200 Yamaha OX66...RPM loss at WOT

[big_b16]

Re: 200 Yamaha OX66...RPM loss at WOT

Well, my ultimate fix might be a fairly cheap one. I got out again tonight with another guy. When the engine starts acting up at idle, the primer bulb is very soft. After pumping it a few times, it settles down and runs fine. At WOT, it ran great after we primed the bulb for the idle issue. I ran it up to 5400 RPMs with no problems at all. On another run, it started coming down and I had the guy pump the primer bulb but it wouldn't ever get firm...it stayed soft. I came down off of plane and had him pump it again, it firmed up a bit and once I gave it more throttle it easily reached WOT RPMs and ran fine. I'm thinking the ball in the primer bulb may either have crap in it or it's just getting bad. I think I'll just replace that and maybe the check valve between the on-board filter and the LP pumps. I know that if priming the bulb fixes low end stumbles, it is usually indicative of lift pump failures, but I've got new ones installed. I'm hoping this fixes the problems...because it runs great otherwise.

 

[mcbassin]

Re: 200 Yamaha OX66...RPM loss at WOT

Big_B16,

The problem your having sounds very similar to the problem I have had with my outboard. I haven't researched mine near as hard as you have yours. I'm interested to see what you found out about the primer bulb/check valve.

 

[big_b16]

Re: 200 Yamaha OX66...RPM loss at WOT

Well, I replaced the primer bulb today (and the coil). I didn't get it out on the water yet, but will tomorrow and I'll update then. After I installed the new primer bulb, I pumped on it a while in order to bleed the air off and after a while it became firm and was full of fuel. What was different now, when pumping the bulb, the fuel level in the on-board filter didn't change at all. It was still near the bottom of the filter element. Previously, when pumping the primer bulb, fuel would rush in and fill the cup up. After running it, the level would drop to it's current position. I'm not sure if this means much, but it is certainly different than before. After I took the old bulb off, I shook it and could hear the check ball rattling around in the bulb. I'm pretty sure it shouldn't do that...so at least now I've found something that was wrong. I pray this fixes the problem...lest I go stir crazy and drive the thing up the ramp.

 

[yaames]

Re: 200 Yamaha OX66...RPM loss at WOT

I had the same problem, went through everything you did and more. I finally broke down and replaced the o2 sensor and poof problem solved. It holds 5500rpm all day now.

Buddy

 

[rodbolt]

Re: 200 Yamaha OX66...RPM loss at WOT

by pumping the primer bulb your bypassing the lift pumps.
the pumping issue you have is normal, the VST is full and the needle valve inlet is closed so it wont pump anymore.
if you still have that stupid check valve between the engine filter and the lift pump remove it,unscrew it, dump out the guts and reinstall it.
3.5" Hg is cutting it fine on an engine that can tolerate 4"Hg at most.
I am thinking you have a restriction between the lift pumps and the liquid level in the tank.

 

[big_b16]

Re: 200 Yamaha OX66...RPM loss at WOT

Thanks Rodbolt. I'll do that with the check valve. I'm sure you're not surprised...but even with the new primer bulb...it still will fall off at 4800RPM. It's hesitating at a low idle also. I just read that in one of your other posts that both are indicative of fuel restriciton. Is it possible that a new Yamaha fuel water separator filter is bad? The one I've got on there has only been on a couple months. Is it alright for me to blow compressed air from the line between the 10micron and the tank? I'm not sure if there is anything on the other end (like a strainer type filter element) in the fuel tanks?

Just blew some crap out of the line between the 10 micron and the tank. I'm holding about 2" Hg when running. I'm heading out to a 40mph lake to see if I can do 65.

 

[rodbolt]

Re: 200 Yamaha OX66...RPM loss at WOT

check your fuel system vacum AT the RPM range your having a problem at. T in your guage between the engine filter and the lower lift pump intake.
some draw tubes have a straner,remove it if its there.
some (most) tanks have an anti-siphon valve.
check it carefully.
on the water seperator, once its filled with debris its bad.
wheter its the day you put it on or a year later.

