# Tecumseh misfiring and doesn't want to start when warm



## afremont (Jul 13, 2015)

New guy here. I've been fighting with a trash-picked mower that looked to be in decent shape. It's got a Tecumseh 6.75HP engine on it. I've cleaned/"rebuilt" the carb completely and that doesn't seem to be having any effect on the problem. 

The engine starts fairly easily when cold, but misfires quite a bit. The spark seemed a tad bit weak on it so I'm going to change the magneto next. It ran lean when I first got it running before rebuilding carb and it got overheated to the point that it "clanked". After rebuilding the carb, it doesn't seem to be running lean any more, in fact the exhaust port and muffler have quite a bit of black soot in them. It's not black smoking though. It misses badly at first and then misses less as it warms up for a couple of minutes. It never completely stops misfiring though. I'm thinking that the black soot is from the misfiring.

Yesterday, I took the carb off yet again and readjusted the float so that it tilted down a bit when the needle was on the seat. It was almost perfectly level. It's my understanding that this would make the engine run even richer, but it runs exactly the same as before.:huh: I also cleaned and checked all the passages and the main jet again. It all looks good to me, but what do I know. 

When the engine is warm, it does not want to start until it cools all the way down. I've read that the overheating can reduce the exhaust valve clearance, but I don't have feeler gauge to check it precisely. It does seem to have a bit less clearance than the intake valve by eyeballing it.

I want to pull the head off and make sure that the gasket isn't blown, and take a good look at the valves and piston top. I will also get a feeler gauge and check the clearance properly. 

This is a nice mower, and being the OCD type that I am, I refuse to give up on this until it's fixed. I'm also a bit worried about the rod since I've seen some metallic glint in the oil after the overheat/clank cycle. It's not knocking so I don't think that's the main problem right now. Outside of misfiring and not wanting to start when warm, it runs fairly reliably and seems to have plenty of power. It doesn't seem like it's seizing up yet and I've run it for at least 2-3 hours since I overheated it at the beginning of this project.

My IR thermometer shows the engine head to be at approximately 250-260F when warmed up which seems a little hot to me. 

Any ideas, thoughts or advice? Am I approaching this problem correctly? I have a fair amount of mechanical experience with cars, but not much with small engines. Any Tecumseh experts out there? Thanks for reading.


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## AVB (Nov 28, 2014)

First replace the spark plug. They do fail and sometimes act this way. 

I just repaired a Stihl FS80 that ran seemly perfect at idle but mis-firing at anything above that. Replaced the plug and it runs like it should at full throttle now.


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## afremont (Jul 13, 2015)

Thanks for the reply, I appreciate it greatly. :smile: I forgot to mention that I did change it when I first got the mower. Is it possible that overheating the engine damaged it? I would think that unlikely since the aluminum didn't actually melt or anything.  I don't know much about small engines though.

I really don't think it's a carb problem because it ran fine for a while, plus I've had it off a couple more times and cleaned it out, but it looked pretty clean each time (no trash or grit in the bowl that I could see).

I've got a whole pile of junk I drug home from craigslist after getting my free mower from the neighbor's trash. I had visions of fixing this stuff up and making a couple of bucks while I learn something. I've gotten half of it running now, but I haven't started on the two-strokes yet. :/ This all started because I got sick of paying my lawn guys to do a poor job, plus I could use the exercise from doing my own lawn again. Though I'm getting a lot more exercise from pulling the starter rope than anything.:banghead:


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## AVB (Nov 28, 2014)

That pile of equipment is similar what I trained on myself and still do from time to time during my spare time. Worst possible problems that other could not fix are good for getting experience. 

Now with the Tec overheating severely it can be just about anything cause problems. Things like sticky valves. And the repairs in this area depends which engine you have. Also be aware that the older original Tecumseh engines parts are harder to come by as they have been out of business for a long time.

As for metal glint in the oil it depends long the oil been in the engine. All small engine do have metal flakes in the oil but high levels do indicate a serious problem. The main sources are the cylinder and bushing wear. 

If you should open the crankcase up I would check the rod bearing on the crankshaft especially since the engine over heated. The bearing surface is actually the rod contact area. A lot of times when engines are run without oil this melts and leaves an aluminum deposit on the steel crank journal. This then starts a serious problem as just keeps getting worst until the rod seizes and breaks.


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## afremont (Jul 13, 2015)

Thanks, I appreciate the support. It's not so much that it's overheating that it just won't stop misfiring no matter what I try. I got a feeler gauge yesterday so I can check those valve clearances. It only majorly overheated when I first got it going, but before "rebuilding" the carb. Doing the carb seemed to cure a lean condition, but it started missing after a while and didn't want to start when warm.

Since my backup proved itself worthy today, I'm going to rip into that Tecumseh and see what's going on with it. I'll start by checking the valve clearance and then pulling the head for look see at the top end stuff, including that gasket. Depending upon how that all looks, I might pull the sump and check out that rod and crank journal. These things are weird to me, there's no replaceable rod bearing, just the rod inner surface. I can get a new rod and piston assembly for about $50, they even have .010 over sets available. If the bore is worn much, I may see if I can hone it out to .010 over and install a whole new assembly. If it looks good, I may just put a new rod in it if it's in rough shape.

