24 replies to this topic

[MFDAC]

Hello everyone, I finally got some time to try and figure out why my MF12G dies when I engage the cutting deck but have not come up with a reason. It drives around fine without the deck engaged. I pulled the deck and there isn't a bunch of debris wound around any of the blade hubs, the gearbox seems ok and turning the whole deal by hand, I'd say for what it is it turns pretty well.

The engine is a Tecumseh HH120 and may be original from 1975. I felt the spark plug color looked ok but I did a compression check anyway. Is 95 psi an indication that the engine is in need of an overhaul? Seems like kind of a low number to me, but I don't really know what is good on one of these.

Thanks in advance for any help!---DAC

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[DH1]

The first thing that comes to mind is too much resistance somewhere when the deck is engaged, could be 1 thing or several things causing a bit of drag that adds up.

 

2nd thing would be lean fuel mixture the plug looks good for sure it's not running rich but maybe 1/2 turn out on the high speed main jet might make a difference. ???

[MH81]

95 would be good, even high-ish... there's a compression release on most of these

Really hard to find compression for sure.

[Gtractor]

My vote is also for too lean of a fuel mixture.  When a load is applied such as engaging the deck - the governor slams the throttle open all at once and the engine just doesn't have ample fuel to run so it stalls out.   ????? 

[HowardsMF155]

The PTO idler pulley, the one on the pedal you use to engage the mower deck, can give you fits.  I'd double check that.  You could also have an issue with timing, that one gave me fits with my massey 12.  Here is a link for that. 

http://gardentractor...ne-lacks-power/

[MFDAC]

 

The first thing that comes to mind is too much resistance somewhere when the deck is engaged, could be 1 thing or several things causing a bit of drag that adds up.
 
2nd thing would be lean fuel mixture the plug looks good for sure it's not running rich but maybe 1/2 turn out on the high speed main jet might make a difference. ???

 
That was my first thought too, DH1, I figured that a wad of wire or twine was wrapped around a hub but everything looks normal. Bearings and gearbox seem ok. It spins well, can't do it with a finger you know, but it don't take much manual effort. Next will be to try to tune the high speed jet better then.
 

95 would be good, even high-ish... there's a compression release on most of these

Really hard to find compression for sure.

 
Yeah I was wondering about that. I remember when I was racing B/S 5HP go-karts that we were told that a compression test wasn't very accurate as the cams had a compression release ground into them. I guess maybe an overhaul may not be what I need, hopefully!
 

My vote is also for too lean of a fuel mixture.  When a load is applied such as engaging the deck - the governor slams the throttle open all at once and the engine just doesn't have ample fuel to run so it stalls out.   ?????

Well Gtractor, that will be my next thing to try to get tuned. I do know the fuel flow to the carb is good, I had the carb off a couple weeks ago and it was clean as could be inside and I checked with the bowl off that good flow came from the tank. I did tweak the idle mixture and high speed screws then by ear but must have missed something.
 
Thanks guys!---Later---DAC

Edited by MFDAC, July 15, 2013 - 10:37 PM.

[JDBrian]

Any safety switches on the PTO of those tractors. If all else fails it may be worth checking for a problem there. 

[larryd]

Sounds like a voltage issue IMO

larryd

[MFDAC]

 

The PTO idler pulley, the one on the pedal you use to engage the mower deck, can give you fits.  I'd double check that.  You could also have an issue with timing, that one gave me fits with my massey 12.  Here is a link for that. 
http://gardentractor...ne-lacks-power/

 
I didn't get time to reply last night, you must have posted while I was typing. I did run a search before I posted but didn't see that one for some reason. I will read it tonite. I have just put a new belt on that idler pulley the same day I started having this dieing issue and broke the starter, if any of you remember that. It had done the dieing deal a couple of times before but then would work ok though.
I posted a couple pics of the new belt disengaged and engaged.
 

Any safety switches on the PTO of those tractors. If all else fails it may be worth checking for a problem there.

There is a a switch right there by that pto idler that doesnt seem to contact anything, and the wires are messed up. I unplugged and it still did the same thing.
 
 

Sounds like a voltage issue IMO

larryd

I will check on that too! It does feel like a car or truck running only on battery power till the battery dies, but it kinda feels like running outa fuel too--can I make this any more confusing??-LOL!
No noticable sputter or backfire to speak of it just starts losing rpm till it dies.

