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Ask Noel Schembri Your Own Question
Noel Schembri
Noel Schembri, Marine Technician
Category: Boat
Satisfied Customers: 1944
Experience:  Advanced technical diploma
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1983 mercruiser 260 will start and idle but will not rev above

Customer Question

1983 mercruiser 260 will start and idle but will not rev above 1200-1500 rpm. Replaced coil and Thunderbolt ignition parts.
Submitted: 1 year ago.
Category: Boat
Customer: replied 1 year ago.
Still waiting almost two days now. I have cancelled my membership thanks
Expert:  Noel Schembri replied 1 year ago.
Hello my name is ***** ***** I will assist you. first thing you need to check is fuel flow to the engine, fuel lines filter etc to be clean, next is to check the carburetor and rebuild if dirty. Most probably te problem is fuel related. Keep me updated. Noel
Customer: replied 1 year ago.
Have now had the carb rebuilt. Fuel supply is strong, fuel is clean. Replaced fuel filter/water separator. Drained this and it was clear. A few small particles of debris but nothing significant. What next? Starts and idles no problem but won't rev. As you apply gas it runs rough and vacuum drops to zero. At idle vacuum is steady at 12 ins. Please respond as soon as you can I have a hard deadline to get this resolved. Tx
Expert:  Noel Schembri replied 1 year ago.
Next thing you need to check is the lift pump, It needs to be gibving aa constant 5-7 psi of fuel pressure, if it drops below that you ll have those symptoms Noel
Customer: replied 1 year ago.
Ok Noel thanks I will replace fuel pump tomorrow.
Customer: replied 1 year ago.
Noel, okay have now replaced the fuel pump. Engines runs exactly the same. I replace a lefty of exhaust hose that looked suspect and checked for an exhaust blockage all seems ok.
Need some help in a hurry here, we are planning on heading out on Friday, please respond quickly I can afford to wait a couple of days.Thanks, michael
Customer: replied 1 year ago.
Make that I can not afford to wait a couple of days.
Customer: replied 1 year ago.
Not sure it make a difference but when this happened, we were motoring along nicely at about 2500 rpm. All of a sudden the port engine slowed down to 1200 and would not go any higher. There was no indication of what may have caused this.
Expert:  Noel Schembri replied 1 year ago.
Ok will try and make up a list for you. So first thing check that the throttle cable is moving the carb linkage all the way when full throttle is applied. Next is to check that there is spark on all plugs and replace any of them that may not have any. If still without spark check the distributor cap for any cracks and or defects and hi tension. Next thing to check is thecompression of the engine, it should be around 150psi on each cylinder. Noel
Customer: replied 1 year ago.
Noel thank you for the prompt reply. I will get on the list tomorrow and let you know what I find. I am also going to hook up the starboard fuel tank to the port engine. Starboard engine runs fine so this will eliminate any fuel supply concerns if the problem persists on what I know to be a good fuel supply.
Expert:  Noel Schembri replied 1 year ago.
Ok keep me updated Noel
Customer: replied 1 year ago.
Noel, well my fuel idea was a bust, both tanks feed into the same supply line that feeds both engines.
I got going on the compression started with no 7 145lbs, no 5 150lbs, no 3 135lbs and No1 0lbs as in zero. I went back down to make sure the tester was tight in the spark plug hole and got Zero again. Not much point in checking 2, 4, 6, 8 is there.
So dropped valve, hole in piston? We had no overheating, oil is clear of water and coolant is clear of oil. There is also absolutely no clatter when the engine is idling. Let me know what you think.Michael
Expert:  Noel Schembri replied 1 year ago.
Hi Michael a hole in a piston is normally caused by detonation, that is excessive spark advance or a very lean fuel mixture. Both these conditions will destroy or melt a piston. At this stage i suggest you open the engine cy;inder heads to check the condition of the bores and from there evaluate if it is worth repairing or getting a complete long block or a new engine since it has some years now. Sorry about that . Noel

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