Newly rebuilt carb, same problem...UPDATE

F

Fish Guts

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Mercruiser 260, carbed. Will not idle. After pumping the throttle until it gets warm, I can idle high and it stays running but surges up and down slowly. I'm thinking vaccuum leak,gonna hook up a gas can to rule out bad fuel. But in doing some reading, could it be a blown head gasket? No water in oil, but... Tune up was fresh at the end of last season, ran fine then. Any ideas??

Thanks,
Richard
 
A vavuum gauge or cylinder gauge would help determine if the head gasket failed without pulling the heads.
 
You mention a carb rebuild, was it before or after the problem started. Who did it? What is their experience level? How about the winterization, was it properly done and include the fuel system or was it just some antifreze in the cooling.
 
Stephen, I will do the compression test.
Mike, The problem was before the rebuild. Purchased from a reputable source, but seems not needed/not the problem. I put Sta-Bil and dry gas in the tank before the winter. The motor was out of the boat and drained while I replaced the whole gimbal/transom assembly.

Thanks,
Richard
 
Oh, one other thing... The rebuilt carburetor came with a thick, almost cardboard type base gasket instead of the thin,"real" gasket. I am thinking it seems like it would almost "breathe" causing a vaccuum leak. So thick, I almost could not thread the bolts down. Does anyone think this could be an issue?

Thanks,
Richard
 
You had the problem with the old gasket so look elsewhere . Bad gas should be eliminated then test vacuum and timing
 
After the rebuild, if your carb allows did you adjust the idle mix needle valves back and forth for highest idle vacuum with idle speed screw backed off, then set the idle speed screw to the engine idle speed spec?
Base timing set to spec in Timing mode then set back to Run mode if that is appropriate for your ignition?
Checked fuel lines and connections and filter seals for vac air leaks?
Are the fuel lines all alcohol compatible marked SAE-J-1527? Not spongy /porous?
Fuel recent & good quality not deteriorated?
Does it look like the throttle plate is closing properly? After the rebuild I'd assume that area is not fouled with carbon.
Checked dist cap & rotor and plugs and plug cable? An inductive timing light attached to each successive plug cable can give an idea of whether all cylinders are firing consistently.Firing order could be rechecked too if the dist cap or plugs have been removed since the issue began to manifest.

Not sure I'd be comfortable doing it on a boat but some aim a non-lit propane torch ( or just WD-40) at the base gasket to see if rpms change or smooth out if they suspect a gasket leak.

As Stephen suggests , a vac gauge test can potentially be very revealing. Click on the various "scenario" green buttons here to get a good idea of that if you are not familiar with the test:
http://www.secondchancegarage.com/public/186.cfm

A quick net search will also lead to utube videos of this test also.
 
Yup, sounds like some kind of leak. Can be intake leak but more likely fuel line bad or hose fitting leaking air into the fuel...like maybe the mechanic yanked on the fuel line while lifting the engine. I'm not sure about not idling unless there's just no fuel getting into the float bowl. Might be fuel pump fault...check for gas in the over flow hose going to the carb.
 
Charlie, I was the "mechanic" and know i didn't pull on anything. Are you saying you think air may be running in with the fuel? When I say not idling, I mean I have to keep it at about 1000 to 1200 rpm to keep it running.
 
Not a mechanic (and did not stay at a Holiday Inn Express last night), but in my youth when I was messing with car engines, carb gaskets were thin, not thick as you described.

I think the thick gasket you describe could be a problem - one that can be fixed relatively easy (replace with a more traditional "thin" gasket). Could also ask a marine mechanic if that gasket sounds right.
 
Some carb gaskets are thick. Some are even wedges. You need to check for the proper gasket by looking up the part from any number of websites with your engine serial number. I'd start there because you can't get an accurate vacuum test if it's leaking there. Another very unscientific test is to start the engine and then begin to cover the carb with a rag. If it doesn't stall immediately, you have a leak. another test is to start and then slowly spray carb cleaner around the gasket. If you observe a difference in RPM, it's leaking there.
 
I am really tending to believe it is the gasket. It is definitely a vaccuum leak, and that is the only place (well, except for the intake gasket). Will stop at the auto parts store to try to find a match, or at least some gasket material to make one.
Stephen, I have hit many part look up sites. They will show the whole breakdown of the carb, but no base gasket.

Thanks all,
Richard
 
What Year is this 260? I take it to be a 350 "Chevy," right? 4-Barrel Rochester QuadraJet?

I was trying to help somebody find gaskets for the carburetor on the Ford engine in an RV, and one of the gasket choices had passages open to the outside air.
 
You got the right gasket. The thick gasket reduces heat transfer.

Intake manifolds are made with a 'wedge' designed into the casting.

You can remove the studs in the manifold and replace with longer studs.

Check for ruptured diaphram in the fuel pump.
 
j-d, it's an 88, 350 mag, quadrajet,yes...

Charlie, would a ruptured diaphram allow the engine to run smoothly at 1100 rpm but no lower?

Thanks,
Richard
 
About that not idling. Was it doing that with the old carb?

That low speed idle is adjusted with the low speed idle jets(2) at the front and bottom and the carb. Could be they're clogged. The idle jets line up with the throttle plate and pull fuel only when the throttle plate is creating the venturi effect/low pressure. This effect goes away as soon as the throttle is opened past the idle jet opening.

carefully close the adjusting screw. the screw tip will deform the seat if closed too far. Open both screws about 2 turns as a starting point. Slowly close one side until the idle falters. Then back out 'til the idle falters again. Final adjustments should be done with a vacuum gauge but can be done with 'feel'. Slowly turn screw in allowing 5 seconds between each 1/8 turn to let the engine 'settle in'. You're trying to acheive the highest vacuum readings and highest rpm in the idle range of 650 RPM. You might have to increase or decrease idle to get to that 650-750 RPM range.

Do the same thing to the other screw. Then go back to the first screw and adjust again.

Check for ruptured diaphram in fuel pump. We really can't advise or diagnose until fuel pump is working properly.
 
Hey Charlie did you ever cross a few wires close the choke and backfire blow out the idle jets??
 
I looked at a few for GMC pickups with 5.7. Last one that was 4V carb was 1987. Later was TBI. Carb gasket looked like this...
2

...and it makes me wonder how effective it is to adjust individual idle jets.
Anyhow, Charlie's comment about the Fuel Pump is intriguing. If a MARINE mechanical fuel pump leaks, it'll send raw gasoline up to the engine intake. That'd make it crazy rich at idle and low speeds. At high speed the V8 is either so busy guzzling gas that you wouldn't notice loss of much more. OR it'd starve because the leak meant the pump wasn't delivering.
 
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