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Post by troy1896 on Jan 18, 2011 21:26:30 GMT -5
Hey guys....I have been lurking here for a little while and really like your site. I'm having a little problem though and need some advice. I have a '08 motomojo which is a re-branded vento zip. I just installed a Stage6 streetrace 70cc, pro replica pipe (derestricted), malossi multivar, 6g weights, white contra spring, red clutch springs, malossi E5 filter with the cover, and b9hs plug. I did a leak down test after the build and no leaks. I am fully pre-mixed at 40:1 with Opti-2 (not the best oil that I have but I was using it for break in). The problem that I am having is that it keeps soft seizing at around 45 mph and I'm losing my new motor. It has happened about 4 times now.  I have tried running with a 120MJ and the clip in the lowest position. Stock was around 70ish. It is so rich in the low rpms that fuel bubbles out of the filter and my gas mileage is horrid, but it still does the same thing. I am in Florida near sea level and the temp is about 60F, so-so on the humidity. The carb is the original 19mm Leader china carb with a 6mm dellorto jet. Question is am I delusional to try and tune this carb? The jets are also not original, but I have heard that as long as it is 6mm it is OK. True? The part that doesn't thread in is a little shorter than the original jet and maybe it runs out of float at higher speeds? Should I give up on the open filter? Maybe the oil is too viscous at 40:1 (and leaning me out) because they recommend about 80:1? I want to try and get the jetting as close as possible before I re-rebuild my engine and any advice is greatly appreciated. |
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Post by Enviromoto on Jan 18, 2011 21:29:34 GMT -5
Did you do a leak down test on the engine after you built it? Sounds like your sucking in air from somewhere causing a lean condition at high rpm.
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Post by troy1896 on Jan 18, 2011 21:34:02 GMT -5
Yes I did. I put in about 10 psi for 10 or so minutes and I was good. But I have not done one since sunday when I finished the build. Just to clarify though, should I have done it at TDC or BDC. Maybe I need to do it again.
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Post by shakerdriver on Jan 18, 2011 21:43:46 GMT -5
also sounds like the carb is jetted wrong thats causing a lean run. not to sure i'd even try to tune a stock china special imo.
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Post by troy1896 on Jan 18, 2011 21:51:32 GMT -5
That's kind of what I was thinking too, but I didn't want to replace a 19mm with another 19mm if I didn't have too. I have 20 6mm jets and a 19mm dellorto takes 5mm jets. I would have to get all new jets as well.
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Post by shakerdriver on Jan 18, 2011 22:34:05 GMT -5
yeah but the dello's a 1000% better carb and jets for it can be bought for like $20 for 5-7 of them.
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Post by shakerdriver on Jan 18, 2011 22:36:24 GMT -5
I do my leakdown test at bdc so i can make sure of the head seal also.
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Post by troy1896 on Jan 18, 2011 22:40:42 GMT -5
OK....cool....that is what I did otherwise it was hard to plug the exhaust with the piston in the way.
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Post by 90GTVert on Jan 18, 2011 22:58:12 GMT -5
If the squish is set tight you can have problems with heat building up from the elevated compression and eventually seizing. I've done that on a couple of builds. Just something else to consider.
:welcome: btw
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Post by troy1896 on Jan 18, 2011 23:05:00 GMT -5
Thanks for the welcome.....I didn't check the squish and I probably should have now that you mention it. I just used the supplied base gasket and head gasket O-ring and didn't think too much about it being a sport kit. I did tighten down the head bolts a little tight though at 15 ft-lbs. Maybe that pushed me into the danger zone?
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Post by stepthrutuner on Jan 18, 2011 23:43:01 GMT -5
The position of the piston doesn't really matter in a leak down test. Position it for your convenience. Air will eventually get past the ring(s) to pressurize the head seal. What is the position of your throttle when it seizes? Did you check the ring end gap(s) when you built it? Would be best to pull it back apart and see how it's seizing. I've been researching single mix ratio oil. Maybe you should back off the mix ratio to 55-60:1. According to what I read about Opti -2 70:1 is the recommended heaviest mix. Oils can have unintended consequences when mixed outside their intended range.
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Post by stepthrutuner on Jan 18, 2011 23:52:51 GMT -5
If you built the cylinder with the supplied gaskets the compression is probably ok even with extra head torque. Seems this is posted in the tech section but if you tear it back down you might find this useful. www.smellofdeath.com/lloydy/piston_diag_guide.htm |
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Post by troy1896 on Jan 19, 2011 6:43:44 GMT -5
The ring end gap was .010 at build time. The oil was originally mixed at 80:1 when I had the 1st seize at about 50mph, half throttle, in roughly a 1/4 mile run. I was doing a half throttle plug chop. When thinking about what could be wrong I realized that I just followed the line on the bottle for one gallon and should have measured it myself. That's when I upped the mix to 40:1 and upped the MJ from 100 to 120 same problem though. It seems like around 45-50mph is the no go zone. Now that I has seized up about 4 times the torque is pretty much gone and I have to lighten the rollers to get going. It will definitely be interesting what I find when I open it back up. Maybe I was too hard on it during break in. Maybe at 15 ft-lbs head bolt torque, the squish became too tight. I am going to spray some carb cleaner around it today to see if it developed a leak somewhere.
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Post by 190mech on Jan 21, 2011 19:45:18 GMT -5
Part throttle seems to be the place that kills them!Its a well known fact that piped motors have a lean 'sag' at part throttle,if its run there she has a meltdown!A thinner jet needle jet or larger needle jet should fix the problem after the teardown and repair inspection has been completed.Dont feel bad,I burned a hole thru the top of my piston while cruising at part throttle!!  |
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Post by troy1896 on Feb 28, 2011 23:49:54 GMT -5
So I just wanted to follow up on your advise and not let this thread die without any solutions. I went ahead and ordered a 19mm Dello and it got lost in the mail! It's been almost a month now and no new carb. I was waiting to post to let you know how it went, but I have been dealing with the post office, etc. Such an ordeal. Also this scoot is my daily driver (200miles/wk) and I had to keep running it.
So 1000 miles later, I got a little too excited on a downhill and ffwwooppp!, I finally buggered the rings or holed the piston. I have some time off from school to work on it so i will keep you guys informed of the progress and what I find inside the jug. I still have to make a determination whether to get a new cylinder, piston, and/or just rings. As well as how it performs with the new carb......assuming I get it this time.
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