MAG 350 loses power after 20 minutes, need help

[onetong8r]

I'm hoping someone can point me in a good direction. I bought a 1994 Scarab 22 with a MAG 350 and Alpha 1 a few weeks ago. The engine has the TBIV ignition, a Carter AFB carburetor, good compression, new plugs, cap, and rotor. The timing advances like it's supposed to. I drained the fuel tank when I bough the boat a couple of months ago and filled with fresh ethanol free. I've blown thru the fuel tank vent and there's no obstructions. When you fill the tank you can feel the air blowing thru the vent fitting beside the fill port. The boat cranks up easy and idles smooth.
I've been working on little things like lights, replacing the fuel sender and trim senders and have only taken it out for short trips and it runs strong. I've taken two longer runs and both times at around 10-15 miles (20 minutes) it has felt like it was losing power, almost like you were pulling the throttle back from 3/4 to 1/4. The first time I thought something bad happened so I stopped at the nearest dock and tied up to pop the hatch and look at the engine. No noises and nothing felt hot or looked out of place so I dropped the hatch and got back up to speed and it did it one more time before I got back to the ramp. This past weekend I was pushing it a little harder running ~45-50MPH and again right around 12 miles of running it started to fall on its face. I dropped down off plane to idle and didn't see or hear anything and then powered back up and it made it back to the ramp which was pretty close by.
I would think if it was a fuel issue it would have shown up before it burned thru 4 gallons of gas. As I said, you can blow thru the vent hose easy with your mouth. Is there anything in the ignition that can act squirrelly after it heats up? The engine doesn't stall so it's not losing spark or fuel entirely. I'd like to get this sorted out before I take the wife and kids out for a ride and get us stranded. Kind of a head scratcher for me. Any thoughts or direction is greatly appreciated.

 

[Grub54891]

Pull the fuel pickup tube and check that. Some have a screen down there and debris in the tank can plug it up. When you shut down the debris floats away from the pickup. Just another thing to check.

 

[alldodge]

Might be antisiphon valve partly clogged or a weak fuel pump.
Make up a Tee fitting with fuel pressure gauge and place inline between pump and carb

 

[onetong8r]

Might be antisiphon valve partly clogged or a weak fuel pump.
Make up a Tee fitting with fuel pressure gauge and place inline between pump and carb

Thanks guys! I assume the anti-siphon valve is installed on the tank at the pick-up tube? I'll pull the pickup tube like Grub recommended and verify the screen (if installed) is clean. Is there any way to check the anti-siphon valve other than visual and sucking on it like a check valve? I think I have a fuel pressure gauge from old project that I can plumb into place. A drawback of this boat is I can't run at speed with the engine hatch part of the way up (gas struts) so I may use some ratchet straps to hold it open enough to watch the gauge and take a spin. Thanks for the thoughts!

 

[alldodge]

I assume the anti-siphon valve is installed on the tank at the pick-up tube?

Yes just a check ball

 

[QBhoy]

I think you usually won’t get away with just draining the fuel tank. Usually needs totally opened up by removing the sender hatch, then sucked out bone dry. Then any crap in the bottom taken out if possible. Pick up pipe pulled out and clean of the entry to the pipe. As said…non return valve then should be proven to be clear too.
That would usually see you right. Only thing other than that might be if you have a mesh strainer on the inlet to the carb too. Check that. Have a check all around the fuel, carb and intake manifold systems for any signs of drawing air too. Seems you have a few abnormal from standard parts too. Check all is well with their function I’d say.

 

[onetong8r]

I think you usually won’t get away with just draining the fuel tank. Usually needs totally opened up by removing the sender hatch, then sucked out bone dry. Then any crap in the bottom taken out if possible. Pick up pipe pulled out and clean of the entry to the pipe. As said…non return valve then should be proven to be clear too.
That would usually see you right. Only thing other than that might be if you have a mesh strainer on the inlet to the carb too. Check that. Have a check all around the fuel, carb and intake manifold systems for any signs of drawing air too. Seems you have a few abnormal from standard parts too. Check all is well with their function I’d say.

I'm still learning all the little gremlins on this boat. The fuel sender was bad and two owners ago had the floor replaced in 2015. They reinstalled hatches at the front where the fill / vent lines connect and the rear where the fuel pickup is but nothing in the middle so I had to cut in a new hatch and glass it in to replace the sender. Luckily I had receipts with everyone's names and cell numbers so the guy was good enough to send me bunch of pictures when the floor was out so I could spot the fuel sender hatch within a couple of inches. I went out last night to pull the pickup tube and anti-siphon and dang if they didn't put the rear bench support blocking ~1/3 of the rear floor hatch (wood screwed down to the floor) so I'll need to take the whole bench apart to open the hatch. I can cut the wood before it goes back in so it doesn't block the hatch but stuff like this drives me crazy.
Side note, I have the OEM mechanical fuel pump and hard line up to the carburetor. Any tricks to install a fuel pressure gauge besides replacing the rigid with UCG approved rubber fuel line?
EDIT - The boat does have a spin on fuel filter / water separator that I replaced when I drained the tank initially. I did not see a mesh screen filter in the line at the carburetor. It has a Carter / AFB marine 750cfm carb. Also, side note, it has a K&N air filter which I was going to ditch for a real flame arrestor but looking thru all the receipts and the K&N filter is a UCG approved flame arrestor. I had never heard of that.