Thanks for all the help, folks! I better do some reading then try some of these helpful ideas. I may not get to really dig into it again till the weekend but I will try.

Later---DAC 

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[HowardsMF155]

Yes, that is the pulley.  When the PTO is disengaged, the deck rotates fine. When you engage the PTO, you don't expect the deck to be able to turn the engine.  But if the idler pulley is frozen, it will put a big load on the engine when you engage the deck.  Hard to find without specifically trying to turn the pulley.

[MFDAC]

 

Yes, that is the pulley.  When the PTO is disengaged, the deck rotates fine. When you engage the PTO, you don't expect the deck to be able to turn the engine.  But if the idler pulley is frozen, it will put a big load on the engine when you engage the deck.  Hard to find without specifically trying to turn the pulley.

The idler and the drive pully both spin real free so that isn't the problem. I re-tweaked the carb last night, put the cutting deck back on and installed a new spark plug. It started fine but it filled the shop with smoke, and was still dieing 15 seconds or so after engaging the blades. It sounds just like that thread you linked me to, but no knocking noise. It always would use a little oil everytime I mowed, having to top it off after 6-8 hours of operation. The smoke was pretty bad this time tho. Think I will pull the head and assess things there. Sure didn't think by the plug color that it was using an exessive amount of oil, but maybe it has just gotten worse. The battery voltage was 12-13 with the engine off and 14.4 with it running.

I did see the link to the parts supplier on that thread that has the parts for this engine too. Thanks again.

Later---DAC

Edited by MFDAC, July 17, 2013 - 06:43 PM.

[MFGray]

It might be worth checking the carb again - the idle circuit is not easy to clean. If the main jet needle is backed out anything more than 2 turns, it might be that the carb has been tuned to run without the idle circuit working (i.e. with the idle rpm stop opened up a bit and the main needle opened up a lot - that allows the engine to idle, but it will bog when it comes under load). It's fairly easy to check how far out the main needle is sitting and that might give you a clue, since the Walbro carbs seem to run well close to the recommended initial setting of about 1 1/2 turns out on the main needle when the idle circuit is working properly.

[MFDAC]

 

It might be worth checking the carb again - the idle circuit is not easy to clean. If the main jet needle is backed out anything more than 2 turns, it might be that the carb has been tuned to run without the idle circuit working (i.e. with the idle rpm stop opened up a bit and the main needle opened up a lot - that allows the engine to idle, but it will bog when it comes under load). It's fairly easy to check how far out the main needle is sitting and that might give you a clue, since the Walbro carbs seem to run well close to the recommended initial setting of about 1 1/2 turns out on the main needle when the idle circuit is working properly.

I took your advice and double checked the idle mixture screw and I had it at 1-1/4 turns out. I did pull the head and the aluminum is washed clean around the back of the piston and the corresponding area of the head. There were considerable crusty baked on deposits and areas wet with oil. The valves look great and have virtually no wiggle in the guides. The bore measures 3.48 before removing a slight ridge that appears to be mostly deposits and the bore is supposed to be 3.5 according to specs. I think I will roll the dice as for about 50 bucks I can get a rebuild kit that comes with the gaskets, seals, rings and a piston. Scroll down to HH120. If I had to go .020 over the piston is over $100!

http://www.psep.biz/...rebuild_kit.htm

Thanks to DH1 for that link in the thread HowardsMF155 mentioned above.
Here are some more pics.

Later---DAC

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[DH1]

When you engage the mower deck do you do it abruptly or slowly? With the deck off the tractor have you removed the belt on the deck and tried to turn each spindle, pulley, gear box separately by hand?

Edited by DH1, July 18, 2013 - 08:01 PM.

[MFDAC]

 

When you engage the mower deck do you do it abruptly or slowly? With the deck off the tractor have you removed the belt on the deck and tried to turn each spindle, pulley, gear box separately by hand?

 
I just ease into it like lettin' out the clutch in my 58 year old pickup-LOL!
I did take the belt off the deck and turned and wiggled the spindles and gearbox. The bearing on the middle blade has just a touch of wiggle but everything spun freely.

I have been mowing with this tractor for 2 years so I think I had a pretty good feel for it.

Later---DAC