# Trying to get my HM80 to run smooth



## hankster1 (Sep 10, 2013)

I have been working at getting the HM80 engine in my 86 MTD machine to run smoother. When I started working on this it only run wide open with the choke fully on. Fuel would drip from the carb. The first thing I did was Put in a new sparkplug, and cleaned the carb and installed a new repair kit. This made some improvement but still dripped fuel and wouldn't run without the choke on. Next I discovered the atmospheric vent on the side of the carb and with a small wire cleaned the corrosion from it. That stopped the fuel from dripping. Then I checked the manifold to head gasket and found it have a section missing so I replaced it. The engine now would run much better and no longer would need the choke to always be on. I adjusted the mixtures and used the machine through several snow storms. When Hot the engine would start to backfire and throw flames from the exhaust and lose power. I removed the head and removed a layer of carbon then adjusted the exhaust valve it had almost no clearance. Now the engine runs smooth most of the time and has plenty of power. However when I hit cracks of bump the machine the engine stumbles and almost stalls out and backfires. after a moment it smooths out and runs well again. Why would it do this?? I almost have this running the way it should.
Hank


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## HJames (Oct 28, 2013)

Did you clean out the gas tank and replace the fuel line when you rebuilt the carb?Is it a metal or plastic tank? Is there a filter on the fuel line? Is it new? Could be that the "bumps" are sending tiny particles down the fuel line or mometarily stopping the flow.


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## hankster1 (Sep 10, 2013)

Yes I did clean out flush and blow out the tank with air. It has the new plastic style tank.


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## Grunt (Nov 11, 2013)

Some Tecumseh carbs came with a dampening spring on the float to prevent this problem with engines subjected to bouncing. I'm sure the spring could be added to any Tecumseh carb and it may help your problem.

Installation, Repair and Replacement of Tecumseh Carb Float Damping Spring


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## scipper77 (Dec 4, 2013)

A long shot but you could have an intake leak at the carb. bumping something momentarily opens up the leak and the air that gets sucked around the carb is not mixed with fuel. I doubt this is happening but you may want to check and make sure the carb is securely attached to the motor.


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## gusgt18 (Jan 18, 2014)

Loose ground / kill switch wire ??


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## fibersport (Feb 6, 2014)

Mine had a bad float, it would run but would leak gas, run very rich and would also run better when I would tilt ot forward. Not saying your float is bad but it could be. A decent ACE Hardware might just carry the float, that's where I got mine. Also, my float came with the spring mentioned above, mine did not have it installed but it could help.


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## nt40lanman (Dec 31, 2012)

Yes, could be a bad or misadjusted float allowing the fuel level to be way high.


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## hankster1 (Sep 10, 2013)

Just to update on my progress with this HM80, I had replaced the fuel line and the carb float when I overhauled the carb. I have tracked the problem to rotten wiring leading to the alternator plug and rotten wiring going to the kill switch on the throttle control.
I plan on replacing the switch and the wiring and see if it improves. I checked the flywheel key and retorqued the nut. I found a mess of spider nests behind there. I think this has been causing most of the problems.
Hank


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## hankster1 (Sep 10, 2013)

I have the problem resolved!! It runs much smoother now. New wire and I replaced the ignition module. No more backfiring and rough running. It also no longer hesitates when hitting bumps or cracks in the sidewalk.
Hank


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## gsnod (Sep 2, 2013)

Having read this post, I was struggling earlier today with the float spring, which was new to me -- meaning I'd not seen such a thing! Now that I've seen the proper way to install it, I'll get to it tomorrow. Thank you all for adding another lesson to my daily agenda!


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