Hey fellas I am still having issues with my '84 mercruiser 260… I have emptied fuel best i can and added new non ethanol fuel. I have installed a rebuilt carburetor. When the boat is cold and first launched it fires up very quickly. but if you cut it off without letting it warm up the only way to crank is to open the front flaps on the carburetor. thats one issue.
The main issue is when running after first starting, the first little bit it runs great. Sounds good gets up to decent speed and whatnot. Then after maybe 15 minutes and I assume when its she gets hot (temp gauge stays around 160 or less) it starts losing power, badly. It wont accelerate wont get on a plane, it seems like it when the secondaries should be goin is when it start dieing, after a while of trying to make it run it will backfire some. Have to slowly cruise right about idle home with it kickin and running rough. Any ideas on what it could be. Also know i have an aluminum plate under carburetor to help with vapor lock, but may need something better. Any suggestions would be greatly appreciated. Thanks
Backfiring is usually due to improper timing. It sound like your distributor is doing something strange. It could also be valvetrain issues or a timing chain stretched out. It could really be a whole lot of things. Without being able to troubleshoot them individually its hard to say.
As Tigerfin said, timing is a critical factor. Mark where you distributor is 1st and then see if it can be turned by hand. If so, it may have moved. Or a plug wire may have come loose.
Fought similar problems on my 454 Mercruiser … ended up being the fuel pump was damaged by ethanol. It pumped fuel like crazy, but the pressure regulator quit and it had too much pressure (very hard to find this problem). Carbs can only handle about 5psi. Any more pressure and the pressure over-rides the float/seal and it just floods the heck out of the engine. I rebuilt the carb and swapped the carb with the QJet from my Corvette - all with the same results, so I finally figured out I had too much pressure. I could run for a bit, and the engine would get so flooded I would have black smoke and then she would die and it would take 30 minutes with the dog house open and the carb open to dry it out enough to even start it again.
May not be your problem, but I checked and fiddled with everything else and that was it.
One test to eliminate the fuel system (and pump) would be to connect a small portable tank with good gas directly to the carb and keep pressure with gravity - with the tank above the carb (off to the side for safety) for test.
Thanks guys, I am hoping to go fiddle with here in a while. If I dont figure it out today I will work on it tomorrow some. I am going to check the fuel pump, put a timing light on it, and check the wires. I will start with those. Thanks for the help I will keep you posted.
Wellcraft V20, 175 Yamaha
Formula 25 SC Express, Mercruiser 260
