What would cause sudden loss of power?

[Lancer76]

1976 mercruiser 233; Ford 351 with Rochester 2bbl carb. Engine starts easily and idles fine. Engine runs great at 35000 rpm for a few minutes and then the engine quickly looses power as if I pulled the throttle back. If I pull the throttle back quickly the engine will continue to run at low rpm, but if I don't throttle back; engine will die. If it dies, it starts immediately. No backfiring so I dont think its ignition. Seems like a fuel issue.

I cleaned fuel tank, replaced filter and fuel line from the tank to the filter. I still have to check the steel line from the pump to the carb. Any chance the carb float is adjusted too high so not opening the inlet valve enough? I did rebuild the carb last year so it's possible the float is maladjusted. Thoughts?

Thanks

 

[Grub54891]

I did rebuild the carb last year so it's possible the float is maladjusted. Thoughts? ​

Did you run it after the carb rebuild? Or is your fuel pump going south...

 

[Lancer76]

I did run it after the carb rebuild. It ran great, but it was late in the season so didnt use it much before winterizing. The fuel pump has crossed my mind too. It's a relatively low pressure pump (6 PSI, I think)

 

[alldodge]

Check your pressure, should be 3-7 psi from idle to wot

 

[kenny nunez]

If after doing AD’s fuel pressure test and the pump is up to spec. Then do the 6 gallon outboard tank test connected to the fuel filter inlet. This will let you know if there are any fuel tank, or an anti siphon valve restriction. A boat that old is bound to have some fuel tank issues. It might just be the anti siphoning valve, if the valve is made of aluminum then get a new one made from brass. I have even found a crack in the suction line from the sloshing around of the fuel in the tank.
You may need a new tank, sometimes the fuel suction fitting threads fail when trying to unscrew the fitting. If you get the suction line out of the tank check if there is a fine filter inside of the the line, they get clogged up easy, pull it out and pitch it.

 

[Lancer76]

Regarding the fuel tank, I emptied, removed, and cleaned it thoroughly last weekend. No in-tank filter or screen. I also installed a new fuel line from the anti-siphon valve to the fuel filter, which is new. The metal line from the fuel pump to the carb is clear, too. I've picked up some equipment to rig up a low pressure fuel pressure gauge between the pump and carb. I will get that set up and see what that yields in terms of pressure readings.

Also, my anti-siphon valve is aluminum. The check valve spring and ball seem to move freely by hand, but if those are known to bind or crack, I will replace with brass as suggested by Kenny.

Update: In search of a new antisiphon valve, I found this on our local marine store we page "MOELLER ANTI-SIPHON VALVE BRASS 3/8 NPT 3/8 HOSE BARB. Use Brass for HP 200 & above and Aluminum for HP below 200."

Also found this info on an old post:

Thank you for your inquiry with Moeller Marine. The CAL number is the calibration of the anti-shipon.
inch of water column (in WC)a traditional unit of pressure, used in plumbing to describe both water and gas pressures. The conventional equivalent of one inch of water is about 0.036 127 pounds per square inch (psi) or about 0.073 556 inches (1.868 32 millimeters) of mercury.

Thus a 15 (WC) calibrated ASV equals .541 PSI or 27.99 mm HG of mercury

30 (WC) calibrated ASV equals 1.083 PSI or 55.99 mm HG of mercury

Industry standard regarding use:

•201 HP Engine and Higher = 15 calibrated ASV
•200 HP Engine and Less = 30 calibrated ASV

I have a 233 hp engine, so I should be using the brass 15 valve anyway.

Thanks for the input. I think I am on the right track.

 

[Lancer76]

Replaced all the filters (again) and rigged up a low pressure fuel gauge (0-10 PSI) between the pump and the carb and put in the brass anti siphon valve.

At idle, was getting 3-3.5 PSI. I was running on the hose so I didnt keep the throttle open for more that a few seconds. Ran it up to 3K RPM a few times and got as high as 4.0 PSI. At 4K RPM I GOT maybe 4.5 PSI. I over revved momentarily to about 5k and and the gauge almost hit 5 PSI.

