8/9.8 hp Nissan peters out on low revs

[Plandeck]

2007 8/9.8 hp Nissan outboard starts only with choke wide open and will run only at high revs. Close the choke and reduce revs and she dies off. Fuel filter, pump and carb are all working fine. Is this an ignition issue?

 

[ondarvr]

Clean the carb, taking extra time on the low speed circuit.

 

[Plandeck]

carb is brand new...just swapped out the 8hp for a new 9.8hp

 

[ondarvr]

The ignition doesn't know or care if the choke is on, all the motor knows is that it's running lean and needs more fuel, which is what the choke helps with.

​Being new doesn't mean it's clean, the low speed circuits are very picky on how clean they are. There should also be a low speed adjustment screw, it might be covered by a brass plug, it's normally on the side of the carb. You may need to drill or pry the brass plug out and adjust it a little richer, they come adjusted as lean as possible.

 

[Sea Rider]

Sorry folk, it's not a dirty carb issue, the hidden screw only adjusts the air/fuel mixture idle speed setting, has nothing to do while OB runs at higher revs past idle rpm as the jets are fixed, non adjustable ones whatsoever.

There's a huge carb/timing advance desynchronization. The 8 comes factory adjusted for timing plate and matting surface to stop at 22? at full throttle and 1.5? at mim throttle while the 9.8 comes factory adjusted to stop at 26? and 2.5? respectively.

What you can do to end your boating frustration is remove cowl, stand at middle powerhead, crouch a little to have timing marks at sight level, with OB -Off full throttle, check at which timing mark does plate stops at. Large stamped mark is 0?, smaller ones past 0? are + 5?, before 0? is minus? territory.

Check which bolt, nut whatever restricts timing plate to travel forward, undo it till timing plate stops at 26?, adjust when there, secondly, need to synch carb to fully open butterfly when timing synch has stopped at 26?, check whatever restrics its movement.In some Tohatsu OB's moving timing advance will advance the carb synch accordingly, check if moves same.

Just in case, before moving anything take pics of the original time advance /carb synch. Can't help any further as don't have at hand nor use a 9.8 horse to the displeasure of some. Probably Paul can add more information about the timing procedure, what to undone, move, unscrew whatever more precisely. Report your findings...

Happy Boating

 

[pvanv]

No Luis. This is a MFS8A3 4-stroke.
The timing is electronically controlled by the CD.

 

[pvanv]

2007 8/9.8 hp Nissan outboard starts only with choke wide open and will run only at high revs. Close the choke and reduce revs and she dies off. Fuel filter, pump and carb are all working fine. Is this an ignition issue?

Peter,

If you are saying the the motor only starts with the choke CLOSED (plate closed, enriched, activated), that would be correct for a dead cold start... BUT. the motor should only need choke for a minute or so at our temperatures this time of year. If so, I agree with ondarvr, that the setup sounds way too lean, and is nothing to do with ignition.

While there is a remote possibility of a defective carb, I have never gotten a bad one from the factory. I suspect something else is wrong, such as the throttle rod setting or similar. Since there are 2 carb base gaskets (separated by the plastic spacer insulator), Check to be 100% sure that something isn't wrong there. It's easy to have a piece drop while installing (particularly working down in your cabin), and especially if it's on the side towards the block, you might not see it, but that would cause a huge vacuum leak, which could cause your symptom. Also, you MUST have the intake silencer installed in order to get the correct air/fuel ratio.

To set the throttle rod, have the carb at dead slow idle, the rod screw unlocked, and the throttle at idle at the RC box. Notice that there is an oval "eye" in the throttle rod where it connects to the bell crank of the throttle mechanism. That allows a manual-choke setup to advance the throttle a little while choking, without causing the throttle bits to bind. In your case of a Remote, you raise the "neutral warm-up lever" on the RC box some to advance the throttle some while cold starting with the choke. Because of that, you don't need much slack in that "eye" on the throttle rod. You just don't want the rod adjusted to keep the carb throttle plate opened when the RC box is set to idle. Then lock the screw.

 

[Plandeck]

I must say I diagnose the problem as Sea Rider does...timing issue...but I'm also aware you can't adjust the timing on Nissan 8-9.9hp (I do remember and not with any pleasant nostalgia the timing light days)...that said, do CD units go bad? I replaced one of the little coils on the fly wheel...exciter coil if I recall...a few years back when she wouldn't start...solved the problem...wondering if the below deck condition for the OB increases the always present humidity and puts an extra load on the electronic parts...I failed a solar controller mounted below deck last year and this never happens...even got a letter of apology from the mfr...How do I test a CD? or just replace?

The open/closed oxymoron has always tickled me...we say "open the choke" in cold weather when starting but really mean close down the air intake to give it more fuel...the new carb is for automatic choke remote steering so you say choke linkage is reversed...haven't had time to play with linkage yet...does clockwise movement open the air intake or close it?...I just know when I turn it fully the engine starts with a bang and runs immediately at high revs so I assume I am closing the airway and increasing the fuel. When I do the opposite turn she won't start even with throttle advanced.

oh...and when I say below deck mounting, I mean beneath the **** pit floor which has a removable aluminum plate hatch making for easy 360 degree open air access to the power head...its a problem to shimmy under the power head and look up (thus I can't spot the oil leak...even with a mirror)...but access from above and all around if far superior to transom mounted OB...don't drop any parts in the drink with me ham hands either...lol

 

[ondarvr]

When going to a larger carb, and if there's a difference in timing between two the different HP motors, then the timing needs to be changed to achieve the full HP gain. It should still run correctly even if the timing isn't advanced though, just not at the full potential increase possible. If it needs the choke to run, then it's too lean, the reason could be exactly what Paul said.

