Logo

Marine power 454 backfires through carb under load?

fishkiller1

New member
A little history. I purchased the boat 10 months ago and initially had an issue while trying to get up on plane. There would be a hesitation or I would even call it a bog. The engine would bog down and then pick up and get on plane. This issue slowly got worse and eventually it started to backfire occasionally through the carb. It is now to a point where it backfires through the carb at around 2500RPM and falls on its face? In neutral at the dock it revs to 4000RPM just fine and has no obvious issues. Being that the boat was new to me and that parts looked pretty worn I started throwing parts at the problem. I initially thought it may be a lean condition so I had the holley carb professionally rebuilt, new fuel pump, and verified anti siphon valves were not plugged. Same issue? I then thought it may be an ignition issue so I replaced the entire distributor, cap, rotor, ignition module, plugs, wires, and coil. Still the same issue? I then started to lean toward maybe cam lobes rounded? I pulled the valve covers and turned the motor over without the coil wire hooked up and could not identify any obvious problem. All the rockers seemed to move up and down full range and open and close valves? Currently I have moved to verifying all wiring is in good condition. I did find the harness at the oil pressure switch was in really bad condition. The yellow wire in the middle was worn through the insulation and most of the wire. I am really surprised it was getting any connection at all. I ordered and replaced the harness and when I put it back on the switch the starter would engage with just the ignition turned on??? I inspected the 3 prong oil pressure switch and determined it had to be bad due to the harness being new. I replaced the three prong switch and it seems to be working properly now. The engine sounds better than ever after basically replacing the entire ignition system. I am really at a loss? My next plan of attack is to inspect the exhaust and determine whether it is plugged or restricted in any way? Any other ideas would be greatly appreciated.
 
I am planning on doing a compression test tomorrow morning. I am doing research now on how to go about doing a compression test. I am not a mechanic but I am mechanically inclined and with the internet these days it makes it much easier.
 
how old is the fuel? did you 'stabilize it' last fall....today's gasoline has a real short 'shelf life' without help...did you verify the correct advance curve is being delivered by the distributor?
 
Back
Top