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Discussion Starter · #1 · (Edited)
I have a generac generator with a Tecumseh HM100 engine. It will start with half a pull when its cold and run beautifully...will not start when it is hot. Since, have replaced spark plug, magneto, carb, and done a compression test. It will hold 150 psi hot or cold,but I noticed anything under 60-80 it will bleed out. I am assuming that this is because it has a auto decompression. i can not figure this damn thing out for the life of me... Please help, Thanks, Jon

ik've also tried hitting it with ether when i got to restart it. doesn't even give me a pop...

I know I posted this in the general forum, but I mean't for it to be here in the 4 stroke forum....
 

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Being that you tried either and it didn't start you've ruled out looking for a gas problem. Might next pull the plug and check for spark. If you have spark then you might check the clearance on the intake valve. Sometimes when you experience valve seat or valve face wear, this results in the valve moving closer to the cam. When the engine is cold and starts there is enough clearance between the valve stem and cam to allow the gas / air charge to enter the cylinder. But when the engine becomes hot, the valve stem expands because it is steel with a high expansion rate. When you shut the engine off, heat builds up more because the flywheel fan is not turning. At this point the valve stem may press against the cam. This would mean that the valve never fully closes, but remains open during all cycles. Gas charge might come in to the cylinder on the intake stroke, but it would just go back out on the compression stroke. When the engine fully cools, the cam tolerances are increased (valve stem cools and contracts) and gas/charge is held in the cylinder. I don't know Tecumseh's gap settings, but I sure some member does.
 

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Discussion Starter · #5 ·
Dirkenshoe....I completely understand what you are talking about and that was what I was thinking as far as my next move. I obviously didnt mention it in the first post, but it is definately getting gas, and definately has spark. as far as fuel I have ruled that out also because I checked flow at the end of the line at the carb. and also the bowl fills and the engine runs fine. It is impossible for the engine to not receive fuel after it just ran fine.... Ive checked all that out. Im really leaning towards a valve clearance issue here, which I have still not checked. This weekend Ill make a point to check that out. My question is, where this is only happening when hot, should I check the clearances cold AND hot to see if theyre tightening up? Btw, I am a welder/fabricator and am somewhat mechanically inclined... thats why im so stumped here...These single cyl. engines have minimal tolerances to run, very basic. thanks for the replies... ill check the valves and let you guys know. Any more input would be greatly appreciated, Thanks, Jon
 

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I hate to belabor this, but how did you test the ignition when hot? Are you using an ignition tester or just pulling the plug or what? Are you confident there is nothing related to the stop switch circuit that might be at fault? As I'm sure you are pulling your hair out, it's hard to fathom that you have gas, spark, and compression and don't even get a pop.

If you have the info, please share the model # of the generator as that could be helpful.

Thanks!
 

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Discussion Starter · #7 · (Edited)
I checked the valve clearances today finally and found that the intake is .008 and exhaust .007... yes Im going by pulling the plug, grounding and pulling the cord. it seems to have ok spark,compression test leaks below say 60 psi but then will hold and I can get it up to around 150+. I know it is getting fuel to the carb because I pulled the line to make sure flow was ok. I am going insane over this. Cold it starts one pull, warm or hot it wont even pop. Because of the scenario I have ruled out low oil circuit, switches, etc as this would strictly be a single wire issue? How could heat affect that? HELP!!!:freak: Also,the model number is: HM100 159436P generac model number: 01356-0 series: sup5ka serial: 03744002

DAWG: Again, the magneto/ignition coil, whatever you want to call it is brand new, the old one/ new one do not make a difference,the old carb/ new carb do not make a difference. Nothing makes a difference, the original part, or the new replacement parts Ive put on it make any difference. Also, the stop switch has 3 wires to it, I believe they are, ground, one for the light up of the switch and a 3rd wire to complete the circuit. How could this possibly change when it gets hot? The thing starts with half a pull when cold. If I keep pulling it long enough till it cools off enough it will start. it's like no matter what is new, (which every aspect that allows this engine to run is), doesnt make a difference! this is rediculous! HELP! :beatdeadhorse:
 

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One other consideration. Possibly you are experiencing gas vapor lock. Somewhere in the fuel delivery system fuel is getting hot enough to boil. Usually happens in the carburetor bowl. This pressure would stop liquid gas from flowing to the carb, but wouldn supply enough gas to operate the engine. Next time if you can get some crushed ice, pack the carb bowl with ice for a few minutes, then try running it. Also the gas in the tank maybe getting too hot,try loosening the cap and see if it stars. A few years back, Oldsmobile sold a diesel. It had a nasty habit of stopping when being driven. Turns out the problem wass an unvented fuel cap.It was easily cured by drilling an eight of an inch hole in the cap. Made its owners happy, but made the EPA unhappy.
 

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You're exhaust valve clearance is clearly too small. The symptom you describe is akin to a burnt valve, or a valve not seating properly.

Aside from the valid fuel vapor lock theory, I'd look at a compression loss when hot.

I don't understand about "anything under 60-80 it will bleed out". Perhaps a leak-down test will tell you more. You cannot ACCURATELY test the compression of an engine with a compression release, only get an IDEA. The HM100 certainly has a compression release, and I'd expect to see only about 60 to 80 Lbs. WHEN COLD, with the CHOKE OPEN, AND THE THROTTLE WIDE OPEN. This is how compression tests are supposed to be done. Again, it's only an indicator given the DEKO system - a leakdown test is accurate in comparison.
 

