Help - 2-stroke carb surging high rpms.

[mpaz]

Hi all,

I've found good advise on this forum, but am stuck on a new problem and was hoping someone may have an idea.

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.

 

[Home Cookin']

Re: Help - 2-stroke carb surging high rpms.

I'd suspect the bubbles show an air leak, maybe at the tank fitting or the bulb fitting. Others?

 

[stylesabu]

Re: Help - 2-stroke carb surging high rpms.

if its sucking air it should also be leaking fuel. surging is usually its leaning out to much,low fuel level in bowl. you can use the bulb to pressurize system. close down vent on fuel tank,make sure gas cap is tight. a little soap and water solution at all fittings,filters and seperators, lines. if there is a leak you will see either fuel or bubbles with soap solution.

 

[mpaz]

Re: Help - 2-stroke carb surging high rpms.

OK, I hooked up a 5 gallon external tank with 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.
So, I'm fairly sure there is no air leak set up this way.

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...

It seems that the plugs 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?

 

[Blown284]

Re: Help - 2-stroke carb surging high rpms.

Did you ever figure this out? I'm having similar issues right now.......

 

[Khowcroft81]

I really wish people that had these issues would post to what The problem ended up being. Because we're trying to search through these forms for answers and do all this reading for nothing. I have the same problem with mine.

 

[GA_Boater]

I really wish people would read the Help Tip on top of the page and all of OUR rules.

Closed.