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Post by 190mech on Mar 10, 2017 4:50:51 GMT -5
Keeping the piston flush with the bottom of the transfer ports allows the mixture stream to attach itself to the piston dome,thus cooling it,,if the piston is a bit high or low it creates a 'step' causing the stream to tumble and not cool the piston as well...
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Post by stepthrutuner on Mar 10, 2017 5:51:28 GMT -5
What niz said. Minimum squish is for optimal race setup to help deter detonation and reduce piston heating when spark advance and compression are being maximized for better torque at less than peak rpm. It is often not uniform about the circumference of the piston in a lot of kits and requires careful machining to get it uniformly perfect.
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Post by AtariGuy on Mar 10, 2017 11:04:48 GMT -5
Brent quoted Bell's book on 2 stroke tuning saying .6 to .8 mm squish is ideal - and we've already presumed he is referencing race setups, but a good squish for daily driving ought to be about 0.9 to 1.2. At least what i've seen for most of brent's project builds and many other daily to sport/street builds, they stick to about that range. Its still a good baseline if you don't yet have access to a compression tester. 190, thats a great point to consider with the level of the piston at bdc. I'm still learning loads of stuff i never thought of before. So much more for me to learn!
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Post by pinkflames420 on Mar 10, 2017 12:38:51 GMT -5
Keeping the piston flush with the bottom of the transfer ports allows the mixture stream to attach itself to the piston dome,thus cooling it,,if the piston is a bit high or low it creates a 'step' causing the stream to tumble and not cool the piston as well... Perfect! Yous are great at answering our concerns will a perfect explanation. I think what stepthru mentioned, about minimum squish, is what I was trying to say about better higher compression. But I assume it's not always good here, especially since I'm not racing lol. I'd like this kit to last me atleast a year or 2! Before I kill it heheee hopefully I'll know enough to switch to a bigger crank and biggERR bores then. So if squish is good(in target range) and say everything else is okay, piston at BDC, TDC, Then do we assume the compression will be fine?(obviously assume no leaks&good comp.) I think I'll go grab the compression gauge today. I was debating , but I shall. PS. For my setup, I wouldn't mind being just a little torquey. Mainly getting out of my driveway which pulls onto the highway:/ (60kmh, limit dw lol) I'll be looking to pull the most top end of this aswell. |
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Post by gsx600racer on Mar 10, 2017 13:00:44 GMT -5
Remember once you get your kit bolted up is to do a leak test. This should be done with intake installed(less carb and exhaust)
I small leak has the potential to cause a lean condition and tuning headaches.
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Post by pinkscoot on Mar 10, 2017 13:25:11 GMT -5
Steer clear of the Harbor Freight Compression tester, They don't have a good reputation. I got one from an auto parts store that wasn't much more than the Harbor Freight.
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Post by AtariGuy on Mar 10, 2017 13:26:05 GMT -5
 I went this route for my diy leakdown tester. Look up a low pressure gas gauge, they should have the dial, brass fitting, and fitted with a schrader valve (bike tire pumps would work well with this valve) for 15 bucks. I snapped up a pvc 3/4 inch male thread to 1/2 inch female fitting for 55 cents then sanded the raised bits on the female outside smooth. Find a suitable piece of flat steel and fab a blocckoff plate that fits the exhaust flange, then add a gasket to it to assure air-tightness. Once fitted, bike tire pump it to 5-10 lbs and let it set for an hour. If it doesnt stay put, use a paint brush with soapy water to check for leaks. |
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Post by 90GTVert on Mar 10, 2017 13:49:29 GMT -5
So if squish is good(in target range) and say everything else is okay, piston at BDC, TDC, Then do we assume the compression will be fine?(obviously assume no leaks&good comp.) I think I'll go grab the compression gauge today. I was debating , but I shall. Basically. Most put them together and hope for the best at that point, and they'll generally be alright as long as you're using a kit that came with a head and supply it with premium fuel. Alternatively, you can do compression tests to see what kind of cranking compression it makes (150-180psi is a common target). You could figure out the actual compression ratios, but most people have no desire to do that and it's not really necessary for the average BBK install. 49ccscoot.proboards.com/thread/5043/calculating-stroke-compression-ratios |
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Post by pinkflames420 on Mar 10, 2017 13:51:58 GMT -5
Steer clear of the Harbor Freight Compression tester, They don't have a good reputation. I got one from an auto parts store that wasn't much more than the Harbor Freight. Thanks, pink. I plan to grab one from our auto store if I can't manage to get something together. I won't say local one because it's the ONLY one. So annoying I gotta drive 2 hours JUST to find a rubber mallet because , of course my luck, the last one was taken the same day I was looking!! ARGGHH. @atariguy , I have a gauge attached to my air brush compressor and another to my big(new) compressor. This will work aswell? Just need to find the thread fitting? My airbrush's compression only goes to 100. I gotta open the box on the bigger compressor.  |