I just got done testing my "backup" mower this morning on some really tough grass. A bunch of it well over 12" high. I mowed over 10,000 sq-ft non-stop without a hitch. I live on a bayou and I neglected mowing the easement for a few weeks and we got some serious rain over that period. Now it's a drought again, go figure. The backup mower is a "frankenmower" I think. It's green, self-propelled and says Roper BP70BR Rally and came with a Craftsman grass catcher that fits it perfectly. I can't find anything online about that thing, but for a 4HP Briggs, it cuts like there's no tomorrow. It's putting out a lot more power than that 6.75HP Tecumseh it seems. It runs absolutely great now. This might become my main mower, but I digress.

I've been watching a lot of youtube vids and reading what I can find online about these engines. Seen some amazing things out there. One guy made a new plastic exhaust lobe for his Ryobi 4-stroke weed eater, and it actually works for him. Seen some stupidity as well, so I don't feel like I could do much worse than some of them. I've learned quite a bit in a short time. So far, I've gotten 4 motors running including the Tecumseh we're talking about, 3 of them well and they were a real mess in the carbs. One looked like it was full of kitty litter in the bowl. I'm finding that you don't need a lot of parts to get these things going most of the time. All of these machines have been sitting for years in a damp environment.


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## afremont (Jul 13, 2015)

Following up. I checked the valve clearances, and they seem okay. I was able to get a .006" feeler between the exhaust stem and "lifter". I don't think a .008" would have gone in at all, though I didn't actually try it. I had to take tension off the spring to get the .006" feeler in there, the .004" went in easily.

Pulled the head, it was carboned up some, but not terribly. The cylinder wall shows some scuffing on the sides, but not the top and bottom (as viewed with the engine mounted to the mower still). It's down far enough that it shouldn't be affecting compression a lot IMO. I may still hone it out and put a .010" over piston assembly in it ($53 total for piston, rings and rod).

My new coil will be here today, but the new head gasket won't arrive for a few days it looks. I'm tempted to put the old gasket back on, and test run the motor with the new coil to see if that was whole problem all along. Some of the sealing surface (pattern) is gone now since I cleaned it all up with a wire wheel so I'm not sure how well it will seal, but it should be alright for a test run I think. I noticed that the head bolts should likely have been tighter than they were.

What is the recommended torque value for head bolts?
Should I worry about the scuffing in the cylinder walls? I can easily see it, but it's very slight to feel. I would venture to guess that they are only .001" deep, if that much at the worst places.


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## afremont (Jul 13, 2015)

Following up again. I changed the plug and the coil. I also put a new head gasket on it. I haven't done a lot of testing yet, but it basically runs the same. It's still misfiring. I don't know if the hot start problem is still there though. 

I didn't take the valves out, so I can't verify their condition, but they seem to be seating okay and from what I can see, they look decent. I'm not getting any smoking at all.

Since I can't seem to find my compression tester, I'm going to borrow one from a mechanic friend of mine. Hopefully he has a leakdown tester as well. 

I'm wondering if I got a sticky valve. Would it run like this if the flywheel key was partially sheared? I'm about to remove the flywheel to take a look, assuming I can get it off with the tools I have on hand. I noticed the service manual says NOT to use a gear puller, but to pry up on it while beating on the end of the crank (through a threaded in bolt).

This has become an obsession. I don't care what it takes to get it running right. I look at this like a learning experience, it's not about time or money. 

I want to order some specialty tools. What is the difference between a Briggs 19163 spring compressor and a Briggs 19063? Are they the same or is one bigger? Which one do I need for small LHead engines like this Tecumseh LEV120?


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## afremont (Jul 13, 2015)

I did a compression test. After 5 pulls, the pressure was at 105PSI. That's pretty reasonable, right?


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## SABL (Jul 4, 2008)

afremont said:


> I did a compression test. After 5 pulls, the pressure was at 105PSI. That's pretty reasonable, right?


Looks reasonable.



> Would it run like this if the flywheel key was partially sheared?


But, before tearing into any small engine with a flywheel key the key should have been one of the first things to check. It doesn't take much distortion of the key to cause timing problems......the key does not have to be completely sheared.


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## afremont (Jul 13, 2015)

Thanks for the reply. That makes a lot of sense, check the easy stuff first. 

I changed the oil, and it looks kinda sparkly. Actually it looks a lot sparkly along with a charcoal gray coloration to it. This was fairly fresh oil too, maybe two or three hours on it. I changed it again to switch to straight 30W instead of multi-visosity oil that I had put in there. I'm thinking the oil condition is a bad sign of significant wear taking place. 

I'm guessing I have a rod failing on me. Would that lead to this sort of misfire? It's not knocking that I can hear, but it sure did when it overheated before. I'm going to rip into it later on and take a look at the rod cap and crank rod journal to see how they look. Hopefully, I can clean up the crank and install a new rod. It's only about $25 for a rod. A whole piston assembly (rod, piston and rings) is about $60 for a .010 over setup. I already have a whole gasket set since it was the same price as just buying the head gasket by itself. Hopefully, I can reuse the new head gasket.

