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Discussion Starter #1
Hello all, Im trying to help a friend get his Kd100 Running. Story he told me was the guy he got it from that it had been parked for a few years but ran when parked. He cleaned the carb and pet **** out, put fresh gas in. Said when he compression tested it, it had around 130 psi And all it will do is pop and backfire out the the exhaust and carb.

That was 2 months ago told him I would mess with it and im at the same spot hes at. I recleaned the carb, put oil into cylinder. Spark is strong enough to be seen in daylight, But all she will do is pop and backfire out the carb and exhaust after extensive kicking I believe the bottom of my foot is bruised.

Saw a similar post on here but that was posted in 07 and he never responded back on if he got it running.


I noticed there is 4 wires dangling from under the gas tank. 2 of them could hook together, I am not able to find any wiring info online about this bike.

Engine # is KD100ME006332
Frame # is KD100M-024669

Any help would be appreciated thank you
 

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It sounds like your engine is suffering from poor crankcase seals and they will need replacing.
At that age, they should be replaced as a matter of course.
The wires may be to add auxiliary lighting if the owner so desired.
 

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Discussion Starter #3
ok thats fine will does this look like all the parts i should need then?

the engine # i am correct that is a 79 if so

on crank case i should need

#2
#4
#6
#8
#26
#27
#28

1979 Kawasaki KD100 OEM Crankcase (kd100-m3/m4) Parts | MotoSport

any other recomendations? dont know if this makes sense but I want to get it running before I put any more money into it. I have no problem with doing it right just want to spend money in the right place.

thank you
 

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What I would do is a tear down, clean and inspection of all parts paying particular attention to the crankshaft big end, piston, cylinder and transmission. ( MX bikes usually take a beating) The reason for this is you do not want to spend the money to re-seal the engine only to discover when you start it up that you have a bad rod knock and need a new crankshaft, which may not be available. Then you would have wasted all your time and money up to that point.
 

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Discussion Starter #5
Good news today! Had my oldest son pull me with our little chinese crotch rocket 110cc bike. Got the bike to fire! First gear you gotta go REAL slow and it doesnt want to rev all the way up in first, just kinda bogs and sputters 2nd gear it clears out and seems like it pulls the whole way then 3rd,4th and 5th it falls on its face in the upper rpms.

But it still wont fire on its own from kick starting, so that does have me believing it needs new crank seals and such.

Im still trying to figure out the carb what is the screw next to where the throttle cable goes? its got a needle type thing that goes into the carb. is it some kind of idle adjustment? it wouldnt change the idle when i moved it around,

But im pretty happy about the bike today!

gave the bike a good run against the 110 chinese bike and from a 2nd gear roll on I left him in the dust!
 

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Hi guys I’ve got a similar problem to this guy. My bike over the last couple years has been running really weird, so basically it used to run perfectly when it was cold and then around 10 min into the run once it was fully warmed up, it would start to pop and backfire really bad and eventually get so bad it would shut off and you wouldn’t be able to start it again until it was room temp. Around last year I took it down the road and it started doing it’s backfire thing and then at full throttle it just quit and i haven’t been able to get it to run since. Like he said it just backfires out of the carb and exhaust and I’ve put a new coil on it and rebuilt the carburetor and tried everything. It only has around 65 psi of compression but it used to run awesome when it was cold with that much compression so i wanted to know if you guys think that the seals in the crank case went bad or you think if I get a top end I could slap it on there and get it running again? I just want to see what you think because I can afford the top end but don’t want to waste my money if the seals are bad. It also won’t start with starting fluid, same result as if It wasn’t used
 

