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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10968
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Chevrolet Mercruiser: , I'm Neil. I just installed a rebuilt

Customer Question

Hi, I'm Neil. I just installed a rebuilt q-jet on my boat , a 454. When I start the motor, it revs high for a second or two, then dies. Vacuum?
Submitted: 2 years ago.
Category: Boat
Expert:  Tan replied 2 years ago.
Hello Neil. My name is***** you replace the base gasket? Some of the base gaskets are not correct for marine use and allow a vacuum leak. Are the throttle plates completely closed?
Customer: replied 2 years ago.
Yes, I replaced them. Two gaskets came with the carb (marine version), one a standard thickness paper (Felpro) and one a harder material about 1/4" thickness. The carb has a wedge because of the motor slant. I put the thin gasket under the wedge, against the manifold, and the thicker gasket between the wedge and carb, no sealant.
Expert:  Tan replied 2 years ago.
Then those gaskets are correct. Are the throttle plates closed? Spray some wd 40 around the base of the carburetor does it change the idle?
Customer: replied 2 years ago.
I'll check the plates to make sure they're closed. As for spraying the base, it won't idle at all. I have to pump the throttle a few times for it to start, and once it does, it revs high (with throttle lever just cracked open) and then dies within a second or so, sometimes popping back through the carb when it does.
Expert:  Tan replied 2 years ago.
Ok what do you have the idle circuit jets at the base of the carb set at? They should be set at 1.5 turns from lightly seated. Then start engine let warm up then fine tune the jets to achieve the best idle. High revving is either advanced timing, vacuum leak or fuel starvation. Now we need to make sure the throttles are functioning correctly opening and closing freely. Gasket can cause the plates to stick and if the wedge is on incorrectly it will cause the same with throttle plates. The wedge is slopped on one side and is 90 degrees on the other. The 90 degrees faces up the slope goes down towards the intake. These are the only things that will cause the issue you are having. Post back with findings and questions. Thank you..Tan
Customer: replied 2 years ago.
OK, I'll check the plates and idle adjust needles, and go from there, and then get back. The wedge is marked with "TOP", and sits right.
Expert:  Tan replied 2 years ago.
Ok many of the wedges didn't have any markings. Post back with findings and we can go from there. Thank you.

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