OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Tecumseh / Kirby Engines Tecumseh HM80 8HP

Folks I picked up a MTD Chipper Shredder with a HM80 Tecumseh powering it. When I looked at the unit the fellow started it and it ran well. Started easily and ran at peak revs well. The only thing it did was that it backfired if decelerated too quick, according to the owner. He said the large flywheel/cutting disc kept the motor spinning so he decelerated it slowly.

Today I started it after bringing it home on the weekend. It fired first pull but ran with the occasional miss. After warming it up I put some clippings through it and didn't sound fine. It was missing and backfiring.

It seemed odd as I putt clippings through it when I picked it up and it didn't miss a beat.

I am thinking that the trip home on the back of the ute may have put some gunk through the fuel lines and causing the syptoms I experienced today.

I do know that the seller said the unit has done very little work but has sat for quite a while. Upon getting it ready for sale he put in a new needle and seat and float. He said the bowl was gummed up from stale fuel.

Today I pulled the bowl off and it was still fairly gummed up. I'll try and clean it during the week.

I cannot find any numbers on the carb itself. Only on the long intake port. Would this be the carb number?

Is anyone able to identify the year of manufacture of the engine with these numbers 8HP Cast Iron Sleeve HM80 155426M SER 1308D?

Thanks
Alf

Pictures would help.
People are lazy and dont like to search, but will help.





Clean it out and try it.


Or buy a new one US$40
http://www.mowers4u.com/oregon-tecumseh-carburetor-hm80-hm90-hm100-p-787.html

Heres an overhaul kit US$10.95
http://www.mfgsupply.com/m/c/22-10948.html

Bruce may be able to get you either of the above in the store if you message and ask.

Some of the mower engines have a fixed size jet others have and adjustable jet. I assume this will have one or the other. If its adjustable it might need tuning.



Theres three different carbs apparently.

Thanks for the links Rodeobob. I was typing stuff into the computer all day trying to find links and you put up two. Cheers.

I ended up buying a second hand carb which is close to mint. I cleaned the external grime, put it on and added a fuel filter this time and it fired right up. It was a quick tune due to the time of day but I'll have a better play tomorrow.

It idled beautifully and ran out to full revs without a cough. There was no backfiring on deceleration too which was good.

Once I removed the air cleaner housing and the intake manifold on the old carb I found it had quite a lot of corrosion. I didn't think that fuel would do that. Perhaps it had some water in it at some stage during it's storage??

If I can get the camera working I'll take some pics.

The only small issue I have to work out is that the new carb must have run a linkage choke lever. The choke is spring loaded. The old choke was a direct mounted lever.

The part number that you require for a new genuine carby is 632351 and cost around $160.00 but it depends on your budget if you want to go for a new one or a oregon one.

Probably ethanol fuel. Tecumseh motors hate that stuff.

Thank you Bruce. I'll be putting the chipper through it's paces this weekend with a stack of Mock Orange clippings.

If it plays up I'll be getting onto you about a replacement carburettor.

I'll also get some pictures hopefully and post them of both the damaged carb and the choke cable job I hope to rig up.

If the fuel contains ethanol and you let it stand for a long time, it absorbs moisture and becomes discoloured. The discoloured fuel will not run an engine properly and is likely to cause partial or complete blockages in the fuel system. Also, you should not leave ethanol in contact with any polymer components of the fuel system, because it attacks them chemically.

Personally I don't use fuels containing ethanol, to avoid these difficulties.

Thanks Grumpy.

I drained the old fuel from the tank and it was very yellow and smelled like old paint.

That sounds like a fairly typical description of old fuel containing ethanol, Aldo: the ethanol has become combined with water by absorption. The yellow smelly stuff seems to be prone to blocking things up. Gum formation in old fuel is a separate problem, mainly relating to fuels with a high proportion of cat-cracked constituents. Olefinic hydrocarbons of that type are not very stable in the presence of oxygen, and the oxidised products include gum.

This oil company bulletin may be helpful (note that the reference to "oxygenates" relates to ethanol, methanol, and certain octane improvers used in the US).

http://www.chevron.com/products/prodserv/fuels/technical_safety_bulletins/ltg_background.aspx

Thanks Grumpy.

I ran the unit today for 30 to 40 minutes and it ran perfectly. Once done I drained the oil and added some replaced it with some fresh 30W Briggs oil. The choke I installed works nicely. It is interesting that I found the replacement carburettor is from a 12 HP OHV Tecumseh. It bolted straight up.

I'll get some pictures in the morning.

A lot of things like that just need minor tuning between engine size some need a jet change

If the 8hp is side valve it would probably need a similar size jet to an OHV 12HP

Good point Bob. The only differences I picked up apart from the spring butterfly choke was the screw in the main jet for the old carb goes to a needle point taper. The newer carb has a taper but then a squared off top. The last 4 to 5 mm of needle is removed.

Here are some pictures of the old carb and the new one

Old carb with the direct choke control



Corrosion in the old carb, sorry for some average pics






The new carb and the choke control I dodged up. It bolted straight up and the machine runs great. I just had to make up some quick brackets and on old cable to get it running.




Cheers

Al

That choke setup looks very well done, not dodgied up at all, I have seen much worse home repair jobs before

Can you take a pic or two of the whole machine for me please.

I like boys toys and you cant get more manly than a machine like that.

Thanks Joe. It certainly does the job even though not original.

Rodeobob, I'm not sure if you are pulling my leg but here you go

https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=29355#Post29355

Nah not pulling your leg.

Sounds like a stir that my wife would give me

Fellas I have a question on a problem that I have solved but curious to know the reason.

