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Post by scootonem on Mar 15, 2013 10:27:05 GMT -5
I finished installing my 54mm bbk today and everything went smooth. I used a torque wrench set at 12ftlbs to fasten my head. I go to fire it up and my piston stops at TDC. Took my head off and looked for clearance. My cylinder lines up perfectly with my piston at TDC and lines up with the bottom of the exhaust ports at BDC and flows smoothly all the way around with mg cylinder on. Threw my head back on and torqued to 12 ft lbs and gave it a whirl... I have a 45mm crank. Any ideas on what this can be?
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Post by 90GTVert on Mar 15, 2013 11:07:54 GMT -5
I've seen 'em hit the head before. Some 54mm kits are made for about 42mm stroke like most stock 90s and some are made for 45mm stroke. Of course the 42mm cylinder would be a little on the short side, and that could cause the issue. If you think it's clearing you can use solder to check your squish clearance to find out for sure. Not much else there to stop the piston unless the head were out of line from being made wrong and it hit the lip instead of traveling into the squish area / combustion chamber.
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Post by scootonem on Mar 15, 2013 12:07:48 GMT -5
How would I go about checking my squish clearance using solder? If my cylinder is too short could I try to double up on gaskets to even it up?
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Post by 90GTVert on Mar 15, 2013 12:43:45 GMT -5
You can take a piece of solder and either tape it to the top of the piston, you'd want it running horizontal, or just bend a piece of solder so it reaches the outer edge of the cylinder when ran through the spark plug hole. Crank the engine over and it'll smash the solder. Here's a vid about squish clearance as well... www.youtube.com/watch?v=ZUe0p0DqFCkReally, I think you've gotta be hitting the piston on the head. The squish check is good to know anyway, because you need a certain amount of clearance to avoid the piston smacking the head at higher revs. I'd like to see at least .9mm of clearance for a 100cc. Most of the time you need more than that just to get reasonable compression unless you've modified the combustion chamber. If it does turn out that you have plenty of squish clearance, take the head and cylinder off. Line up their mounting holes, preferably with studs/bolts or something in each hole, and then look into the bottom of the cylinder to see if the combustion chamber is aligned properly or if you have part of the gasket sealing surface hanging over into the cylinder. |
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Post by Fox on Mar 15, 2013 17:11:08 GMT -5
You can stack two or three base gaskets on the cylinder to give it clearance. You'll lose compression some but it's gotta be sky high as it is.
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Post by scootonem on Mar 15, 2013 23:28:56 GMT -5
I ended up fixing this with another base gasket at 3x the size of the first. Instead of the solder method I used what was handy. Tootsie rolls. I softened them up in my pocket while I was cutting out a new gasket. After I installed the new gasket I threw everything back together. And rotated my flywheel to ensure proper rotation and boy was that thing hard to turn at TDC. Then, mashed a few tootsie rolls along the squish band, and kicked it over really good. I hardened the tootsie rolls in the fridge and measured. .08 read on my gauge. Now I'm unable to start the motor. The electric start won't turn the ridiculous amount of compression so I've been kicking and kicking and started for about a second, revved really high and then shut off. I'm running a uni filter with a 133 main. I know its high but I really want to start rich and dial in slowly working down to about 118,108. Any suggestions? Beautiful weather in these parts lately, it's killing me.
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Post by Fox on Mar 15, 2013 23:39:05 GMT -5
More base gaskets!
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Post by scootonem on Mar 16, 2013 0:20:36 GMT -5
Even with the squish clearance being so close to .9?
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Post by 90GTVert on Mar 16, 2013 7:10:06 GMT -5
So I assume the first post of .08 is a mistake, since you posted it's almost .9mm just now. You may have to push the squish up more to get reasonable compression. Possibly another .5mm or so. That or machine the head. I've opened up a few of my combustion chambers 2-3cc more with good results. There are a couple of approaches here. Simple is to just add more spacing on the base, all the while upping your port durations (could be good, could be bad). More complicated is to take a bunch of measurements and make the most educated guess you can. Time consuming, could be worth a little extra power... or you could end up deciding that more base gasket spacing is fine. lol You'd wanna know your corrected compression ratio and your port timing with different base gasket setups. You can then decide if modifying the head may be the best choice or if spacers will serve you well. Make no mistake, it takes way longer to do the measurements and math, so if you feel modifying the head is farther than you wanna go then just add more at the base (although it can be done with a dremel, drill press, or I had one done at a machine shop with a ball mill for about $20). Here's the process for finding an accurate corrected compression ratio... 49ccscoot.proboards.com/thread/5043/calculating-stroke-compression-ratiosHere's a printable degree wheel to help find timings... 49ccscoot.proboards.com/thread/2229/printable-2t-specific-degree-wheel |
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Post by scootonem on Mar 16, 2013 13:36:54 GMT -5
Threw on another base gasket. She started up revved real high, got my idle right. Went for a .25 mile ride and died on me. Plug was white I was super lean. Feels like I have less compression. I walked it to work. I'm at work right now, what possibly happened?
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Post by 90GTVert on Mar 16, 2013 18:02:26 GMT -5
Sounds like a soft seize. When it runs lean it gets hot. The piston will expand faster than the bore and seize. Should start once it cools, but the lean condition needs to be remedied ASAP.
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Blowin Smoke
Scoot Enthusiast
Posts: 289
Location: Port Orange, Fl
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Post by Blowin Smoke on Mar 16, 2013 20:25:07 GMT -5
Couple of thoughts:
Are you sure the piston is not hitting the head gasket as it comes up?
Do you have an intake air leak?
Have you put a compression tester on it and if so what is the compression psi?
What is the total thickness of the added base gaskets?
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Post by moofus02 on Mar 16, 2013 21:12:18 GMT -5
Air leak? Would explain being lean
Sent from my HTC Evo 3D using proboards
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Post by scootonem on Mar 17, 2013 17:41:44 GMT -5
After working an 11 hour shift, i ended up having to walk my scoot all the way home about 3miles. I woke up this morning and took my head off and found the loss of compression... Blew a hole straight through my piston... You can imagine I'm very frustrated. I'm one of those people that learn the hard way... I ordered the 54mm hoca tall bbk to accommodate for the longer stroke and prevent this from happening again. I'm in the process of taking my cases apart to properly clean it out. The real big pain in the rear is the shrapnel from the piston is spread throughout my reeds, crank, and exhaust. I also ordered a dremel because some of the shrapnel was welded on to my cases; so, I figured I would also port and polish my cases. What chemical is safe and should use to get all of this gunk out and how do I get it out of my exhaust? All of you guys are awesome, and I really appreciate the help! Definitely my new favorite scooter forum.
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Post by moofus02 on Mar 17, 2013 18:13:05 GMT -5
Sorry to hear that I've only holed one piston and not all the way through so I can't help much but someone here will know what to do
Sent from my HTC Evo 3D using proboards
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