Rebuilt two stroke refuses to start - I am stumped!

[erik_w]

Having recently made a major overhaul of my Izh Jupiter two stroke (350cc, 180 degree twin) it now simply refuses to start. Startgas, bump start, flooding it, not flooding it, simply NOTHING works.

New is seals, bearings, pistons and rings.

It doesn't sputter, doesn't cough, just nothing - I might as well be trying to start it with the ignítion off. Ignition system works thou, nice fat blue sparks, have tried two different systems (from working bikes) with no results.


I suspect the problems are compression related.

- Dry compression is 80 psi
- Wet compression is 160 psi
- This would suggest badly worn rings, but they are brand new and haven't had a chance to wear in since the Engine doesn't start.

I did manage to get it to run once, after pouring oil into the spark plug holes, the kicker felt much stiffer and the Engine started on the first kick, ran for 0,5 seconds and then died.

What am I doing wrong here, is there something I am missing? Are my cylinder walls to far gone and need a overbore? They were honed prior to putting in the new rings.

[BPRM]

80 psi is indeed too low.
Did it run before the new rings?
Correct end gap gaps not in line?

[Triple Jim]

The fact that adding oil in the cylinder made it run briefly does point to the compression being too low for the engine to run.  A good engine with new rings and a hone job should have decent compression before the first time it runs, although maybe a little below its final numbers.

If you put 1/2 a teaspoon of gasoline (normal gas-oil mix) in the cylinders through the plug holes, does it run for a couple seconds?

What's the skirt-bore clearance?

[erik_w]

Yes, the cylinders were from a running Engine (a bit worn thou), piston are new, due to the design of the pistons the ring ends can't line up.

Negative on adding gasoline or start gas directly into the plug holes, it has no effect. Not sure on the clearence, but I suppose it is a bit bigger than spec, between 0,05 and 0,1 mm. 

[ratguzzi]

For the curious minded....
JB

http://www.southbayriders.com/forums/threads/98763/

[Triple Jim]

Not sure on the clearence, but I suppose it is a bit bigger than spec, between 0,05 and 0,1 mm.

That means about 0.002" to 0.004", which is fine.  But measuring it would help find out what's going on.  You can use feeler gauges between the piston skirts and bore to get an approximate idea of the clearance.

Anyplace else that could be causing the low compression?  Like something odd at the head gaskets.  Did you hold the throttle wide open and kick until the gauge quit rising when you did the test?

[erik_w]

Compression test is done like this:

Both plugs out, throttle wide open, 5-6 decisive kicks and then taking the measurement. It might go a little bit higher if I kick it more, but after 5 kicks the gauge virually stops rising with each additional kick.

Since adding oil makes such a difference, I would not suspect the head gaskets, both of which are new.

I'll try to get a measurement tomorrow of the piston / bore clearance.

[Triple Jim]

Yeah, something is certainly odd.  Sometimes spark can be weak, so it fires a plug lying on a head, but not under compression, but you tried two different systems.   Finding out why the compression is so low would seem to be the next step, even if it's not the only problem.

Assuming it's not a wasted spark system, could the plug wires, or something upstream, be reversed?

[Groover]

Sounds like fun, not! (kick starting and pushing). Are the pistons in the right direction? Just thought I'd ask, you never know. If not, then if there is an intake port/window on what would be the intake side, it would then be facing the exhaust side and that would mess up the entire order of things.

Sounds like you know what you are doing, so I'm just throwing it out there because you've tried everything else to no luck. I think even low compression (more than none that is), it should still fire up especially when you pour gasoline in the spark plug hole.

[Peter from Sch'dy]

If your crankshaft seals were worn it might do this..scavenging would be compromised.

Best,
Peter

[sparkman]

Might be a good idea to pressure test the engine. Plug the intake and exhaust ports and pressurize the engine through the spark plug holes. I made a handy pressure tool using the fitting from a compression gauge and a small regulator. Start with the regulator set to 0 psi and slowly open it up - no more than 8 -10 psi. Soapy water sprayed around the crank seal(s) and sealing surfaces will show any leakage.
That low dry compression number is the culprit. Remember, suck, squeeze, pop, fooey all need to be in order.

[erik_w]

the ignition system is a wasted spark so it doesn't matter how you put the plug wires... it is all set up correctly and sparking really strong.

pistons are the right direction...

the bores did look a bit scruffy even after the hone, but since they were from a running bike to start out with I thought it wouldn't be a problem.

could it be that after the hone the bores are too damaged to seal to the new rings?

