350 backfires and loses power when warmed up

[marlinempress]

hi I have a 22' marlin empress with omc 800 outdrive I just rebuilt and a chevy 5.7 with holley projection 2d fuel injection system and edelbrock performer intake. The first time I took out the boat this year was a couple days ago and it was running great, good acceleration, good idle, good WOT. But then it warmed up and it started running like crap, backfiring through the intake if I gave it too much gas and missing all the time. I fiddled with the timing and it seemed to help, but I don't know if that was just because I had given the engine time to cool down. The gauge says it's running about 170-180 when warmed up. I'm thinking it's an ignition system problem but I'm not sure and definitely open to suggestions. It's the original ignition system I believe. Thanks in advance! <br /><br />Matt

 

[Bondo]

Re: 350 backfires and loses power when warmed up

My 1st Guess would be,<br />It's Running Lean..........<br /><br />Check,+ Verify the Fuel Pressure + Flow........

 

[marlinempress]

Re: 350 backfires and loses power when warmed up

Would a lean condition cause it to start running like crap after about half an hour but be fine for the first half hour?

 

[marlinempress]

Re: 350 backfires and loses power when warmed up

I was reading the holley manual for this and it says the fuel pump should be mounted below the lowest point of the tank because it's gravity fed. Well, mine isn't, it's mounted right above the engine. Could this possibly cause the symptoms I'm having?

 

[TwoBallScrewBall]

Re: 350 backfires and loses power when warmed up

Sure, especially since it's happening after you run the level of the tank down a bit. Move the pump.

 

[crazy charlie]

Re: 350 backfires and loses power when warmed up

Check the inside of the distrib cap for any condensation.that would cause it to do about the same thing.Charlie

 

[randycc58]

Re: 350 backfires and loses power when warmed up

something I tried that worked for me when I used i/o engines and had moisture under the cap. Lightly spray wd40 under the cap. Old mechanics trick used for years.

 

[crazy charlie]

Re: 350 backfires and loses power when warmed up

Be careful with an old can of wd-40,they were highly flamable!!!!!!!The new formula is not.Charlie

 

[marlinempress]

Re: 350 backfires and loses power when warmed up

Sweet thanks for the advice, I'll probably get some time to work on the boat later on today and I'll look under the cap for moisture and move the pump, hopefully being able to take the boat out tomorrow and I'll give you an update. <br /><br />Matt

 

[marlinempress]

Re: 350 backfires and loses power when warmed up

Any other suggestions to try or check while I'm at it?

 

[crazy charlie]

Re: 350 backfires and loses power when warmed up

If there is no moisture when you check under the cap,run the motor until it acts up and then check it again.I had a Searay that ran fine for about 10 minutes,then it lost power and backfired if you tried to keep it on plane.Charlie

 

[Bondo]

Re: 350 backfires and loses power when warmed up

Any other suggestions to try or check while I'm at it?

Checking the Color,+ Condition of the Plugs is Always a Good Way to find out What's Going On in the Combustion Chamber...........<br />

It's the original ignition system

Coil ??<br />Points ??<br />Wires ??

 

[marlinempress]

Re: 350 backfires and loses power when warmed up

Well I took it out, and to no avail it's still having the problem I looked under the cap when it was cold and warm and didn't see any moisture, but I cleaned it good and put some wd40 on it anyway. I moved the pump to a lower position but that didn't seem to help either, the hose outlet of the fuel tank is at the top of the tank anyway. I checked the plugs and they look fine, I checked them after idle though when it was fine. I checked the resistance of the ignition coil and it passed. I think my next move is to check the gap and dwell on the points. I know how to check the gap but can someone fill me in on the dwell? I really need to get this problem figured out and fixed by the end of this week while I have time. So any suggestions or information would be great!<br /><br />Thanks,<br />Matt

 

[marlinempress]

Re: 350 backfires and loses power when warmed up

oh yeah and also this last time I took it out it didn't really backfire, but it was missing a lot when it was acting up. If I could get it up on plane around 4000 RPMs the engine seemed as though it would cut out for a bit, missing much more than just one cylinder, then it'd kick back in. Getting up to plane it would miss a lot too so it took a while. I dunno if this helps diagnose anything but I just thought I'd add it

 

[butch290]

Re: 350 backfires and loses power when warmed up

I'd change the dis.cap and rotor button then wires and the plugs. my .02 cents

 

[trpcham]

Re: 350 backfires and loses power when warmed up

Mine has been doing the same thing! Since it runs perfectly for 30 minutes, I am figuring it has got to be something classic, not anything mechanically major. I chased it down for water, still has the problem. I let it sit a day and took it out, ran fine for 30 minutes then started pooping out. I see someone else on the forum who had the same issue and resolved by changin a blocked carb filter. So, I am going to try that. Let me know if you find the remedy! <br />Terry

 

[PeteHarris]

Re: 350 backfires and loses power when warmed up

I had a similar problem, and it turned out to be the wire from the coil to the center of the ditributor .... so I replaced all plug wires with marine type good stuff and haven't had any problems since.

 

[Don S]

Re: 350 backfires and loses power when warmed up

Check the fuel tank vents guys. If they are plugged up you will be able to run for a while then the tank builds up a vacuum and the fuel pump can't pull fuel.<br />Just go for a ride and leave the fill cap off. After an hour put it back on and see what happens. If it dies after 30 minutes, then you need to clean or replace the vent and hose.

 

[Scaaty]

Re: 350 backfires and loses power when warmed up

Put in new points, condenser, and invest in a new coil. Bad points or condenser will heat up the coil too and run like you descibe