Obviously, the numbers are low and point toward a new fuel pump. The pump that is on there could be 43 years old so I'm good with that. Would you agree or should I verify while on the water to be sure?

Thanks.

 

[Grub54891]

You have it all set up, may as well dunk it. After putting a new pump in, check it again. Comparative numbers are good reference.

 

[kenny nunez]

Another thing about aluminum anti siphon valves. When the valve is screwed into the fuel tank elbow it is possible that the pipe threads can make the internal bore of the valve tighten just enough to cause the ball to get “wedged”. Any time I found an aluminum valve it got replaced with a brass one.

 

[alldodge]

Don't see an issue with your pressure, if motor keeps 3 psi at wot your good, but does need to be in the water under load to determine

 

[Lancer76]

Okay, before I put the boat in the water I checked the carb float. That measured fine,but I noticed that the little carb filter was in wrong. The filter was put in first ( by me), then the spring and than the fuel line against the spring. I switched the spring and filter.

on the water the boat now runs great. About 4 psi on the fuel pressure gauge and no loss of power. Could it have been the little filter?

 

[alldodge]

Yes, the filter causes a high flow restriction, it does fine a low rpms, but higher and under load it needs much more fuel. It also depends on which filter your have, the old stone filters go in the other way

 

[Lancer76]

Quick check on those diagrams, Alldodge. This video shows the opposite of what you show as "correct". It makes sense to me that the one-way valve end of the fuel filter should be flush with the fuel inlet fitting with the spring behind the filter (i.e., between the filter and the carb inlet) to keep the filter snug against the fuel line inlet.

Put another way, the diagram you label as "incorrect" seems correct to me and vise versa.

 

[alldodge]

Well if its on you tube it has to be correct right? I'm going to stick with my above post, its your boat, your decision

 

[Lancer76]

True enough. It is my boat. How does one weigh the validity of a you tube video over a forum post?

I do appreciate you input. Thank you.

 

[Lancer76]

The only diagrams I find in manuals or on line show the spring, filter and then fuel line fitting.

 

[alldodge]

All paper/media type filters pass liquid from the outside to the inside. This is done to give the greatest surface area and also so the pressure differential is inward and not out like a balloon. The guy is installing the filter so fluid comes into the center and then out (same as your pics).

When carb was first manufactured it used a Bronze type (some call stone) filter, the paper type filter didn't come out until later. The old drawings have not been updated, just the part numbers being used. In the link below note this is a 1987 Rochester 2GC carb and the filter is shown on the other side of the spring.

https://www.mercruiserparts.com/bam/...31719/1014/180

Would also add that if you have a 10 micro canister filter installed, there is no need to the carb filter

 

[Lancer76]

Hmmm. Maybe the difference lies in the type of filter being used. Seems that the link you provide (thank you) shows a screen type filter. The type that I have is the paper filter with the one way valve in it. The paper filter with the one way valve wouldn't work as you describe (fuel entering the through the outer surface and out the center) because the one way valve would be slammed shut, if you will. Result - no fuel. However, the strainer screen doesn't have the one the way valve so the alternate configuration would work.

Interesting discussion. Could be, too, that I am using the wrong filter in the carb. Should have a strainer type instead of a paper filter. That would explain the difference in the drawings.

I do have the canister installed before the pump, too. Thanks.

 

[Rick Stephens]

Direction of filter is dependent on the outlet (to needle valve) port location and sealing surface. Most of the filters I have seen use the outside of the screen to catch debris and the port to the bowl is in the middle of the filter pocket. The spring goes as shown in the lower photo.

The stone type filters go the other way round and the outlet is on the outer edge of the pocket. You can confirm that by removing the filter and eyeballing that outlet port location.

 

[alldodge]

one way valve

Whats the part number

screen type filter

Same as paper and depends on part number. Merc (Rochester) started using the screen filters later on and then just like when the Bronze filters went to paper they just changed part number in the drawing. The link was just for orientation info.