Rebuilt two stroke refuses to start - I am stumped!

[Kentktk]

Except on a two stroke you need compression on the crank (bottom) side of the piston so as to force the air/fuel mixture out of the crankcase through the transfer ports and into the combustion chamber.

 https://www.youtube.com/watch?v=xNLE8G3pC0k

[erik_w]

Everything was apart on this Engine, crankcases, gearbox ... So if it is put together wrong somehow, that is entierly my fault! :D

The crankcases are seperate for each cylinder, sealed up with two seals and one o-ring. It is possible that there is such a huge leak it won't even start at all, but it would then have to be on both sides, otherwise the Engine should at least fire on one side.

I have tried changing the plugs around, but not after every starting attempt. Can anyone suggest a good drill for this?

[Groover]

Try delaying the timing maybe as a test. Maybe when you put the oil in the head it delays ignition enough to make it start.

Could be a number of things. As some as suggested, could be bad coils as well, bad condensors, etc. if you have bad condemsor(s)?, you'll see a spark at the points when they open. (They spark a little, but excessive spark means bad condemsor). Do the test in the dark. Coils...: Those are tricky. I had s similar issue with my p125x and finally resolved the issue by changing points, condemsor, and coil, even though everything seemds to be ok.(what appeared to be normal spark, etc).

Good luck.

[Thunderbutt]

Erik-there is a great article in last months issue of "Motorcycle Classics" magazine on conducting a "leak down test".  They used a Yamaha RD350 for the article.  You can also find the article in their on-line site.  I cant recall what the compression was on their test bike but the leak down indicated a bad crank seal s well as bad carb/reed valve surface issues.  Having raced and ridden 2 smokes back in the day I found the article very informative. 
You say you installed "previously used" cylinders.  Were they from the same year and model as your?  If a different model year the port specs may have been changed which would throw off your piston port timing.  Just a wild as$ guess.  If it were my bike, do the leak down test first, (this is the least expensive), remove the cylinders and check the port measurements between the originals and new to you.  Have the newer cylinders bored or honed to proper specs.  This may involve another set of pistons and rings but you are eliminating a possible problem and at least will have a fresh top end for the engine.

  Good luck.

Jerry

[Groover]

I agree with that as well. It's possible that the Pistons are wrong for those cylinders. Definitelty take a close look and understand the flows and make sure everything is correct. Just the up and down motion is not enough to say it should work (if/when mixing and matching in 2-stroke engines)

[Thunderbutt]

That just occurred to me also, I should have mentioned to check/compare the newer and old pistons while you have it apart.  Check for overall height, ring placement, wrist pin to piston top measurement as well as pin to skirt bottom.  Some bikes had transfer ports in the intake side of the skits, if so compare sizes and locations.   
You stated you already checked ignition timing but did you install new points and condensers?  And I would throw another set of new plugs at it when the other work is completed.  The oil you poured into the cylinder my have fouled one or both of your plugs when it initially started.   Just trying to help you go thru a process of elimination.

Jerry

[Groover]

Also be sure that the ring ends fall on the continuously solid part of the cylinder sleeve (No ports) otherwise they'll catch on the port and break messing up a lot of stuff along the way.. This ilocation is driven by the piston having the pins in the ring grooves, which is another reason to have compatable pistons.

Anyway, that's just a side note to all side notes.

Good luck!

[erik_w]

yes, that there might be something with the pistons occurred to me too, but they are suposed to be the same for all model years (on russian bikes the spec changes every 20 years instead of minutes as on a jap  )

i have now ordered a complete set of new pistons and cylinders, which is actually cheaper than doing the bore job on my old ones. I should have it all set up in a couple of weeks

no, the condensers are fine to my knowledge, both ignition systems I have tried have come from working bikes

[acogoff]

    Does that thing have two crankshafts connected mid-engine? And a rotary valve setup. If so, very "impressive" ???? engineering and is indeed a puzzler.
You may be on the right track going for better compression. Interesting project.

[erik_w]

the Engine has two seperate crankchambers, one in each Engine half. the cranks are connected by the flywheel but there is no valves or anything else to connect them. The carburettor plugs straight into the crankchambers without reeds.

it is a very simple design that easy and fun to work on - but it is hard to find quality spare parts...

[Triple Jim]

The carburettor plugs straight into the crankchambers without reeds.

Do the intake ports get covered by the piston skirts when the pistons are down?  The intake has to be valved somehow.  In this case, I'd expect a piston-valve setup, where the piston skirts cover and uncover the intake ports when they travel up and down, or reed valves between the carburetor and the intake ports.

[Groover]

Eric, feel free to post more pics of the engine; You've got us all intrigued! From the photos it looked like the intake manifolds were going into the cylinders, which would indicate a piston controlled intake interval as Triple-Jim descibes, but you mention crankchambers (crankcase?) which to me indicates a rotary valve which would be one of the crankshaft shoulders controlling the timing. (the 2-stroke Vespa design is this way). If rotary, then you may need to inspect that as a shot rotary pad will make it impossible to tune (though terrible low end performance and idle, it should still fire at least, so that may still be a different issue).


Edit: I looked at your photos again, and it certainly looks like a piston/cylinder driven intake system (not rotary pad),

[lrutt]

if that is a piston port 2 stroke, which I suspect it is, 80 psi is plenty to get it running. Are the plugs getting wet? if not then you know you're not getting fuel. If they are wet, and it's not water, then maybe your timing is out of whack. Not sure of that engine, and on most 2 strokes it's pretty hard to get timing way out of whack, but it could be enough after TDC to keep from kicking off. If it were before I'd think you'd be getting some serious kick back.

2 sets of points I assume? is there an advancer? or just points on mag?

