1997 yamaha pro 75 help

Hopefully somebody can help me out with this one?

I cleaned the carbs this winter. This spring I put the muffs on it and ran it. Ran great. Went to the Rainy river last Friday and ran great to first fishing spot. Started right up and went to next spot and ran great til I let off the throttle and it started running rough. Looked it over, had spark in all 3 plugs but couldn’t find anything wrong. Fished for awhile and then decided to move. Ran rough still until it got to wide open throttle and then run fine. Let off throttle and ran rough again.

Well we went home and I looked it over closer but couldn’t find anything wrong other than a cracked fuel line on middle carb. Fixed line, hooked muffs up and started it. Same thing, ran rough. I decided to manually choke it with the emergency choke and started running better but not smooth which makes me think not enough fuel. Turn the choke off and running rough again. Decided to pull sparkplug cap off one at a time and put cap back on. The bottom two cylinders didn’t make any difference but top one made it run worse when pulled off. But would stay running…barely. I have the carbs off cleaning them again. Took the filter off too and cleaned it. Was a little bit of crap in filter but not much of anything to make it plug.

Anything else that I should check before I put it back together? I’m stumped on this one.

Also the compression seems good also.

Thx
Woody

Are you running fresh gas? Is there any chance you have water/condensation in the tank from winter storage? Do you have a water seperator filter installed?

-J.

Well it had about 2/3 tank of gas in it and then filled it last Friday on the way to the Rainy. It doesn’t have water separator. I was thinking water or gas got bad over winter? It has stabil and sea foam from last November in it.

I’m thinking maybe getting too much gas? I think I was choking it when I was actually not choking it manually. I’m going to get it back together and try it again. Possibly bad choke? Will let you know.

Thx for the input

not super familiar with the yammies,but will give it a shot,most multiple cylinder two stroke engines have balance tubes between cylinders to help balance intake charge pulses,they are normally a plastic line pushed onto a fitting near the crankshafts center line.

when these break on a merc for example,it will run lean,hence choking it making it better,but if not found soon enough,it will burn a piston due to being too lean.

look for these lines on your motor,make sure if it does have them that they are not cracked/broken or popped off of the fitting.

another thing I have seen is oil fouling of the spark plugs,do the bottom two plugs look oily wet or shiny?? if so,they are fouled and need replaced,the may show good spark when not in the engine,but the spark will blow out under compression,try moving the top plug to one of the bottom cylinders and see if that changes which cylinder runs best,if it does,then you know that you have a plug problem.

this happens a lot when bad fuel/oil are splashed onto the plugs when cold and wont clear up when warm.
good luck and let us know!!

Does it run rough at idle while in neutral?

FDR

I learned last year that you can clean carbs but if there’s more particles in the tank or fuel line, they go right into the carb you just cleaned.

iowa_josh wrote:

I learned last year that you can clean carbs but if there’s more particles in the tank or fuel line, they go right into the carb you just cleaned.

good point iowa josh,its a good idea to flush the fuel hoses clear up to the carb( s ) before hooking them back up.
while you are at it,clean the screen at the fuel pump when done flushing it out.

Different animal but your symptoms sound just like my harley a few years ago. Ended up that the pilot jet was bad. To me it sounds like a jet issue if you are getting smooth performance at WOT.

Thx for the responses. I figured it out. It was the diaphragm for the choke. It was super brittle and tore when I tried to take it apart. Don’t know if it was tore before.
Thx again
Woody