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The first thing that should be done with any old two stroke before spending any more money or effort is a pressure test. In a four stroke engine all breathing, air and fuel management, takes place above the piston. So a four stroke with a crank seal, base gasket, crankcase mating surface leak may make a mess but it will as long as one continues to top off the oil, run fine. A two stroke relies on air management below the piston. The air feel mixture be drawn into the crankcase under low pressure and then a the piston descends and crankcase pressure increase the mixture is forced up thru the transfer ports. Crankcase pressure constantly fluctuates between lower than atmospheric to higher than atmospheric. A leak almost anywhere - crank seals, crankcase halves, intake downstream of carb, base gasket, cracked/damaged transfer port casting - will cause a loss of carbureted mixture and the intake of uncarbureted air. This will cause the mixture to go lean. Two strokes use the fuel not only for combustion but also for lubrication and cooling. Two strokes can never be jetted for absolute power, they must always be jetted rich enough to provide cooling. Any who have run a chain saw have experienced how they rev much higher than normal as they run out of gas. This is the mixture going lean. Even if you get a two stroke running, a pressure leak will result in eventually melting the piston. You may have other problems but the very first test after verifying that the engine turns over should be a pressure test. Plug intake, remove exhaust pipe, plug exhaust port, introduce compressed air thru spark plug fitting. Engine should be able to hold 6psi after air supply is turned off. Do not go higher than 6psi as you can blow seals out of crankcase. By the way an engine that is seriously misfiring can also, by means of high pressure combustion gasses pushed down thru the transfer ports, also damage crank seals. This is such an important factor that though I would need to look up the official documents, I believe Rotax lists a 5 year time life on their two stroke aircraft crank seals.Start with this test, if your engine passes repost and we can go from there.
 

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The first thing that should be done with any old two stroke before spending any more money or effort is a pressure test. In a four stroke engine all breathing, air and fuel management, takes place above the piston. So a four stroke with a crank seal, base gasket, crankcase mating surface leak may make a mess but it will as long as one continues to top off the oil, run fine. A two stroke relies on air management below the piston. The air feel mixture be drawn into the crankcase under low pressure and then a the piston descends and crankcase pressure increase the mixture is forced up thru the transfer ports. Crankcase pressure constantly fluctuates between lower than atmospheric to higher than atmospheric. A leak almost anywhere - crank seals, crankcase halves, intake downstream of carb, base gasket, cracked/damaged transfer port casting - will cause a loss of carbureted mixture and the intake of uncarbureted air. This will cause the mixture to go lean. Two strokes use the fuel not only for combustion but also for lubrication and cooling. Two strokes can never be jetted for absolute power, they must always be jetted rich enough to provide cooling. Any who have run a chain saw have experienced how they rev much higher than normal as they run out of gas. This is the mixture going lean. Even if you get a two stroke running, a pressure leak will result in eventually melting the piston. You may have other problems but the very first test after verifying that the engine turns over should be a pressure test. Plug intake, remove exhaust pipe, plug exhaust port, introduce compressed air thru spark plug fitting. Engine should be able to hold 6psi after air supply is turned off. Do not go higher than 6psi as you can blow seals out of crankcase. By the way an engine that is seriously misfiring can also, by means of high pressure combustion gasses pushed down thru the transfer ports, also damage crank seals. This is such an important factor that though I would need to look up the official documents, I believe Rotax lists a 5 year time life on their two stroke aircraft crank seals.Start with this test, if your engine passes repost and we can go from there.
Ok, what would you recommend Sealing the intake and exhaust with?
 

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An automotive freeze plug will generally work to plug the intake after the carb is removed. A piece of inner tube sandwiched between a piece of scrap metal and the exhaust mount will generally seal the exhaust port. Remove mag cover, use some dish washing soap mixed wither in a spray bottle. Pressurize engine, lock off air supply, if pressure Gage starts to drop use spray to locate leak. remember to spray all areas including crankcase seams. There are some areas not accessible, a small portion of the crankcase mating surface which borders the transmission cavity and the right hand crank seal. However as these areas are exposed to trans. fluid should they leak they will usually suck trans fluid and produce white smoke.
 

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Just to cover all bases. It is theoretically possible due to the construction of crank seals to have a situation where the seal acts as a reed valve and only leaks under low pressure. Thus passing the pressure test but still sucking air under crankcase low pressure. This is certainly possible and should be understood but in fifty years I have never encountered an actual example of this. I have however rebuilt many 2 strokes that had top end melt downs from crank seal failures.Any time we have an engine which will not run or runs poorly, a logical and sequential set of diagnostic tests is the quickest way to the lowest cost remedy. Of course some will get lucky, cut corners and find success, but in the big picture accepted diagnostic procedures will save time and money. An exception is a known design weakness, in that case past experience with a particular problem will prevail.
 

