Kymco ZX50

[sidthesloth]

Yes, 40 mm back from the end plate there is a series of spot welds.

[Shaka]

Sid, the distance from the piston at BDC is not really an important measurement.  Unless you want to get into changing the port heights the only measurement you really need is the deck height and the squish clearance.  I think Zombie was more asking for a pic of the inside of the cylinder with the piston at BDC.  It really only matters if the ports are not fully exposed, or there is a large gap beneath them.  Those would be tell tale signs there is something "off" with the porting.  I have been doing a lot of reading lately on port mapping because I am about to do some modifications to my cylinder.  I probably won't change any of my port heights unless they are way off, but I am going to clean up the transfer tunnels, exhaust port, and match the case to the jug.  I also want to make sure my squish clearance is set right.  That is the main reason I'm going to pull it all apart again.  I just want to make a spacer to set the jug at the right height.  Here is a good link to how to do some good port maps.  It's very in-depth, but you just need to get the jist of it all.  http://www.macdizzy.com/cyl_primer.htm.  Here's another good one on measuring deck height, http://www.2strokeheads.com/tech1.htm

I think Zombie may be onto something with looking for the spot welds!  It sounds like there may be some type of restrictor in your exhaust.  Is it 40mm from the inlet or outlet side?  If it's the inlet, you should be able to see something in there.  It could just be a washer that needs to be ground out.  Did you get any info on the pipe?  Post a pic if you can.
 

[sidthesloth]

G'Day Shaka, the spot welds are 40 from the end plate at the back of the exhaust. I thought zombie wanted the measurements from the piston at b.d.c. When I lowered the barrel I had an enormous increase in detonation and a large drop in power, I did rub the head down a little with some 400 wet and dry and finished with 1200. When I restored the original base gasket the detonation was still there but not as bad, I put a 0.4mm gasket in with the original one and detonation is now gone but power is still a little off. I was thinking of going back to the original base gasket and using a second head gasket, (when I say original I mean the one supplied with the kit). Using a piece of solder that is 1.2mm thick I can't get any squash between the head and piston, which seems strange from what zombie is saying and what I have read in Gordon jennings book. With this set it may not be what is right but what works.?.

[Shaka]

When the piston is at BDC are the ports fully exposed, or is there a gap between the bottom of the ports and the top of the piston?  That is weird you are getting detonation with a large squish.  I am wondering if the shape of the combustion chamber in the head is too small?  Next time you take the head off take a pic. 

If the spot welds are at the back of the exhaust it may just be holding on the outer canister to the inner expansion chamber.  For that matter, I guess we aren't even positive that it is an expansion chamber design.  I would be trying to contact the manufacturer if you can figure out who and get some clarification!

Hopefully Zombie comes up with some porting software soon, hint hint!   I think we could both use it!  I will buy one probably by Tuesday if all else fails and can run some numbers for you.  Once I figure out how to use it that is!

[sidthesloth]

I do have some pics of the head on this thread. the ports are not fully exposed, the top of the piston is just above the bottom of them. the exhaust is the one the seller recommended, hmm, seller.

[Shaka]

Oh yeah, forgot about the head pics.  I just re-looked.  I see that your piston has a positive deck height also.  I'm not certain on this, but I've done a lot of reading.  Most people don't like to run a positive deck height!  It usually makes things screwy with the squish and the combustion chamber volume.  Which I thing is exactly what you are experiencing!  It's also not good to have the ports not fully open at BDC.  The piston should ideally sit at the bottom edge of the ports.  I'm hoping you are not running into the same problem as Josh over at Scooter Invasion.  He is talking about scraping the cylinder before even installing it because the port timings are so far off they are not fixable!  I think he may have a cylinder by the same manufacture as yours.

If you can take some real precise measurements, I'm sure we could get one of the guys on SI to run the numbers through the software for you.  You may end up have to raise the jug up to get rid of the positive deck and allow the ports to fully open.  Then either run with no head gasket and/or shave the head to bring the squish down to where it needs to be.  You may also either have to run a different head or have the combustion enlarged a bit depending on how much may need to be shaved.

