Yamaha L4 3.0 - Top end power boost issue

MORLOK

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Aug 9, 2013
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3
I seem to have a weird top end surging problem that is different from what I have found in searches.

The boat pulls fine from a start, planes out, then builds a few rpm and then it will surge in power. It is a constant surge, like a Vtec or something.
The surge seems to be very RPM specific (don't know which rpm because the tach broken), after I plane out, I will keep WOT throttle, hear the rpm's build, then it will boost in power. Then I can back out of the throttle, rpm's will start to fall, and it will find that rpm again and loose power. So it seems to me to be independent of throttle position.

I have changed fuel filter (inline and carb).
Fully cleaned the carb.
Checked choke and throttle plate operation, and all seems to be normal.
Changed plugs. (old plugs looked fine)

The power increase is quite significant and sudden. It seems to be too sudden to be a fuel thing, I would think if it was going lean, the surge would be slower.
I am less familiar with engine ignition systems (especially on boats), is there something non-mechanical that adjust the timing based on rpm?
Also, WOT power is great, drives smooth. But after I drove it hard for a while I had starting issues (probably a different issue).
 

Bondo

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Apr 17, 2002
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70,470
Re: Yamaha L4 3.0 - Top end power boost issue

Ayuh,... Welcome Aboard,.... Sure sounds like a fuel delivery issue to me,...

Could be the tank, fuel pump or carb,...

Test the flow, 'n pressure, at the carb,...
 

MORLOK

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Joined
Aug 9, 2013
Messages
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Re: Yamaha L4 3.0 - Top end power boost issue

Ayuh,... Welcome Aboard,.... Sure sounds like a fuel delivery issue to me,...

Could be the tank, fuel pump or carb,...

Test the flow, 'n pressure, at the carb,...

Thanks for the help! I don't have the tools to test fuel pressure or flow, is there a cheap way to do it, or do I just need to buy something? Or perhaps something else I could check?

Do you think I am running lean at low rpm and then it finds fuel pressure and riches up? Or the other way around?
The boat will run for extended periods at any RPM so it seems like the fuel pump is able to keep up.

Has no body ever heard of a engine running like this before?
 

Capt Ken

Commander
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Jul 30, 2002
Messages
2,269
Re: Yamaha L4 3.0 - Top end power boost issue

Change your sparkplugs and plug wires. Sounds like you are losing a cylinder and when it cuts in you get the power surge.
 

MORLOK

Recruit
Joined
Aug 9, 2013
Messages
3
I ended up figuring out the power boost problem. It was a timing issue. I started messing with the distributor and advanced the timing a little and it runs great!
I assume that the timing must have retarded a bit from wear on the timing gear. I did not see any place to check the timing with a timing light, but I barely turned the distributor and listened closely for any knocking or pinging and did not hear anything.

But this got me thinking... It says to run at least 91 octane, which would be required with full timing advance, but if I wanted to run cheaper gas (and sacrifice a little power) couldn't I just retard the timing a little bit and then be safe to run 87 octane? Is there any other reason I would need to run 91 octane?
 
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