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Mf12G Power Issue


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#16 MFDAC OFFLINE  

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Posted August 20, 2013 - 10:33 PM

Hello everyone, time for an update. There were two problems with this engine so far and I have gotten the smoking situation solved and was able to mow long enough tonight to seat the rings on the new piston.
The old piston had about .020" of collapse in the skirt and it was slightly cocked in the cylinder. Fortunately the block honed out nicely with a minimum of metal removal.

GTractor and MFGray were right on in their thinking too, about the fuel mixture leaning out. The carb is clean but the throttle shaft has so much play that the choke needs to be 3/4 closed to run well. You can measure the play with a yardstick, not a dial indicator!-LOL The shaft is half cut thru and the bores in the carb are egg shaped. Gonna have to order a new carb, but they aren't as expensive as I expected. I mowed a half acre tonight with the choke 3/4 shut and the oil still looks fresh! No smoke in the shop when I was getting it going, except for a couple puffs from the assembly lube.

I got a pic of the piston damage. The scoring on the top ring land corresponds with the area where oil was washing the top of the piston and the combustion chamber. When I get a new carb I will show pics of how loose the throttle shaft is.

Later---DAC

Attached Thumbnails

  • HH120 piston & valves 019.jpg


#17 MFDAC OFFLINE  

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Posted August 21, 2013 - 09:58 PM

I re-torqued the head tonite and decided to do a compression test again with the new piston and rings. Maybe some of you remember before it was 95 psi. Now it is about 126. Now wether or not that means anything I don't know with the compression release potentially being a factor.

I decided to put a dial indicator on the carb throttle shaft too and it was .050". That's a pretty bad vacuum leak in my opinion. Got the new carb ordered tonite.

Here's pics.

Later---DAC

Attached Thumbnails

  • MF12 compression2.jpg
  • HH120 piston & valves 022.jpg
  • HH120 piston & valves 024.jpg

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#18 HowardsMF155 OFFLINE  

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Posted August 22, 2013 - 07:30 AM

Thanks for the followup.  I know I've compression tested two of mine and gotten right around 30.  Whether that is due to compression release, or if the rings are just in that bad a shape, I don't know. 


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#19 MFDAC OFFLINE  

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Posted August 22, 2013 - 06:31 PM

 

Thanks for the followup.  I know I've compression tested two of mine and gotten right around 30.  Whether that is due to compression release, or if the rings are just in that bad a shape, I don't know.


You're welcome Howard, from what I have read here, some of these engines have the compression release, and some don't. I guess if the 30 psi engines run, that's the main thing! If I remember correctly a lot of the B/S 5 HP engines we used to run on go-karts would test around 30 or 40 psi and they did have a compression release ground into the cams. Our rules required stock cams.

I have had a setback, Pat's Small Engines left a message on my phone that the carburetor I ordered is now discontinued. Probably going to have to hunt for a decent used one and that is a little scary price and condition wise I imagine.

Later---DAC

#20 MFDAC OFFLINE  

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Posted August 26, 2013 - 08:21 PM

Hey everyone, time for another update. I couldn't find a carb or any parts locally so I had to make something happen since having to run with the choke part closed wasn't going to hack it.

I decided I had to build some kind of bushing or shim to repair the throttle shaft and get the hole in the carb round again. The "speedy sleeve" kept entering my mind but far as I know they don't exist for this type of thing. To make a long story short, I measured a variety of pieces of tubing but everything that was close was around a quarter inch inside diameter, but I needed something a quarter inch outside diameter with pretty thin walls. What I finally found was a section of a broken car antenna from my daughters old Camry. I cut a little piece of the right size section with a tubing cutter, deburred it well and shoved it up the throttle shaft using some Locktite I had left from some automotive speedy sleeve installation, probably a vibration damper for the front main seal.

Then I drilled the carb bore out to 1/4 inch. Since the "speedy sleeve" was the same size a little clearance was accomplished by wrapping some 360 wet or dry sandpaper around a smaller drill bit and just taking a little time honing the bore till the shaft slid in tight but the spring on it would close it smoothly.

I put it on after lunch today, changed the break-in oil and went mowing! It ran great and I did the entire acre only stopping once when a twig tossed the belt on the cutting deck, in 95 degree heat.

So far so good, here's some pics of the carb work.

Later---DAC

Attached Thumbnails

  • HH120 carb speedy sleeve 003.jpg
  • HH120 carb speedy sleeve 006.jpg
  • HH120 carb speedy sleeve 005.jpg
  • HH120 carb speedy sleeve 002.jpg
  • HH120 carb speedy sleeve 008.jpg
  • HH120 carb speedy sleeve 009.jpg

Edited by MFDAC, August 26, 2013 - 08:24 PM.

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#21 MH81 ONLINE  

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Posted August 26, 2013 - 09:30 PM

Great follow up, thanks for the idea.
BTW what is the suggested color of locktite to put a chromed brass sleeve into a cast aluminum carb?

Just kidding, looks like you just gave that carb a brand new lease on life. :thumbs:
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#22 HowardsMF155 OFFLINE  

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Posted August 26, 2013 - 09:51 PM

I'm not sure where you were looking for tubing.  You certainly found a cheap source that worked for you.  I do want to add that hobby shops stock a variety of sizes of brass tubing and aluminum  tubing as well.  It may also be available in different wall thicknesses.


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#23 MFDAC OFFLINE  

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Posted August 27, 2013 - 06:50 PM

 

I'm not sure where you were looking for tubing.  You certainly found a cheap source that worked for you.  I do want to add that hobby shops stock a variety of sizes of brass tubing and aluminum  tubing as well.  It may also be available in different wall thicknesses.


Since I hoard, I searched thru some of the junk and garbage I have here instead of driving into town spending 10 bucks worth of gas for a dollar piece of tubing that I would only use 3/8" of. I am very familiar with K&S brass and aluminum tubing, I still have some around from slot car racing and model car building days. Didn't have what I needed tho until I measured that broken antenna. Yeah I'm all about cheap--LOL!

Later---DAC

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  • KS brass.jpg

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#24 DH1 OFFLINE  

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Posted August 27, 2013 - 07:14 PM

I did the same thing with a carb I had problems with only I used a piece of soft 1/4" copper tubing.

 

http://gardentractor...f12g/#entry4776


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#25 MFDAC OFFLINE  

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Posted August 27, 2013 - 10:01 PM

 

I did the same thing with a carb I had problems with only I used a piece of soft 1/4" copper tubing.
 
http://gardentractor...f12g/#entry4776


Wow, that ain't good with carb parts getting sucked into the cylinder! I tried 1/4" copper but it was much too big inside diameter so the piece I used was exactly 1/4" O.D. I already forgot what the I.D. needed to fit the shaft tight was. What I built is a part of the throttle shaft, not a bushing installed in the carb body, because the shaft had a bad gouge ground into it and the carb body hole was also oblong. The carb body is still the bushing like original just bigger.

Pretty cool that you were able to re-use the carb. I dammnear gave up and was trying to buy a new carb till I found out you can't get 'em then desperation kicked in!

Later---DAC
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