F40 motor bogging when accelerating fast

[Hrvatska41]

Hi all, im new to this forum so i dont know if i was supposed to post this in a specific section or not. Anyways i have a yamaha f40 outboard (3 cylinder carbed) which starts, idles , *****s and even runs at wot fine but when i am going a slow speed then suddenly accelerate full throttle it boggs down sometimes turning off but if i pull the lever back quick enough i can manage to keep it running. It starts back up fine and if i ease the throttle up it doesnt have a problem its only when i push it up fast. Has anyone had a similar issue with this motor? Only thing i can think of is a blocked jet or accelerator pump ( dont know if these motos have accelerator pump or not). Anyways any help would be appreciated

[boscoe99]

What model/year F40?

They are not all the same.

[ausnoelm]

Best guess seeing as it’s an “old” carb model, then a proper carb clean/rebuild is in your future.

[Hrvatska41]

What model/year F40?

They are not all the same.

My bad, its a 2009 F40BET. I have only owned it for a month or so. The problem was apparent from the first time i took it out. Since i took it out last i have changed the inline and water/fuel filters and added fuel stabilizer/cleaner. I will update if theres any difference once i get a chance to take it out again

[99yam40]

I think I have read about rejeting those 40s to help if the proper cleaning does not help

[ausnoelm]

My bad, its a 2009 F40BET. I have only owned it for a month or so. The problem was apparent from the first time i took it out. Since i took it out last i have changed the inline and water/fuel filters and added fuel stabilizer/cleaner. I will update if theres any difference once i get a chance to take it out again

Fuel stabiliser/cleaner is going to fix nothing.

[DennisG01]

My bad, its a 2009 F40BET. I have only owned it for a month or so. The problem was apparent from the first time i took it out. Since i took it out last i have changed the inline and water/fuel filters and added fuel stabilizer/cleaner. I will update if theres any difference once i get a chance to take it out again

I don't see that model listed - can you go to the Yamaha link at the top of this page and find your model and copy and paste that link?

Regardless, though, as noted above - sounds like a good carb cleaning/rebuild is in order.

It won't hurt to use heavy doses of cleaner/stabilizer, but it's unlikely to help - hopefully, but unlikely.

[boscoe99]

An F40BET model was not offered in the US by Yamaha USA. But it is similar if not identical to the US F40BTLR model. Which indeed has an acceleration pump.

Remove the intake silencer. Look at the inlet to the carburetor. Specifically at the horizontal nozzle. Quickly advance the throttle. If you do not see gasoline spray out then the carburetors need to be cleaned. More than likely in a heated ultrasonic cleaner. Completely dismantled. By someone that knows what they are doing.

Easier written than done. Good luck.

[Hrvatska41]

An F40BET model was not offered in the US by Yamaha USA. But it is similar if not identical to the US F40BTLR model. Which indeed has an acceleration pump.

Remove the intake silencer. Look at the inlet to the carburetor. Specifically at the horizontal nozzle. Quickly advance the throttle. If you do not see gasoline spray out then the carburetors need to be cleaned. More than likely in a heated ultrasonic cleaner. Completely dismantled. By someone that knows what they are doing.

Easier written than done. Good luck.

Thats probably the reason im having trouble finding a service manual because my model is specific to australia. The old owner did have the carbs cleaned and balanced a year ago (which i can confirm) because of a similar issue but said he hasnt had a problem since. Judging by the picture and description you sent it sounds like it could very likely be an accelerator pump issue. I did run it yesterday on the muffs for about 15 minutes since i put the stabilizer and new filters and seems to be idling slightly higher although im pretty sure my taco is off because it shows a reading of 9900rpm at wot which is unlikely

[Hrvatska41]

I thought id post an update incase abyone reads this one day with a similar problem. Took the boat out today. Warmed it up and when to punch the throttle and it started to hesitate so i backed off then went up slowly. Drove it at wot for a few kms and stopped for a fish then tried again and was noticably better. By the end of the day it was running perfect. Thanks all for your help anyways

[DennisG01]

Hopefully that luck continues! Keep in mind that using additive (successfully) to fix that type of problem is not the norm. Glad it seems to have worked out for you! Must have been just a little gummed up. Usually there's more going wrong. A couple more outings should tell for sure.

Keep running the additive all the time.

[Hrvatska41]

Hi all, i seem to have a new problem coming up. I noticed the water trail when running the boat on muffs seems to have carbon particles floating around in it. So i pulled the plugs and noticed #1 and #2 plugs were quite black and #3 still looked brand new. Im not sure if they are wet with oil or fuel but im really hoping its fuel with carbon soot. Any ideas? I changed all 3 plugs about a month ago and the previous ones looked fine

[panasonic]

Is it burning any oil?

[Hrvatska41]

Is it burning any oil?

Im not quite sure its hard to tell. I took the lower unit off yesterday to check on a gear case seal i changed the other week and noticed there was also engine oil on the drive shaft spline, so now i also have a leaking crank or oil pump seal which wasnt there last week. Could the fouled plugs and oil leak have anything to do with each other or is this ust a coincidence?

[panasonic]

As for your plugs...run the engine hard for awhile and see if the carbon clears up. Perhaps the fellow who had it before done alot of idling with the carbs out of whack. I would take those carbs off and clean them properly. Then see what you get. Try running Yamaha ring free additive as well.