mun2frio Posted August 22, 2009 Report Share Posted August 22, 2009 When I first got the bike last year, I did a compression test and got 110/115 psi. After two months I replaced just the rings and the compression went up to 120/125 psi. I just got my cylinders back from Jeff with a fresh hone and bore, new pistons, top end gaskets and new seals. I got done earlier putting the motor back togheter and when I did a compression test I got 160/165 psi. My question is, what could be causing the 5 psi difference? I did a leakdown test and it hold 6 psi for 6 minutes. Also since my compression is around 165 I went and got a five gallon can of power mist t105 racing fuel which claims to be 109 octane. Should I run a 50/50 mix? Mods are: +4 timing 20cc domes FMF Gnarly pro flow with K&N and lid on VF3s stock carbs, 290 main, 27.5 pilots, needle on stock position airscrew 1 1/2 turns out sea level 80 degrees temperature Thanks for any advice guys. Quote Link to comment Share on other sites More sharing options...
TwistedSheeRida Posted August 23, 2009 Report Share Posted August 23, 2009 you should be able to get away with pump but it wouldnt hurt to run a 50/50 mix. as long as the comp is within 10 i wouldnt worry about it. Quote Link to comment Share on other sites More sharing options...
spurdy Posted August 23, 2009 Report Share Posted August 23, 2009 you should be able to get away with pump but it wouldnt hurt to run a 50/50 mix. as long as the comp is within 10 i wouldnt worry about it. Stock porting and stock head? Could be inconsistencies in the stock casting. Exhaust roof height and dome size also play a part in your compression besides how well your rings seal. SP Quote Link to comment Share on other sites More sharing options...
mun2frio Posted August 23, 2009 Author Report Share Posted August 23, 2009 Stock porting, and the first two times I did a compression test I had the stock head, now for the fresh top end I put a pro design cool head with 20cc domes. It's just weird that with a fresh bore and new pistons and it still has a 5 psi difference between cylinders. I've never used or mixed race gas before, so if I go with a 50/50 mix that means if I put 2 gallons of pump gas I have to put 2 gallons of race gas right? Thanks guys, I'm still learning as you can see. Quote Link to comment Share on other sites More sharing options...
Onesickcrewcab Posted August 23, 2009 Report Share Posted August 23, 2009 5 PSI is nothing to worry about. I believe the spec is that it should be within 10% of each other. Don't quote me on that, but I do think thats correct. I've never seen a motor have the same exact compression in each cylinder. Quote Link to comment Share on other sites More sharing options...
mun2frio Posted August 23, 2009 Author Report Share Posted August 23, 2009 I guess I shouldn't worry about it then. You guys think I should run a 50/50 mix or a 25/75 mix? Anybody have any experience with Power Mist T105 Racing Fuel? They also make a premix oil that claims to be the ultimate 2stroke oil. Quote Link to comment Share on other sites More sharing options...
spurdy Posted August 23, 2009 Report Share Posted August 23, 2009 I guess I shouldn't worry about it then. You guys think I should run a 50/50 mix or a 25/75 mix? Anybody have any experience with Power Mist T105 Racing Fuel? They also make a premix oil that claims to be the ultimate 2stroke oil. Never heard of it and I'd bet you'd be ok with high octane pump gas. I run Sunoco Turbo Blue. SP Quote Link to comment Share on other sites More sharing options...
mun2frio Posted August 23, 2009 Author Report Share Posted August 23, 2009 Thanks for the info guys, I'm about to go and try to start the bike and do the break in. Quote Link to comment Share on other sites More sharing options...
bad 350 shee Posted September 9, 2009 Report Share Posted September 9, 2009 with a leakdown test it should hold longer than 6 min i think it should hold like over night i think Quote Link to comment Share on other sites More sharing options...
mun2frio Posted September 10, 2009 Author Report Share Posted September 10, 2009 with a leakdown test it should hold longer than 6 min i think it should hold like over night i think [/quot Yeah, I left the air in for like ten minutes and then released it to keep putting the motor back together. I'll prolly be doing another one in like a week to make sure nothing is leaking. Quote Link to comment Share on other sites More sharing options...
Building my First Posted September 10, 2009 Report Share Posted September 10, 2009 You can lose a psi and still be fine. The test is 6 for 6. It probably would be good to do another leak down test after a couple heat cycles. Recheck your touque on the cylinder bases and head too. Quote Link to comment Share on other sites More sharing options...
bbcmudtruck Posted September 10, 2009 Report Share Posted September 10, 2009 Recheck your touque on the cylinder bases and head too. This is a very important step! Quote Link to comment Share on other sites More sharing options...
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