Chip M Posted March 31, 2012 Report Share Posted March 31, 2012 (edited) I picked up a new compression tester today. I have been having some trouble with thinking I am lacking the power I should have. I posted a topic called "plug question" yesterday that listed all the details and I got some really good clues to help me out. I see the manual says 6.5:1compession which did not do me much good as a starting number. I checked without carbs or pipes on and had the plug in opposite side when checking. I do not know if that is an issue but my compression was 162 psi on the left and 165 on the right cylinder. It is a new rebuild. These numbers seem really high from what I read. I do not know what size domes are in my coolhead and did not find numbers on them anywhere. From reading I gather that my domes control the compression along with the condition of my bore, pistons,ect. Can someone tell me how to determine the size of the domes I have and if my compression is close to where i should be? I run 93 octane gas only. Thanks for your help. 96 Banshee-64.75 mm bore V Force Reeds/Cages ProDesign Cool Head FMF Gold Series Fatty's- Powercore 2 Silencers K&N Air Filter-stock box w/lid-no snorkel Timing +4 Presently 300 main jets-27.5 pilots @ sea level-80 deg. F Hard trail / MX style riding Edited March 31, 2012 by Chip M Quote Link to comment Share on other sites More sharing options...
Chip M Posted March 31, 2012 Author Report Share Posted March 31, 2012 Also found this link to my ratio vs. P.S.I. issue. This helped me out alot.....ummmmm NOT!! http://mgaguru.com/mgtech/power/pp105.htm Quote Link to comment Share on other sites More sharing options...
cjc Posted March 31, 2012 Report Share Posted March 31, 2012 if you remove the domes from the coolhead ,the CC size should be stamped on them. you will more than likely need a set of orings to reassemble it. looking at PSI #'s ,if your she has stock port timmings you should have around 20 cc domes Quote Link to comment Share on other sites More sharing options...
cjc Posted March 31, 2012 Report Share Posted March 31, 2012 if you keep your A/F tune safe you should be able to run 93 ,but your on the edge of the SAFE limit ! have you checked your squish clearence ? Quote Link to comment Share on other sites More sharing options...
Chip M Posted March 31, 2012 Author Report Share Posted March 31, 2012 if you remove the domes from the coolhead ,the CC size should be stamped on them. you will more than likely need a set of orings to reassemble it. looking at PSI #'s ,if your she has stock port timmings you should have around 20 cc domes I looked when I rebuilt it but I will pull the head and look again. If they are not stamped and I was to buy new ones, is 20's what I should try first?. The cylinder has not been ported but I will be sending to Jeff at Fast for a hard trail port job. Aiso, do I need to get a new set of O-rings if I just replaced them when I rebuilt a month ago? Quote Link to comment Share on other sites More sharing options...
Chip M Posted March 31, 2012 Author Report Share Posted March 31, 2012 if you keep your A/F tune safe you should be able to run 93 ,but your on the edge of the SAFE limit ! have you checked your squish clearence ? No I have not checked the squish clearance. I have read a little about it but I need to find a link to give me good instruction. I assume that the solder needs to be near the edge of the piston where it comes up to the head, not in the center where the bowl shaped dome area is. Can this all be done from through a spark plug hole or do I pull the head, lay it in, assemble, squish and then pull back apart. I hope to be able to find a video of it. I do not know what A/F tune is unless you are talking about my carb jetting. 300 mains 27.5 pilots. Kinda new at this but I really appreciate all your help Quote Link to comment Share on other sites More sharing options...
n2otoofast4u Posted March 31, 2012 Report Share Posted March 31, 2012 Pull the head off, and inspect domes closely. Despite if you find numbers ir not take a piece of solder that if the size of the bore and lay it on top of the piston paralel with the wrist pin. Reasemble the head and BY HAND roll the motor over so that the piston "squishes" the solder between the head and piston. Remove that peice of solder and measure... Once you do that report back your findings. If you cannot find numbers on the domes, your builder is gonna want that peice of solder, and your existing domes to look at before cutting new ones to help assure that you get what you want. Quote Link to comment Share on other sites More sharing options...
cjc Posted April 3, 2012 Report Share Posted April 3, 2012 If your careful you can do it thru the plug hole,just use a LONG piece of solder, bent at the end . be sure to place the solder parallel with the wrist pin CL ,and get it against the cylinder. Quote Link to comment Share on other sites More sharing options...
cjc Posted April 3, 2012 Report Share Posted April 3, 2012 I looked when I rebuilt it but I will pull the head and look again. If they are not stamped and I was to buy new ones, is 20's what I should try first?. The cylinder has not been ported but I will be sending to Jeff at Fast for a hard trail port job. Aiso, do I need to get a new set of O-rings if I just replaced them when I rebuilt a month ago? If the Orings arn't frayed or otherwise skruffy looking, you may get away with it ,but I wouldnt chance it ! TDR has the Orings and their in FL, you can get then the next day ! BUY SPARES !! Quote Link to comment Share on other sites More sharing options...
Wildcardracing Posted April 5, 2012 Report Share Posted April 5, 2012 Chip, When doing a compression test, you should test the engine with the throttle wide open and kick untill the needle stops moving. I usually do this test warm with both plugs removed. Warm or cold doesn't really matter too much just as long as the test variable are the same for subsequent tests. Yamaha's 6.5:1 ratio is the trapped ratio figured using the volume above the exhaust port. I agree that you likely have 20cc domes in there and would recommend 100 octane for that ratio. I would recommend switching out to 22cc domes to be safe on the 93 octane you run. -Brandon Quote Link to comment Share on other sites More sharing options...
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