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MFS6C starts fine, idles fine, 1/4 throttle fine, full throttle runs bad

B

Bike4Him

New member
2014 6hp with low hours.
New fuel filter.
Valves properly adjusted.
Compression at 45+ with decompressor (I believe)
Carburetor ultrasonic cleaned.
New spark plug.
New impeller.
Good 87 octane no ethanol fuel.
Oil at proper level.

Anything over 1/4 throttle and it shakes and sputters like fuel starvation.

Before I drop it off for 3 weeks at a mechanic I'd like to try again to fix it myself.

Thank you.
 
Verify timing. Re-check carburetor. Never replace just an impeller - always do a complete wp kit.
 
pvanv said:
Verify timing. Re-check carburetor. Never replace just an impeller - always do a complete wp kit.
Click to expand...

Frankly I'm tired of removing and cleaning the carb:)
It's been decades since I used my timing light. May not even be able to find it. Any suggestions on what one to buy?
I assumed the timing would be ok if it ran great at idle and 1/4 throttle. Just ran it again and it's more like 1/3 throttle until it runs bad.
 
Who cleaned the carb, with what method? I always do at least a 6 hr dunk in real carb dip in a heated ultrasonic tank, followed by a whole can of aerosol cleaner to flush all passages. If the power circuit is wrong (sounds like it), you may have missed installing something properly.

I use a D-cell timing light like from electronics specialties.

Is there a chance that the oil is above the full mark?
 
My first cleaning was with about 1/2 can of carb cleaner and there was no change in the performance.
I then soaked the carb in carb cleaner/degreaser in a heated ultrasonic tank overnight and then about 2 hours of ultrasonic cleaning, then 1/2 can or so of cleaner, then air.
The mechanic said he would clean the carb as his first attempt. He may be able to do a better job than I, but I believe it is clean and I also believe I installed everything properly.
Oil is not above the full mark.
 
I verified the timing. close to 0 degrees TDC at idle and close to 25 degrees BTDC at full (rough running) throttle.
I'm back to cleaning the carb again unless there are any other thoughts.
I may buy a $30 can of Gunk Carb dip and use that instead of the carb cleaner/degreaser I have.
 
I only have a small tank at home.
Is there an issue with testing in a tank in neutral or does there need to be a load on the prop?
 
Yes, use real carb dip. Neutral is ok for running, but you would ideally be able to put it under load. You need at least 100 gal tank for that.
 
Used real carb dip for about 8 hours at 130 degrees then 2 hours ultrasonic then flushed with 3/4 can of carb cleaner.
Unfortunately no difference. Idles great. Blip the throttle and it responds. Hold is open past 1/3 and it sputters and runs very rough as it did in the beginning.
 
Is there any chance the float height is wrong? Usually blipping the throttle is an issue due to being too lean...
 
Probably used the wrong term. By blip I mean I turn the throttle rapidly open then closed like warming up a motocross bike before a start.
It doesn't stall or sputter when I do that.
The float height is around 10mm depending an where I measure.
 
OK, so if oil is OK and timing is on, then you are either too lean or too rich. When you motor runs so poorly, stop and check the plug. Wet=rich... dry=lean.
 
I warmed it up then ran it full throttle for 15 seconds then pulled the plug.
It didn't look wet.
I have tried to insert a photo. I'm not typically technologically challenged, but this one got me.
The plug is black.
 
Float height is 9-10 mm depending on where I measure it. This is in line with service manual.
I don't know about the jets except that the motor worked well prior to me owning it and nothing was changed.
 
Probably needs a Proper carb cleaning then. Yes, sometimes it has to be done 3 times to get all the crud out of the tiny passages.
 
I dropped it off at a Tohatsu dealer/mechanic. He ultrasonic cleaned the carb and used a tiny bit to clean one of the jets on the upper side of the carb (slow jet?).
I don't have a large tank and his was not 100 gallons like you mentioned, but it seems to run a little better.

Before I get on the lake, I have one question on tank testing. Can you run it full throttle in a small tank for a short period to test it or does it have to have some load on it to be able to really know what's happening at full throttle?
 
I am new to the forum, and thank you for allowing me to join. I have an MFS6CS (same motor with a generator), and have a similar but more severe problem: Starts fine, idles forever, will run under load up to "restart" indicator, then runs fine for about 30 seconds and then dies...like you turned it off! I ran it dry before fogging and storage last fall and this spring it exhibited this from the get-go. Don't mean to intrude, but have been following this thread, and am guessing I have a fouled carb. One question: what is REAL carb dip? Thanks for letting me 'pop in'.
 
You really should start a new thread, but in my experience wrenching on these for 20 years, it sounds lean. Could be a leaky o-ring if running from external fuel tank, a weak fuel pump, or again the filthy carburetor. Real carb dip comes in 1-gallon cans at the auto parts houses. After it "shuts off", open the drain screw in the carb and see whether there is fuel in the bowl.
 
Paul, thank you very much. I am brand new using a forum, and apologize for not starting a new thread (is that the right term?) I searched Auto Zone and their dip is non-flammible. Walmart has Gunk dip and it is flammable. Which (if either) is better? I'll check the bowl. I already replaced the filter in the housing. I'll start at the (external) tank and work my way through the fuel system. Thanks all for your understanding.
 
My tank holds about 3-4 gallons, and I have a couple of different brands mixed in. The new stuff is nothing like what we used to get back in the 70's and earlier. That stuff would take the hyde right off your hands, even after washing. Odds are you need to completely disassemble your carb, clean it, then reassemble.
 
Thanks, Paul. I'll get back to you when I've followed through on your suggestions. It is wonderful to have you, with all your experience, on this forum. I think we're very fortunate.
 
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