20hp Merc bogs out when trying to go to full throttle

[dpski]

Hello,<br /><br />I have a 1989 20hp Mercury Tiller, manual start with the Walbro WM carburetor. It has the combination pull to choke/turn to adjust idle knob. I at first had a problem starting until I read that the knob is used like a primer and it is actuated several times before pulling starter.<br /><br />Still doesn't start great, cold 2-6 pulls, once warm 1-2 pulls.<br /><br />My problem now is once running it idle's o.k., and low speed operation is o.k., when trying to speed up, past 1/3 throttle, it will throttle up and run fine, then as rpm's climb the motor will cough, spew a cloud of smoke, almost sounds like it's backfiring and kind of choke itself then quit. Someimes it will just bogg down, choke, cough and quit, but more often it does the first description as you try to go to full throttle. The motor has been sitting a while. I'm guessing it might be time for a carb rebuild, but I'm hoping for a simpler answer. Any ideas?<br /><br />Thanks,<br /><br />DP

 

[riptide116]

Re: 20hp Merc bogs out when trying to go to full throttle

I had the same problem with my 20Hp merc,get some fresh plugs,mebbie some fresh gas,I have learned when I rinse out motor,I disconnect the fuel line and run out the gas thats in the carb,..me thinks this is good cuz there is no 'standing' gas in carb that will build into 'crud',other than that I dont know didely about em...

 

[Laddies]

Re: 20hp Merc bogs out when trying to go to full throttle

Yup you need a carb kit, before reinstalling the bowl ck the primer function--Bob

 

[kfpemc]

Re: 20hp Merc bogs out when trying to go to full throttle

You should first eliminate the primer ball, fuel pump and fuel lines as a possible cause. I have seen these items cause similiar problems. The fuel pump diaphram can be purchased as a separate item from the entire carburetor kit. If the problem persists, I would then recommend the carburetor kit. You also mention that this engine has been "sitting a while". The oil mixed with the fuel can leave a sticky residue in small carburetor passages and mixing jets along with hindering the operation of the carburetor float if the engine hasn't run for a long period of time. You also mention that the engine runs at 1/3 throttle or less. As one last resort before rebuilding the carburetor, you may just try running through at least 3 to 6 gallons of fuel. This may actually dissolve any residue within the carburetor. This actually happened to me very recently with a Mariner 25HP. I had a similiar issue at full throttle. When running the engine beyond 1/3 open, the engine would lose power and stall. The only way I could run the engine beyond 1/3 open throttle was with the engine's mechanical choke in the full choke position. ( the choke knob pulled out ) After running through about 3 to 6 gallons of fuel at less than 1/3 throttle or by manipulating the choke beyond 1/3 throttle, the problem went away. It runs from a "hole shot" to full throttle now without any problems at all. It's a nice smooooooth running engine now...wouldn't you agree Don?!!

 

[dpski]

Re: 20hp Merc bogs out when trying to go to full throttle

OK, just for future reference in case someone searches the forum, here's how I fixed it.<br /><br />Took apart the carb and cleaned it all out. It wasn't that dirty, except I noticed the float needle was sticking ever so slightly. There was a tiny bit of crud around the base of the main jet. Cleaned it all up and put it back together with the orignal gaskets. The float needle was not worn so it looked fine. Trickiest part after reassembly is adjusting the slow speed mixture screw along with the throttle linkage. There doesn't appear to be a fixed location, you kind of have to hear, feel, smell what's the best combination, not too rich, not too lean, not too fast, not too slow, is about as best as I can describe it.<br /><br />Previously the pull to choke/turn to adjust RPM knob did nothing regarding the RPM's. After fiddling and adjusting the throttle linkage I noticed that the wire connecting the knob to a plastic link under the flywheel wasn't really moving much. After repositioning the wire further back within this white plastic link piece it now can actually cause the throttle linkage to move which in turn cause the butterfly valve to actuate. So now I can actually adjust the RPMs.<br /><br />For cold starting, pull knob all the way out just ONCE, turn it fully to the right (clockwise), one pull she fires right up. After 5-10 seconds push knob in, then as it warms up turn knob to the left to slightly lower rpms. Once it's fully warmed up turn knob all the way to the left or until the idle is just the way you want it. Runs like a champ now, I'm hoping it stays that way.<br /><br />I'm still a little fuzzy on the best slow speed air/fuel mixture jet/screw setting. The plugs seem a tiny bit wet after some low speed running in the driveway, haven't pulled them after a long high speed run on the water. She runs great so I'm inclined to leave it alone and just add in some carbon cleaner to the fuel.

 

[laker925]

Re: 20hp Merc bogs out when trying to go to full throttle

How often do you need to run the carbon cleaner thru -especially if you fuel mix was a bit on the rich end for a while?<br />And what type would I get?

 

[laker925]

Re: 20hp Merc bogs out when trying to go to full throttle

Any suggestions?