Two Stroke wont start.

[Rustydog2010]

I have a late 70's, early 80's Yamaha YZ400. 400cc Two stroke. Somewhere along the line, someone has build their own exhaust, which was more like a four strokes. I have now modified that and added in an expansion chamber, originally these bikes did come with one.

Now the bike wont start. It has reasonably low compression, but it did run previously. Since adding the new exhaust the bike will back fire about every dozen kicks.

It has spark, and cant see any thing wrong with the carb. I have tried a bit of fuel in the spark plug whole and it just does the same thing as above, this denotes to me that it is getting fuel. My understanding is the it would at least run, it may run a bit lean or a bit rich, but it would run with the new exhaust.

Any ideas? I haven't touched the timing. The only thing I can think of is that the difference in back pressure between the two pipes are doing something?

Any help would be appreciated.

Jeremy.

 

[Russ2251]

The only thing I can think of is that the difference in back pressure between the two pipes are doing something?

Very unlikely. Compression being low may be the problem. Compression has a serviceable limit. Possibly, you are below the limit.
Service manual will tell you what the limit is. Will require a compression tester to verify.
There may be a couple of other ways to get it running besides the kick starter.

 

[Rustydog2010]

Service manual will tell you what the limit is. Will require a compression tester to verify.
There may be a couple of other ways to get it running besides the kick starter.

Other ways to start? Im open to suggestions, tried role starting to no avail.

Its easier said than done trying to figure what the bike actually is, the manuals I have say what codes should be on the bike, but these have know relevance to that of the serial numbers on the engine and frame. Im only guessing at been a YZ, but pictures I have seen marginally clarify my thoughts.

Edit: And yes the compression could be the problem, but it was running only days prior?

Jeremy.

 

[Russ2251]

but it was running only days prior?

There is a point of no return.

tried role starting to no avail.

That was my first suggestion.
Try closing the gap on plug(s) to about 2/3 of spec...that is, if .030" is called for, close it to .020". Try this first. Let me know if still no start.
Hopefully, you have tried new plugs of the correct heat range.

 

[The Mechtician]

Very unlikely. Compression being low may be the problem. Compression has a serviceable limit. Possibly, you are below the limit.
Service manual will tell you what the limit is. Will require a compression tester to verify.
There may be a couple of other ways to get it running besides the kick starter.

I agree. Try push starting the bike, or hill starting it. Incidentally, how low is "reasonably low"? I've seen a running two stroke with 78 psi compression, but that's an extreme case. Anywhere south of 100 psi basically means it's time for a top end.

 

[Kaptain Krunch]

like everyone else said, how low is compression?

 

[Rustydog2010]

100PSI

I realize that it is in need of a top end rebuild, but it still doesn't explain it running only a few days ago.

Russ: New plugs and smaller gaps didn't make a difference.

Mechtician: Hill starting is hard as it just locks up the rear wheel.

Jeremy.

 

[Russ2251]

Hill starting is hard as it just locks up the rear wheel.

Try the next gear up.
100 psi is enough.

 

[The Mechtician]

Try the next gear up.
100 psi is enough.

x2 How sure are you that the carb is good? Also, does it have fresh gas in it, or old stinky stuff? What mix are you running?

Incidentally, I use top gear when bomb starting. 5th or 6th gear in most bikes will be an overdrive gear (eg; less than 1:1 ratio) which when driven in reverse will actually provide some gear reduction, making it easier to turn the engine over with the rear wheel. Give her a try

 

[newrider3]

Did you happen to stall it out the last time you were riding it? You could possibly have a sheared flywheel key.
If nothing else, put the exhaust back the way it was, and see if it is able to start again.
Then you'll know there's something wrong in your exhaust design. Unlikely, but worth a try.

 

[Rustydog2010]

I tried starting it in 1,2 and 3 will try a higher gear later. The Carb has fresh gas in it and was working last time, I have done nothing to it since.

No, the last time I turned it of by the kill switch. The old exhaust is gone. lol

Jeremy.

 

[Russ2251]

Try straight exhaust...that is, no pipe...nothing at all.
It'll be loud as Hell, but won't hurt anything for a short while.
This will tell you if pipe is culprit.
My gut feeling is that there is a scavenging (flooding) or ignition problem (spark blowing out).
Avoid resistance plugs.

 

[outdooz]

Starter fluid?

 

[newrider3]

Starter fluid?

How about NO. That sh!t is good for wrecking parts.

 

[Rustydog2010]

How about NO. That sh!t is good for wrecking parts.

+1 I have never used it to start an engine, its not going to fix my problem, still going to be hard to start the next time.

Anyway, tomorrow is a new day!

Jeremy.

 

[bighead]

I would try open exhuast. Did you add a silencer? Did you check the new pipe to make sure no mice were nesting inside? I had a few 2 stroke pipe I had to bake in a camp fire to get all the crap out of them. What does your plug look like? Wet? Oil? Dry?

I would try open header. Then dump a 1/2 teaspoon of gas in the plug hole.

What type of back fire?

 

[sexyvicta]

i can confirm that 2 stroke lawnmowers which are low on compression will be easier to start if you remove the exhaust/muffler

 

[Cam]

shes prolly just flooded mate, crank it over till piston top is level with bottom of exhaust port, spray heaps of crc in there and leave it overnight, should evaporate any gas or crap in there.

 

[Rustydog2010]

Does CRC evaporate fuel? I know it displaces moisture but didn't know that. I'll try some of these suggestions, if it doesn't work she'll be stripped for a rebuild.

Jeremy.

 

[Russ2251]

Problem is this...what happens if after a rebuild you discover that the problem still exists?
The time to solve it is NOW!!!
Something very basic is plaguing the engine.