Logo
FULL SITE MENU

1994 40HP Yamaha 40TLRS 2 Stroke

K

kboutboard

New member
"Last fall when I was winteriz

"Last fall when I was winterizing I put in new plugs (its a triple), new fuel filter and a new fuel line with primer bulb (from external tank to engine).

This spring the engine is running terribly. It runs OK under 1/2 throttle. If I go to full throttle it runs OK for about a minute and then it starts to surge, alternating between full power and reduced power. It doesn't die it just keeps surging. If I throttle back to about 1/2 it seems to run normal. It also dies in neutral a lot.

I have a tech manual so I've checked the throttle linkage (for timing), and I've adjusted the external mixture screws to the recommended 1 1/8 - 1 1/4 turns out. I've also adjusted the idle screw. I also have sprayed carb cleaner into each of the three carbs while running. None of this seems to help.

I dont know what to try next. Could it be my fuel pump?? Could it be a venting problem?? Could the heat range of my plugs be inappropriate?? Something else??"
 
"Kevin, it could be a number o

"Kevin, it could be a number of things but the first thing I would do is change the plugs again (you may have got a bad plug) and make sure the connections on your gas line are tight (new line may have an air leak).

I have chased my tail on a number of occasions when a bad plug was the culprit. If that doesn't make a difference then start troubleshooting the fuel system..."
 
"Graham, if it had a bad plug

"Graham, if it had a bad plug would it still run fine at full throttle for a minute?? I would think a bad plug would give rough performance thru the entire throttle range. I thought about the air leak. The connections seem tight at the tank and engine. How can I detect an air leak??"
 
"Seen plugs do some weird thin

"Seen plugs do some weird things - bad plug wires too - work when they feel like it - so wouldn't rule it out completely.

Easiest way I know to check for an air leak is disconnect the gas line from the motor, pump the bulb up as hard as you can get it and inspect the line from tank fitting on for gas dripping..."
 
"Graham, thanks for responding

"Graham, thanks for responding. I checked for air leakage. I did what you suggested and didnt see any gas dripping. I also had another person drive the boat at full throttle, after about 30 secs of normal behavior it started to surge, I pumped the primer bulb to see if the surging would stop and it didnt. I have read other posts and they suggested that as a thing to try to eliminate fuel pump / supply issues.

I have not tried the spark plug replacement thing you suggested, that is the next thing I will try. Do you have any more suggestions??"
 
"Ok, so sounds like the fuel p

"Ok, so sounds like the fuel pump is ok.

I would check to see if you losing spark when it surges. If your buddy can help out again and you can get hold of a timing light, it is fairly easy to check.

But it really still sounds like a fuel problem - however I would test the ignition and maybe do a compression check, just to be sure..."
 
To check for the loss of spark

To check for the loss of spark do I have to perform timing while the boat is at WOT while it is surging out on the lake?? Am I looking for a variation in the timing or something else??

If it was a compression issue would the boat run at WOT at all?? How could the compression be OK for a time and then be bad??

I had other boats that they would run WOT for a small amount of time and then die out. Those always turned out to be a fuel filter issue. They would perform fine until the filling of the carbs could not keep up with the draining. The problem is I replaced the Yamaha fuel filter last fall. I cant imagine I could have screwed that up.
 
"just-in-time, do my symptoms

"just-in-time, do my symptoms sound like a stuck float or a plugged needle valve?? Why at WOT do I experience this and why does it run fine at WOT for about 30secs and then start to surge if the carbs are bad?? Wouldn't it not go to WOT at all??"
 
Back
Top