TOPIC: Another Merc 800

Hey All,

Back to the drawing board or actually back to the work bench with the old Merc 800 FGS.
Been running it on Mama Mia! for 8-years now and been taking lessons all along. We have logged over 3-thousand miles and rebuilt everything other than the power head which has never been opened.

I started having a problem keeping it idling a couple years back. At first intermittently but last summer it became progressively worse and it was time to rebuild the carbs. Previously I had replaced the fuel pump diaphragms and hoses and float valves and crankcase bleed check valves but never pulled the carbs off.

Last fall I yanked em off and cleaned ultrasonically and finished by spraying carb cleaner and compressed air through all orifices and passages.

Put it all back together and went out on the lake. Following the Merc manual on the idle adjustment procedure I dialed them in and she ran better than ever all summer. Nothing beats a flawless idle when docking or maneuvering through anchored boats and swimmers.

We went up early on Thursday for the Geneva Lakes Show and put her in the water for some motor boating before the show opened on Saturday. Got her launched and it started right up but I knew something was not right as I tried to idle out of the harbor at no wake.

It seems to idle up and back down and soon dies. Checked tank vent right away was running a fresh tank of gas (ethanol unfortunately) although treated with Seafoam. Try switching tanks but it made no difference so I never made it out on the lake and missed the boat parade on Saturday.

I did change the water separating fuel filter at the show but that was a waste of time and money because the original had no water in it.

See Video:


Just getting started checking it out and the plugs look fine and compression tests 95 to 105 which has not changed since I got the motor. Distributor was rebuilt last year and all the wiring has been replaced so I suspect a fuel delivery problem.

Thoughts, ideas, comments welcome.

Thanks,
Terry

does it pick up if you do a QUICK CHOKE? back the idle adjustment screws out 1/4 turn.

Thanks Dave,

Before I rebuilt the carbs that QUIK CHOKE maneuver is what I used to help keep it running. It was a good indicator that it wasn’t getting gas and that the QUIK CHOKE richened the mix.

I am sure the idle screw adjustment hasn’t moved and she ran great the last time out a few weeks ago. If the mixture is the problem now I think there must be something else going on beyond a simple adjustment. But what the hell do I know?

What’s strange is that it revs up momentarily without touching the idle lever then just stops. I never heard it do that before.

Another video:



Terry

yes but a small piece of dirt can be there and re adjusting the carbs could dislodge it also check the idle tubes for cracks. just a thought.

Thanks Dave,

So you’re suggesting opening the idle screw temporarily to try to blow it out if in fact there is an obstruction? I can try that.

When I pulled off the carbs last fall they looked pretty clean to me. I guess what happens though is the buildup is internal where all the passages merge under that metal button. The bowl strainers were clean too and I can't see how dirt would pass through those. My tanks have been coated and are clean and I use a Merc water separating filter too.

Non ethanol fuel is not available in my area and I been burning 10% ethanol for years.

What do you mean by idle tube.

Terry

It's either carburation or dropping spark on one set of points at idle. How do the plugs look?

If you've run thousands of miles without having to touch the distributor, it might be due for some maint; at minimum a points clean/adjustment then setting the dwell on both sets of points.

If you have an inductive timing light or firing indicator, you can use that to check spark energy while it's running.

The first vid sounded more like a carb issue, but in the 2nd vid it sure sounds to me like it's dropping cylinders then picking back up. But it's starting to miss before it drops out. More like an ign issue.

One way to check the carbs is to "tweak" on each idle mixture screw, one-at-a-time, top-to-bottom. See if turning the screw in or out affects idle speed, and if it's running well at the existing # of turns out for each carb needle.

If the engine responds like it should for each carb needle, then likely it's not a fuel delivery problem.

If there's one that isn't responding, likely a problem with trash in the idle circuit. Pulling the needle & blasting it with carb cleaner & compressed air, as previously suggested, might clear it out. Doesn't hurt to try!

Not many of us are running our old motors for that many hours, so you're certainly gonna experience some wear-related issues beyond the "norm".

And the points rubbing blocks are definitely a wear item!

Let us know what you find..........ed

Thanks ed,

The distributor has been rebuilt a number of times over years and it was last done 2-years ago. I found this post where I had questioned the condition of the cap.

www.fiberglassics.com/glassic-forums/home/mercury-outpost/merc-800-distrbutor-cap

Yesterday I checked the points with a meter and with the plugs out. Turning the motor by hand both sets appear to open at TDC and measures .5 ohms closed.

The plugs were also changed 2 years ago and don’t look too bad although 1 and 4 appear as though they got hot. Put them in the boots and cranked over and spark looks good.

L to R, 6 thru 1


4 & 3

2 & 1

Raining here today but going to go pick up new plugs and start it again.

Thanks,
Terry

Plugs look a bit gnarly, especially that strange "glob" deposit on #5.

Maybe try a set of NGK B6S plugs, they seem to work really well in the older motors.

Number (5) does look like it needs to be cleaned or replaced, but I am also concerned with the lighter deposits on number (4) & number (2).
That looks very suspiciously like melted aluminum caused by water intrusion.
I agree with Master Ed,...Replace the existing plugs with NGK B6S, and then,...Get the beast up over 4000 RPMs, and see if a failure develops.
The reason I say 4000 RPMs, is that seems to be the magic number at which water intrusion rears its ugly head if that is an issue.
A water pressure issue.
doc

Hello All,

Finally back to work on the old Merc and thanks for your suggestions.

Put in new plugs and took a close look at the old plugs. Not sure what you see in the pics Doc but it appears to be carbon build up and it wipes right off.

Went to start it and when I pumped the primer bulb I noticed gas pouring out of the top carb. Pulled the cowl top off and I can see the gas is not coming out of the float bowl overflow but rather it appears to be coming out of the air inlet opening.

I have never seen this happen before and if I continue to pump the gas keeps coming. I rebuilt the carbs last fall and she ran great up until now. Any ideas?

Thanks,
Terry

Terry, if its coming out of the air inlet hole your float setting is either too high or the needle is not seating. The only way fuel gets that high is if the needle is not closing off the fuel.

Thanks Dave,

I thought if the float did not close the needle valve then gas would overflow out through the opening on the side of bowl cover.

Terry

She started right up and sounds much better and even idles.



I did find one thing other than the suspect plugs. The coils common negative terminal nut was not fully tightened. Not sure that was it but what a dumb ass!

Oh well, I put Mama Mia! up for the winter yesterday so further investigation will have to wait til Spring.


We had a good summer on the water and Mama Mia! was a winner at Famous Freddies in July.


Terry

Very cool & rewarding! Congratulations Terry!!

Yuppp,...If the float doesn't float, or the needle doesn't seat, or the needle links are set too high, the beast can flood out of either place mentioned.
Another thing to consider though when the fuel floods out of the throat (air intake area) of a carburetor, if there is a fuel pump mounted on the block at those two related cylinders, be sure and check the condition of the cork gaskets and diaphragm for ruptures in that fuel pump.
The cork gaskets in the Sierra Marine kits (18-7804) are a very poor grade, and I'm seeing more & more failures from them.
I have returned to exclusive use of the Merc Marine kit (23014A1)
I still prefer using Sierra Marine needle & seat kits (18-7057 for your Tillotson carburetors) and Sierra Marine floats (18-7208)
doc