Crud in carburetor, is it a problem?

[Rick Stephens]

At least it's a really simple carburetor to disassemble and reassemble. You will find that the instructions in the correct Mercruiser Service Manual for your motor, on carburetor rebuilding and adjustments, are if anything, too detailed. It is your essential guide to this rebuild - not the piece of paper that comes with any kit. Nice to have a complete set of instructions when you find some portion of the carburetor that is damaged or mis-installed.

Pay particular attention to getting every passageway squeaky clean. You won't regret time spent on a carb rebuild.

 

[Scott06]

At least it's a really simple carburetor to disassemble and reassemble. You will find that the instructions in the correct Mercruiser Service Manual for your motor, on carburetor rebuilding and adjustments, are if anything, too detailed. It is your essential guide to this rebuild - not the piece of paper that comes with any kit. Nice to have a complete set of instructions when you find some portion of the carburetor that is damaged or mis-installed.

Pay particular attention to getting every passageway squeaky clean. You won't regret time spent on a carb rebuild.

good advice especially the last paragraph...

link to carb manual http://www.discount-marine.com/club/files/carb.pdf

 

[Scott06]

It makes sense and other's thought timing would have fixed it too.... but tested the engine in the water by making slight timing amusements all around the spec, although there was some improvement, it didn't fix the problem. Then over time it still progressively got worse.

I replaced the fuel filter, it was very tricky to get it reassembled. Having this concern, I also tried removing the filter at the bottom of the pump and replaced it with an in line filter instead. This didn't help so I put back the stock filtration set up.

When disassembling the carb, I noticed the accelerator pump plunger seems swollen, does not travel smoothly, and will not reach to the bottom of the cylinder without a lot of finagling. The new one from the "no name" kit seems excessively small or loose in the pump cylinder. Let's see what the Walker kit looks like.

The boat is 15 years old and the carburetor was never rebuilt, hoping this will resolve the issue.

i meant once you verified the timing is ok likely this is related to carb being plugged up Or some type of fuel starvation. If you get it successfully cleaned out and are feeding it with clean fuel you should be ok after that

 

[ronaldreagan]

During rebuilding the carb I found three things that might contribute to the problem.

1. Crud
2. Swollen accelerator pump plunger
3. Float adjustment was off by about 4mm.

Testing it out tomorrow, let's see how it goes.....

 

[Rick Stephens]

Those are plenty to cause issue! Good job finding them.
Good Luck.
Rick

 

[ronaldreagan]

That did not fix it.... So it was not the carburetor.

Still pinging and back firing, when it can actually get to top speed it's still about 25 mph.

 

[nola mike]

That did not fix it.... So it was not the carburetor.

Still pinging and back firing, when it can actually get to top speed it's still about 25 mph.

Did you check your compression?

 

[ronaldreagan]

Did you check your compression?

Yes, it's about 120psi on all 4 cylinders.

Would tight valves have these symptoms?

 

[Rick Stephens]

Certain firing order is right? Always worth double checking.

 

[Scott06]

In addition to what Rick mentioned .

Can you measure fuel pressure at the carb at or close to wot rpm? wonder if carb bowl is empty

I wouldn’t run it too much or too hard if it is detonating you will beat up your pistons.
when you rebuilt the carb did u replace the power valve and did the actuator that releases the power valve operate smoothly? Does this only happen under load or heavy throttle ?

 

[ronaldreagan]

Certain firing order is right? Always worth double checking.

99% certain the firing order is correct, 1-3-4-2.

 

[ronaldreagan]

In addition to what Rick mentioned .

Can you measure fuel pressure at the carb at or close to wot rpm? wonder if carb bowl is empty

I wouldn’t run it too much or too hard if it is detonating you will beat up your pistons.
when you rebuilt the carb did u replace the power valve and did the actuator that releases the power valve operate smoothly? Does this only happen under load or heavy throttle ?

Yes, the kit came with a new power valve. I did clean in out the actuator and it moved smoothly.

It's a hard line between the fuel pump and carb, it would be difficult to measure the pressure without some serious rigging. I cranked it a couple times with fuel line disconnected to sample the fuel for moisture, no water in the gas. It seems strong but not certain if it's the same at higher rpms.

It seems like the bowl is empty or the fuel is not leaving the carb. It only happens under load and heavy throttle. It can go 15 mph consistently, no problem. During acceleration it pings and backfires, maintaining higher speeds it's not steady and pings, cannot reach the previous high speed at all.

I'm with you on the concern with the pistons.

 

[Rick Stephens]

Man, still sounds leaner n hell.

Things that make lean:

Plugged something in carb
Low float level
fuel pump poor output
vacuum leak

You know what might help diagnose is to run some propane into the intake and see if the pinging goes away momentarily. If it fixes the ping and adds acceleration you kind of know something on the above list is off.

 

[Scott06]

Man, still sounds leaner n hell.

Things that make lean:

Plugged something in carb
Low float level
fuel pump poor output
vacuum leak

You know what might help diagnose is to run some propane into the intake and see if the pinging goes away momentarily. If it fixes the ping and adds acceleration you kind of know something on the above list is off.

That’s a good list I’ve also used starter fluid sprayed at gasket joints like carb base and intake to head.

for testing fuel pressure I’ve seen Ts that thread into the flare fitting on the carb and have a female flare for the fuel line sandwiches in between line and carb

points to if firing order is ok, fuel supply is ok and no vacuum leaks then maybe still some plugging in carb?