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An automotive freeze plug will generally work to plug the intake after the carb is removed. A piece of inner tube sandwiched between a piece of scrap metal and the exhaust mount will generally seal the exhaust port. Remove mag cover, use some dish washing soap mixed wither in a spray bottle. Pressurize engine, lock off air supply, if pressure Gage starts to drop use spray to locate leak. remember to spray all areas including crankcase seams. There are some areas not accessible, a small portion of the crankcase mating surface which borders the transmission cavity and the right hand crank seal. However as these areas are exposed to trans. fluid should they leak they will usually suck trans fluid and produce white smoke.
Ok thank you I will try it when I get the chance
 

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So I just tried it today after getting some parts/ tools and I can’t lock the pressure to see how fast it leaks out due to the type of regulators I have but if I put what would be as close to 6 pounds as my gauges can read and I spray it with soapy water, I don’t hear any hissing and I can’t find any spots that seem to be obviously leaking/bubbling with the soapy water I sprayed. At one point the gauge crept up to 10lbs and I caught it but I still didn’t see any bubbles forming from a leak. And both gauges were the same reading which leads me to believe that either the leak is incredibly slow or that it isn’t leaking. I also did a compression test after this and it only has 59 lbs of compression on the Gauge. So I guess now do you guys think I should send it to a shop to test for leaks again or slap a top end on it and see if it will run without Spitting and backfiring once it’s warmed up like it did before it stopped running?

another thing I will add is that every time I used to start it back when it ran great it would smoke out the entire yard and would do that until it warmed up and then the smoke would clear up to a normal amount, would this also indicate crank seals? I’ve never had an issue with having low gear oil levels though.
 

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Hard to say without the ability to isolate the and lock off the pressure, You should be able to readily find a small valve to install before the pressure gauge. If you lock off the pressure, find no leaks with the soapy water but the gauge will not hold pressure then you would have to suspicious of the right crank seal which would also let the engine ingest trans fluid. Unfortunately I can not see the amount of smoke you are talking about. A certain amount of very visible smoke on a cold start is normal with a two stroke. On the previous shut down a certain amount of oil condenses out of the mixture and pools at the bottom of the crankcase. You can also remove the exhaust system, put the piston at bdc, shine a light in and inspect the cylinder walls. Then raise the piston just enough to see the rings. Use a piece of wood such as a pop sickle stick to reach in and push on the rings. You should see and feel a very small amount of spring or give to the rings. If you look you may also see a bit of movement in the residual oil trapped around the rings. Also check for any scuff marks on the piston. If the engine has ever experienced a seizure it would most likely have occurred on the exhaust port side of the piston. Often the engine will still run but the aluminum of the piston will have seized the rings.
 

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Should have mentioned this long before now. With the carb off slowly rotate the crank and check the condition of the leading edge of the rotary valve.
 

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Some two stroke background. The difference between a properly jetted good running two stroke and one that if facing seizure is only about 200 degrees. Air leaks are the number one cause of seizures as they allow the mixture to go lean and the EGT's to rise. The first place to suffer is the exhaust port side of the piston. This is already the hottest spot on the engine due to the flow of exhaust gasses. When the mixture goes lean the EGT's rise and the aluminum piston starts to melt. The aluminum is then smeared across the rings seizing them.
 

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Do you think then maybe the reason it wasn’t running very well was that the rotary valve wasn’t Sealing properly, Causing a lean condition? And then maybe running lean caused the top end to blow? If you kick the bike over with the side case off and the carb on it will blow air out of the carb almost like a backfire and also the housing for the carb always used to be soaked in fuel mix when you took it off
 

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Could you take a photo of the leading edge of the valve? Was there a lot of dirt in the carb chamber? Was the cover on when you got the motorcycle? The rotary valve should not let air blow back out of the crankcase. The valve should be closed as the piston descends so the crankcases will pressurize and push the mixture up thru the transfer ports. Whey work great but do not like dirt.
 
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