 

[pvanv]

The entire MFS8 and MFS 9.8 are 100% identical mechanically, except for the carburetor. Period. The timing is controlled by the CD. Period.

Yes, it is possible to check timing with a timing light, but unless the gear belt that drives the cam shaft is damaged or installed incorrectly, the timing will be correct.
Maybe 1 out of several thousand CD boxes could fail on these. I have only had one fail in all of my time turning wrenches on these 8/9.8 motors, and it was subjected to very severe conditions. Electronics don't wear out; they just quit. With the possible exception of coils (such as exciter, trigger, alternator, or ignition coils, which can drift off spec over time.

Odds are the linkages are not set correctly. The choke plate on the electric-choke version of that carb swings in the reverse direction of the manual choke version. The choke plate on the solenoid-operated version of the carb has a cutout in the choke plate so that even fully choked (when the solenoid engages), there is still some air allowed through.

No, "we" don't ever refer to "opening" the choke when cold starting. Only someone who does not understand the purpose/operation of the choke (or a luddite) would say that.

 

[Plandeck]

false alarm...guess i had the reverse choke on the new carb all confused...now purring like the kitten she always was...but i do have a question on the auto choke, since tiller model is installed where do i attach blue wire from choke? not sure of the theory here...what engages the choke in cold weather and not hot weather? How does it know not to engage?

 

[Plandeck]

No Title

...just thought I would show off the 360 access to the power head with the under deck OB installation...

 

[pvanv]

false alarm...guess i had the reverse choke on the new carb all confused...now purring like the kitten she always was...but i do have a question on the auto choke, since tiller model is installed where do i attach blue wire from choke? not sure of the theory here...what engages the choke in cold weather and not hot weather? How does it know not to engage?

Glad you got the linkages straightened out. The blue choke wire gets activated to choke when cold. The person starting the motor does this himself; it's not an automatic choke. On RC key switch controls, you "push to choke", and that activates the solenoid; If no "push to choke" key switch is being used, you can use a normally-open momentary contact push button switch, as is used for horns, etc.

 

[Plandeck]

No Title

...tach is installed with a little plywood and epoxy remodeling of the cockpit...but electrical hookup not working...instructions diagram are for tiller model...trouble shooting guide recommends switching instrument power supply switch on to jump needle if gauge is inoperative...no such switch on tiller model. I have tach yellow and white going to alternator yellow and white, oil lamp red going to oil lamp red/white and oil lamp light green going to oil lamp light green, and tach instrument light blue goes to oil lamp red/white and tach instrument light black goes to alternator yellow...but get nothing when engine revs. Whats problem here?

 

[pvanv]

You are trying to power the light with the tach signal. The instrument blue is "running lights" on your boat. Black goes to ground.

 

[Plandeck]

Light the tach when motor is not running?...good power source for the tach light (and oil lamp) running off the motor only would be?...red off the rectifier?..seems I should be able to do this with 4 wires rather than 6...

 

[pvanv]

Most people connect the instrument backlight to the boat running lights. In the case of a sail boat, you can also put a shutoff switch to turn off the tach when not actually running -- conserving battery. The red from the rectifier is likely hot any time the battery is connected to the motor.

 

[Plandeck]

yes rectifier will be hot...but yellow tach runs to lanyard black which cuts the ground when not running (if I'm correct) which would also un-ground the instrument light.(?)...oil lamp is similar..the ground is switched...thus 4 wires. (guess the same result would be to run off the hot wire on running lights...but its a completely separate system on my boat.

 

[Plandeck]

Finally bought the bullet gave up trying to run the tach hot wire off the engine electrical system and connected it instead to the running lights as recommended...she appears to be working fine now, however the engine still will not make 5000 RPMs even with the new 9.9hp carb...she tops out at about 4800...nothing I can do about this right?...should I go back to the 3 blade prop instead of the 4 I put on this spring even though losing control running in reverse will be the consequence?...I think it's more important to have power available for rough seas at high RPMs than low speed control...also the tach oil lamp stays lit continually...running half full with oil only half way up the dip stick as recommended by you (this seems to help the leak issue)...but concerned about the constant lamp light on condition...is it my wiring? As I explained the boat has 2 distinct electric systems that are separate from each other. One battery lights the boat and instruments etc. The other battery starts the motor. Running the tach hot wire from the oil lamp and the tach light to the hot side of the 2nd battery and all the other wires to their respective wires on the engine's electrical harness....is this causing the oil lamp to remain lit?

 

[pvanv]

The carb is not for a 9.9; it's a 9.8.
Yes, you want the WOT RPM to be 6000.
If you are running a 7-pitch HT prop, you will want to go to a 5-pitch HT prop to get the RPMs up.
You are wired incorrectly if the oil light is always on, even though the LED on the cowl is not.
If you are worried about draining the house battery with the tach light, wire that circuit to the engine battery.