350 Mag cutting out at WOT
 

Hey guys,

I got the boat out for the first time this year, and she was running like a top! Started out nice and easy, idled for about 5 mins, then another 20 mins of ~2500 rpm cruising, then a 2 min wide open run at ~4500 rpm. Everything was running perfectly up to that point. I cruised around for another 20 mins or so at ~3000 rpm, and then went to make another WOT run. As the RPM's came up, the motor started missing, I checked the temp and oil pressure, and they were fine. I dropped down the throttle, and it started to smooth out the closer I got to idle. I just idled back to the launch with no problems.

Any ideas? I'm thinking it may be a fuel filter or pump, but I'm not sure. I'm going to swap the filter in any case, it got skipped during this seasons maintenance.

Since last season, I did the following maintenance:
Cap, Rotor, Plugs and wires (Plugs gaped correctly)
Compression test (155-160 in each hole)
Oil
Drive lube
Impeller

Thanks guys
-Rob

Could be a little water in the fuel. Change the fuel/water separating filter and dump the contents into a foil pan and look for water bubbles at the bottom of the pan. Are you running fresh fuel or last year's gas? Also, you should be turning 5,000 RPM at WOT on a 350 MAG (assuming it is an MPI motor).

I'll check out that separator. It was last years gas with Sta-Bil.

Sorry, forgot to include my boat info!
It's a 1990 Scarab 21, original 350 Mag (Carbed)

The RPM issue is because the previous owner put a 4 blade Merc Rev4 21 pitch prop. Just a little too much pitch for this thing. I'm looking for a Mirage 21 3 blade to try out, but can't seem to find one locally.

Thanks
-Rob

If it was valve float it will come back again once you get over 4000 rpms and get progressively worse. I think you are on the right track with checking the fuel

Definately put a new water separator filter in it. Change your regular fuel filter also (they are cheap). Just for piece of mind...pull the distributor cap again and see if there is any corrosion on the brass tabs on the inside. Sometimes they get "frosting"on them from condensation. Its worth a look sir. :drink:

I think 4,800 is what you would look to turn on the old carb 350 MAG's.

Well, I swapped out the fuel separator and the fuel filter. I did notice that when I removed the filter, there was some sediment and sand in it.

Another thing that I noticed after removing the separator, and unscrewing the fittings for the filter, is that there was quite a bit of pressure built up in the line between the separator and the carb, even after not being started for a couple of days. It sprayed fuel out under pressure when I removed the line, so I'm going to guess my pump is in ok shape.

I dumped the separator into a pan, and didn't notice any bubbles, just looked like straight fuel to me.

I checked out the cap, that still looks like brand new.

Knot 4 Me: That's what I heard I should be running, 4800 to 5000. I've been looking for a prop to borrow, or pickup cheap, but haven't found one yet.

Thanks for the help guys, I'll be water testing this thing again once the weather clears up here.

-Rob

Best thing to do with the separator is to pour it into a glass jar and let it sit and see if any water separates out. Obviously you would need a way to put a top on it

Fixx
 

ok does it have a carter afb style carburetor??

if so you will need to remove the top of the carb,remove the 2 fuel floats and unscrew the needle and seats,under the seats theirs a little tiny screen that filters the fuel before it goes through the needle and seat,,clean those screens,,they are probably all plugged with chit..

if its a Q-jet their is a rock filter that is probably plugged with crap restricting the flo of fuel..

When it cuts out how much do the tach(s) drop.Is it a slow drop or do the lose 500 rpms instantly?

It's a Q-jet carb, and I just replaced the filter last night, haven't had a chance to test it yet.

I'm honestly not sure how fast the tach dropped. I heard it start to chug, and immediately checked the Oil, temp and voltage gauges as I throttled back. They were all solid, and as I throttled back, it started to run properly. I was only about 200 yards from the launch, so I idled in without testing anything else out. I didn't want to blow something up before I had a chance to poke around and check the basics.

Thanks
-Rob

Fixx
 

so you checked the filter at the carb then? i remember you saying when you unscrewed the in line filter it was under pressure and its not suppose to be which indicates then the carb filter may be plugged.

I saw that pressure when I unscrewed the inlet into the filter that's built into the quadrajet to change it. I haven't tried it with the new filter yet. If that filter shouldn't be under pressure, I guess I found my problem!

Thanks
-Rob

Just an update for anyone that might find this thread in the future.

I replaced the Fuel Filter and Water Fuel Separator, and she's running like a top!

Thanks guys
-Rob

Feels good again doesn't it ? :drink:

Sure does!

https://lh3.googleusercontent.com/_E.../s800/Boat.jpg