'78 Merc 200/20hp - bogs, then dies at full throttle

[EclecticNeophyte]

I have a 1978 Merc outboard model 200 (ser# 4954879), that's rough at idle, runs great at mid-throttle, but dies after 15 to 20 seconds once reaching full throttle (hope that makes sense). I can re-start, but the last time it did this, it was a struggle to get it fired up again. The carburetor and fuel pump are integrated, and I plan to rebuild both.

I've checked the coils and plug wires as best I know how, by taking continuity readings with a digital multimeter. The coils checked ~ 990 and 1000 ohms between high voltage and ground, and zero between + and - terminals. Plug wires were solid zero ohms.

The plug in the top cylinder was pretty oily/dirty but other than that looked ok. The bottom plug was pretty clean.

I rebuilt the water pump, cleaned the discharge tube and the motor pee's a solid steady stream even at an idle.

The gas tank, hose and primer bulb, etc., are only a few weeks old. I've made certain the vent is open, but the motor still dies at full throttle. Not a lot of coughing, sputtering, etc.. the RPM's simply fall off, then eventually dies.

Once I get it going again, it runs fine as long as I don't try to max it out...

Am I on the correct path thinking this is a carb/fuel pump issue, or are there other things I should look into? I've read through the Clymer repair manual, and while it has very good 'how to' info, it's not much help in the trouble shooting dept. (no symptoms >> check this ).

Thanks, and apologies if this is a frequent question. I searched the forum, but didn't find a similar thread.

-Vern
-Kennewick, WA

 

[EclecticNeophyte]

...one additional. I'm running a 50:1 mix, using mercury brand oil. Again, the engine has good power all the way up to full throttle, then as I said above, 15 to 20 seconds into the game, it all goes away... Not sure what other info I can provide that might help with diagnosis... I don't have a vast amount of experience with outboards, so thanks in advance for any help.

-V.

 

[RCO]

Sounds like a lack of fuel from your description. I would be willing to bet that a carb/fuel pump rebuild will solve the problem . I would check compression and spark on both cylinders just to be on the safe side. The factory manual is well worth the investment.

 

[EclecticNeophyte]

Thanks RCO. I'll report back any findings...

-Vern

 

[EclecticNeophyte]

Did some small amount of investigating (all I had time for), and pulled the strainer bowl that is on the side of the fuel pump. Except for some very slight discoloration, the strainer appeared to be as clear and clean looking as the day it was made. I was hoping that was the issue, but no love there. Will tear it (i.e., carb and pump) apart this evening and see what is what.

 

[444]

Sounds like it's running out of fuel. I would rebuild the carb/fuel pump and try again. Change your fuel lines and make sure you have good gear style hose clamps on it because if there's a fuel restriction or if the fuel pump sucks air it's going to run dry at full throttle. Check to make sure your tank vent is clear, fuel lines have no obstructions, carb is clean interally, fuel pickup in the tank is clear, etc.

 

[EclecticNeophyte]

Things I noticed during the rebuild:

* Cork gasket under the fuel pump strainer had been used and re-used, to the extent that it looked (to me) to be home made. Mashed out and ragged I.D. to the point that I'm not confident it would provide adequate seal.

* Float valve had the rubber tip. Didn't look bad, but it wasn't sealing. You could squeeze the primer bulb and force fuel out the intake. The old plastic float appeared to be fine, but I stowed it away as a spare and put a new one in.

* Missing gasket under the idle tube. I put a new one in.

* In the exploded view (parts breakdown), there is what looks like a gasket, but is labeled 'pump diaphragm'; and fits between the main carb body and the 'fuel pump body', which to me looks like a simple intermediate plate. This is in addition to the other mostly solid 'diaphragm' that fits between the 'fuel pump body' and the 'Screen strainer body'. Anyway, the diaphragm that looks more like a gasket, has what look to me to be two little semi-circular 'flaps', which would lay flat against and cover a pair of ports in the pump body. I took a guess and decided these must act as 'check valves', and put the part in. It was missing when I tore the assembly down...I included/replaced both 'diaphragms'. I'm hoping this is the crux of the issue. It's shown as item '21' in this exploded view: http://www.*****************/parts/me...bly-complete/

* At first, I thought there was varnish inside the fuel bowl, but realized later that it was epoxy of some sort, which had been applied to seal a hole worn in from rubbing against the inlet fitting. There were flecks of the stuff in the bowl, so I cleaned the thing as best I could, and removed as much of the lose stuff as would come away...

