Teardown Compression/Leak down results

[blacknred03]

I have a 03 with a FAST dune/play port, stock stroke, v3's. Noss head, 20cc domes, fattys, stock carbs, 360 main, 27.5 pilot, tors elim kit, welded crank, K$N no lid, + 4 timing.

 

Recently the bike was missing on the top end so I pulled the head and saw on my right cylinder the crown of the piston was a little pitted, but the heat had not spread to the exhaust ports. i felt the piston was salvagable.

 

So I put the head back on and did a compression test 150 in each cylinder, ice cold. I also attempted my first leak down. i used rubber stoppers but could not get one the exhaust ports to seal well. I eventually weged a screwdriver against the frame to put some pressure on it and it held 6 psi for 5 mins then my arm got tired and it lost pressure.

 

So this leads to my questions, I run 40:1 motoul 800. Should I richen the mix, go to 21 cc domes, mix 50/50, buy a 4 stroke?

 

I tried a squish test and all I had was some pretty thin solder. I got something like .053 squish, I know this is high. What size solder do I need? Please help

[Machwon]

The pitting is from detonation and with 20cc domes you should run at least a 100 octane or 50/50 mix. Basically you need more octane or less compression to make it stop. The thinner solder will improve the result, make sure you make the end of the solder like an L. Do the squish test over the wrist pin on the right or left side of the cylinder (better results if you do both sides at the same time) and I always feel the side of the cylinder with the solder end and let the piston bring it to the top for squish. You should have around .045 if everything is right.

[blowit]

Sounds like jetting might be a hair lean, heat range of plug may need bumped to a 9 if it is hot outside, and we target .035 for squish to retard detonation. I would want that squish number down. BANK on this, you do NOT need 50/50 mix or straight race fuel, or diesel fuel to retard detonation. We have motors running every weekend at 175-190psi on pump fuels. 150 is not even close. Feel free to go pump up a brand new 125/250 dirt bike for comparison and those are to be run on 92oct fuel from the OEM. Point being, there are other factors besides compression that dictate a detonating condition.

 

 

That being said, you will never hurt the engine by running 50/50 fuel and it will give a level of protection but I would not go straight race fuel. That will be a waste. I think if you are experiencing detonation right now, you need to look else where than fuel.

 

 

 

Brandon

Edited August 2, 2008 by blowit

[blacknred03]

I think I am goind to go to a 21cc dome. The bike has ran fine for about 10 tanks and then all of the sudden one cylinder leans out. I thought the main got clogged but it was fine when I took it out. My bowl were a few mm of but I set them back to 21. Should I take out any timing?

[blacknred03]

Help I found a deal on domes but is that my problem??????????

[THE GRIM RIPPER]

Help I found a deal on domes but is that my problem??????????

 

Sure.. Step up dome size youll have less detonation if thats what you were having in the first place.... You said you leaned on one side.... Did you check for an air leak on that sides intake setup?

[Machwon]

Help I found a deal on domes but is that my problem??????????

 

 

blacknred03, you obviously got conflicting info here which is not uncommon. Your best served by giving your motor builder FAST a call. A 360 main is right on, so its not lean. A 20cc dome will be at least 13.5 full stroke compression which most builders on a Banshee won't go over 13:1 on pump gas. Relying on cranking compression to determine fuel octane is asking for trouble. You can have a motor with a 15:1 compression ratio which will have 175 psi and another with more porting only have 145 psi. Here's a link to a fuel chart which is another guideline, but in my experierience compression ratio plays a huge role in determining the octane requirements.

 

http://www.msnusers.com/RBRacingTwoStrokeP...uel%20Chart.jpg

 

The bottom line is the pitting on top of the piston=detonation. You should either increase the dome size or increase the octane. Retarding the timing will help but it likely won't be enough to eliminate the pitting. Hope this helps.