Logo
FULL SITE MENU

Mercury 9.8 Fuel pump issues

M

mjh8300

New member
Hello all,
So this mercury 9.8 110 has been giving me some issues,
1) After priming it will start and run fine unit the carb bowl runs out of fuel then it will die.

2) on first start it doesn't make any weird noises for about 30 seconds or until you give it some rpm and then it will make a clicking, clacking sound that I cannot pinpoint with a mechanics stethoscope, seems like an internal thing/coming from throat of the carb, it does not always seem rpm dependent however often is.

3) is it normal when pressing the primer bulb (after it has already been hard) for gas to come out of the front of the carb on the nozzle pointed out of the throat

I am not sure if these problems are at all related.
The carb is perfectly clean and has all new gaskets. The motor seems to be a later model as the fuel pump is built into the side of the carb, when rebuilding the fuel pump the carb kit I ordered had an extra rubber gasket (similar to the brown one that I think is the check valves). I originally left it out of the rebuild because it was not there to start with. After putting the carb back on and still having the issue I went back through it and put the rubber gasket in it to still have the issue. During the second carb takedown I checked the float position and barely made an adjustment. I have replaced all the hoses and primer bulb, I am also using a proven gas tank. Today I pumped fuel into the carb using the primer bulb, and pulled off the hose that runs over the top of the carb and put my finger on the hose to find that there is the slightest bit of suction if you leave your finger there for 30seconds. When the carb is full of fuel this motor runs like a top, super responsive to throttle and doesn't miss a beat, will start up first pull after running. I have read that these motors can have bad crank seals but I would like to check anything else before messing with that. I am at my wits end with this any help would be great. last thought, is there a way to check any sort of vacuum or crankcase pressure on these motors.
 
Pull the flywheel.----Inspect the 4 bolts that hold the upper bearing housing.----Have seen those loose.----And look at the upper seal.----Have seen those come loose as well.----The gasket between the carburetor and block is installed correctly with PRESSURE pulse hole lined up.
 
What were the symptoms before you did the carburetor work? Did the problem begin after an extended storage period? Agree with Racer to check those things....that "pulse hole" to carb/fuel pump is often the problem as the gasket may block it off. Then fuel pump has no chance to work. You can fit a vacuum gauge to that little pulse hole and pull motor over. I have an adapter, you can make one too. See what you get. What are your compression numbers? By the way, welcome to the Forum.
 
Last edited:
racerone said:
Pull the flywheel.----Inspect the 4 bolts that hold the upper bearing housing.----Have seen those loose.----And look at the upper seal.----Have seen those come loose as well.----The gasket between the carburetor and block is installed correctly with PRESSURE pulse hole lined up.
Click to expand...

I did take the flywheel off to inspect before, everything was tight. the gasket for the pulse hole is brand new, odd how it’s got the two holes along the bottom, but not another hole for the other port. It’s the same gasket I took off that was tore up so...As far as symptoms before, I picked up the motor with a Jon boat I bought knowing it needed carb work but I was not expecting this amount of trouble. I have not checked compression numbers yet which will probably be my next step as well as using a vacuum gauge on the pulse hole...tried last night but my Chinese gauge was junk lol.
thanks for replies all will report back soon
 
Yes, forgot to mention, my vacuum gauge is also a pressure tester. It reads to 8 psi.....dah! There is no "peg" on it. Have had it for 46 years....back when the only thing you could buy that was Chinese, was food. Don't think they make 'em anymore. Racer, your one sharp dude.....don't miss much.
 
Last edited:
Ok so here is what I have figured out...... Checked compression, top cylinder 105 bottom 100. Not the greatest not the worst. took carb off again, I could not find anything to check pressure from the pressure port. So I grabbed a can of carb cleaner sprayed some in the hold and pulled. Out came carb cleaner in a mist.... ok so the carb has to be the issue. After pondering for 30 min I took the fuel pump apart and realized that the diaphragm that came with the kit was A LOT thinner than the one that I replaced the first go around. So I double cleaned everything and put the old diaphragm on and now its pumping fuel. I presume the lower compression and the thinner diaphragm both played a part in this but no longer an issue, however still have the odd noise.

Noise isn't there on start up till the motor gets warm or if you rev it past idle before it warms up. Any ideas on that part??? I have watched a couple videos of them just running and can't seem to hear the noise on those, so its probably not a normal thing.....it does run great though
 
How high are you revving it in neutral, without a load on it high revs can cause internal damage what you might be hearing is a rod knock or piston slap.
 
Not any higher than it will let you in neutral. Wouldn't rod knock or piston slap be easily detectable with the stethoscope? Here is a video of the issue, keep in mind this is after its warmed up and started making the noise, it ran for about 2 minutes before the noise started the first go around. Here I start and stop it to show the issue....
https://youtu.be/RJ1gZQOfcQQ
 
That sounds more like a buzz; there's a neutral interlock that contacts the flywheel - maybe it's hitting and vibrating.
 
Back
Top