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Discussion Starter #1 (Edited)
for quite a few years I've been cleaning out carbs on car/truck engines, and a lot of small engines i.e. tractors, lawnmowers, snowblowers, chainsaws, weedwackers, generators, tillers, etc.

most of the small engines were Briggs and responded well to cleaning. a few needed new needle/seat. a very few needed new floats, only a couple.

I read a lot of Tecumseh carb problem threads online, and many put kits in those carbs but they simply don't respond sometimes and the carb must be replaced. this made no sense to me as the Tec. carb is so darn simple, there's almost nothing inside it. until recently every Tec carb I cleaned out responded and ran well.

that was until now, when this AMF Dynamark HM80 threw me a new challenge like others have had- a Tec. 8HP carb that simply doesn't respond to a rebuild.

this particular carb was so carbon-ed up the throttle was stuck wide open. It took about an hour of soaking in gas and spraying with PB blaster just to free the throttle up. that is a bad sign, meaning the rest of the carb is just as bad.

the passage behind the idle mixture screw was plugged, and the tiny idle passage in the idle jet was plugged solid. I poked both of them out.

the float had pinholes and had gas inside it, I replaced the float. Also the gasket, needle, seat.

the engine would still hunt, idle fast, then stall, could not set mixtures or idle. WTH ??

so this carb had to go UNDER THE KNIFE to expose the IDLE JET passage that is inside the main nozzle well riser. this passage is drilled from the side, then plugged from the factory, so you can't get at it to clean it- unless you grind or drill the passage open again.

I ground it open and whattyaknow, it was blocked with a ton of crap on the INSIDE. I cleaned that jet out good. Now the trick will be closing up the passage I ground open.

truth be told sometimes you have to break something the first time to learn how to fix it right, so I may have to get a new carb. But I'm pretty sure my expert welder friend can TIG or MIG this shut again.

this passage goes into the threaded part of the main well where the needle jet threads in, and you can't see it from the top. You may be able to poke it out with a allen-wrench shaped pin, but that will just push the debris into the idle circuit.

the correct way to fix this, is put it in a small drill press, drill the side plug out, clean the passage, then tap the hole you just drilled with a small tap, and screw in the proper set screw to seal it back up

see pics. there are threads online where the guys fixing these say they drilled out the "ball plug" on top. don't try it, that plug is hardened and won't drill out. you have to drill the soft side plug, the tiny one. the main jet inside is .030" and lets fuel from the main jet bleed off into the idle circuit, then get controlled by the mixture screw above in the carb body, and mixed with air and emulsified, then discharged into the idle discharge ports, or which there are 3.

if this inner port is blocked, and many of the old carbs ARE, you can rebuild it and soak it until **** won't have it, and the carb won't work. compressed air may not even remove it. the stuff that I dug out of this carb was like CONCRETE. that small port was completely blocked and piled up with debris inside.

you don't get to this point without a lot of R&D, trial/error, experience, time, grease, grime, dirty hands, etc.

so Merry Christmas, fellas. clean this passage and keep those old Tec. engines going, with the original carb, rather than bolting on a Chinese carb.

BTW I noticed the new Chinese main jets have small idle ports in them than the original main jets. the new Chicom ones are only .015", the originals are .020" clean the old jet and re-use it, it is calibrated richer



 

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Discussion Starter #2
other passages

be sure to get a complete kit with the welch plugs, pull the side plug, it covers the (3) idle and off-idle circuit DISCHARGE HOLES, and also accepts the end of the IDLE AIR BLEED. this is where the idle fuel mixes with air then is discharged at idle into the carb bore. make sure all 3 of these tiny holes are clear. as the throttle opens, another hole is exposed and begins to pull fuel. if one is blocked, there will be problems i.e. stalling, surging, etc.

