does that little bowl thing every pull the rod that is connected to it?(Which hooks to the secondary).

It should have a hose running from it to the base of the carb, for vacumn. If it's never doing anything, I would start with replacing it. I believe they are under 20 bux at O'Reilly.

I don't know how the q-jet would know difference between neutral and in gear. I'd unhook your throttle linkage and rev the carb by hand with the engine breather off. Watch the vacumn advance, and the secondaries and go from there.

Re: Need help with Mercruise and Quadrajet carb

The vacuum canister is a choke pull-off. The air valve will not open reving in neutral. What is it doing at 4krpms on the water.

Re: Need help with Mercruise and Quadrajet carb

secondary air valve windup spring adj. usaually this requires going out on the water and under a load. make small adj. until they open. if boat is under propped sometimes they will not open. messing with this adj requires some mechanical exp. seek out a good mechanic if you have any doubts

Re: Need help with Mercruise and Quadrajet carb

If you are running the bowls out of fuel, and have checked everything suppling the carb, time to rebuild, or replace, as Bondo suggested. If you are mech inclined, the Merc service manual is fairly easy to follow the rebuild and adj procedures. A carb kit plus new float, plus very thorough cleaning, using carb dip. Also, there are some well plugs that may be leaking, that will need some epoxy.

I wouldn't buy a rebuilt unless you know or have high confidence in the rebuilder.

New, the Edelbrock 1409 may require an adapter for your intake, and will require fuel line change. Holley has a marine 650cfm spreadbore (vac 2nd) that may bolt right on. While the 1409 will be the closest to a bolt-on, all will require idle mixture, idle speed, and choke adj. But, internal rod/jet changes may be required.

Re: Need help with Mercruise and Quadrajet carb

Joe, I would check to make sure the secondary throttle shaft isnt stuck,with the engine off unhook the throttle cable,hold throttle wide open,hold secondary flap open on top, look down at the throttle plates and turn the shaft. make sure it moves freely and opens all the way. I would also help to know if the boat pulls hard getting on plane and then stalls or is there a lack of power as soon as you go wide open?

Re: Need help with Mercruise and Quadrajet carb

Thanks for the response. We tried a tip yesterday that seemed to work. Boat has a right and left fuel tank on a manual switch valve. I have only ever ran the boat on one tank. I switched tanks, hooked up the earmuffs, and ran the boat for 20 minutes from idle to gunned up to 5 grand and everywhere in between, never missed a beat. I waited a half hour, switched back to the other tank and repeated. within 5 minutes it coughed and stalled out. The mechanic that installed the fuel pump thinks there is debris blocking the pickup screen in that tank. Does this sound like a possibility? Also, Skipper2b, it pulls like a monster with no hesitation, then just dies like you flipped a switch.

Re: Need help with Mercruise and Quadrajet carb

sounds like you might have found the problem,I would stick it in the water and try it out. secondaries not opening shouldnt cause the engine to die. also 5000 rpm on muffs is not advisable.check out the sticky at the top of the forum page and do the fuel system checks.

Re: Need help with Mercruise and Quadrajet carb

I think that may be it, I ran it this morning at low rpm for 30 minutes, no hiccups at all. I think I will look at the pick-up screens in both tanks as long as I am there. I have a few questions. The valve to select right or left fuel tank is an arm with a paddle on one end and a point on the other. I am assuming the point end, in the up or down position, "points" to the hose that is drawing fuel, I am also assuming that the middle position is fuel off. Am I correct there? The other possibility could be middle position has both tanks drawing fuel, but I don't know. Next, how hard is it to remove the fuel pick-ups from the tanks? They are original Wellcraft aluminum tanks, and probably have anti-syphon valves. I just want to know if I need any special tools before tearing into it? Lastly, my engine idles nice at 900 RPM, but alot of reference is saying 6 00 or 700 is correct, does it really matter? Thanks

Re: Need help with Mercruise and Quadrajet carb

Please see questions in my last response, if anyone can answer I would really appreiciate it. I'm trying to get her straight by the end of this weekend, thanks.