 

[big_b16]

Re: 200 Yamaha OX66...RPM loss at WOT

Thanks for the inputs Rodbolt. I was able to run it today. It pulled hard all the way through 5400RPM without hesitation the few times I got it up there. I made several full speed runs and never had a high end problem. It ran fine at idle when I was pulling away from the dock and around the lake. On the way in, it stumbled on idle and died a couple times when just putting it in gear to get on the trailer. My warm idle is still only around 600-700RPM. Only problem I've got now is that the debris I blew off the end of the pickup is still in the tank somewhere. So...I'll run this tank dang near dry and give it a thorough cleaning. Is the anti-siphon valve on the pickup in the tank or on the fill hose (not sure if it's anti-human siphon, or anti-engine siphon)? Just looking for an idea of what I should see in the tank (and not see) when I pull it out. I can't get to the sending unit where it is now. I have to drain it and pull it out from the back deck several inches to gain access (it's on a bass boat).

 

[big_b16]

Re: 200 Yamaha OX66...RPM loss at WOT

Pulled the tank, only a few particles in it...nothing else I could see. What I thought was blowing something off the end of the pickup was likely just the fuel in the line blowing back in the tank. Pickup screen is clear. Still have an intermittent problem of WOT loss of RPMs. What can be causing this intermittent problem? I ran it off an external tank and it ran fine...but it also ran fine that day on the main tank. The only thing I haven't checked at the problem RPM is the O2 sensor and the low fuel pressure (after replacing the primer bulb, at idle I'm getting 2"Hg of vacuum). Like I mentioned before, I'm still getting 35psi out of the high pressure pump when the engine dropped RPMs. I also still have a problem with the engine stumbling intermittantly at idle (usually after I come off of a WOT run down to a low idle...like a no wake zone; also it'll do it if I start it after it cools off...like first start and start after fishing an hour or so). If I start it after a half hour or so, it runs fine, but will usually sneeze a little bit once I come off plane. Any ideas what these symptoms sound like? I'd hate to spend the $250 on an O2 sensor, but that's all that I haven't checked at WOT or replaced (the rest is relatively cheap). I did swap out the 10 micron filter with a new one before tonight. Could a fouled (I cleaned once with a ring free soak) O2 sensor cause these problems? My plugs have been darker than usual and I had to clean a bunch of carbon off them while I've been tracing this problem. Any ideas Rodbolt? If you think these symptoms are still a fuel starvation issue (or a flooding problem based on the plugs), I can try and get a low pressure fuel reading at WOT when it looses RPMs. I'm not sure I can check the O2 reading at WOT without causing a short or 2 using the paper clip method...and I don't have the O2 sensor test harness. I'd hate to buy one if I end up selling this motor in the near future. I pulled the two plugs on the VST; at idle I'm not getting any fuel out of the VST into the recirc system.

 

[big_b16]

Re: 200 Yamaha OX66...RPM loss at WOT

Finally got new O2 sensor installed. Ran it today and pulled all the way through 5400rpm and stayed there all evening. Now that it is running great on top end, I've got problems of it stumbling on low end when idling in gear. Runs fine in neutral. I'm thinking that I have to do a good de-carb and replace the plugs since there was so much oil from running rich. When I give it a little gas, it'll stumble. If I feather the throttle, and get it above 2000rpm or so, it'll run strong. Just at low RPMs does it hesitate. The plugs were all fouled since the O2 sensor started acting up. Rodbolt (or any other guys who've dealt with O2 sensor problems), is it common for engines to get carbed up when the O2 sensor starts acting up? I've done a link and sync and the oil pump linkage is right, I'm also not noticing a huge use of oil. Granted, I don't make long runs to gauge if I'm using a little more than normal.

 

[Barry2980]

Re: 200 Yamaha OX66...RPM loss at WOT

my motor is doing the exactly the same thing as yours! did it help when you put a new O2 sensor? where are they located at? thanks