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Tecumseh Engine Lacks Power


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#1 HowardsMF155 OFFLINE  

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Posted June 03, 2012 - 08:15 PM

I've got a Tecumseh HH120 engine in a Massey 12 Hydro that runs well but lacks power. I've tried cutting my grass with it a few times and it really won't mow much faster than a slow amble. Going up hills and inclines really lugs the engine down and causes problems. I tried rigging a wire directly to the throttle and checking to see if the throttle was wide open or if it might be a governor issue. The throttle is definantly opening all the way, but the engine RPMS drop anyway. I also tried using a carb off another MF12 (one that has run that same deck). The mower deck has been used on a MF12 G model to cut the same lawn, with no issues related to mowing speed. The blades have been sharpened very recently. I was running the tractor late this evening, nearly dark, and noticed that the exhaust was a dull red. Is that normal for these engines or is it a clue to why the engine lacks power? I probably need to check valve clearances. I don't see a lot of blowby out the crank case breather, though it does seem to be burning a bit of oil, with puffs of smoke when I shut it down.

Anybody got any suggestions?

Thanks

#2 olcowhand ONLINE  

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Posted June 03, 2012 - 08:19 PM

Before I got to the point in your post, I thought "valve clearances" being this is a known good carb & deck. Might have a burned valve too.
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#3 HowardsMF155 OFFLINE  

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Posted June 03, 2012 - 08:25 PM

I know that the dark makes it easier to see glowing exhausts; if the valves are getting that hot I'm sure they are burnt.

#4 timtractor OFFLINE  

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Posted June 03, 2012 - 09:00 PM

The two other thing to check is the condition of the air filter?

If the air filter is pluged with dirt the motor can't get enough air to run the engine right.

The other thing is to check to see the condition of the head gasket, or to see if the head is at a flat plane surface.

With the head gasket having a pin hole in it, the motor will still start and run, but it won't have enough compression like it should.
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#5 HowardsMF155 OFFLINE  

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Posted June 03, 2012 - 09:05 PM

The air filter is brand new, the old one was dirty, but that didn't change the power.

#6 MH81 ONLINE  

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Posted June 03, 2012 - 09:25 PM

Just wondering, what version of the HH120 is it? Does it have points? If not, does it have the pins in the flywheel?

I'm sure you're right, and it's valves or mixture, but timing will rob a lot of power if it's not right.
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#7 DH1 ONLINE  

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Posted June 03, 2012 - 09:36 PM

This is your MF12Hydra that had the cracked frame?

#8 HowardsMF155 OFFLINE  

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Posted June 03, 2012 - 09:43 PM

I'm pretty sure it is the solid state ignition, but it might be firing on the wrong pin. I'll have to pull the sheet metal and put a timing light on the pins to see which one is firing. More troublesome would be a problem with a partially sheared key. I'd have to pull the flywheel to find out.

No one has commented on the muffler being dull red. Is this reasonably common? I rarely operate a small engine hard around dusk. Not only was the muffler dull red, but so was the pipe in places.

#9 HowardsMF155 OFFLINE  

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Posted June 03, 2012 - 09:45 PM

This is your MF12Hydra that had the cracked frame?


Yes it is. And I am so happy about the operation of the hydro lever now, it keeps speed without constant monitoring.

#10 olcowhand ONLINE  

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Posted June 03, 2012 - 09:48 PM

Not so unusual about the dull red pipe & muffler, though usually this will only be visible in more darker times & after pulling hard for a while. That's one reason I lean to a leaking valve as it will heat up the exhaust sooner.
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#11 DH1 ONLINE  

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Posted June 03, 2012 - 10:05 PM

If I remember right that tractor has an SSI engine in it, no points and the timing is not adjustable.
But there are 2 trigger pins on the flywheel, taller one for starting, and a shorter one once the engine has started.
The shorter one advances the ignition timing.

Adjusting the main jet on the carb will effect power but you tried 2 carbs, one was a known good one.

I think the head needs to come off and see what it looks like inside.

Head gasket, I had one where some of the material between the 2 metal layers was missing, really hard to see but I could feel the air leaking out when it was in place.

Clean up the carbon and lap the valves and test there seal.

This is the best place for parts, they have a good selection > http://www.psep.biz/...umseh_parts.htm

edit I see I missed a couple of post before I posted this one, slow connection tonight

Edited by DH1, June 03, 2012 - 10:09 PM.

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#12 timtractor OFFLINE  

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Posted June 03, 2012 - 10:49 PM

About the orange exhaust.

What might be going on is that the exhaust valve is the problem and is letting the fuel mixture enter the exhaust chamber before the cylinder has a chance to fire it.

When the spark happens it fires the unburnt fuel that entered inside the exhaust area instead of firing all of the fuel inside combustion chamber.
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#13 HowardsMF155 OFFLINE  

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Posted June 04, 2012 - 08:02 AM

Ok, thanks guys. I think I'll proceed as follows:
1) check valve clearances
2) find an adapter so I can pressurize the cylinder and check for leaks.
3) act on anything found in #2
4) check timing
If anything turns up and gets fixed, I'll try again and see if it has the power it needs.

#14 MFGray OFFLINE  

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Posted June 04, 2012 - 03:47 PM

Your decription of the exhaust sounded hot, until I checked just what temperature that glow might mean - this table came off the internet:
C F Color

400 752 Red heat, visible in the dark
474 885 Red heat, visible in the twilight
525 975 Red heat, visible in the daylight
581 1077 Red heat, visible in the sunlight
700 1292 Dark red
800 1472 Dull cherry-red
900 1652 Cherry-red
1000 1832 Bright cherry-red
1100 2012 Orange-red

C= Centigrade
F= Farenheit

That suggests you might have had the exhaust somewhere around 800 degrees F if you could only see it at twilight. That still sounds hot, but a recent EPA study on small engines found that a Briggs & Stratton generator had an exhaust port temperature of 1250 degrees F when new and less - around 1150 degrees F - after 250 hours running, (but about 40 degrees F higher if it was run with 10% ethanol). All of which suggests that as has been said above, the glow is pretty normal and the engine is designed to run at that sort of temperature without hurting the valves.
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#15 HowardsMF155 OFFLINE  

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Posted June 04, 2012 - 08:23 PM

Excellent chart, thank you. I knew that red in the dark is not as hot as red during the day, but again, could not recall whether a small engine would normally reach that sort of temperatue. Still, when I can I will proceed as I outlined above since the engine does lack power. I included the red muffler as a symtom in case it might better match someones memory.




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