What would you do if...

stevolandis

stevolandis

New Member
...you had a 1983 yamaha mx100 that wouldn't start, although an hour earlier it was running fine? I checked the spark, it was fine, also the carb, all jets were clean and the float was good.

I roll start it, it does start and run for about 30 seconds, but it is very weak. Then it won't start at all. The gas is fresh, the oil is fine(it's an automatic oil-mixing two-stroke). Ground wires for kill switch aren't touching, choke is half-out. Head bolts are torqued, exhaust pipe is on good, it has a new air filter.

I noticed the plug was wet when I checked it, but not too black.

My guess is the ignition(although the spark plug looks fine- it sparks) or compression.

BTW my neighbor and I just bored it and put a new piston and rings in.


Any ideas anyone?
Thanks.
 
Re-check the spark right after it dies!!! The coil could be bad.
Or maybe it's not flowing enough gas to refill the carb?
 
something i just remembered from the old days. some of those old bikes had a bridge across the exhaust port, when you put in a new piston you had to drill an 1/8th in. hole in the skirt that lined up with the bridge or the new top end would burn up prety fast.
 
Thanks everyone for helping out!

Camlifter-
My exhaust port is open- there is nothing there up to the piston- does this mean there's no bridge?

I did blow through the exhaust to see if it was clogged- is this a good enough test?

Venice-
I will check the fuel to make sure it flows nice.
Hopefully the coil is okay- i hear they're expensive!

Nougat-
I have no idea how to check the crank seals- do you know of any other symptoms to see if it's the seals? Thanks!

Update- It won't start at all- it did combust once or twice. Yesterday it made a loud coughing noise from the exhaust- maybe that is a problem.

Thanks everyone for their help!
 
Yup, Mine's a two-smoker. I think the clutch and ignition are on the same side- the clutch cable and ignition wires go into the same side- maybe because mine mixes oil automatically.
I'll check that sometime(hopefully tomorrow)

BTW I added myself to the zeemap!
 
if the exhaust port is one large round port then it's one that dosen't have the bridge. the best way to check a 2 stroke for leaky seals and gaskets it to take off the exhaust and carb, plug the exhaust port, make a cap with a tire valve in it and put it on the intake, pump it up with a tire pump to 3 or 4 psi and spray every where with soapy water, any bubbles means a leak. since you just rebuilt the top end i'd pull it back off and make sure every thing is ok. i'd also take off the exhaust and see if it would start without it.
 
Okay- thanks guys. I'll be going away this weekend, but maybe next week I can work on the bike.

I did check the fuel filter- it works great. I bought the gas sometime in September, so it could be old.

It looks like we have it narrowed down to a few steps-
1.Try new gas
2. Try without exhaust
3. Find a manual somewhere.
and finally
4. check crankcase seals or other leaks(use camlifter's idea) .

Did i miss anything?

Thanks!
 
Camlifter knows... pressure test the engine before you tear into it. Also, I might add, use a compression tester on the cylinder. Screw the tester in the spark plug hole,open the throttle all the way, and push start the bike to get the crank turning. Ordinarily I would kick it over 5 or 10 times, but your motor may not have a kick start. The idea is to let the tester see the max amount of pressure the cylinder can make. I'm guessing that with a stock motor you would see about 125 psi. Anything lower than 90 means trouble. These numbers are relative. As a new motor breaks in ,compression can increase. The thing to look for is big drops in pressure. Since you put new rings in, I would check those if you tear it down. Once you take the exhaust off , you can use a flashlight and a small inspection mirror to check the walls for scoring. Hope this helps.
Sincerely, Gary P.
 
Thanks guys-
Gary- Mine does have a kick starter- but I have a nice steep hill right where I pull out of the basement- it's easy just to roll right down.
I was planning on testing the compression- then depending on the results either try a new spark plug and new gas, or if there's little compression, find a manual and check the top(rings, wall) then the bottom(crank seals).


Goat- when we honed it, we put the cross hatch in with that. To break in, I took a lap around my yard slow, then a lap medium-fast, then slow again, etc

P.S. I noticed a trend in the engine trouble- it started to sputter at high-ish revs, then act normal then sputter. Pulling a bit of choke seemed to help. Then one night it stalled and wouldn't start(continuous roll starting), but the next day it started right up.
The next day it started well, but then it lost power and finally quit and hasn't started since.

I took apart the carb and cleaned the jets, the float is good every channel is open. The throttle works. The gas flows from the tank through the filters, the cap vents. The air filter and spark plug are new, it has spark, the kill wires aren't touching.
That leaves the head, compression, or maybe the oil mixer not working right.
I will check the head bolts torque and compression. That's all that's left, i think.

There was that coughing sound from the exhaust twice, and it has combusted once or twice.
 
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