[Jurgen]

Even low compression engine will fire and pop if the timing is correct.  Check that the spark is coming at TDC or slightly before.  Did you mention that it DID pop with fuel into the plug holes?  80 psi sounds ok at slow speed cranking and should generate a few revs running.  Double check bottom end seals.  Jurgen

[erik_w]

Jurgen,

the timing is set at 2.5 mm before tdc. it does not even pop when putting gasoline or startgas right into the plug hole.

even if the bottom end is leaky it should still fire and run when p
I put start gas into the cylinder?

[acogoff]

   a stab in the dark here.  Base gasket covering passage to port or missing so it sucks air, center seal bad or missing??

[erik_w]

all gasket and seals are there at least, if they are leaking I don't know. but even with a bottom end leak, it should start and run?

[Kentktk]

Two strokes with low compression wiil not even try to start. Had a Honda CR with lightly scored walls, it was enough so it wouldn`t start. New cylinder, pistons and rings installed and it did.

[erik_w]

have a four stroke bike that will start and run with 60 psi ...

[canuguzzi]

There are two rings on a two stroke. When you put the new rings in did you happen to line up the gaps? If so, that might be your problem. Make sure the gaps are not aligned.

Experience with Kawasaki's 250 triple.

[Peter from Sch'dy]

have a four stroke bike that will start and run with 60 psi ...

There's a good place to start. Here's what I think. The four stroke will work with very low compression because it still has enough VE to fill the cylinder to the point the engine will "run". With a 180 degree two stroke twin however, if the crankshaft seal between the cylinders is poor, the bike probably will not run more then a couple seconds on plug hole fuel because it is incapable of generating enough pressure in the crankcase half to get good mixture back into the cylinder. This could be because it's leaking into the other crankcase half instead of being sent to the cylinder. So you have one piston descending and pressurizing it's crankcase half and the other piston rising creating vacuum in it's crankcase half and there's nothing to impede the flow of gasses between the two. Based on what you've told us I can't think of any other reason for this motor to fail to run. I am therefore, probably wrong

Best,
Peter

 

[erik_w]

Peter, I deras you being right, if the crank seals are bad I have to split the cases again ...

but since it won't even fire on startgas I'll start by sorting out the cylinder walls , if it still won't run - then apart it comes again.

[jlburgess]

Could be camshaft failure?  It's going around ya know. 

[Peter from Sch'dy]

Or, it could be tinnitus. 

Best,
Peter

[Two Checks]

"It ain't got no gas in it!"  Karl Childers 

[rbond]

Bore it to first over with new pistons and rings. Wipe the cylinders and pistons with two stroke oil. Make sure baffles are not clogged up in mufflers too. Check compression, should be close to a little over 90 psi. Next be sure both carbs are full of gas, set choke and kick!! I have worked/ ridden two strokes for forty years.

[Rich A]

I don't know anything about these bikes, but there aren't any safety (sidestand/kill) switches are there?

Rich A

[Idontwantapickle]

There's a good place to start. Here's what I think. The four stroke will work with very low compression because it still has enough VE to fill the cylinder to the point the engine will "run". With a 180 degree two stroke twin however, if the crankshaft seal between the cylinders is poor, the bike probably will not run more then a couple seconds on plug hole fuel because it is incapable of generating enough pressure in the crankcase half to get good mixture back into the cylinder. This could be because it's leaking into the other crankcase half instead of being sent to the cylinder. So you have one piston descending and pressurizing it's crankcase half and the other piston rising creating vacuum in it's crankcase half and there's nothing to impede the flow of gasses between the two. Based on what you've told us I can't think of any other reason for this motor to fail to run. I am therefore, probably wrong

Best,
Peter

Agreed, inability to pressurize in the case.

[erik_w]

no, there ís nothing to stop it from working, no sidestand kill function or anything like that.

No, i didn't curse the bike at all in fact - bad manners!   

I guess the question is really this:

If compression is only 80 psi - will this make the bike impossible to start during anything condition or only given that the crankcase preassure is ALSO too low?

It didn't start even with a drop of gas down the spark plug hole.

[Robert]

Once I fouled the plug on a new, right out of the shipping crate Honda 250 Elsinore, with only 2 kicks.

Plug looked New, Perfect.  But NO firing at all.

Boss said try a new plug.  That did it.

?????

[Groover]

I agree on changing the plugs as well. In regards to the compression... I don't think that is the problem. For example, there are many piaggio mopeds, 50cc 2-stroke that have a compression relief lever which you pull when you pedal to start the engine. The lever when pulled opens a valve on the cylinder head that basically lets most of the compression out so you don't have to pedal so hard to start the engine. That's why I don't think conpression is the culprit in this chase, as in the case of the mopeds you basically drop all compression as part of the factory designed startup process.

I suspect something else, possibly electrical. Did the bike rum before the rebuild? was the engine completely disassembled, or did you just do top end? (Not touching other areas). Knowing that will help us trouble-shoot with you.