Feul system/carb problems on tohassu MFS6B

[kpgraci]

Tohatsu 6hp external tank - bough used, previous owner used it as a backup, he says it had about 10hr on it, motor looks in showroom condition. Did not idle well so I did a complete cleaning of carb, removed all jets and soaked, then it ran perfectly in test tank. Put on boat warmed up, motored out at 3/4 throttle, no problems. Then the motor missed once, then again, then it backfired and quit. Attemtps to start only resulted in some backfires and then only with 3/4 throttle. Reopened carb bowl expecting to find debry, it looked clean. Blew out carb, and after a few pulls where it backfired it now runs fine. Fuel filter is like new, clean and clear, fuel tank is clean, gas is new, tested ethonol and water free, but I repleaced the fuel anyway. I have a fuel filter on order. I'm sure this is a fuel system issue but there is no debry in the fuel system that I can see...am I missing anything to do here?

Motor is currently running fine in the tank, but it stranded me so I have a low trust level with it right now...

I'm thinking that this carb is pretty touchy...in reading abround this forum I've come to some conclusions:

Only use fresh gas (< 30 days old, guilty on this one)
Only pure gas (which I can get and do use)
Run carb dry at end of day (I do this)

Before I put the motor back in service I will

flush out all lines
replace fuel filter
complete re-clean of carb

One thing I forgot to mention: before I did the first carb cleaning for the idle problem the float got stuck (overflowed when priming) so I field striped the carb on the boat and got that fixed. I had since forgottn about that but there was an indication of something gunky in the carb. On that first celaning I found some gel-like grit in the bowl - just a few specs - the cleaning I did was pretty good but I did not remove the mixture screw - I suppose there was still some junk somewhere in the system. But when I bouth the motor (2 months ago) the fuel was new, so sure it violates the 30 day rule but not by much.

 

[TOHATSU GURU]

Re: Feul system/carb problems on tohassu MFS6B

"am I missing anything to do here?"

No.

 

[kpgraci]

Re: Feul system/carb problems on tohassu MFS6B

I read about someone that adds seaform to the gas (no idea how much, or little) for normal operation. Is that a good idea?

BYW - Your avatar is creepy and I find it disturbing that I like it.

 

[pvanv]

Re: Feul system/carb problems on tohassu MFS6B

The gel/grit junk is telling that some fuel went bad. I have seen stabilized E10 with water contamination do that quite often. You probably need to re-clean the carb to be sure everything is OK. On the 6B, there is a pilot screw (capped for EPA compliance -- and you typically don't need to pull it if you do a proper soak in carb dip), but be aware that there is also a slow air jet, which should be removed (externally accessible) and cleaned, when going through that carb.

If the needle was jamming open, it may have some oxidation on it, causing it to be "sticky" in the seat bore... I've seen needles where the tip was perfect, but the metal body had a "velvet" feel, which was oxidation. Typical in water contamination cases. If so, a carb kit (or at least a new needle) wouldn't hurt while cleaning the carb.

YMMV with additives. Some swear by them, some swear at them. If the fuel is fresh, stabilizer isn't really needed -- it slows the "rotting" of the fuel. In many cases, we call that stuff "snake oil", after the 19th century elixirs that were promoted as curing everything from lumbago to dandruff.

Some things that some of my customers add to fuel:
Marvel Mystery Oil
Star-Tron stabilizer
Sta-Bil stabilizer
K100MG stabilizer
Chevron Techron cleaner
Sea Foam

I have seen the K100 actually help bond water to gas, so it could possibly "get you by". Likewise, I am testing Techron in massive (10x) doses, to see whether it helps un-varnish a carb that has minor buildup... so far, the answer is "maybe"... that carb is slowly working a little better... hard to say whether the regular detergents in the fuel are slowly washing the varnish away.

I would go through the fuel system one more time, and be sure you flush out the fuel pump, as that grit or gel could get lodged in there.

 

[kpgraci]

Re: Feul system/carb problems on tohassu MFS6B

Right. I did remove the 'slow air jet' and suspect that was causing the poor idle I originally had. I don't plan on messing with the fuel/air mix and am glad to here it's ok leave it alone. Following your advice I'll soak the carb for 4 hours. thx.