8hp Nissan OB misses in high revs

[Plandeck]

My little 8hp Nissan outboard runs great except sometimes she misses in high revs. Engin is constant in low and medium revs but if I push the throttle she starts missing. Opening the choke helps this but should this be necessary? I've changed the fuel filter which seemed to work at first, but the problem re-emerged yesterday after running on high revs for half an hour. Also, I've installed remote throttle; when pulling the throttle back the motor doesn't respond immediatly but takes a few moments to slow down. Any reason for this?

 

[pvanv]

Re: 8hp Nissan OB misses in high revs

What is your actual model/serial?
Are you over-revving and engaging the ESG?
Do you haver a tach to determine at what RPM it misfires?
Do you mean CLOSING the choke (richening) seems to help?
Did you synchronize the throttle cable to the RC box so that idle is idle and WOT is WOT?
When was the carb last cleaned correctly?

 

[MattFL]

Re: 8hp Nissan OB misses in high revs

Could you be hitting the rev limiter? Like pvanv asked, do you have a tach to see how high it is revving?

 

[Sea Rider]

Re: 8hp Nissan OB misses in high revs

Is that a 2 or 4 strokes engine ? Probably could have a partially clogged carb, fuel pump diaphragams with starting issues, worn, dried, cracked, not well torque head gasket letting small water droplets pass into combustion chamber and engine missing specially at high revs ?

Happy Boating

 

[Plandeck]

Re: 8hp Nissan OB misses in high revs

Is that a 2 or 4 strokes engine ? Probably could have a partially clogged carb, fuel pump diaphragams with starting issues, worn, dried, cracked, not well torque head gasket letting small water droplets pass into combustion chamber and engine missing specially at high revs ?

Happy Boating

...well I did remove the head when installing the engine. Thing is it has been running fine at high revs and now not. Could it mean that after engine heats up thoroughly, say after 1/2 hour operation at high revs, there is some gasket failure?

As far as the choke goes, I mean pulling out the choke (I think that is richening it) the problem is somewhat relieved.

If I were surpassing the high rev governor some of you seem to be alluding to, it would not have functioned properly at the start, right? It really sounds like either H2O somehow getting into fuel mixture or not enough fuel getting through carb. Carb was removed, dissembled and cleaned this winter. Did not replace the jets or gaskets. Engin is 4 stroke NSF8A2EF2.

Could it be the timing? Or is timing a thing of the past? I remember on my college sports car if the timing was off it was fine at idle but would not rev higher.
Throttle linkage returns properly, but engin revs do not immediatly follow. Takes a few moments to decrease speed.

 

[pvanv]

Re: 8hp Nissan OB misses in high revs

Grab a Factory service manual for the 8A3 and 9.8A3, and run through the ignition tests. Do it when hot. You can verify timing with an inductive timing light. Verify crankcase oil level. Verify carb is really clean. Initial timing is set by aligning the marks on the crank and cam pulleys with the block marks. Advance is designed into the CD, so it not adjustable.

 

[Sea Rider]

Re: 8hp Nissan OB misses in high revs

When you are at speed and you throttle down to a stop and shift into neutral, the engine revs up for some seconds and then lowers down to idle rpm. Been wondering why 2 strokes engines being pure mechanical does so ? Could the CD be the one taking charge of this momentarily faster cooling situation ?

Happy Boating

 

[Plandeck]

Re: 8hp Nissan OB misses in high revs

Grab a Factory service manual for the 8A3 and 9.8A3, and run through the ignition tests. Do it when hot. You can verify timing with an inductive timing light. Verify crankcase oil level. Verify carb is really clean. Initial timing is set by aligning the marks on the crank and cam pulleys with the block marks. Advance is designed into the CD, so it not adjustable.

False alarm...the fuel line was kinked or not fully plugged into the tank so there was too little fuel at high revs...but I'm still not happy with the reluctance of the revs to decline when the throttle is pulled back...the tank is mounted above the engin power head. Is it possible this is forcing fuel and making the engin race even when throttle is cut?

 

[Sea Rider]

Re: 8hp Nissan OB misses in high revs

If you're reffering to "engine race" when throttle is cut, meaning slight over revs when at a stop after been running for some time, don't have a tech answer for this one, will see if someone more technical chimes in and provides an understandable response.

Fuel pump is what controls the fuel rate, assume not an issue if fuel is fed by gravity though as the hose internal diam is not that thick to over increase pressure. My tank is placed at deck and with over revs when at a complete stop when shifting to neutral after being runing in forward. Some seconds latter, engine drops to sweet idle rpm... next time will measure lag time.

Happy Boating

 

[bcbit01]

Re: 8hp Nissan OB misses in high revs

Sounds like you fixed the primary issue. Another case of "check simple stuff first." PVANV is right. Calibrate your cable to the RC throttle box.