engine questions

[jif102]

I have a merc mcm228 (GM 305) engine ss# 4930267. I know that these have had cam problems in cars. My boat reaches about 3000 rpms and then backfires. I have no problems when out of gear or when I accelerate from idle. Some people think it might be a worn out cam. Is there anyway to tell. It has a new plug wires (carbon core), new distributor and coil, and new plugs.

thanks for any help

 

[thrasher]

Re: engine questions

Back firing at high engine speed, is often a sign of an incorrect fuel/air mixture, or incorrect timing. Have you checked the distributor and its timing? If you have a mechanical centrifugal advance, it might have seized up and that will mess up your high speed running..

Just some ideas.

Gary

 

[jif102]

engine questions

It is a brand new distributor I put in this weekend. The idle timing is good and the advance timing looked good when out of gear at 2000 rpm. I can run at 2800 rpm going 23 mph with no problem. I just can't get anymore rpms.

 

[Don S]

Re: engine questions

That problem comes up almost everyday on this forum. I have never heard one having a bad cam that ran ok up to 3000 rpm then goes bad, if a lobe was gone, you would also have a miss at low speed.
Here is a list of possibilities for low WOT rpm.

Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation (Full tuneup done recently)
• Remote control cables and linkage for proper travel to open throttle plates fully.

 

[Bondo]

Re: engine questions

My boat reaches about 3000 rpms and then backfires.

Ayuh,........... 9 out of 10 times,.....
That's a sign of Fuel Starvation in Boats,......
Usually caused by Water,+ Crap in the Fuel Filters, or Carb........

Change the Fuel Filters,+ see if it goes away.........

 

[paulrfrancisco]

Re: engine questions

Lesson time... (for me that is....)

How can fuel starvation or water in the fuel cause a "back fire"? I can understand it "mis-firing" where it missed the firing of the fuel due to it being too lean or too much water, but to fire when its not supposed to... sounds ignition to me... How does this happen? (I apologize for hijacking your thread... You may send a private email if you wish...)