YMIS Question! Ray Neudecker or Rodbolt around?

I have a 1990 225ETLD. I just got the motor and I am trying to figure a few things out. I know these things have no timing linkage and the timing is controlled solely by the computer. I've read the service manual cover to cover, but it really doesn't cover this.
While cruising anyhere between 3500-4500 it will just all of the sudden speed up a few hundred rpm's like you hit the throttle. It will usually settle back down and may do it again. At higher speeds it seems to do the opposite. It will drop real quickly just for a second. Feels like hitting the rev limiter only I am between 4500-5500 and the limiter is 6250. Otherwise it runs smoothly and doesn't seem like it is starving for fuel. The carbs were brand new just before I got it and it does not seem like a fuel issue because the rpm changes are too quick. But then what do I know.
I know this motor has a bunch of sensors that affect timing. I suspected a flat spot in the TPS but it ohms smoothly through the full range. Any ideas?
THX

Re: YMIS Question! Ray Neudecker or Rodbolt around?

when your doing your ohms and output voltage tests on that TPS always use an analog meter, digitals average to much and wont find most intermittant issues.what your describing does sound like a possible fuel issue.
I would replace the two lift pumps and if that model uses the enrichment pump with the A and B fuel solinoids rebuild that pump as well.
you can also use the diagnostic test lamp on that engine,best I remember.
timing is controlled by the ymis computer based on various signals from the CPS,TPS and coolent temp sensors.

Re: YMIS Question! Ray Neudecker or Rodbolt around?

Thanks. I will rebuild the lift pumps...can't hurt. No solenoid pumps on this one. I will try an analog multi-meter on the TPS. What exactly am I looking for? Would the needle jump or stay put on the bad spot?

Re: YMIS Question! Ray Neudecker or Rodbolt around?

I dont think that system supports the diagnostric test lamp.
the analog meter needle should rise and fall smoothly with throttle angle.
a hiccup will make the needle jump one way or the other then recover.
also monitor pulser coil and crank position sensor voltages at the problem RPM.

Re: YMIS Question! Ray Neudecker or Rodbolt around?

Update:
Rodbolt, It is looking like you were correct in that this is a fuel related problem. The TPS ohms out fine with an anolog meter. I rebuilt the 2 lift pumps (everything looked perfect but I carried on anyway since I had the parts in hand).
Back in the water the problem persisted and at times was even more pronounced. Seems to run fine up to 3500 RPM and then it starts to surge. Above 5000 the surging was quite bad and it would not go up to full RPM as it rose only for few seconds at a time and then dropped back to 5000.
I pulled the wire off the detonation sensor to see if it was that kicking in, and it made no difference. Heat sensor and crank sensor ohm out fine.
I was told by the dealer I bought the motor from that it had brand new carbs, but he also told me it had a brand new water pump which was not true, so I don't believe him. What next? Fuel filter is clean, bulb is good, fuel tank is clean. Should I strip the carbs and clean them since I don't really know if they are new or not? Any other thoughts? Where else would I look for a fuel restriction that only affects higher RPM?
THX

Re: YMIS Question! Ray Neudecker or Rodbolt around?

Further info:
I really want to eliminate simpler problems before I go to the trouble of pulling the carbs. (And water testing the boat is a big deal at this time of year in Canada. It snowed 4" yesterday.) I double checked the fuel pumps, made sure check valves were functioning. They were. I noticed this model has a Fuel Feed Assembly (check valve). I have read elsewhere on this forum that these can be a problem. It opens when you blow into it but it requires a pretty good blow with your mouth. I have removed the guts and it is now unrestricted. Is it too much to hope for that this is the problem?

Re: YMIS Question! Ray Neudecker or Rodbolt around?

A fuel pressure/vacuum guage test will tell you if you have a restricytion or low pressure and keeps you from just shooting at it blindly and replacing parts that may not need to be replaced.
Without being able to reproduce the problem and testing it when it is having the problem, the only thing you can do now would be to clean the carbs. That old of a motor will probably need it if you do not know when it was done last.