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Post by AtariGuy on Mar 10, 2017 15:03:42 GMT -5
It -may- work as long as it doesnt suffer from a slow air compressor leak common to 98% of compressors on the market. If it can hold a steady air pressure for 1 hour by itself, then i'd say try it out on your case. You dont want to use much more than 10psi or risk blowing out gaskets, seals, or anything else. You do have to make sure you have an airtight seal at the intake where the carb usually fits and additionally at the exhaust
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Post by pinkflames420 on Mar 10, 2017 16:06:56 GMT -5
It -may- work as long as it doesnt suffer from a slow air compressor leak common to 98% of compressors on the market. If it can hold a steady air pressure for 1 hour by itself, then i'd say try it out on your case. You dont want to use much more than 10psi or risk blowing out gaskets, seals, or anything else. You do have to make sure you have an airtight seal at the intake where the carb usually fits and additionally at the exhaust Cracked open the new compressor. Same brand as the small one tho. So I can stick the end of my manual basketball pump into the other Side? Hope I can get this thing off there.  |
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Post by niz76 on Mar 10, 2017 20:34:44 GMT -5
6 psi is as about max amount of pressure you'd want to put into your engine for a leak test so that 100psi gauge may be a bit difficult to read. Many of the gauges meant for everyday compressor stuff doesn't even start until 10psi so can be difficult to use with the such low psi we're working with for a leak test...
Wow I think it's awesome that you's are learning so much and diving so deep into this 2t tech! I was just happy I got my first BBK running and the carb dialed in haha. It took me much longer to learn and understand about port timing, squish clearance manipulation, and leak down tests so KUDOS!
Btw, here's a vid I made doing a leakdown test on my Vespa engine... totally different engine than our scoot engines but I use this same tester in the same way with my 49cc scoot engines...
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Post by pinkflames420 on Mar 10, 2017 22:04:43 GMT -5
6 psi is as about max amount of pressure you'd want to put into your engine for a leak test so that 100psi gauge may be a bit difficult to read. Many of the gauges meant for everyday compressor stuff doesn't even start until 10psi so can be difficult to use with the such low psi we're working with for a leak test... Wow I think it's awesome that you's are learning so much and diving so deep into this 2t tech! I was just happy I got my first BBK running and the carb dialed in haha. It took me much longer to learn and understand about port timing, squish clearance manipulation, and leak down tests so KUDOS! Btw, here's a vid I made doing a leakdown test on my Vespa engine... totally different engine than our scoot engines but I use this same tester in the same way with my 49cc scoot engines... Thanks Niz! I always enjoy your vids.  And I appreciate yours and everyone's patience here because I really didn't know anything when i came here! This bbk install has and will teach me a lot, aswell as a new hobby! I will grab a new gauge instead of tearing them off my compressors lol. I can get one close by(surprisingly) Sounds silly now that I know the compression is set so low. Definitely go with^^^ Made sure my bike was running and took me awhile to file that dang nub sticking out of the port transfer. Wish me luck |
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Post by spaz12 on Mar 10, 2017 22:15:08 GMT -5
I went this route for my diy leakdown tester. Look up a low pressure gas gauge, they should have the dial, brass fitting, and fitted with a schrader valve (bike tire pumps would work well with this valve) for 15 bucks. I snapped up a pvc 3/4 inch male thread to 1/2 inch female fitting for 55 cents then sanded the raised bits on the female outside smooth. Find a suitable piece of flat steel and fab a blocckoff plate that fits the exhaust flange, then add a gasket to it to assure air-tightness. Once fitted, bike tire pump it to 5-10 lbs and let it set for an hour. If it doesnt stay put, use a paint brush with soapy water to check for leaks. Nice leak down tester! |
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Post by AtariGuy on Mar 10, 2017 22:51:16 GMT -5
I went this route for my diy leakdown tester. Look up a low pressure gas gauge, they should have the dial, brass fitting, and fitted with a schrader valve (bike tire pumps would work well with this valve) for 15 bucks. I snapped up a pvc 3/4 inch male thread to 1/2 inch female fitting for 55 cents then sanded the raised bits on the female outside smooth. Find a suitable piece of flat steel and fab a blocckoff plate that fits the exhaust flange, then add a gasket to it to assure air-tightness. Once fitted, bike tire pump it to 5-10 lbs and let it set for an hour. If it doesnt stay put, use a paint brush with soapy water to check for leaks. Nice leak down tester! i'd hope you'd say that, its your design! Proof i been doin my research |
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