Merc 470 - fuel or spark issue?

[dn010]

Between replacing floors and the trailer, etc. its been over a year since I've been on the water. The last time I went out-a year ago-I was running WOT for a few minutes before the engine started to shutter, pop and slow itself down - I immediately reduced speed and all was well.

So last night, I took the boat out finally and it still can't handle WOT, not even for a brief moment without popping, and reducing its own speed. Then when I was returning to the ramp, it couldn't do anything over 2800-3000 rpm or it would backfire and sound terrible.

I've rebuilt the carburetor and it's been great up until the end last year; but for the past 3 years when it's resting it leaks down so I am going to replace it.

My question is - does this sound consistent with carburetor issues or does it sound like I have something more going on that I should start looking at?

Thanks for any info

 

[saltydogjeff]

Re: Merc 470 - fuel or spark issue?

It sounds like it could be a vapor lock issue. Mine has had the same problem, where the higher revs generate a higher heat temperature. I've tried resolving this by putting a spacer between the engine and the carburetor. If there are no problems when running at low RPM, then there should not be a spark issue. The high RPM leads me to believe it is a fuel delivery issue, engine timing or cleanliness within the cylinders. A compression test can help identify dirty cylinders with high compression numbers. Are there problems when you run the engine, then cut it off, and turn it back on shortly thereafter?

 

[dn010]

Re: Merc 470 - fuel or spark issue?

Thanks for the response. I'll get a compression test going here in a little while. There are no issues running at all until I hit 2800+; the engine starts every time however I usually pump the throttle a few times prior to hitting the key, if I don't it just takes a little longer for it to turn over. I've always viewed my 470 as bulletproof because it just seems to run forever - it just doesn't like higher RPMs anymore LOL. My timing is set at 8 degrees - the factory original before they changed it to 4. I left it at 8 because I've never had issues throughout the years until now.

As far as your efforts to battle vapor lock, I was looking at phenolic spacers for the Rochester 2g, did you use something similar? And did it work for you?

 

[Bondo]

Re: Merc 470 - fuel or spark issue?

Between replacing floors and the trailer, etc. its been over a year since I've been on the water. The last time I went out-a year ago-I was running WOT for a few minutes before the engine started to shutter, pop and slow itself down - I immediately reduced speed and all was well.

So last night, I took the boat out finally and it still can't handle WOT, not even for a brief moment without popping, and reducing its own speed. Then when I was returning to the ramp, it couldn't do anything over 2800-3000 rpm or it would backfire and sound terrible.

I've rebuilt the carburetor and it's been great up until the end last year; but for the past 3 years when it's resting it leaks down so I am going to replace it.

My question is - does this sound consistent with carburetor issues or does it sound like I have something more going on that I should start looking at?

Thanks for any info

Ayuh,.... Ya gotta make sure the tank is givin' up it's load, 'n the fuel pump is pumpin',...

ya should have good flow, at 4 to 6 psi,...

If ya don't have a canister type fuel filter, Add 1, 'tween the tank, 'n fuel pump,...

 

[dn010]

Re: Merc 470 - fuel or spark issue?

My fuel line runs from the tank to a separator, then to the pump with filter and then the carb with filter. I never suspected the pump but it is the original pump from 1978 so I'll put a gauge inline for testing purposes and see what it's delivering. I'm all for old school technology but I'd really love to go with electric pump. Not happening for a while, but I do have a spare pump at least if my numbers aren't good. Thank you!

Ayuh,.... Ya gotta make sure the tank is givin' up it's load, 'n the fuel pump is pumpin',...

ya should have good flow, at 4 to 6 psi,...

If ya don't have a canister type fuel filter, Add 1, 'tween the tank, 'n fuel pump,...