 
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Discussion Starter #3
air bleeds

each circuit in a carb, be it small engine or car, has an AIR BLEED, usually located at the entrance to the carb bore. make sure they are all clear

one bleed is for the MAIN BOWL
one if for the IDLE CIRCUIT
and the hidden one behind the choke plate shaft, is for the MAIN CIRCUIT and goes directly to the main well. this one is hidden and you have to look at it from the side to see it in there, behind the choke plate SHAFT itself. if you close the choke one or 2 notches, you can see it easier from the side

the 2 on the left in this picture, go to the float bowl vent under the welch plug in top of float bowl, and idle circuit air bleed that goes to the idle discharge port area under the side welch plug. again, if these are plugged, she ain't gonna run

 

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Discussion Starter #4
finally, check the MAIN WELL AIR BLEED found behind the choke shaft, hidden by the shaft unless you look sideways in there at an angle with a light, and close the choke a notch or 2.

you can barely see the passage in these pictures, but it's there

if this one is blocked, ditto, she ain't gonna run !

look closely at this picture, in the bottom of the carb bore just behind the choke shaft, is the main well air bleed

 

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Good to know, thanks for the tip!

I've historically avoided HM80s at all costs because of weird carb issues. I've had a good success rate, but there were one or two that I just couldn't get totally right, which is probably the issue that you outlined.
 

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I went through that last year with my carb that wouldn't idle. I ended up cutting the entire end of the idle passages and recreating it with JB weld. Started perfectly last week.
 

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That was a great write up greatwhitebuffalo . Thanks for taking the time and effort to document it.
 

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Discussion Starter #9
Good to know, thanks for the tip!

I've historically avoided HM80s at all costs because of weird carb issues. I've had a good success rate, but there were one or two that I just couldn't get totally right, which is probably the issue that you outlined.
this type of carb is used on ALL the old Tecumsehs, from 3.5HP up to 10HP that I know of,including 4-5-6-7-8-9HP

all the old "HM" series engines have that design.

if the clogging in the idle circuit isn't too bad or not hardened from sitting over years, you can soak the carb, and blow the highest pressure air down the main well, with the idle mixture screw removed, and you may get lucky most of the time and dislodge and blow the debris out of there

but if it's hardened like calcium concrete, then you have to pick open every passage with a pin or drill bit, no way around it. you can soak the carb until the cows come home, and not clear that passage if it was dried up in there
 

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Discussion Starter #10
I went through that last year with my carb that wouldn't idle. I ended up cutting the entire end of the idle passages and recreating it with JB weld. Started perfectly last week.
good show ! nice work.

another thing you can do, just let the existing passage be if it's blocked, let it blocked, drill another .030" hole straight through just below the existing side plug for the original idle passage. then plug the outer hole you just drilled, leaving the inner hole. presto new idle jet and no hole to tap and replug.
 

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this type of carb is used on ALL the old Tecumsehs, from 3.5HP up to 10HP that I know of,including 4-5-6-7-8-9HP

all the old "HM" series engines have that design.

if the clogging in the idle circuit isn't too bad or not hardened from sitting over years, you can soak the carb, and blow the highest pressure air down the main well, with the idle mixture screw removed, and you may get lucky most of the time and dislodge and blow the debris out of there

but if it's hardened like calcium concrete, then you have to pick open every passage with a pin or drill bit, no way around it. you can soak the carb until the cows come home, and not clear that passage if it was dried up in there
Same basic design, but I've had a lot more surging issues with the 8 and 10 HP carbs than I've ever had with the smaller engine carbs.
 

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Please forgive me if I take this thread off the direct topic for a sec, but you're obviously an expert on this carb, and I have a quick question.

On idle mixture screws on the HM80 style carbs, they seem to be not adjustable. Either you turn them all the way in, or you adjust them but they're loose and end up possibly falling out...as opposed to the mixture screws on the carbs for the smaller engines which are spring-tensioned and adjustable. For the life of me, I've never understood that.

Mods, please feel free to remove or delete if inappropriate.
 

