Loss of power & backfire issue

[bbgun24]

Hello all,

I have a 1990 OMC 4.3L HO w/ the spitfire electronic ignition. I just rebuilt the carb, ran the boat on the hose and it ran very well. Took the boat out to the lake and ran great for about 10 minutes at 2500 rpms. It then started to slow down and when I tried to give it more throttle it would backfire. I slowed down and the boat idles just fine and I can plan back out again for about 30 seconds then it will start slow down again. I looked at my temp gage and it was close to 200, which I know is too high. I replaced the thermostat and impellar as both probably needed to be done anyways. Im curious if the slight overheating would cuase this loss of power and backfire issue?? Could it be something in the carb??(power valve maybe??) It seemed to run fine until it started getting hot. I ran the boat on the hose last night and it will idle and rev. up all day with out a problem. Gonna go to the lake tonight for the true test. Thanks in advance.

 

[Idlespeedonly]

Re: Loss of power & backfire issue

I would check the timing and fuel supply. When th power valves go they will usually run really rich. If the timing is of it can cause what you mentioned. But also fuel starvation can cause it also. You said it runs good for 30 seconds. The first thing that comes to mind is you are running the carb out of gas.

 

[Scott Danforth]

Re: Loss of power & backfire issue

backfiring can be from ignition or fuel. things in the ignition that will cause it - bad cap, bad wires, bad coil, bad distributor.

an advanced ignition timing will cause the engine to run warm, and cause backfiring (an probably detonation).

check your base timing.

running it on muffs just means it turns, there is no load on the engine. it will do that with half the plug wires removed. you need the load of actually having the boat in the water underway to determine if you fixed the problem

 

[bbgun24]

Re: Loss of power & backfire issue

The boat will idle around the lake all day. Does that mean I should lean towards not enough fuel delivery?? Would it still idle fine if there was an ignition problem?? Since it has the lovely S***fire ignition all the timing is electronic so Im hoping fuel delivery is the problem.

 

[Idlespeedonly]

Re: Loss of power & backfire issue

No it could still be ignition related. I am leaning towards fuel. But Scott is correct also. People dont realize this, but the higher the combustion pressures are the harder it is for the spark to jump the gap. So at idle with such low pressures a misfire or faulty ingnition system can easily be hidden. As soon as the pressures get up there is when ignition problems will start to develope.

 

[Scott Danforth]

Re: Loss of power & backfire issue

the base timing is still set by you. put a timing light on it and find out what your advance is.

agreed, if it runs great for 30 seconds then bogs down, you have a fuel delivery issue.

 

[bbgun24]

Re: Loss of power & backfire issue

Ok, I will check the base timing. Im going to change my fuel filter as well. Thanks for the quick responses.

 

[Silvertip]

Re: Loss of power & backfire issue

When people mention "fuel dellivery problem" they mean not enough fuel is being delivered and the fuel filter is not the only reason that can happen. A weak fuel pump is also a likely cause as is a stuck anti-siphon valve (if the boat has one), as is a clogged pickup in the fuel tank, collapsed fuel line, etc. Pressure and volume test the pump. Since you will also need to check timing, you also need a service manual specific to your engine. Without one you will be back here repeatedly and getting a multitude of opinions - some which may actually be valid. (I say that tongue in cheek folks -- don't go ballistic). I've been wrong before -- once I think!!!!!!!!!!