[Triple Jim]

Fixed timing in the neighborhood of 23- 25 degrees before TDC is normal for a 2-stroke.

[wymple]

While a compression release may make it easier to pedal that moped, it's akin to dumping the clutch. The compression gets thrown back on for start up. It takes, I think, about 75-80 lbs compression for a 2 stroke to run at all. I could be wrong, but as I recall, 60 psi is a dead duck, 70 is weak, 80 should run, over 90 is best. Ideally you should get 110 or a bit more. Also, you need more rpms to build the reading than just the couple of turns needed on a 4 stroke. Crank pull engines take as much as 5-6 pulls to accomplish this. You can gain 40 psi from pull 1 to pull 5.

[Triple Jim]

Right, and that's why I asked "Did you hold the throttle wide open and kick until the gauge quit rising when you did the test?" above.  Eric pretty much said he kicked until it quit rising.

[lrutt]

Fixed timing in the neighborhood of 23- 25 degrees before TDC is normal for a 2-stroke.

what????? I've had several 2 strokes, including 2 right now, and neither one of them are even close to 23 degrees BTDC. Try 3 degrees or so. If you were running that much it would not start or would kick you back badly.

As a matter of fact, some smokers are timed so close to TDC they can actually run backwards.

[erik_w]

The Engine is quite cool, it is basically two 175 cc Engines put together - that is to say the cylinders don't interact in any other way then getting fuel from the same carb, but were I to set it up with two carbs, the two halves of the Engines would be completely independent.

Yes, it is a piston port design - as the pistons rises it allows air fuel mixture to enter the crank chamber, on the downstroke this is then compressed and pushed into the compression chamber via ports on both sides of the cylinder. The pistons work at 180 degree opposities of each other and there is one ignition cam driving two breaker Points.

Last night I charged the battery up to full (it was already good), just to see if this would help. Still nothing!

After doing the priming procedure (kicking it with full choke and ignition off for 2 kicks), both of the plugs are wet with fuel but still it is not firing.

I Think the manual calls for a compression of around 130 psi, and currently it is between 70 and 80...

Yes, I had both spark plugs out and throttle wide open, then kicked it five times, at which Point the needle wasn't Rising by any siqnificant amount... Wet the compression came to well over 150 psi and dry to under 80...

[erik_w]

what????? I've had several 2 strokes, including 2 right now, and neither one of them are even close to 23 degrees BTDC. Try 3 degrees or so. If you were running that much it would not start or would kick you back badly.

As a matter of fact, some smokers are timed so close to TDC they can actually run backwards.

The manual calls for setting the Point of ignition to between 3 and 2.5 mm Before TDC depending on that gas you are using.

[Groover]

In regards to the timing, maybe it depends on the engine design and CC. For example, the Vespa p125x is 21 degrees BTDC, and it's 23 degrees BTDC for the P200.

http://www.scooterhelp.com/scooters/VNX1T.p125x.html

http://www.scooterhelp.com/scooters/VSX1T.p200.html

Hope that helps some.

[lrutt]

The manual calls for setting the Point of ignition to between 3 and 2.5 mm Before TDC depending on that gas you are using.

That would seem right, at least from my Suzi and Yam 2 strokers. Once i set mine with a dial indicator, I then verified the mark on the flywheel to ensure it was spot on for future timing and it was. That way I didn't have to muck about with dial indicators any more.

Hard to say what your problem is then. I still have to ask, and didn't read it anywhere. Is your plug wet when you take it out? If you are kicking that much and it's not firing, that plug has to be wet. If not, you got a different problem.

[twhitaker]

After doing the priming procedure (kicking it with full choke and ignition off for 2 kicks), both of the plugs are wet with fuel but still it is not firing.

[Groover]

Eric, have you tried putting it in second gear and pushing the crappola out of it? I know that's not what you want to do normally, but sometimes that's what it takes for a problematic 2-stroke to get it going then if it starts you'll know what and where to tweak.

Having a second person help with pushing will be very helpful....

Good luck.

[erik_w]

Eric, have you tried putting it in second gear and pushing the crappola out of it? I know that's not what you want to do normally, but sometimes that's what it takes for a problematic 2-stroke to get it going then if it starts you'll know what and where to tweak.



Having a second person help with pushing will be very helpful....

Good luck.

tried it and it didn't work ..

[tris]

....... . The pistons work at 180 degree opposities of each other and there is one ignition cam driving two breaker Points.....

It can't be timed 180 degrees out can it?

Did that (360 degrees) with a Ford Escort lump once 

[lrutt]

It can't be timed 180 degrees out can it?

Did that (360 degrees) with a Ford Escort lump once 

It is possible, fires every time up and cylinders alternate. with 2 sets of points they could be off but don't know what the thing looks like behind the flywheel. My Suz twin would be very hard to get it wrong though, that's my only point of reference on a twin smoker.

[Jurgen]

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?

Eric, If the spark is ok with plugs out of the engine (fat blue spark) and the plugs are NEW, then they should fire in the engine.  I'd try one other thing: instead of gas in the plug hole, try starting fluid (ether) in the carb inlet (let your buddy spray it in) while kicking the engine over.  This way fuel actually gets into the inlet, thru the bottom end and up the transfer ports.  If it does not run for a few strokes that way, there is something wrong with the ignition, because some fuel will get to the plugs.  Jurgen

[twhitaker]

Going out on a limb here but I thought it ought to have been mentioned. Could the plugs be getting too wet to fire?

[Jurgen]

Eric, be sure to let us know what the problem was when you finally get it running.  Lots of conjectures here, we'll all learn something.  Jürgen

[erik_w]

Eric, be sure to let us know what the problem was when you finally get it running.  Lots of conjectures here, we'll all learn something.  Jürgen

yes! absolutely I will let you know.