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Post by stepthrutuner on May 22, 2010 18:57:47 GMT -5
I think 12:1 would be a pretty practical limit. Isn't stock qmb139 c.r. 10.5:1 and regular gas works ok?
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Post by 90GTVert on May 23, 2010 1:11:26 GMT -5
Manufacturers usually quote it at 10.5:1 and that's what I came up with as well. 87 works fine with them.
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Post by 190mech on May 23, 2010 4:56:43 GMT -5
Can you hear a spark knock with that pipe?Dont think that little motor would appreciate knocking!!
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Post by 90GTVert on May 23, 2010 6:54:11 GMT -5
I dunno. Never head it spark knock yet, but I've never seen any signs of it on the plug or anything else either.
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Post by 90GTVert on May 28, 2010 20:13:32 GMT -5
I got the 50cc insulator/spacer port matched to the head and intake and stuck the motor back in. When I firt hopped on the low end grunt made me . Kinda feels like a 150 on the initial hit. I took it around the yard but I don't think I got over about 5MPH just kinda... GRUNT .... GRUNT ... GRUNT ... :rofl: I took it out on the road and it's got that initial hit, but then it dies off and doesn't rev very high. It still got to 35MPH in my little tuning distance, which isn't bad at all for the SunL. I suspect it may be too rich, because it was jetted for a setup with larger ports. I can't remember if I had 87 or 93 octane in it, so I drained it out and I'll wait to try anymore until I'm sure I've got premium gas.
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Post by stepthrutuner on May 28, 2010 22:32:47 GMT -5
To me the only way to get much out of these tiny engines is to forget about tractability and let them rev.
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Post by 90GTVert on May 29, 2010 5:14:44 GMT -5
My first thought last night was that I shoulda left the NCY cam in there. I went with stock to keep the compression as high as I could, now I'm seeing that I really do have plenty or possibly more than enough. Not sure that I'm motivated enough to pull the engine back out and swap it right now though. My piston to valve clearance is OK now, but if the duration of the NCY cam closes that up at all I'm not so sure it's safe to go deeper on the piston.
I just haven't been able to make this thing rev like you. I'll have to see if I can't find out what other 80cc scoots are revving to. All the big parts and porting and I still can't get it to like revving to even 9000. Just feels like the peak power is low to mid 8000RPM range.
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Post by stepthrutuner on May 29, 2010 8:46:40 GMT -5
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Post by 90GTVert on May 29, 2010 13:35:02 GMT -5
The worst part is, I've tried damn near any head combination you could imagine on a GY6 50, but 90% of my results are skewed by the timing being so far off. I will say that my next highest speed was about 40MPH with the timing way off and that was done with the head I burnt the valve on. It beat the other setups by around 5MPH IIRC. I thought I was screwed for a machinist that would take care of it around here and threw it away... then the machinist tells me he woulda done it no problem. :doh: Anyway, that was port matched to the carb outlet (19mm from head inlet to the port, intake and insulator matched accordingly). The exhaust was opened up to match the header and mirror polished. I had worked the whole ports over really well and spent tons of time on that one. That's why this head got matched to the carb outlet diameter and the exhaust was opened up a little and smoothed. I didn;t go all the way with the exhaust this time just after reading about the function of the step in Bell's 4 stroke book. I just didn't feel like dealing with the hassle or expense of going over the head the same way again then having the seats recut. Small stock ports didn't show me much promise. Big ports didn't give me great gains or high revs either.
I haven't read that page yet, getting ready to help a friend move (hooray).
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Post by 90GTVert on May 31, 2010 9:28:38 GMT -5
Can you hear a spark knock with that pipe?Dont think that little motor would appreciate knocking!! I can now tell you for sure, yes I can. The high pitched sound cuts through the pipe's tone. I filled up with 93 octane and took a test run just as it was setup. I went about 3/4 mile away and back. Max speed of 37MPH. Felt rich and didn't wanna rev above 7,300RPM. I know it revved nearly 1,000RPM more with the same CVT setup last time. Plug looked rich. I pulled the bowl and I had a 109 main in there. That would do it. With big vavles and big ports and a bored out intake it liked a lot more fuel. I swapped down to a 95 main. I took the same course and got up to 41MPH at 7,900RPM. By the time I was 3/4 of the way to the spot I turn around in I heard a very slight tick and let off. I turned around and gave it gas and it made that kinda piston slapping sound when I rapped the throttle (I'm used to that sound from adventures with my 99cc and high compression). I eased it up to 30MPH and then tried to go WOT but the spark knock was getting worse. I ran it home at 30MPH. I'm gonna have to pull the engine back apart and try to add some volume to the combustion chamber. I'll probably shoot for something in the 12-12.5:1 range like Reg suggested earlier. The only reason I say I may look for 12.5 is that I'm not measuring out the valve relief volume and I'm sure my 3 valve reliefs will knock it down at least 0.5:1. It doesn't take much with such a small displacement.
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Post by 90GTVert on May 31, 2010 9:36:23 GMT -5
Looks like I need about 7.1cc total volume above the piston to get 12.5:1. The head gasket and distance from the piston to deck is about 1.1cc, so I need to take the combustion chamber from 5cc to 6cc. It's amazing how much difference 1cc makes in these little engines. I think I might put the NCY cam back as well, assuming it causes no more piston to valve clearance issues. Obviously I have enough compression that loosing a little effective compression to squeeze out a few revs won't kill me.
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Post by 190mech on May 31, 2010 19:46:42 GMT -5
More cam duration will lower compression,that NCY cam might make the difference..Ya plan on taking a bit out of the head with a carbide ball cutter?Wonder how you figure how much to remove?1cc is'nt much!!
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Post by 90GTVert on May 31, 2010 20:15:19 GMT -5
I figured I'd do just a little and then cc it again. Rinse. Repeat. Doesn't sound like fun, but whatever works.
You think I should try just the cam without head mods? I think it will help, but I would still think it would be over the edge for something I commonly ride for many miles WOT.
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Post by 2strokd on May 31, 2010 20:42:11 GMT -5
I admire your persistence with this, i woulda a long time ago! You da man
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Post by 90GTVert on May 31, 2010 21:31:44 GMT -5
I'm probably about done. I've heard (maybe in this thread? 34 pages is a lot to look through) Jeremy (using him as an example because he's about my weight) mention just bolting on 72cc kits and going 50MPH. That must mean I have no idea what I'm doing, so I'll just get it going 45 again and quit. lol 45 is still so much better than 35 at least.
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