Yamaha 8hp 2 stroke bogging/surging above 3/4 throttle

[2shallow2sail]

I got this motor last year, I have done a bit of work but am still having an issue with top end power. When I have the motor on the muffs/bucket it starts on one pull and idles perfect. Once in the water on the zodiac, the motor will run fine as i throttle up to WOT and then after around 20 seconds running perfect up on plane, the motor starts surging/bogging (apologies I am not sure of the proper term). The motor will momentarily lose most of its power very abruptly and then suddenly regain that power. This causes a lot of head jerking for the driver and gets worse the longer you try to run above 3/4 throttle. Once I throttle down to idle and wait a minute, I can throttle back up to WOT for another 20 seconds before the symptoms repeat. The symptoms do not seem to improve if i pump the ball on the fuel line while running.

This weekend i rebuilt the carb and it idles even better than it did and is smooth to power up to 3/4 throttle, but still has the same surging and loss of power issues on the top end. I have fresh fuel, and brand new fuel hoses from a 1 yr old 6 gallon portable tank. The motor pumps water fine and does not run hot (new t-stat and water pump last year).

Does anyone have an idea? I believe that it is getting either too little or too much fuel. Will adjusting the needle screw on the carb allow more gas to flow? Or is the needle screw just for Idling. Also thinking it could be the fuel pump, but thought that pumping the bulb underway would help if that was the culprit. Could it be electrical or timing issues?

Sorry for the long winded post, I did a lot of forum reading before posting so I wanted to make sure I gave all the info I had. Many thanks.

 

[2shallow2sail]

Re: Yamaha 8hp 2 stroke bogging/surging above 3/4 throttle

FYI it is a year 2000 model 8 hp 2-stroke yamaha with a tiller. Sorry for not including that.

 

[robert graham]

Re: Yamaha 8hp 2 stroke bogging/surging above 3/4 throttle

Sounds like a classic case of insufficient gas tank venting(vent in fuel cap), possibly other fuel relates issues like weak fuel pump...double check fuel system, tank, fittings, fuel filter, etc.

 

[2shallow2sail]

Re: Yamaha 8hp 2 stroke bogging/surging above 3/4 throttle

Thanks for the quick response. I definitely have the vent on the tank open while underway, but I too still think it is a fuel issue. The tank is one year old and the hose and all the fittings on it are brand new. If it were the fuel pump, I was under the impression that pumping the ball would help? but it didn't (I even unhooked the hose from the tank while up on plane and held it above my head to pump fuel down towards the motor). Fuel Pump was my next project to tackle but was hoping to more concretely nail the problem down to the pump before I take it apart and buy the parts. There is a small filter inside the cowl just upstream from the fuel pump, I took it off and was able to blow through it but cannot see inside. If i had fresh gas would it hurt to bypass that filter for a test run to see if it made a difference? thanks again

 

[99yam40]

Re: Yamaha 8hp 2 stroke bogging/surging above 3/4 throttle

Test the fuel system with a fuel pressure/vacuum gauge to see if it has a restriction and putting out the nessisary fuel pressure by teeing in the gauge at different points to see what is going on.

Look in the frequent asked questions part of the forum to find a good post on testing fuel system

 

[2shallow2sail]

Re: Yamaha 8hp 2 stroke bogging/surging above 3/4 throttle

I printed the fuel test page in the FAQ and just ordered the vacuum/pressure combo gauge. This weekend hopefully I will find air bubbles or confirm the vacuum from the pump is weak. I will post what I find, I appreciate any useful tips for this type of testing this in the meantime. cheers.

 

[BaileysBoat]

Re: Yamaha 8hp 2 stroke bogging/surging above 3/4 throttle

You should also check the fuel pump diaphragm. If its leaking into the crankcase can cause the problem as well.

 

[rickjm1001]

Re: Yamaha 8hp 2 stroke bogging/surging above 3/4 throttle

Strange I think I'm having the same problem as you only mine is an 86 9.9hp 2 stroke.
I'm thinking it's fuel related as well, I have checked everything. Reed valves, rebuilt carb.
It seems a bit better but still surges.
I tried squeezing the primer bulb but its very firm so it can't be the pump.
I was thinking maybe it's the timing, to advanced and pre igniting?
It's a CDI ignition and I was under the impression that it's not adjustable
But I have no idea.
If you find out that its a fuel pump issue then I guess I will just follow your steps, because we seem to have identical issues here.

Thanks!

 

[rickjm1001]

Re: Yamaha 8hp 2 stroke bogging/surging above 3/4 throttle

How do you check the fuel pump?
I was under the impression if it's a pump problem that the priming bulb would start to feel soft.
But even so why is it after a few seconds I'm not sure of this 8hp but on my 9.9 it starts to run fine.

 

[Euphoria]

Re: Yamaha 8hp 2 stroke bogging/surging above 3/4 throttle

I have the same problem with my Yamaha 8 hp 4 stroke. did you solve it? if so do you mind pointing me in the right direction.

Thanks,

Thanks for the quick response. I definitely have the vent on the tank open while underway, but I too still think it is a fuel issue. The tank is one year old and the hose and all the fittings on it are brand new. If it were the fuel pump, I was under the impression that pumping the ball would help? but it didn't (I even unhooked the hose from the tank while up on plane and held it above my head to pump fuel down towards the motor). Fuel Pump was my next project to tackle but was hoping to more concretely nail the problem down to the pump before I take it apart and buy the parts. There is a small filter inside the cowl just upstream from the fuel pump, I took it off and was able to blow through it but cannot see inside. If i had fresh gas would it hurt to bypass that filter for a test run to see if it made a difference? thanks again

 

[2shallow2sail]

Re: Yamaha 8hp 2 stroke bogging/surging above 3/4 throttle

I rebuilt the fuel pump but that wasn't it. To test it i got up and running on my inflatable then unhooked the gas from the tank, so just the line was hooked to the engine. I then held the fuel hose over my head to see if gravity would act as a fuel pump. It didn't seem to help but I rebuilt fuel pump anyways. In the end, I just adjusted the carb screw a bit more and it seemed to pretty much solve it. Since it is the kicker on my sailboat I am rarely above 3/4 throttle so i cant really provide much of a current update. Now for the hole in the block from the water jacket corrosion, that's more current. Getting by on JB weld, not sure what junk aluminum Yamaha used on my engine but its going to be an anchor pretty soon.