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Post by stepthrutuner on May 15, 2016 21:29:39 GMT -5
Now that's a good head's up - blackening of the xfer ports. Would a more shallow taper on the diffuser section help this? Of course I understand that pipe design is dictated by piston face-to-mid baffle cone length and high rpm pipes would thereby limit diffuser length by that shorter distance.
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Post by niz76 on May 15, 2016 22:38:56 GMT -5
Keep at it!Part throttle is a 2T killer,,They love WFO,but part throttle allows exhaust gasses to wash back into the transfers before they flow causing mixture dilution and lean meltdown..Even the worlds top tuners have this problem,only cure is part throttle rich(stutter),proper head and pipe design..Indications of exhaust wash back are blackening of the transfer ports.. Awesome info as always John. Thank you! Ok I'll keep a lookout for this when I teardown and will post pics. |
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Post by niz76 on May 15, 2016 22:44:56 GMT -5
I've been eliminating issues one by one but still can't seem to get this thing tuned just right. Grrr- very frustrating! I'm wondering now if my modified stock head w/head gasket isn't a tad too small cc wise... Hmm. not many issues left I need to solve but she certainly isn't cutting me any slack! Man Niz! That's tough. I thought you had this all sorted out. I hope you find a blown head gasket but you may be on something with the head being shaved. That may be what happened to the original piston. Let us know what you find. Ya FA this thing sure is a challenge! I've changed my mindset a bit and don't mind keeping at it until I get it right. There's still a few more goodies on my list that I want such as a Kytronik Smart Booster which will let me choose a custom timing curve. I'll get it eventually! IDK what happened to the original piston as I modified/shaved this head myself for the Polini install. Oh that reminds me- I need to add the correct Polini head to my shopping list    |
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Post by 190mech on May 16, 2016 3:55:24 GMT -5
This is a quote from Jan Thiel about the RSA125 over at PitLane.biz; When EGT goes up and up there should be some serious problem with the engine. We never had pistons seize during our steady state tests. Working on the dyno continuously 5 days a week! I am 100% convinced our engine could have run for 6 hours at max power without seizing. The problems arise when you close the throttle, or run part throttle! The piston is mainly cooled by the transfer flow. And at part throttle there is less transfer flow, causing detonation (auto ignition) The entering fresh charge is ignited by the remaining, hot, burned gases! You can see very severe damage to the piston after maybe 10 seconds at 20% throttle. This still is an unresolved problem! I was thinking about a way to reduce engine power without closing the throttle. But how can you do this? I did not find a solution before I retired. And nobody else was really interested. At 100% throttle the engine was undestructible! By making the transfer ports as wide as possible we had very good piston cooling. www.pit-lane.biz/t3173p60-gp125-all-that-you-wanted-to-know-on-aprilia-rsa-125-and-more-by-mr-jan-thiel-and-mr-frits-overmars-part-2-locked |
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Post by niz76 on May 16, 2016 10:48:22 GMT -5
...so basically this scoot is now too fast! It only takes 1/2 to maybe 3/4 throttle to stay within the 55/60 mph speed limit. Hmmm...
Im hoping fattening up the needle and installing the Kytronik Smart Booster with adjustable timing curve is the way to go to solve this issue?
I honestly rarely see the need to drive WOT at 70mph...
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Post by 190mech on May 16, 2016 18:56:39 GMT -5
Just need to address your part throttle problems,,Did it really melt the piston?Stuff to check is head geometry,piston to cylinder clearance,ignition curve,needle jet/jet needle sizing(richer= thinner jet needle/bigger needle jet),tail pipe diameter or restrictions in pipe..CHT gauge would help too..
Youre not running an all out racer like Jan Thiel,I just wanted to show even the best of the best have the same problems!!
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Post by niz76 on May 16, 2016 19:04:42 GMT -5
I'm not quite sure what happened as I haven't opened it up yet. Hopefully I can get to it tonight. I do have a CHT and was quite surprised that happened at only 325 degrees... No ignition curve- just the crappy stock Vespa CDI where you set it (18 degrees BTDC) and it always fires there which is why I'm so interested in that Kytronik Smart Booster add-on.
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Post by niz76 on May 17, 2016 0:20:16 GMT -5
Italian wasp strikes again! Confirmed what I was hoping not to see; a holed piston  .  Sigh. Looks like about a little north of $250 will get me back up and running again. On the bright side, this is with the correct Polini head that should help tons in the "shouldn't have re-profiled my stock head by hand without ever really knowing the correct compression ratio" department. It'll also get me a big upgrade in the piston department which I keep reading over and over again is the weak link in this Polini 208 setup resulting in scores of stories just like mine online. I opted to go with the Grand-Sport piston (friggin expensive!) which is just as good as going "Polossi" (running a Malossi 210 piston in the Polini 208) or supposed to be actually better as it's custom fit for this cylinder already with huge transfers cut into it and extra support, protects against heat seizures- yada yada yada. Here's the lowdown on it;  plus a couple other little odds n ends I need...  I guess it's a rite-of-passage for my scoots to disintegrate a piston into the engine before deciding to be good little machines... Another piston for the "Wall of Shame" I suppose! So I checked out my helmet cam and sure enough I caught the failure on camera. Not much to see really, but you do get to see a bit of my walk of shame at the end hehe. |
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Post by spaz12 on May 17, 2016 0:52:34 GMT -5
Damn, sorry to hear that Niz. Looks like the new parts together should work out well though.
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Post by niz76 on May 17, 2016 1:02:40 GMT -5
Thanks Spaz. I'm thinking I better find another flip scoot soon if I want to keep up this destructive way of tuning |
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Post by stepthrutuner on May 17, 2016 4:49:38 GMT -5
That bigger boost port window will help cool the piston.+++ Most likely the holed piston had already developed a "mushy" spot at the center. I've never holed a piston but I've taken one out that had the concave "mushy" spot. The beauty of consecutive power strokes unfortunately is accompanied by ugly consequencies when the fuel, air, spark, compression, timing and right wrist support team suffer prep, monitoring and coaching deficiencies.  |
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Post by niz76 on May 19, 2016 18:33:02 GMT -5
Ya Reggie unfortunately this one suffered from my tuning deficiencies. I need to be much more careful with this thing. Still surprised it happened at only 325F but I need to take 190mech's advice and run the needle real rich. I think the new proper head will help a lot also as it has some extra cooling fins underneath and not to mention I modified the original head myself and have a feeling I didn't make the combustion chamber big enough. Luckily Brent has well documented the procedure for figuring out the correct ratios. Now I just need to apply the knowledge and do a lot less "winging it!"
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Post by niz76 on May 24, 2016 1:27:55 GMT -5
So the cases have been split and cleaned for a few days now just waiting on parts. All the bearings are clean and spinning freely, however a few tiny fragments of aluminum found their way to the rotary pad and gave it a couple tiny new scratches. I skimmed it with JB weld using a razor blade. Should seal up better than ever now...  |
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Post by drc174 on May 24, 2016 10:10:10 GMT -5
Just go reed valves and PHBH Carb.
Way more fun!
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Post by Florida Antique on May 24, 2016 10:17:24 GMT -5
Just go reed valves and PHBH Carb. Way more fun! He already has a Polini carb/intake on it now. I believe from the looks it is a Mikuni TM series flat side carb (or maybe a Keihin D slide). From what I understand, reeds are expensive to setup on a PX case. It's much easier to deal with them if you have a LML like your Stella, there the reed block is already setup so it's easier to mod. |
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