1991 225 Yamaha drops 700 RPM's

[phreddp]

I've got a problem that's driving me nutts. My 1991 225 HP Yamaha will start-up and idle fine. One time it will feel like it's not at full power and only get up to 4600 rpm at full throttle then while running will jump up to 5300 rpm and run smooth. Then back down to 4600 then back up again. The next time you crank it, it comes right up to power and runs great for 5-10 minutes then start the same up and down 700/800 rpm's. I've rebuilt all carb's ( twice ), replaced fuel pumps, replaced fuel filters and water seperater filter. Replaced plugs twice, ohmed out crank position sensor and throttle position sensor. They were corroded but with-in limits. I cleaned all ground leads to the box.
HELP please. It hasn't quit on me yet, but I don't trust it to go off shore like it is. Any idea's??

 

[cousinabe]

Re: 1991 225 Yamaha drops 700 RPM's

Does that model have resistor spark plug caps?
Have you checked to make sure the caps are within specs?

 

[Ed R]

Re: 1991 225 Yamaha drops 700 RPM's

sounds like carbs are running out of fuel, at high speed. !!!!!!!!!!!!

 

[ghind]

Re: 1991 225 Yamaha drops 700 RPM's

You are losing a cylinder.

Why is the question. If it is electrical the motor won't quickly blow up from this. If it is fuel, it will quickly blow up.

Checking the plug caps (if you have resistive caps) is a five minute job. It could be that. They unscrew from the leads and you measure their resistance. They should be about 5-6K ohms. If any one or more are up around 10K+ ohms this can cause the problem you describe.

Otherwise it is most likely to be fuel system related. Is the bulb collapsing? Is there any possibility of an air leak in your fuel line? Have you replaced the bulb and line recently? You've already done the pumps and the carby rebuild which are among the most common issues.

After that you might have to move on to fuel system vaccuum and pressure tests.

 

[phreddp]

Re: 1991 225 Yamaha drops 700 RPM's

Thanks for the responses, I have replaced the fuel blub and tried pumping it when it was running at the lower RPM's. It doesn't seem to be a fuel problem. About the plugs being resistent type, they have an "R" in the #, but I'm not sure about the plug wires. They have a coil on each plug wire. Is this what your referring to? I'll test each and see what I'm reading. But the trouble is intermittent (spell?) could a coil go open and then back to norm? It does feel like I lose a clyinder then it comes and goes. Doesn't have a pattern to relate to heat.

 

[ghind]

Re: 1991 225 Yamaha drops 700 RPM's

The spark plugs screw into the heads. The plug caps are then pressed onto the spark plugs. Those caps, simply unscrew off the leads. You can then measure the resistance of them with a multimeter (from the bit that locks onto the top of the spark plug to the bit that screws into the plug lead). If your motor is really a 91 model, it looks like it does have these caps http://www.boats.net/parts/search/Yamaha/Outboard/1991/225TXRP/ELECTRIC PARTS 2/parts.html
Let us know what you find, it is a 30 second test. Check each. It is possible that some, all, just one or none could be bad. On a motor of your age, I'd expect to need to replace them but check first.

 

[cousinabe]

Re: 1991 225 Yamaha drops 700 RPM's

the caps will unscrew off the plug wires. Peel back the black boot from the cap, hold the wire and unscrew cap off of wire. Test ohms across cap between screw threads and tip. Should be between 4-6k
On a defective cap, you may get a high reading or none at all.

Keep us posted on your progress.

 

[phreddp]

Re: 1991 225 Yamaha drops 700 RPM's

You guys are sharp. I found out two things. First, my 225 is a 1990 225 ETXD
and second, three of the plug assembles ohms are the same, 4th was reading off scale "open" and the 5th was much higher than the first three, the last one was so corroded it broke off at the wire. I've ordered three new caps and I'll let ya know what happen's. I've got a good feeling ya'll are right -on the problem. I 've been fighting this problem for over two years... THANKS alot.
hopefully
gone fishin'

 

[ghind]

Re: 1991 225 Yamaha drops 700 RPM's

Good work phreddp

What readings did you get on the first three and on the fifth one?

I think you are on the right track but I'd be replacing all six. They are only 10 bucks each and with 3 of the 6 faulty you might just as well deal with it.

These are great motors, worth looking after.

