Logo

92 8 hp mercury starts only when holding throttle plate open

bigjohnson

Regular Contributor
Bought a 1992 8 hp 2 stroke and rebuilt the carb, fuel pump etc.
Started and ran for a while on the bench but then not. Not certain of the correct setting for the SLOW NEEDLE. I tried 1 1/2 turns. Played with it a bit.
Anyway, it's getting fuel but it will only start if I hold open the THROTTLE FLAP by hand. Also, not certain of which way the DIAPHRAGM COVER goes on OR if it makes a difference. Guy before me worked on the motor and may have gotten it backwards. Any ideas?
 
Does the knob on the front of the motor push on the primer when it is pulled out? I will bet it doesn't because it looks like that assembly might be put together wrong. Let me know. That arm by the primer should drop and push on it. If it doesn't move far enough you need to take the plastic bits off of the front of the carb. The pull the knob out of the carb assembly make sure you remove the clip and screw. When the plastic block comes out of the carb it will come out in 2 pieces. You will see a slot in the block for the primer arm to ride in. When you reassemble the carb push the primer arm all the way down on the primer and push the block back in. Make sure it rides in the groove.
 
Last edited:
Thanks for the response but, I seem to have found the issue.
The characteristics of this motor seems to be different then say evinrudes I've worked on. If I CHOKE this motor AT ALL it will flood. So, DON'T CHOKE IT.
The components you are referring to are ok. I pull the knob and it does push the primer......MAYBE TOO GOOD since it seems to flood so easy. If you look at the second pic it shows a small hole which was actually turned around when I removed it. I saw a video of another guys motor showing it like my pic and his motor ran fine.
It idles fine now in a tank. I'm still wondering about the position of that hole.
 
Last edited:
Here is what your starting procedure should be. The hole position is not important it is just a vent for the primer diaphragm
#2 pull primer knob out and leave out
#3 turn knob all the way to the right.
#4 start motor and wait for it to start bogging down.
#5 push knob in until the motor starts to rev up
#6 turn knob back to the left until it is idling.
 
Last edited:
Did you rebuild the primer (assembly) as part of the carb rebuild?

There is a very small ball bearing that (should be) on the end of the spring under the primer plunger. If that gets lost or dislodged it will cause a flooding condition when the choke knob is pulled. Worth a check...
 
I did rebuild the primer and I saw the ball bearing and reinstalled it the way it came out with a NEW spring.
Fuel pump as well.
If I pull my choke out it seems to flood.
Don't pull the choke and it starts and runs fine.
Do you know the EXACT low idle screw adjustment? Is it 1 1/2 turns out and then adjust accordingly?
Oh yea, I was wondering about the small O-RING that the ball sits on. My rebuild kit did not come with a new one. Can you buy them separate?
 
Last edited:
The 1 1/2 turns is a starting point. There is an adjustment procedure in the service manual.

There is supposed to be a rubber ring (held in place by a tiny c-clip) that the ball bearing seats on.
The rubber ring comes pre-installed when you buy a new float bowl.
There is also a small check valve between the float bowl and the primer bowl that comes pre-installed.
 
The ball bearing sits in its own seal no clip on it. It comes in the $108 dollar carb kit. Do you turn the knob all the way to the right when starting.
 
Here's a photograph of the underside of the primer bowl.

If the throttle plate is closed, there is a high vacuum in the intake and if the rubber ring is not present,
there will be a free-flow of fuel through the primer into the intake and resultant flooding.

If the throttle plate is open, there is no vacuum in the intake and flooding will not occur.

Makes sense?

The spring and ball bearing prevent free-flow.
marine engine photo.jpg
 
The ball bearing on mine "MWC" carb, sits on a o ring. The o ring has probably NEVER been replaced but still looks ok.
As of late, I just screw the choke at the half way mark and I do NOT PULL it out at all and it STARTS on half a pull. Just does not seem to like being screwed in to the right and pulled out. I'm still playing around with the "LOW IDLE" screw 1 1/2 to 2 turns. Don't have a manual on this motor but I'm looking to buy one so I can DIAL it in.
(If the throttle plate is open, there is no vacuum in the intake and flooding will not occur).
Yea..... I guess that's why it would start when I opened the throttle plate.
I'm wondering about that o ring though. Wish I new the EXACT size so as to not buy the $100 kit just for that.

 
WHITEGREG56
You are correct, I saw the small c clip holding the o-ring and the check valve. I figured it came from the factory that way.
 
New info: So the motor will run at idle but it idles really slow. It runs just fine at any other speed it's just a idle issue I'm trying to figure out. Also, I can put it in forward gear (before starting) and then start it while moving the throttle forward and it starts and runs just fine. But once I put it in neutral the idle goes slow and almost (and sometimes) stops. If you see the first pic at my index finger at the screw......this screw is in all the way. It appears if I was able to screw in further it would advance and open the throttle plate a bit causing it to idle faster. This position of the SCREW is where it was when I bought the motor. Also, if I twist the choke to advance the idle nothing happens. The idle does not increase.
This SCREW seems to be in way too far. Why would it be in so far. Any more and it would fall off of the stop????
I pull the choke all the way and it does push the PRIMER component.
 
The ball bearing on mine "MWC" carb, sits on a o ring. The o ring has probably NEVER been replaced but still looks ok.
As of late, I just screw the choke at the half way mark and I do NOT PULL it out at all and it STARTS on half a pull. Just does not seem to like being screwed in to the right and pulled out. I'm still playing around with the "LOW IDLE" screw 1 1/2 to 2 turns. Don't have a manual on this motor but I'm looking to buy one so I can DIAL it in.
(If the throttle plate is open, there is no vacuum in the intake and flooding will not occur).
Yea..... I guess that's why it would start when I opened the throttle plate.
I'm wondering about that o ring though. Wish I new the EXACT size so as to not buy the $100 kit just for that.

Throttle plate position has nothing to do with the primer. The fuel pump fills the primer from the float bowl. If the seal is bad or the ball is pushed into the spring, Then the motor floods because the fuel pump will push fuel through the primer. Have you done a link and sync on your motor. You need to adjust the low speed mix screw and the idle screw as a pair. This must be done idling in gear on a warm motor.
 
Something is not right if that screw (idle timing screw) is turned all the way in!
Went the knob is turned clockwise, the timing should advance and the throttle should open up a little bit.

Is your idle wire installed properly? Make sure it isn't upside down.
The factory service manual has the adjustment procedure for idle mixture, idle speed, idle wire adjustment, idle timing, etc.
Did you get a factory service manual?
 
Something is not right if that screw (idle timing screw) is turned all the way in!
When the knob is turned clockwise, the timing should advance and the throttle should open up a little bit.

Is your idle wire installed properly? Make sure it isn't upside down.
The factory service manual has the adjustment procedure for idle mixture, idle speed, idle wire adjustment, idle timing, etc.
Did you get a factory service manual?
 
Last edited:
Back
Top