Mercruiser 5.7 Stalling above 3000 RPM; able to restart immediately

[StevNimrod]

The engine is a Chevy 383, carbureted, set up like the Gen + (low pressure electric fuel pump).

I am having a problem with the engine wanting to stumble and die when operated above 3000 RPM. The first time it happened was just after getting up on plane. At about 3200 RPM it stumbled down to about 500 RPM (according to the tach) and died. Turned the key and it fired immediately. Then it ran 5 minutes on plane and it happened again. Then it ran 10 minutes before doing it again. Each time I was able to immediately restart.

I ran a compression test:
1 - 145 psi 2 - 150 psi
3 - 145 psi 4 - 150 psi
5 - 150 psi 6 - 150 psi
7 - 150 psi 8 - 150 psi

I have a pressure gauge plumbed into the fuel line and the reading at idle is 6 psi (I set the regulator to this setting at idle); at 3000 RPM the reading is 3.5 psi.

The engine has a new cap, rotor, spark plugs, and wires. The ignition coil is also new. The carburetor was rebuilt over the winter. I pulled the fuel water separator and found no water.

Here are pictures of the spark plugs from cylinders 1, 3, 5, and 7:



and pictures of the spark plugs from cylinders 2, 4, 6, and 8:



Given that the problem seems to happen only under load, and the plugs are white, I'm leaning towards a fuel issue. The only thing that doesn't make sense is that I can immediately restart the engine. I'm planning on starting with the fuel pickup and working my way back towards the carb with my next tests.

In the meantime, does anyone have any suggestions or similar experiences given the symptoms?

 

[vipzach]

Re: Mercruiser 5.7 Stalling above 3000 RPM; able to restart immediately

I am no expert, I'm having my own carb issues now too. It sounds like maybe the float isn't set properly.

 

[mnypitboat]

Re: Mercruiser 5.7 Stalling above 3000 RPM; able to restart immediately

If it restarts imediately, without cranking to get the fuel back in the float bowl, then I am thinking maybe its not fuel. Could be I guess, but.......if it just dies, I doubt it. Lack of fuel would result in a sputtering I would think.

I had almost exactly the same thing toward the end of last season. Just died, no sputtering, no put put, no nothing, just died. Started right back up imediately. I thought it was the distributor, so I replaced it all with a complete MSD system. Seemed to be fine til the beginning of this year when it wouldnt start at all. That made it easier to find the problem. Turned out I had a broken ignition positive wire. At the end of last year it was not completely broken. It must have been just about touching and under load, or on rough water it would come apart, but seal back up.

So in the end I put in a new wire in place and no more problems. Nothing at all. It starts and stays running without issue.

Just some ideas. I was at a loss too and would have never found it had it not completely broken finally.

 

[Bondo]

Re: Mercruiser 5.7 Stalling above 3000 RPM; able to restart immediately

I'm leaning towards a fuel issue.

Ayuh,... I agree, 'n so does yer fuel pressure gauge, 'n sparkplugs...

 

[StevNimrod]

Re: Mercruiser 5.7 Stalling above 3000 RPM; able to restart immediately

Moneypit,

That's the only thing that gives me pause about it being a fuel issue - it restarts immediately. And now that you mention it your situation - it did this twice a few weeks apart at the end of last season. It never happened again (due to winter layup) and ran fine for the rest of those days so I wrote it off to unknown cause. Gives me one more thing to check so I'm on board with that.

Bondo,

I tend to agree, the pressure gauge and spark plugs are saying fuel.

I guess the only way to find out is to find out. I'll bring gauges and meters, charts and diagrams and see if I can't get to the bottom of this.

 

[Bondo]

Re: Mercruiser 5.7 Stalling above 3000 RPM; able to restart immediately

I guess the only way to find out is to find out.

Ayuh,... Couldn't have said it any better myself,...

 

[StevNimrod]

Re: Mercruiser 5.7 Stalling above 3000 RPM; able to restart immediately

Just to wrap this one up in case someone in the future has the same symptoms - it ended up being a bad anti-siphon valve.

The sleeve inside the valve had collapsed towards the barb so the check ball inside had a limited range of motion. As fuel demand went up, supply couldn't match.

It didn't "fail" the fuel system vacuum test either in terms of showing high vacuum.

And no, I didn't just "delete" the valve.

 

[mnypitboat]

Re: Mercruiser 5.7 Stalling above 3000 RPM; able to restart immediately

Glad you found it. Thanks for letting us know what it ended up being. I am sure that will save somebody someday.