I picked up some fuel filters a while back. They were petrol filter which were an inline job that had a clear canister which housed a cylindrical pleated paper element. I use them on my push mowers as I feel they filter better.

I put one on the shredder and the trouble started. It would run fine but if it stopped, it would never restart. You had to pull the fuel line from the carb and fuel would run freely. Reconnect and it would fire right up. If you stopped it would not restart and had to repeat the process.

Why does it run in the push mower yet give problems in the ride on and shredder?

What is the push mower???


Perhaps the volume of the little filter cannister has something to do with it. Im thinking a vacuum lock.
It could be the head pressure. How high above the carb is the tank on the HM80?? As opposed to the mower??

You could maybe try a slight enlarging of the breather holes in the fuel cap.
On a different cap is probably a good idea.
Or rather than removing the filter/hose, plow some pressure in the tank before trying to restart.

Is the motor running lean?? Perhaps the fuel delivery when running is not good enough to keep the float level right.



Im sure its got a filter in the tank, perhaps filter the fuel before or when you are filling the tank.

Hi Bob the push mower is a Quantum 55 motor. I reckon you are on the money with the fuel cap call.

For the record the tank carb height difference is greater on the ride on and chipper than the push mower.

I have switched to the red Briggs filter and they have run perfectly so I'll stick with them. The problematic filters did seem to collect a lot of fine particles that the others let through.

Theoretically the filter only has to filter out anything that would not fit though the smallest passage in the carb.


As a precaution as the fuel should be clean.

Bob, it doesn't really work that way with fines in the fuel. There is usually a small amount of gum some of the time, and the gum glues the fines together. This all happens in a small passage of course, especially right upstream of a seldom-adjusted mixture needle, where a sudden drop in flow rate causes particles to get deposited (like on the inside of bends in a meandering river). Once a few grains of fines stick together, they make a base for a blockage that just grows like a vegetable from a seed. That is why carb cleaner has such a profound effect: once you take the gum out, you just have loose grains that will fit through the passage under air pressure.

The Briggs red filter is intended for gravity-fed fuel systems, and the black one for systems with a fuel pump. You shouldn't expect to be able to use a black filter in a gravity system: there is just too much restriction.

Hi folks, just a question on whether or not I am following a correct fault diagnosis path.

The HM80 has not been run in over a year. Last time I used it it ran fine for around 20 minutes then cut out and would not restart. A day later I checked purely to see if it would fire and it did on first pull so I turned it off drained the fuel and put it away.

Today I had a lot of stuff to put through the machine. I put in some fresh fuel and it fire and ran from the third pull.

After about 20 minutes of running fine it stopped as if running out of fuel. After 20 or so pulls I gave up and decided to check a few things.

Fuel in the tank
Carb was getting fuel
There was spark
Plug burn looked perfect
Fuel was purchased less than 3 days ago
Magneto air gap was fine
Compression felt strong

I decided to check the valve clearances. They were minimal at best, about .005 thou. By specs exh is .020 and inlet is .010

I pulled the valves and adjusted them accordingly.

Could the valves being tight have caused the problems I experienced? I have yet to start and run the machine as I had to make a gasket up for the valve cover.

The valves looked good as did the seats and were tight in their guides. The bore looked excellent.

Hopefully I will get to run the machine by the weekend.

There isn't much to go on in that description, Aldo. I'd begin with three possibilities that are easy to deal with.

First, tappet clearance just about always decreases when the engine warms up, due to metal expansion at high temperatures. That is the main reason to have tappet clearance in the first place. Since the engine stopped when it got warmed up, there is a fair chance it was holding the valves slightly open, at those highly inadequate clearances.

Second, have you removed the cowls and checked that the air intake passages and cooling fins are clear, and the flywheel has all its fins? Poor cooling air circulation is prone to causing the fuel to boil in the carburetor, whereupon the fire goes out in the engine due to lean mixture.

Third, have you checked the rate of flow of fuel into the carburetor? You can get upstream problems that cause the fuel flow to stop after a while - in particular, blocked vent in the fuel tank cap, or a floating mass of grunge in the tank or outlet pipe that moves down the pipe when fuel flows, but goes back up when the engine is stopped. By checking that fuel flows full stream out of the needle-and-seat when you remove the float bowl, and keeps on flowing while you empty the tank through the needle-and-seat, you can check for that problem.

There are other things it could be, in particular high temperature breakdown of the ignition system, but there we get into stuff that is more difficult to test for, and way more expensive to fix. Would you mind checking those three things, making another test on the engine's behaviour after a long run under load, and reporting back please?

Thanks Grumpy I will look at those. I did try the fuel cap but no change.

I had a play and I think, as you say Grumpy, there was a fuel restriction blokage of some sort. I have a fuel tap and inline filter and removed the filter and it ran just fine. Started and then retarted it many times with no issues.

Its a gravity feed and when I pulled the hose from the carb inlet fuel would only very very slowly dribble out. I used a second piece of hose with no tap or filter and the fuel poured from the hose at any level below the tank level. Very strange. All these tests were done with no fuel cap.

We have had a couple of floating-mass fuel hose blockages just like that reported here in the past couple of months, aldot. I think it begins with water in the fuel tank, which grows something like algae. The algae dies when the water is drained and the tank is filled with petrol, but it continues to drift around in the tank. Eventually, when the machine is run out of fuel, it finds its way through the outlet fitting into the hose, and then it causes problems - it doesn't float back into the tank, but stays in the hose.

IIRC, we had one that lodged in the fuel tap and another one that plugged up the in-line filter. Both were delayed-action problems: the machine would run normally for a time, then stop mysteriously, but would start and run again after the floating mass had floated back up the fuel line.