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Old 08-25-2009, 05:18 AM
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Default Yamaha surging

I have a 2004 Yamaha 150 TXRC 2-stroke carb outboard on a boat purchased at the begging of the season. For the first few months it was running fine, but then started rough idling and stalling at idle. I cleaned/re-built the carbs with the yamaha carb kits and installed a 30GPH Racor fuel water separator. It took care of the stalling and rough idle, but now I have a new problem.

At high rpms, it starts surging and dosn't seem to reach full rpms. I have tried a number of things, but the problem seems to be getting worse, not better.

I have:
1) replaced the two fuel pumps.
2) replaced the on engine fuel filter.
3) replaced all on-engine fuel lines (under the cowling).
4) I empty the racor before each use, but have never found any water.
5) tightened all of the fuel line fittings.
6) Replaced the primer bulb.

I have noticed that if I pump the primer bulb when it starts surging, I can see air bubbles in the clear racor bowl, until the primer bulb gets hard again. And the surging seems to get better for a very breif period of time (maybe 30 seconds to a minute).

However the rpms still seem low, but may this may be the prop. First time out after cleaning the carbs I scraped up my prop a bit and switched from a 17p 3 blade to a 16p 4-blade, but the rpms were over 5200 and now only 4600. When it surges, it drops to 3800 and back to 4400-4600ish. So, due to the timing, I don't know if the low rpms are a prop problem or an engine problem.

It seems like an air leak somewhere, but I can't find anything. Is it possible it is the racor? I have the primer bulb before the racor. Seems correct, but don't know. Really running out of ideas.... just can't find anything obvious.
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Old 08-25-2009, 05:32 AM
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Primer bulb should be between Racor and the engine. Check all connections and clamps between engine and tank, it sounds like a vacuum leak. A pressure and vacuum test should show the problem. Less the 2-3 inches of vacuum indicate a leak. Your engine should run 5500 RPM with an average load. Lack of fuel could cause lower RPM and engine damage. Find the problem and fix it before you run again.
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Old 08-25-2009, 08:56 AM
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OK, I'll check the pressure.

Also, why would the primer be after the racor? The racor has a drain screw, and if I empty it, don't I need the primer before the racor to refil the bowl?

Is it possible having the primer before the racor is causing the problem, or is this a seperate issue?

Thanks!
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Old 08-26-2009, 06:38 AM
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The primer will pull fuel through the Racor to refill it. The only way to find out if the primer location is the problem is to change it.
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Old 08-30-2009, 03:31 PM
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I stopped by today. I didn't have a vacuum gauge handy, but did have a 5 gallon external tank with me. I put fresh fuel in it and plumbed it directly to the motor. As I said, I just replaced all the hoses on the motor and today I went through and tightened all the connections.

I just took it out for about a minute or so - still surging....

I removed the spark plugs, they were very black and were quite oily.

I had extras, so I changed them, tried again for another minute or so and again, still surging. I removed some of the plugs and they already were fairly oily again after only a few minutes of use...

I'll try to get a vacuum gauge before I run it again, but the plugs seem to suggest it's running rich not lean.

Any thoughts?
I was thinking maybe the oil pump is faulty or needs adjustment. However, would this cause surging?
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Old 08-31-2009, 05:25 AM
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The oil pump meters the amount of oil to the engine based on RPM and throttle opening. The amount of fuel going into the engine does not effect the amount of oil. I don't beleive the oil pump can cause surging. The fouling of plugs may be caused by oil, not fuel. You seem to have covered all the easy things, it is time to get help from a good Yamaha tech.
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