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Discussion Starter #13
Same basic design, but I've had a lot more surging issues with the 8 and 10 HP carbs than I've ever had with the smaller engine carbs.
I've had that same problem with my 3.5 and 5 HP Tec. engines, on the Ariens and Snowbird, but a cleanout and spray fixed it. I didn't have to drill it out.

I don't think it's a matter of what size or HP the engine is...IMO it's the fuel used and how long it sits in the carb. If the machine is put away with a carb full of gas and it just sits until it dries up, and then sits a few more years- Vegas house odds the passage may be blocked.

or if the engine is run with dirty fuel, fuel lines, or no fuel filter
 

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Discussion Starter #14 (Edited)
Please forgive me if I take this thread off the direct topic for a sec, but you're obviously an expert on this carb, and I have a quick question.

On idle mixture screws on the HM80 style carbs, they seem to be not adjustable. Either you turn them all the way in, or you adjust them but they're loose and end up possibly falling out...as opposed to the mixture screws on the carbs for the smaller engines which are spring-tensioned and adjustable. For the life of me, I've never understood that.

Mods, please feel free to remove or delete if inappropriate.

all the old Tecumseh and Briggs carbs have both idle mixture adjustment and WOT power adjustment, and both screws are spring tensioned. If you have one without a spring, it's just missing. Even the base idle and high speed governor setting screws on this carb are spring tensioned.

HM80 idle mixture certainly ARE adjustable ! the further they are turned in, the less fuel goes to the 3 idle discharge holes under the side welch plug. the passage that gets blocked is ahead of the idle mixture screw, so if it's blocked, the idle mixture screw doesn't get any fuel to adjust in the first place. the idle circuit just goes dry.

then the engine is trying to run on the power WOT jet, and they get a high idle, surge, and hunt all over the place, then stall

in all confidence, the Flo-Jet carb on the Briggs is a lot better carb from reliability standpoint, than any Tec. carb. but the Flo-Jet also has a skeleton in the closet- the main jet tube has a flathead screwdriver head, and those tend to strip. then the jet is stuck in there and boogered up, and can't be removed to clean it, without using some sort of easy-out and destroying the jet. their kit also comes complete with a new jet/tube but it's a leaner calibration. I always try to clean and re-use the old jets in any carb.
 

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Discussion Starter #15
it worked

I plugged that grind mark hole with a piece of plastic, cut from a Sharpie marker pen cap- and glued it in with Super Glue. And it worked- for now.

machine started up today and was able to set the idle and WOT mixture.

but I am going to pull it back apart and reseal that area with gas tank 2 part epoxy from the auto parts store.
 

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Discussion Starter #16
this carb is fixed, running

this carb is repaired and running back on the HM80 Luminaire blower.

the steel stick automotive gas tank repair, 2 part epoxy, is holding up.

it comes in a package that looks like a big tootsie roll, the inside is the hardener the outside is the epoxy. slice a piece off, mix the 2 parts together like putty between our fingers, then push it into the repair area

let it dry overnight, it's hard as a rock and gas won't take it off

don't look pretty, but it fixed the carb. next time I do one of these carbs, it will be drilled out neatly on a small drill press, and then tapped/plugged with a set screw

but if you find yourself in hot water, in a pinch, this steel stick stuff, does work



 

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This is awesome and great advice. I restore alot of the Snapper rear engine riders (Forest Gump Mower) and there equipped with these carbs. They have been putting up a fight. This should make things alot easier as I've been shying away from nice machines because of these darn carbs.
 

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Discussion Starter #19

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Discussion Starter #20
hey you're the guy that fashioned up a carb from JB Quik-Weld from scratch...like throwing a pot on the potter's wheel... I think we spoke before...:D:cool:

plugged carb idle jet...oh well.... nothing an axe and body grinder can't fix...:D;)

just kidding...as long as she runs, we learn as we go... I found (2) HM80 carb cores for dirt cheap and still run the patched up one for now. I just want to see how long it will last.

a careful drilling and tap, and plug with an allen set screw, would be a precise repair next time we run into this...but now we know what's inside there:p
 
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