stroking Posted October 11, 2009 Report Share Posted October 11, 2009 I POSTED UP A TOPIC ABOUT THE COMPRESSION on this motor [120psi] a while back and today i decided to take the head off to have a look at the setup....well the head is completely stock 64mm.. and i rechecked the squish and its .110 on left and .108 on right...wayyy to big for a alcohol motor i think..... i have a pic o how much needs to be cut out of this head to fit the cylinder.....oh and the piston was hitting the head on the very edge also.........good thing i tore it down to have a look.......why would someone put a stock..UNCUT head on cub cylinder i don't know. Quote Link to comment Share on other sites More sharing options...
stroking Posted October 11, 2009 Author Report Share Posted October 11, 2009 I POSTED UP A TOPIC ABOUT THE COMPRESSION on this motor [120psi] a while back and today i decided to take the head off to have a look at the setup....well the head is completely stock 64mm.. and i rechecked the squish and its .110 on left and .108 on right...wayyy to big for a alcohol motor i think..... i have a pic o how much needs to be cut out of this head to fit the cylinder.....oh and the piston was hitting the head on the very edge also.........good thing i tore it down to have a look.......why would someone put a stock..UNCUT head on cub cylinder i don't know. this motor was bought like this,so i guess the seller just threw a head on there to make a quick sale Quote Link to comment Share on other sites More sharing options...
bansheeseat$$ Posted October 11, 2009 Report Share Posted October 11, 2009 Thats a big problem,One being a banshee piston has a 13 degree dome and a 68mm blaster piston has a 9 degree dome.My sugestion to you would be spend the money on a aftermarket head and buy the correct big bore 9 degree domes for it. Quote Link to comment Share on other sites More sharing options...
jbooker82 Posted October 11, 2009 Report Share Posted October 11, 2009 And blaster pistons are shorter from the wrist pin to piston head. Quote Link to comment Share on other sites More sharing options...
stroking Posted October 25, 2009 Author Report Share Posted October 25, 2009 i got a set of 68mm 18cc domes and the compression is now at 162psi,think i should have gotten 16 or 17cc domes. but the bike runs ALOT better now....just can't get it to shift under a load Quote Link to comment Share on other sites More sharing options...
RILS Posted October 25, 2009 Report Share Posted October 25, 2009 i got a set of 68mm 18cc domes and the compression is now at 162psi,think i should have gotten 16 or 17cc domes. but the bike runs ALOT better now....just can't get it to shift under a load That sounds low to me with those domes whats the squish? How far are you from sealevel. Quote Link to comment Share on other sites More sharing options...
bansheeseat$$ Posted October 25, 2009 Report Share Posted October 25, 2009 Something is still wrong with your compression #'s.With 18cc domes you should be more then 162 psi Unless your at 8000 ft above sea level.I run 18's in mine at 4500ft and have 185psi. Quote Link to comment Share on other sites More sharing options...
RILS Posted October 25, 2009 Report Share Posted October 25, 2009 Something is still wrong with your compression #'s.With 18cc domes you should be more then 162 psi Unless your at 8000 ft above sea level.I run 18's in mine at 4500ft and have 185psi. I agree im at 1000ft with 18s on a 65cub and got 193 cold the other day. That 68mm should be way up there with those domes. Quote Link to comment Share on other sites More sharing options...
stroking Posted October 25, 2009 Author Report Share Posted October 25, 2009 im below sealevel in some area and 20ft above in others......both cylinders have the same comp so it can' be the rings. Quote Link to comment Share on other sites More sharing options...
RILS Posted October 25, 2009 Report Share Posted October 25, 2009 (edited) im below sealevel in some area and 20ft above in others......both cylinders have the same comp so it can' be the rings. You have something going on for sure. I wouldnt event put those domes in a 68mm cub at that elevation. Check the squish. What kinda gauge are you using? Might try another one. With that setup u should be way above 200psi cranking and I would guess 18/1. After reading the whole post starting to sound like you have a 4mm cub on a 54mm crank if the squish is .110. It should be around .050 or less. Edited October 25, 2009 by RILS Quote Link to comment Share on other sites More sharing options...
okbeast Posted October 26, 2009 Report Share Posted October 26, 2009 I've also got 18cc domes in a 4mm cub and am around 160-165 psi. 1000 ft. elevation. I'm not worried about it though. Got a .020 base gasket in and squish around .050 I believe so I'm happy with it. Does great as an all-around cub motor. Did I think the compression should be higher? Yeah, but I'm not too worried about it. Quote Link to comment Share on other sites More sharing options...
rubberneck Posted October 26, 2009 Report Share Posted October 26, 2009 The problem is that most compression gauges are garbage. I have a snap on gauge that has been tested and is accurate, and at sea level with 20cc domes on my 4milx68mm cub it kicked 185psi. That was with .050" squish and cleaned up ports. My guess is that your gauges are off Quote Link to comment Share on other sites More sharing options...
dajogejr Posted October 26, 2009 Report Share Posted October 26, 2009 My guess is that your gauges are off That, or the threaded end is not as long as the spark plug or he tested it incorrectly.... Quote Link to comment Share on other sites More sharing options...
stroking Posted October 27, 2009 Author Report Share Posted October 27, 2009 That, or the threaded end is not as long as the spark plug or he tested it incorrectly.... my threaded end is not as long as the plug on my gauge Quote Link to comment Share on other sites More sharing options...
dajogejr Posted October 27, 2009 Report Share Posted October 27, 2009 You can safely add 10 to 15PSI if not. You start jacking the compression way up, it's going to take away from top end (the motor is fighting itself) and you're gonna put undue stress on the bottom end and bearings.... Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.