90hp Yam no power after low speed circut?

Hey just replaced to a new power head on a 1990 yamaha, this was a complete and new old stock motor. I had to clean the carbs just to get to idle when I took to the lake it would run at low speed fine and once I tried to open the throtle it would just shake and sound like it was going to die. I can rev the thing fine and it sounds strong. I though it was the carbs and tried another set of carbs and got the same results, once I start to crack open the throtle plates it just craps out and sounds sick. Any ideas??
Thanks Mike in nc

Re: 90hp Yam no power after low speed circut?

ideas? many
formost is simply trouble shoot it and fix the faulty subsystem.
start with a spark and compression test. if compression is good you eliminated a raft of mechanical issues, if spark at cranking speed is good you now know the ign coils and secondary wires as well as the wiring from the CDI to the coil primarys are good.
was this actually a new fully dressed never used powerhead from japan, a new block assy or simply a new to your frame motor?
20 years is a stretch that you got a fully dressed new powerhead from yamaha.
sounds more like a craigs list or E bay junker.
try to clarify new or new to your frame powerhead.
test the ign and compression, if all is well clean or repair the fuel system as nessasary.

Re: 90hp Yam no power after low speed circut?

this was a new old stock fully dressed. Yes it was an ebay buy.
I will check the comp tomorow, its has spark at all cylinders, and the carbs were very clean and the seals fresh. For sitting 20 years it looked good, but not good enough.

Re: 90hp Yam no power after low speed circut?

If it has good compression and spark at the correct time, it has to be a fuel/air ratio problem.
You can check fuel delivery with a fuel pressure/ vacuum gauge for problems and make sure the timing is doing what it is suppose to be doing with a timing light.
Did you do the sync and link procedure from your manual after installing carbs?

Still sounds like carb problems though.

Re: 90hp Yam no power after low speed circut?

Ok here is My update for today.
Copression was even 115 on all three cyl.
I checked the thermostet with an ohm meter and warmed it up to make sure it was working and its good. The other carbs I put on came off the bad powerhead and had just been gone threw less than 6 months ago. I removed the reed plate and carbs togethere for this. Could the power pack be involved?
Thanks, Mike

Re: 90hp Yam no power after low speed circut?

What happened to the other power head? Did you find out why it died?
Did it crater because of running too lean because of a fuel restriction and now you are trying to run the same carbs on the new power head.
Do the simple things like make sure the carbs are good and clean, all passages, and adjusted properly.
Check fuel delivery for restriction or low pump pressure.
Use a spark board to check spark and a timing light to see if the timing is correct
get a manual and start testing the electronics with the proper tools if you find out it is a spark problem.

You cannot test a thermostat with a ohm meter, but you can a temperature switch. What temperature did the switch close its contacts at and at what temp did they open back up?

Re: 90hp Yam no power after low speed circut?

In the shop today, I will give an update as to the problems found. It may have been a timing issue all along..