question about compression

[LX8VD69]

hello i wonder if someone can help me. i scored a boost ported head kit off someone on this forum for bugger all and put it on my other CAG with the stock bottom end, including the piston. the head kit is supposed to only bump the compression up to 13:1 but when i try to start it its really hard to pull the pull cord. i didnt check the squish, i just bolted it up and screwed a plug in and made sure it turned over freely. if it even starts itll idle for about 15 seconds and then cuts out. could this poor idling be caused by having too high compresion and if so can i just ad another squish ring to lower the compression to make it easier to turn over?


cheers adam

[Benny88]

hmmm compression is good. You may have an air leak somewhere. check the squish. Mac90 will sort this out for you. hes all over this cag engine stuff

[mac90]

it sounds a little like a leaky crank seal inspect behind flywheel & clutch for damp or oily patches

but i would make sure the squish is not too tight, you can stick a bit of bluetack on the plug electrode first to be sure its not touching the plug
just double up on your base gasket as this will raise the cylinder .3 to.5mm

[Benny88]

see, all over it. Hey Mac while we are on the topic, theres no problems with doubling up base gaskets in banshees? will it weaken the seal or anything?

[mac90]

i cant really see a problem with it on a 2 stroke engine as long as it seals
you can use a little gasket goo between them to almost glue them together tho its not really needed when using new gaskets

the only other thing is it also raises all the ports so you can safely add gaskets until the piston crown at BDC is in line with the exhaust port

once you line it up you can measure squish & adjust at the dome if possible

[Benny88]

unfortunately I i dont have a dome. I have a polini rep head that im going to put on and the standard cylinder thats too long to be able to make serious adjustments. Ive been told that the polini rep head is good tho and will lower the squish enought to get some room for port raising. If I was to machine the cylinder down from the top then we would be talking. what sort of dome head would fit a banshee? Also do you know anyone I could take my cylinder too to cut it down in melbourne? Also im not sure what a BDC is. Is that the bottom of the stroke? do you want the crown of the piston exactly in line with the bottom of the exaust port at its lowest point? Massive thread jack here sorry..

[mac90]

yes that's the basic idea
BDC= bottom dead center of the stroke

set it up like this & then see what the squish is, if its close then it will be fine but you can remove a base gasket or too if need be without having to go to the trouble of machining your cylinder