I hated it when I had this problem - it took forever to find on mine and cost me well over $1000. I think (and hope) you have it sorted.

Greg

 

[triumphrick]

Re: 1991 225 Yamaha drops 700 RPM's

Would I expect to find these caps on a 2002 150 two stroke??

Similiar subject, don't want to hijack thread...

 

[ghind]

Re: 1991 225 Yamaha drops 700 RPM's

yes. They are used on my 2002 225 VMAX EFI also. They were also used on 97 models so I assume they are on all but the oldest motors. Check on the boats.net parts guide (follow my previous link and go from there)

 

[phreddp]

Re: 1991 225 Yamaha drops 700 RPM's

Greg,
I'm reading 5K ohms on three of the caps and 8k on the other. The one that was corroded also was reading 5K. The last one was off scale above 10K. I've also put over $600 in chasing this rabbit. I've ordered 3 more and plan to change all 6. I'll let you know how it turns out. And thanks again for putting me on the parts location, it's much appreciated.
Fred

 

[scottcmb]

Re: 1991 225 Yamaha drops 700 RPM's

An easy way to tell if they are resistor packed caps is they will be orange in colour...non resistor are black......careful when handling the resistor caps....when unplugging from a spark plug never grab and twist off!!! that can crack the carbon resistor inside.....allways grab at the opening of the cap (where the spark plug goes into) and gently pull straight off with no twisting....
I have one of these engines and had same problems a while ago and had 5 caps that gave no reading at all!!!!....so i replaced all of them and got a more resposive and smoother top end.
Another thing i would suggest you check is the six recirc check valves that are on the sides of the block (they are a one way valve) if they flow both ways then they will need replacing( dont confuse them with the nipples that are located on the reed block plate and intake manifold plate these flow both ways normally!!!)..i found i had to replace all six!!!.....this reduced plug fouling at idle.
to remove them simply lock a pair of vice grips onto the valve and twist back and forth whilst pulling outward..they just twist off.....dont remove good ones with this method as you will end up ruining a good one!!!!.....install new ones by gently tapping them into place.

 

[phreddp]

Re: 1991 225 Yamaha drops 700 RPM's

OK,
I've replaced the caps. On test run, it cranked-up and idled smoother, but on excelleration it was under powered (like it was on 5 cylinders again). After 2 /3 min. it would jump from 4600 RPM to 5200 and then back. Then it started running at 5200/5300 at full trottle. Problem still intermittent, but smoother!!!
I've just ordered a new crank position sensor.
Scott,
how do you check the valve's you were talking about without removing them?

 

[scottcmb]

Re: 1991 225 Yamaha drops 700 RPM's

OK,
I've replaced the caps. On test run, it cranked-up and idled smoother, but on excelleration it was under powered (like it was on 5 cylinders again). After 2 /3 min. it would jump from 4600 RPM to 5200 and then back. Then it started running at 5200/5300 at full trottle. Problem still intermittent, but smoother!!!
I've just ordered a new crank position sensor.
Scott,
how do you check the valve's you were talking about without removing them?

your intermitant problem on a good chance could be a fouled plug!.......i would look at replacing them after you check the valves....
To check them there is the easy nasty way.
Go to the nipple that is on the side of the reed block this nipple will have TWO tubes connected to it, one tube leads to the nipple on the intake manifold and the other leads to the one way valve on the cylinder block......disconnect the one that goes to the check valve and place into your mouth and blow first!!!.....then suck back on it.......if air flows both ways the valve needs replacing!!!.....you might get a bit of fuel juice in ya mouth ...thats the nasty bit lol.....a cup of coffee will fix that lol.....
remember if you find that you need to replace them make sure that the same tube gets connected to the same cylinder it was disconnected from No1 cylinder check valve is NOT line connected to No1 carby.......there is a special sequence....the best way is to do it is one at a time...(disconnect..remove and replace valve then reconnect the tube and move on to the next one)
I personally would do all this and replace the plugs before running the boat again....that way you then can test your top end issue properly.......
If you find that after all this you are still fouling plugs especially the port side bank plugs then its more than likely the oil pump shaft seals need replacing!
I went through the same issues as you did last season and have since rectified the problem by doing all this and replacing all these parts.....motor is running sweet as a nut now.......PM me if you need more help or tips

 

[cousinabe]

Re: 1991 225 Yamaha drops 700 RPM's

was this ever resolved?