 

[alldodge]

The K&N isn't going to be an issue with SBC. A boat really doesn't need any filter with it being on the water, just something to keep big things from going in.

Fella (Smitty - articfriends) on OSO did a dyno test with a (if memory serves) 600+ BBC and tried different filters, Flame arrestors and the K&N was the worst in reducing HP. Best was nothing, but right next was the Merc factory arrestor

 

[stresspoint]

try a new coil , coils will play up as they get old.

 

[onetong8r]

First, please don't be jealous of my Miami Vice purple rubber diamond plate flooring. I got to the hatch and found no anti-siphon valve. There's a ball valve that seems to be functional but not entirely sure why it's there. I pulled the pickup tube and it does have a screen up inside the tube but I didn't see any debris in the tank or in the tube that would plug it up. The rubber hose from the pickup to fuel pump is heavy wall marine hose and in decent condition. I don't have a good way to install a fuel pressure gauge in the rigid pipe between the pump and carb but I do have a spare running merc 350 in my shed so I may just swap the fuel pump and ignition coil and see if that clears up the issue. I hate throwing parts at a problem but I'm still stumped. If I'm missing anything obvious let me know. Thanks!

 

[dingbat]

I got to the hatch and found no anti-siphon valve.

Are you sure about that? Required by law on I/O boats

Most ASV are housed in hose bibs. Check the fitting between the valve and the hose

 

[alldodge]

I like the purple. If you want to see a work of art is a 6 year restore on OSO. The last pages is were its almost ready for the water
https://www.offshoreonly.com/forums/scarab/336665-back-dead;-81-scarab-38-resto.html

To check fuel pressure use a length of rubber gas line and temporarily replace the steel line. Can get fittings from local auto parts store. Use loaner program to borrow a gauge

 

[onetong8r]

Are you sure about that? Required by law on I/O boats

Most ASV are housed in hose bibs. Check the fitting between the valve and the hose

I'm sure it's not there. All it has is a little full-port ball valve with MPT connections on one side and a 3/8 barb on the other. I think I'll replace the isolation valve with an anti-siphon valve to be safe. I'm not sure why you'd even need a ball valve there.

 

[onetong8r]

I like the purple. If you want to see a work of art is a 6 year restore on OSO. The last pages is were its almost ready for the water
https://www.offshoreonly.com/forums/scarab/336665-back-dead;-81-scarab-38-resto.html

To check fuel pressure use a length of rubber gas line and temporarily replace the steel line. Can get fittings from local auto parts store. Use loaner program to borrow a gauge

I read through that thread a few months ago and it is amazing what they did to that boat. The paint and everything is amazing. I'd be scared to death to take it out

 

[onetong8r]

So I don't know if anyone cares but I may or may not have solved the problem. I rebuilt the carb to make sure the jets were clean and everything looked pretty good. I put in new float needles and floats and got everything adjusted again. I am still waiting on some parts to install a fuel pressure gauge. I took it out and drove for 10 minutes with no issue and then it stumbled so I quickly shut off the engine and looked it over and had fuel running over the top of the carb into the secondaries. I let it sit for a few minutes and it fired right back up. I watched the carb and no overflow. I ran it for another 10 minutes or so to get back to the ramp and had it running smooth at 4,000 rpm @ 53mph on GPS and not full throttle so it's making good power. Got back to the ramp and still no stumbling. I got home and pulled the top off the carb (its a marine AFB / Edlebrock) and the floats are still adjusted correct and blowing into the fuel inlet the needles seem to open and shut off properly. It has a mechanical pump so I have no idea why the pressure would vary but that's what I'm leaning towards. I'm going to put a regualtor along with the gauge so I can set it at 5psi and see if it happens again. Maybe that will fix it or maybe it's just one more gremlin down and 25 to go.