You can see in the pictures above that there is a little bit of scuffing on the cylinder walls, but the cross-hatch is still visible over a lot of the surface. I'm thinking that I can hone it .010 over and install a whole new assembly and have it running like new again. Of course I'll lap the valves too. To me, it's worth the cost of the rebuild just to gain the experience.


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## afremont (Jul 13, 2015)

Oh yeah, one more thing. Should I buy the equivalent of the Briggs 19063 or the 19163 spring compressor tool? I assume that one is larger than the other, but that's just an assumption.


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## SABL (Jul 4, 2008)

AVB is the professional tech on small engines......I'm just an innocent bystander.......:laugh:. I do what I have to do and only when I absolutely have to.....strictly a home mechanic.

Most times, gray oil indicates the presence of moisture.....if it's light gray. But, in your case I would guess the gray would be from the rod wearing when engine overheated. 

Sparkly is not good in any oil unless it is after initial break-in and was to be expected. I had the seals go out on a mower pump......hydraulic drive system. Simple replacement of seals didn't happen when the hyd oil was sparkly......pump failure at 465 hours even with scheduled maintenance being performed. I learned how to rebuild a pump real quick......only $200 for parts and fluid/filter. 

Sounds like you are going to get the engine running again in the right way. Some of these older engines are worth repairing......if you can find the parts. Make sure to take many detailed photos as you go along......you may need them for reference unless you have a tech manual.


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## afremont (Jul 13, 2015)

Yeah, I'm afraid that the engine is on its way to failure judging from the oil. I'm glad I have a good backup mower. It's a lowly Briggs 4.0HP, but it cuts like there's no tomorrow. It's really makes a lot of torque.

I'm going to order a couple of tools and rip this thing apart and get a look at that rod and crank. I've been watching a bunch of youtube videos and reading what I can find of service manuals on-line. Saw some pretty interesting things that I would never have thought of on my own. Saw a few other things that I would never have tried and still won't.  Such is the nature of youtube. 

I think I'll try my hand at porting and polishing while I'm in there. Is that ate-up or what?


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## AVB (Nov 28, 2014)

Sorry been in the shop learning an ATV repair job. I don't normally do these repairs but it is for one of my best customers.

A couple comments. 

When you hone the cylinder hopefully you will be using a rigid hone as a flex type will only amplify an out round and/or tapered condition. Also I would go ahead hone the cylinder before ordering that .010 over as it may need to go to .020 over instead.

Second if the crank journal has aluminum transfer from the rod don't use an emery cloth to remove it as can cause a need for under sizing of the journal. The preferred method of removal would be the use Muriatic Acid_. _This stuff can very dangerous if used indoors as it is very sensitive humidity and produces toxic fumes. Once used it must be neutralized which can done by using large amounts of water. Once your through cleaning make sure you oil the crankshaft very well as it will rust easily if not done immediately after cleaning. Do not get the Muriatic Acid on aluminum surfaces as it will destroy them.


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## afremont (Jul 13, 2015)

Just following up. Aside from the metal in the oil, the mower runs like a top now. Turned out to be a partially sheared flywheel key. I guess the main symptom that I never mentioned was how it would try to rip your arm off at times when pulling the rope. That's probably why it was tossed into the trash. That really gets old quick.

Another resolved problem is that the brand new coil I got from e-bay stopped working after using the mower once. I don't know why, but there is no spark coming from it. I wasted a lot of time yesterday after changing the flywheel key by trying to figure out why it wouldn't run. I finally swapped the old coil back on to the engine and it started on the second pull. It runs super smooth and can be idled down to very low RPM and it idles great.

I had to switch the governor linkage back to the original hole in the throttle butterfly because it was running way too fast after fixing it. I should have known not to mess with that since it should have run right in its original position.

I got a complete gasket kit for it and I will be buying a new rod before blowing it up. Any suggestions on cleaning up the crank the right way since it surely has aluminum transferred to it. 

I mowed the front yard with it yesterday and it ran like a top. Started instantly after shutting it off and then started on the first pull after letting it sit for a couple of hours to cool down. It doesn't act like it's going to fail, but I don't want take any chances on damaging the block if the rod seizes up. Am I being paranoid, since it's not knocking or sounding odd? Will it start knocking long before it seizes, or it will it just start bogging down and then lock up in the course of a few seconds?

I won't run it any more if you guys think I shouldn't. I really want to rip it open and get the experience of changing the rod. I'd like this mower to last a lifetime while I rub it in my neighbor's face for throwing it away. We don't get along, never have. He's always been extremely rude to me and my wife since we moved in 22 years ago. A couple of months ago he backed into my car and then took off without saying anything. Thank God for security cameras.  

A new rod is about $30 and a new oversized piston is about $60 (w/rings and new pin). It's tempting to do the whole thing and make it new again, but I don't think it really needs a new piston and rings since it makes plenty of compression and the cylinder walls aren't scuffed up too much. (see pictures above and let me know if you think I'm wrong about that). I am going to lap the valves and polish the intake port up some for the heck of it since I'm a gearhead by nature.


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