* The short length of fuel hose between the fitting and the strainer housing appeared to have been very recently replaced, but I put a 'newer' piece in it's place. Clamps were the worm screw type, I made certain they were installed properly.

The rest appeared to be ok. I flushed brake cleaner through every orifice and passageway I could find, including throat of the carb; then blew compressed air through everything. I then slapped it all back together with the new parts, replaced on the motor and connected everything back up. I just finished running it (garden hose and muffs), and it fired right up, then died, but then after a little coaxing, fired up and ran. I adjusted the idle up a bit in an attempt to smooth it out, ran it though forward and reverse, etc.. At the end, I disconnected the fuel hose and let it run the fuel out.

It will be a few days before I can slap the motor back on the boat for a 'real' test, but I'll try to report back after that's taken place.

Thanks for the advice. It's good to not have to forge ahead in a vacuum!

-Vern

 

[EclecticNeophyte]

For whatever reason, the server removes the guts of the link (i.e., marineengine dot com)...

 

[EclecticNeophyte]

Sounds like it's running out of fuel. I would rebuild the carb/fuel pump and try again. Change your fuel lines and make sure you have good gear style hose clamps on it because if there's a fuel restriction or if the fuel pump sucks air it's going to run dry at full throttle. Check to make sure your tank vent is clear, fuel lines have no obstructions, carb is clean interally, fuel pickup in the tank is clear, etc.

Thanks 444. I've done as you suggested (except for the high speed test of course...).

-Vern

 

[merc850]

Set the idle in water in F gear just as the throttle opens, set the lo speed needle 1 1/2 turns from seated then turn it slowly in then out until the motor is running smoothly. Accelerate to full throttle to check for stumbling - if this occurs turn it out 1/4 more and recheck. Make sure the vent in the tank is open or engine will stop at full throttle.

 

[EclecticNeophyte]

Set the idle in water in F gear just as the throttle opens, set the lo speed needle 1 1/2 turns from seated then turn it slowly in then out until the motor is running smoothly. Accelerate to full throttle to check for stumbling - if this occurs turn it out 1/4 more and recheck. Make sure the vent in the tank is open or engine will stop at full throttle.

Finally got a chance to take it out again...took my eldest son with. I first ran the motor with the boat still on the trailer, but deep enough in the water to submerge the lower unit/water intake. I had to turn the low speed needle out an extra half turn before the motor would start. But once it did, it fired at the leading end of the first pull. After letting it idle for a bit, I put it in forward and tried adjusting further to smooth it out. No luck there, so I gave it more throttle until it was as far as I could twist it open. This time the engine ran...and ran and ran! Feeling much better about things, we launched the boat and took it for a high speed run. Everything worked great. I wouldn't say the motor ran great, but it ran, and that's the main thing.

Only bad thing that happened, was as we were docking, I tried to shift out of forward into neutral, and broke the shift handle off! Before I could grab it, it hit the water and was gone...No worries, I have a small hobby aluminum foundry, and a mill and lathe. So making a new one out of an Audi intake manifold won't be too big a deal.

The other Bozo move was...in the distraction of everything going on, I forgot to open the fuel tank vent. But I didn't realize that until we returned home! It's a small plastic tank, and the sides had started to collapse. I opened the vent after my son backed it into the driveway, and heard the 'hiss' of air rushing into the tank. So the fuel pump is definitely working!

Thanks again to everyone for your help and support. I spent the last two years building this little boat, and was really ready for some play time.

-Vern