Tecumseh hm80, can’t get it to idle

[skeer]

So the engine on this Drift breaker won’t idle. I’ve cleaned the carb, replaced float valve and seat, main jet and cleared all the passage ways even behind the Welch plugs although I did not pop the plugs out.
Pulled the top of the head and cleaned a massive amount of grease and carbon. When I first got it I put a couple cap fulls of new oil thru the plug hole just to make sure since there was water in the crank case.
Valves move like they should, but I don’t know what the clearances should be. Piston sleeve and top are very nice and smooth.. no scratches or marks.
(30 minutes later)

It will idle now only at full throttle, but backfires a few times a minute.. can see a tiny bit of flame out of the muffler. And is hunting slightly the entire time.
If I drop the throttle any little bit the hunting Increases and it bogs out and dies.

I tried adjusting the main and then idle screws and while I can make things worse I can’t make them better. Hopefully the video works whenever it’s finished uploading.

Maybe timing? The video I watched mentioned that tightening the two bolts midway in the slots is a safe place but might not be 100%

 

[The_Geologist]

The popping and backfiring sounds like it is running lean (and I know you talked about adjusting the screws already).

Sometimes those old carbs can be cleaned and cleaned again several times and still not run correctly. I've gone the route of finding a replacement carb on Amazon and haven't had issues the 8 - 10 times I have done so thus far. I usually go to Jack's Small Engines and find the part numbers of the carb I need (based upon the engine model/serial numbers) and then go to Amazon.

 

[skeer]

Yeah that was a thought.. replacement carbs are cheap. But I did have the points/coil assembly off the engine. Can't remember if I rotated the crank at all during that time though.

 

[Zeke]

Sounds to me like a classic intake leak. Anyway to do a little low pressure air into the spark plug hole and rotate the engine slowly? You'd hear a leak at 5 lbs. pressure

 

[skeer]

Yeah man.. no idea tbh. I might could rig something up pulling a vacuum.

I've had intake leaks before but never had something backfire b/c of it.

 

[skeer]

Maybe related question.. if you hold the governor stationary and the idle sounds good then it's not fuel, right? That'd also eliminate an intake leak yeah?

 

[Zeke]

I would think. You may have installed the gov linkage wrong. Bad intake valve can cause backfire. Mixture too but you said you did that.

 

[Motown]

Those bigger Tecumseh's have a habit of hammering the valve seats down. You probably need to adjust the valve clearance. And might as well lap the seats while you have them out.

 

[The Cobbler]

there are some carbs that just won't clean up.
is it an adjustable carb or pre set jets? as they get older they need more fuel & the presets can't accommodate that

 

[skeer]

@The Cobbler nope, I mentioned above that I replaced main jet and idle restrictor screw.

@Zeke linkages are correct.. I hope it’s not valves since I’m not familiar at all with that.

Let me ask, I pulled the flywheel to check the timing. Gap was a bit closer than 20k, but when I went to check stator position I used my MM and ended up reading continuity between the points and ground when at TDC…points open. Pretty sure that’s a no no.

So I pulled the stator, no faulty wires shorting I can see yet. But I haven’t yet pulled the points, condenser and coil off.

 

[The Cobbler]

you have to take the condenser out of the circuit or you will get a reading with points open

 

[The Cobbler]

I watched the video. it's most definite lean .classic pop, fart, etc on a tecumseh
I had a preset carb that I had apart at least 4 times, couldn't get it to stop popping & farting.even with a drilled main jet ... a $12 ebay carb fixed that and still runs fine

 

[skeer]

Ha sob! I watched the official Tecumseh video earlier. Forgot the condenser part.

 

[PoorUB]

There is a fuel passage in the very bottom of the carb, where the main jet assembly screws in. If you look from the side you will see a small round metal ball plugging the passage from the side. You will need to take a tiny stiff wire, like from a wire brush, and poke around in the threads until you find it and open the passage with the wire. A bit tricky to find, but trust me, it is there!

Look closely at the top picture in this link,

https://www.tecumsehcarburetorguide.com/troubleshooting-manual/

 

[skeer]

Ok so was able to set the stator finally last night. Clamped an indicator above the piston and measured .080 BTDC. Then set the gap at .020 at TDC. Reassembled it all and now it refuses to start, I did something wrong. Don’t believe I’m getting any spark but it was hard to tell 100% at midnight.
I’ll also check for that passage, I for sure didn’t look for any passages inside the main jet thread area.

 

[theoldwizard1]

The popping and backfiring sounds like it is running lean (and I know you talked about adjusting the screws already).

If it is not the idle jet (these have been non-adjustable on small engine carbs for more than 20 years) then there is an air/vacuum leak. Also, there is usually an idle air bleed passage. Very tiny.

Less than 50% of the Chinese aftermarket carbs actually resolve the problem !

 

[PoorUB]

If it is not the idle jet (these have been non-adjustable on small engine carbs for more than 20 years) then there is an air/vacuum leak. Also, there is usually an idle air bleed passage. Very tiny.

Less than 50% of the Chinese aftermarket carbs actually resolve the problem !

That snowblower is 50 years old. The carb has adjustable jets.

