Mercruiser 5.7 Stalling and backfiring. Chasing a gremlin.

[Baka123]

I have a 91 SeaRay 210 with a Quadrajet carb. I have been chasing this problem all summer and am looking for some much needed advice on what to try next The boat ran flawless at the beginning of the season, logging 15-20 hours a week. The boat will start and idle fine. Coming off of the lift, it will run wide open and get up on plane for about 500 yards and then it falls on its face like its starving for fuel. If I reduce the throttle it will get better. I can run at about 2k rpm with little to no problems for hours. (its been the only way I've been able to enjoy my boat this summer.) But when I hit the gas, it will never get as far as it does when it comes off of the lift, without falling off.

So far I have tried the following: Replaced ignition coil, distributor cap, new plugs, new rotor, new fuel pump, replaced the fuel lines, replaced anti siphon valve, blew compressed air through the tank vent, pulled the fuel pickup and checked for debris in tank and on pickup filter, have gone through and cleaned the carb 3 times. The 3rd time adding the components from the rebuild kit (which did not include a float, but checked the float by putting it in a cup of gas) Also, I am no pro when it comes to carbs, I did not adjust any of the adjustment screws. Also replaced the fuel/water separator. I am to the point where I think I should either send my carb off to be rebuilt by a pro or buy a new one... I am not sure what else it could be at this point.

Any help is GREATLY appreciated as the tourists have started to go home and the lake is back to being a peaceful place!

 

[alldodge]

I don't think the fuel pump can keep up with the need.
Place a fuel pressure gauge in line with a Tee and see if pressure stays up

 

[Baka123]

That is one thing I have not done as up until the new fuel pump went in, I had metal lines. The problem was completely unchanged by replacing the fuel pump though... Fuel pump is mechanical BTW

 

[kenny nunez]

Just to be sure why not get a 6 gallon outboard tank and connect it to the fuel filter and take it for a run. This will separate a tank supply problem from an engine problem. Did this problem start all at once ?
When you rebuilt the carburetor are you sure about the filter in the inlet nut is It paper or the gold looking rock material. If the paper filter is installed wrong that can also cause a problem.
You said that the tank suction tube had a filter on the end, there may also be a fine tubular filter inside the tube.

 

[Baka123]

Hook the outboard tank up to the fuel water separator input, just bypassing the tank and pickup line? Actually now that you mention it, the problem was very mild at first, and became progressivly worse. After changing the cap and rotor it went away for a while. I thought that fixed it... I have checked those components since and they appear to be in the same condition as when purchased. Filter in the inlet nut was replaced with the rebuild kit. There was no debris in the previous filter or anywhere in the tank or pickup filter. I believe the new filter is a mesh like material. It is the filter pictured although that is not the kit I used.

 

[kenny nunez]

As long as the open end of the filter faces the incoming fuel and the spring is behind and holding it against the inlet nut That is good.
Yes bypass the tank with the outboard tank hose.