The measurements I think you need to run the numbers are deck height, squish, port heights from the top most edge to the top of the cylinder, port opening height (from bottom edge to top edge), and possibly head gasket thickness.  Ideally, you want these measurements to the tenth of a mm.

[zombie]

The piston should be lined up with the bottom of the ports. If it is lower then there is a small zone where nothing is happening. If you put a degree wheel on it you will see the total duration of wasted energy in the cycle. Turn that into percentage of the cycle, and you will see how much it will impact performance. My ex wife has been staying at the house (?) so I haven't had the opportunity to focus on the forum the way I should. Sorry Guys. I promise to get on this tonight, and figure how to correct the worst of it. I can see on your pipe how hot the engine is running. Sid can you put some #'s on the map. Distance from the top of the ports to the top of the jug will help alot. Also if you have access to a good welder, adding another exit tube to your pipe will cancel some of the resonance that I believe is pushing burning gases back into the jug. My guess is there is a cone shaped baffle inside. They make a taper on expansion pipes to allow the gases to expand as the travel. The tube shape on yours allows for a flash expansion that just may be bouncing back because it cannot get out fast enough. I just saw the post w/ the #'s. I'll look at it and get back

[zombie]

http://www.scooterinvasion.net/forum/viewtopic.php?t=2028&start=0    Here is the thread for the build josh is undertaking. It is the same Manufacture. Something here may click w/ your build sid. You may have to set up an account/user name to see the photos.

[sidthesloth]

Thanks a lot zombie, I have an arc welder but that may be a bit crude, could give it a go though. I have seen the pics, Josh has posted them here and I have seen them on scooter invasion, there is a whole lot of sweet looking gear going into that one, I hope he doesnt have to scrap the jug.

[zombie]

I was thinking of all the work he did to match the jug to the cases. I don't believe there is another "bolt on" 125cc kit available. I think an arc welder is too much for the stamped steel, but I am not proficient enough w/ one to make that call. Hold off on that for the moment to get the jug sorted out anyway. The main focus should be figuring out the Squish for now. If you can bring that to approx. .8-1.0mm you can then work on raising or lowering the jug to where the ports work best. Lets say you have NO gasket on the base, and 3mm gasket on the top; (squish set to 1mm) You can then transfer gaskets to the bottom, and mark the results. It will be working in a predictable pattern that you can control. Once you get the best from that you can get into modding the pipe.

[Shaka]

Sid, If you're not getting the ports fully exposed, then wouldn't running no base gasket as Zombie suggested make the problem worse and make the piston stick out the top of the cylinder even more?  Seems that way to me.  It may get your squish where you need it, but throw off the port timings even further.  You should try taking a squish measurement with a thicker piece of solder to see exactly where it sits right now.  That will give you a better idea of how much needs to be changed to get it right.

[sidthesloth]

G'Day zombie and Shaka, when I tried with no gasket the piston was out the top a quite a lot, (I must learn to measure), the base gasket I am using now is the original plus a 0.4, after having been in service they compressed to 0.88mm, my new piston kit turned up today and the new gasket is 0.72mm unused. Yesterday I took some photos of the piston at b.d.c. with the jug bolted down, I will post these for you but it may take a couple of posts due to size restrictions.

[sidthesloth]

Here is pic no. 2 (trying again)

[sidthesloth]

Annd, pic 3, exhaust.

[zombie]

That's much better than I thought. The reason I suggested lowering the jug, is sid had stated the scoot ran better with the jug higher. I am thinking the squish will be easier to set if all stays relative. Don't run it w/ the jug down just try to find the squish. Then transfer the thickness to the bottom , and remove from the top. The known squish will remain, and the jug timing will change to the range that is working for him. I would try too work this out w/ the current piston, and save the new one for a more proven set up.