Ok so was able to set the stator finally last night. Clamped an indicator above the piston and measured .080 BTDC. Then set the gap at .020 at TDC. Reassembled it all and now it refuses to start, I did something wrong. Don’t believe I’m getting any spark but it was hard to tell 100% at midnight.
I’ll also check for that passage, I for sure didn’t look for any passages inside the main jet thread area.

You need to be carefully how you route the wires for the kill switch and condenser or they will rub on the flywheel. I also make sure the points are spotless after adjustment. A miniscule particle of dirt will ruin your day.

 

[skeer]

That snowblower is 50 years old. The carb has adjustable jets.

You need to be carefully how you route the wires for the kill switch and condenser or they will rub on the flywheel. I also make sure the points are spotless after adjustment. A miniscule particle of dirt will ruin your day.

Yeah so that was a question… the fuel line routing behind the flywheel seems odd. But there’s the accessory power feed that was routed along the fuel line to the starter side. It seemed crowded so I ended up moving it to exit on the carb side.

I think I’ll end up putting loom over the kill wire that exits the stator area to be safe. I really need some alligator clip jumper wires.

 

[bb29510]

china making replacement carbs that are so cheap and good, for about $20 you can get a new carb that already adjusted, just bolt on and run. I have done this with big rider, weed eaters, chainsaws

 

[skeer]

The desired result is getting this operational then, most likely, flipping it. So the less I put into it the better.

But yeah.. small engine carbs are so damn cheap anymore.

 

[theoldwizard1]

Yeah so that was a question… the fuel line routing behind the flywheel seems odd.

No. That is typical on Tecumseh engines. You have use special fuel line to snake it through. The typical 1/4" ID fuel line has a OD that is too large to fit.

 

[Mick56]

I would def check the valve lash. Most old engines I work on need the clearance increased by grinding a little off the bottom of the valve. If there is no lash, the valve does not seat properly and can pop back through the carb, or shoot flame out the muffler. The lash is usually in the range of .006 to .010, should be easy to find online.

 

[FredWanaker]

also on these old engines, some of the intake runners can crack and let in air. I would check the valve clearance because if it has a lot of hours on it, they will be tight.

 

[laser3kw]

the fuel line routing behind the flywheel seems odd

common on flat head Tecumseh's.
When the fuel line goes bad, hope the hose hasn't corrosion welded itself to the block. If it hasn't, you can use the old line to pull the new line through. If it has, then disassemble the shroud, flywheel to replace the hose. And... as long as you're that far, and you removed some of the head bolts (possible broke the head gasket seal), might as well pull the head, check for head and block warp and replace the head gasket.

 

[firebirdparts]

Let me ask, I pulled the flywheel to check the timing. Gap was a bit closer than 20k, but when I went to check stator position I used my MM and ended up reading continuity between the points and ground when at TDC…points open. Pretty sure that’s a no no.

I guess technically both sides are grounded, but I never thought about it much. One side is grounded through a reaaaalallllly long wire that goes through the coil. When the flywheel comes around that becomes hot for a little while, but there is a path to ground there. Ya'll got me curious about the condenser in that situation, but hopefully I'll never need to know, ha ha. The only thing I have left here with a points magneto is the Rokon.

Anyway, if the thing idles with the throttle on the idle screw, then it's idling through the "idle circuit". If it won't do that, my guess is that the idle circuit is plugged up and that's not a strange thing at all. The governor doesn't play a role in idling.

 

[skeer]

So yesterday I re-adjusted the stator timing, this time opting for an .080 BTDC based off a Tecumseh doc I found for the HM80. Also regapped the spark plug, but today it just refuses to fire up.
I bought a spark plug tester a few days ago.. the light that plug inline with the plug. When I turned it over before putting the flywheel cover back on the light was strong after the cover is back on it’s real faint. Makes me think maybe the kill wire is shorted but I haven’t found anything yet.

 

[skeer]

Ok well I’m out of ideas. Other than possibly a bad comdensor , but idk the symptoms of that. Either way damn thing just won’t catch the spark.
Checked last night, the tiny transistor ignition conversion kits won’t work for the larger, aluminum Tecumsehs with magnets covering the entire diameter of the inner fly wheel.

Not idea why yet though.

 

[The Cobbler]

so you have good spark with the fan shroud off, and no or little spark with the shroud on?
bad plug wire getting shorted on the cover ?

 

[skeer]

so you have good spark with the fan shroud off, and no or little spark with the shroud on?
bad plug wire getting shorted on the cover ?

That’s what I had thought.. after removing the flywheel and stator again last night with the intention of checking for shorts..found none. Put it all back on, and I’m getting intermittent..weak spark. The bulb lights really intensely randomly but kind of dull mostly.

So that means coil right? Possibly a weak magnet (or two)?

 

[laser3kw]

^^ strong spark with shroud off, weak / intermittent with it on? How crusty is the spark plug wire itself? Is it pinched / compressed under the shroud? Can you unplug the shutoff wire from the coil to eliminate the possibility of a downstream problem? Just temporary for testing.

 

[skeer]

Yeah the later tests were with the shroud off, results were unchanged. The stator assembly.. the plates, are rusty but not bad. I mean I've seen worse on external coils that worked fine. I haven't ohmed out the coil itself yet, nor done the screwdriver-magnet "test".
My cylinder doesn't have the bosses to mount and external coil so I'm kinda up the creek on that.
I am going to also try pulling the alternator and kill wires off the stator entirely to rule those wires out.