1991 5.7l - Carb Problems

[Incorrigible]

Engine is a 1991 5.7l

Issue is this: Engine quit producing full power. Tach read 4400rpm WOT in the beginning. Now reading 3600rpm MAX WOT. This is IN THE WATER and in gear while running under normal conditions. Out of gear engine will more than redline.

Have checked alignment and lower unit.
Have checked tach.
Have replaced wires/plugs/coil/ignition sensor.
Have completed compression test, spark test all cylinders.
Engine is properly timed.

SECONDARIES on the ROCHESTER 4MV carb do NOT open.... I'm guessing, but I think this is where my problem lies.

For what reason would my secondaries fail and how would I go about testing this?

Also, when holding the secondaries open, in water, in gear, engine still doesn't reach power (assuming just holding the butterflies open doesn't increase the fuel flow from the secondary full metering rods?)

Any help, or point in the right direction? Greatly appreciated.

THANKS

 

[Bondo]

Re: 1991 5.7l - Carb Problems

Ayuh,... Sounds like it's time to rebuild or replace the carb....

 

[DACEAC]

Re: 1991 5.7l - Carb Problems

What about the vaccum pod that is connected to the secondaries. Maybee that is bad. happened on my 4mc

 

[John_S]

Re: 1991 5.7l - Carb Problems

When you are saying secondaries, I think you are refering to the air door that sits above the secondaries. The secondaries are mechanically linked to the primaries. With the engine off, if you move the throttle all the way forward, you can push open the air door and look down and make sure they are verticle.

The air door will open as the engine develops enough vacuum under load and the throttle is forward enough to have the secondairies open. They won't open just by reving the engine. At 3600rpms the air door is probably just starting to try to open. By observing the secondaries as mentioned above, you can find the exact throttle position that they start to open.

Did the engine bog when you pushed the air door open?

Is the ignition advancing properly?

Overall, it is still probably a carb or fuel problem, but no or little advance can cause those symptoms.

 

[Incorrigible]

Re: 1991 5.7l - Carb Problems

Okay: First "Ayuh.... sounds like a to rebuild or replace the carb" - WHY? What is it you think is functioning incorrectly, or failing to function?

Second: The vacuum pod (assuming we're talking about the same item) is part of the choke system and I believe is functioning properly. Will test and get back to that one. DECEAC? When that failed for you, what were the symptoms?

Which brings us to: Secondaries: Yes and no.
The air door on the secondaries doesn't open under power. You're correct, the secondaries are mechanically linked to the primaries, and when the throttle is fully advanced, the secondary throttle plates (the butterfly valves in the bottom of the carb) should be opened. This in turn should cause the secondaries air valve to open through suction. In the middle of the air valve butterflies for the secondaries is a plastic cam which, when the secondary air valve opens, should actuate a lever which lifts the fuel rods, allowing additional fuel to flow.

I will verify that the throttle plates open mechanically when the throttle is advanced.
I will also verify that the ignition is advancing properly. Good call.
At 3600rpm the secondary air valves were not opening. This is all the further I can get the engine to rev.

I did install an external fuel tank with fresh fuel directly to the air/water separator, eliminating the internal fuel tank/fuel and the same problem exists. As well, the external tank has a primer bulb, so I was able to increase fuel pressure by pumping the bulb while running in an effort to determine if the fuel pump wasn't delivering enough fuel pressure. I'm of the belief that it's not the fuel, the fuel tank, or the fuel system.

If the engine bogs down under load when at full throttle (now around 3600rpms) when the secondary air valves are opened, what does this indicate?

Thanks much for the feedback. Has me looking in a few additional directions.

 

[Incorrigible]

Re: 1991 5.7l - Carb Problems

UPDATE: Throttle plates open and the fuel metering rods (secondaries) are indeed lifted by the cam that is internal to the air valve (secondaries)

SO: I'm begging and pleading that it's NOT ignition advance, as this is controlled by the ignition amplifier: NOTE This is the Thunderbolt IV ignition system and is therefore EXTREMELY Expensive. I'll go through the checklist and determine if that's the case and verify with the timing light/adjusting the distributor to see if that will solve it before replacing it.

I know that the air valves in the carb secondaries are opened by engine vacuum, and this in turn raises the fuel metering rods. Is there a chance that the reason the engine is not developing enough suction to open the secondary air valve due to a leak anywhere else such as the intake, valve, head gasket, etc?

Thanks!!!

 

[Incorrigible]

Re: 1991 5.7l - Carb Problems

Okay. Not the ignition. Not the carb (glad I didn't "ayuh" rebuild/replace that)

Leaking valve.

 

[DACEAC]

Re: 1991 5.7l - Carb Problems

My symptoms were exactly the same. I rebuilt carb and still had same problem. pulled that vaccum pod and found the diaphram in it ruptured. got a new one from napa and after the 10 minutes it took to install it, motor was back to normal. new pod was like 17.00.

 

[Incorrigible]

Re: 1991 5.7l - Carb Problems

Unfortunately? My vacuum problem is coming right from the intake manifold. We took measurements from both the port on the top of the intake manifold as well as the vacuum line connecting from the carb to the vacuum pod. I will certainly keep this in mind in the event that my next steps don't resolve the problem....

 

[Don S]

Re: 1991 5.7l - Carb Problems

Assuming for a second that the secondaries are working properly, is any fuel coming out of the secondary discharge nozzles?

 

[Incorrigible]

Re: 1991 5.7l - Carb Problems

I cannot verify that without taking a look. I know that the vacuum pressure coming out of the carb (vacuum line that runs from the carb base to the vacuum actuator for the choke) is around 9-11psi and isn't steady. This should be considerably higher and definitely stable. As well, there is a decreased compression reading in a few cylinders. The secondaries (throttle plates) open vertical, but there is insufficient vacuum to open the air valves. As such, the metered fuel rods would not be opened (the cam that initiates the opening of the secondary fuel rods is built into the shaft the secondary air valves are mounted to) I will be working on it again this weekend (been playing with the other boat lately) and will come back with what I find out....