 

[alldodge]

Might have it figured out
The one time it had fuel leaking out might have just been a speck of crud that got caught in needle seat. Might be good to go now

 

[QBhoy]

So I don't know if anyone cares but I may or may not have solved the problem. I rebuilt the carb to make sure the jets were clean and everything looked pretty good. I put in new float needles and floats and got everything adjusted again. I am still waiting on some parts to install a fuel pressure gauge. I took it out and drove for 10 minutes with no issue and then it stumbled so I quickly shut off the engine and looked it over and had fuel running over the top of the carb into the secondaries. I let it sit for a few minutes and it fired right back up. I watched the carb and no overflow. I ran it for another 10 minutes or so to get back to the ramp and had it running smooth at 4,000 rpm @ 53mph on GPS and not full throttle so it's making good power. Got back to the ramp and still no stumbling. I got home and pulled the top off the carb (its a marine AFB / Edlebrock) and the floats are still adjusted correct and blowing into the fuel inlet the needles seem to open and shut off properly. It has a mechanical pump so I have no idea why the pressure would vary but that's what I'm leaning towards. I'm going to put a regualtor along with the gauge so I can set it at 5psi and see if it happens again. Maybe that will fix it or maybe it's just one more gremlin down and 25 to go.

Good that you’ve likely solved the issue. Nice one.
Can I ask a couple things around your boat ? Is this the scarab 22 that there was also models of in the early 2000’s ?
I know two guys with scarabs on the Loch over here. One is a 22 with a worked 8.2l HO and the other is a 1990 scarab 21 excel, that had a 7.4 carb and now has a later 7.4 mpi in her.
The 21 is a close friend of mine. And might be compared to your 22 perhaps. You mind me asking what prop and drive ratio your boat has ? And what kind of performance you can see from her top end…as in GPS and at what rpm? Appreciated, if you can tell me a few of those details. Thanks

 

[onetong8r]

So mine is a 1994 Scarab 22 with a MAG 350 / Alpha 1 gen 2 drive with a Mirage 21 3-blade prop. The previous owner had the engine rebuilt and added a marine grind Erson roller camshaft, Edelbrock marine RPM intake, AFB 750 cfm carburetor, and IMCO powerflow exhaust, thru-hull. It was super loud and my wife thought it was obnoxious idling thru the no-wake zone by our house so I bought 4" elbows and a strauight piece of 304SS and made turn-downs to put the tips under the water at idle. I've been chasing a few issues with the boat since I recently bought it so the fastest we've had it was 53mph @ 4,000rpm and it felt like it wanted to keep going. I still not 100% that I have the timing right so I haven't pushed it to the limit yet. I think the engine made 250-260HP stock so maybe it makes 300HP with the mods? The PO said it needs a bigger prop so I may try a Mirage Plus 23 and see how it does. If I can get it to run 50MPH at 3,500rpm I'd be very happy. We ride on the St. Mary's River 99% of the time and that's plenty fast for me. I looked at another identical make and year when I bought this one and it was pretty beat down but it had a 7.4 / Bravo combo and the guy said his would run 70MPH but he said it handled very sketchy over 65 and I'll take his word. I haven't had it in the ocean yet but it rides really nice. I guess you could sleep two people under the front if you needed to but it would be tight. I've only seen them from 1994 thru 2005 but they may have made them earlier. I've been a Ski Nautique guy forever so this is my first Scarab. I really wanted something we could cruise ~20 miles down the river in to eat or hit a sand bar over at the ocean (by Fernandina) and I think it will be good for that.

 

[alldodge]

So mine is a 1994 Scarab 22 with a MAG 350 / Alpha 1 gen 2 drive with a Mirage 21 3-blade prop. The previous owner had the engine rebuilt and added a marine grind Erson roller camshaft, Edelbrock marine RPM intake, AFB 750 cfm carburetor, and IMCO powerflow exhaust, thru-hull. It was super loud and my wife thought it was obnoxious idling thru the no-wake zone by our house so I bought 4" elbows and a strauight piece of 304SS and made turn-downs to put the tips under the water at idle. I've been chasing a few issues with the boat since I recently bought it so the fastest we've had it was 53mph @ 4,000rpm and it felt like it wanted to keep going. I still not 100% that I have the timing right so I haven't pushed it to the limit yet. I think the engine made 250-260HP stock so maybe it makes 300HP with the mods? The PO said it needs a bigger prop so I may try a Mirage Plus 23 and see how it does. If I can get it to run 50MPH at 3,500rpm I'd be very happy. We ride on the St. Mary's River 99% of the time and that's plenty fast for me. I looked at another identical make and year when I bought this one and it was pretty beat down but it had a 7.4 / Bravo combo and the guy said his would run 70MPH but he said it handled very sketchy over 65 and I'll take his word. I haven't had it in the ocean yet but it rides really nice. I guess you could sleep two people under the front if you needed to but it would be tight. I've only seen them from 1994 thru 2005 but they may have made them earlier. I've been a Ski Nautique guy forever so this is my first Scarab. I really wanted something we could cruise ~20 miles down the river in to eat or hit a sand bar over at the ocean (by Fernandina) and I think it will be good for that.

Hey not cool using another thread, start your own, sorry my mistake

 

[Scott06]

Hey not cool using another thread, start your own

He is the OP