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Discussion Starter · #10 · (Edited)
Hey guys... I have tried to start it with the fuel cap off actually... no dice. I havent tried ice around the bowl yet though... And also when I did the compression test, the throttle was held wide open because it is always held wide open because of the governer because it is a generator. When I did the test i would pull it a couple of times and it would go up to about 60 to 80 with a couple pulls and instantly leak down to nothing. If I kept pulling more than just the few, like 10+ times, i had 150 psi in there no problem and it would not leak, period. The strange thing that still throws me for a loop, is that if you put a shot of ether in the carb when trying to restart hot, it wont even pop. Which leads to spark issue, but this cant be the problem because everything is brand new set correctly and Ive checked for spark. Keep throwing the ideas at me I really appreciate and am still trying all of the ideas you guys are giving me. I really appreciate it and I dont want to buy a new one because of this stupid problem. :eek:

If it IS definately a burnt out valve, what can I do about this problem? I know my way around engines, but have never had the valve train apart on one of these guys.... Im assuming there is no way to adjust them, and if the need to be re seated or re cut, I dont think that is a job I can do. Thanks again please keep the ideas coming!
 

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As paulr44 stated,.007 clearance on the exhaust valve is to close.As the engine heats up,the exhaust valve is expanding and not allowing the valve to seal tightly when hot.To confirm the problem,check the valve clearances when the engine is hot and more than likely,the exhaust valve will have zero clearance.To correct the problem,you will have to remove the head and then remove the valve from the engine to shave off a "tiny" amount from the valve stem.If you take off to much from the stem,you will need a new valve and start all over.Here is a link to help explain the problem.Hope this helps.

 

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In response to USMC, I digress. You don't need a new valve if you grind too much off the stem and end up with a lash too big, you can cut the seat down a bit if you have seat cutting tools, which will lower the valve which is to say bring it closer to the tappet/cam, reducing the lash. If you only touch the stem (at a 90 degreee angle) to the side of bench grinder wheel a bit at a time, you should be ok. I ALWAYS use the smallest size (per spec.(EG.004" blade on a .004" to .006" spec.) feeler gauge, this way if you grind a bit too much off you should still be within spec. If you don't have the seat cutting equipment, then you'd need a new valve as the GIrene indicates. (I was going to say "ex-GIrene," but remembered that "once a Marine, always a Marine." Us'ns who served in the Army have respect given they're the first in).

In addition for those inexperienced with valves and valve jobs: When replacing a valve, or doing a valve job, or have an issue with valves seating, not only should you either cut the valve face (being mindful of the margin remaining (google valve margin)) or replace it, you should cut the seat. Lapping valves is over-rated. Lapping is done to eliminate grinding patterns and smooth things over. I worked in an automotive machine shop and did triple-angle valve jobs and we almost NEVER lapped valves. We WOULD however, put each valve in the head with no springs etc. and with the valve about a half inch open smack the head with a few fingers so it'd hit the seat (AND REPEAT THIS STEP A FEW TIMES), and look at the impact ring on the valve face. This would show us both how complete the seat contact patch was, and equally as important WHERE the valve face contacted the seat. Given we used an interference angle (46 degrees vs. 45 between valve face and seat), we'd look for a "high" contact patch which means closer to the stem vs. the margin (or head area), and as it "seated in" it'd complete the contact area to a wider area, and be centered in the middle of the face (ideal). The amount of contact area is important as too little or too much can cause valve or seat burning, or valves leaking (also resulting in burning). So, either a too-wide or too narrow a contact patch is bad. Valves "pound" in whether you like it or not, and over time that pounding rounds the valve seat which is why you want to cut them. The valves end up with a depressed contact area, which is why you want to cut or replace them. Given an ICE engine relys on "air, fuel and spark UNDER COMPRESSION" valves are critical to their operation. Whew!!! I've rendered enough valve experience for one thread!
 

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Discussion Starter · #14 ·
Guys greatly appreciated... I just wish I knew this was definately the problem, but as it is out of spec I think om going to pull it apart and do so. I completely understand what your talking about, as with high output 4 stroke engines I see this issue of tightning up of valves a lot also due to lack of maintainance. I just wasnt sure how to deal with it on a small engine. I know I can adjust the valves now that I have the knowledge to do so. Do ytou need any special tools to take out/ put back in the valves/ springs? Im definately not going to re-seat or re cut anything, that is out of my area. Instead of just grinding, would the use of a bridgeport or lathe be more accurate? anyways thanks for the responses Im looking forward for more input. ill let you guys know where Im at if I end up taking it apart. I just need to know If I need any special tools to take apart/ put back the valve spring/valves etc. Thanks again, Jon
 

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I've never used a bridgeport, but a standard lathe turning on the slow side could be used to cut valves. The seats are a different matter, and when they're cut your tooling needs to be centered in the valve guide - a mandrel or pilot is used.

The valve spring retainers can be persuaded off the groove in the valve for disassembly, but getting them back on without a spring compressor while not impossible is rather difficult.

The image shows you a the 3 designs commonly used for valve spring insertion.
 
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