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Post by tansen on Nov 24, 2013 16:01:02 GMT -5
Hey Everyone, new member here.
I'm helping a friend with his CPI Oliver City 49cc 2 stroke. Neither of us is inexperienced with mechanic work but this one has us stumped. In utter frustration he finally dropped this bike off with me hoping I could figure it out.
So here's the rundown: it kickstarts quite easily and idles fine. If you pull the throttles it bogs and if you keep it pulled it dies. If you let go in time it returns to idling fine.
He says he's replaced the stator and cdi and bought a new carb for this particular bike. Symptoms all remain the same. So I assumed pickup coil should be good as it comes with the new stator, jetting should be fine since everything is stock and cdi is new. Of course the new cdi may be defective and who says the carb he ordered is the right one?
So here's my troubleshooting:
1. Compression is 120, cold engine, throttle pulled wide open. Thought that maybe insufficient compression would idle but not be enough so suck in more mixture to run at higher rpm. Thing is, what's the minimum compression to be able to rev it up?
2. Plug comes out black and sooty. Not shiny oily, just rich sooty very dark.
3. Reed valve looks fine, no light visible through petals and metal flaps right back in place when pulled away. I had thought this might be the problem given the plug reading.
4. Spraying carb cleaner around intake has no effect. Plus given the plug reading it's a rich issue anyway and vacuum leaks should show lean conditions.
5. Vacuum petcock line does not appear to be leaking, plus symptoms remain when running on 40:1 on an external tank with petcock vacuum line sealed as well as oil feed nozzle on carb body.
6. Pulled clutch housing and belt in case there was anything jamming up. Put cover back so I could kickstart since starter is no good. Symptoms remain the same. Rollers had a little flat spot but variator didn't look to bad. Plus the black plug tells me that it's an engine issue.
7. On to the carb. Cleaned it, of course. Passages are all clean and carb cleaner shoots right out with compressed air. Plus idle circuit seems fine given how it runs. Slider slips back and forth properly when pulling throttle cable. Needle height had no effect when going leaner and of course also when going richer. Still bogs and dies. Luckily main jet has a size on it: 47.5 !! Is this stock? I am at sea level in Hawaii. Sorry for the super long post. Any ideas from the 2 stroke experts?
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Post by 90GTVert on Nov 24, 2013 18:23:40 GMT -5
 If the old carb is around, see what jet was in it and swap if it's different. Not all stock carbs are jetted the same. The range for 2Ts is huge from around 50 to about 80 for the main jet stock, although most common is 70-80. Hard to believe it should be rich on a 47.5 jet. Is that a new plug that's coming out black? Does it improve if you remove the air box or partially cover the inlet of the airbox? |
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vaporz
Scoot Enthusiast
IF YA AINT FIRST..... YOUR LAST SUCKA!!!!!!!!
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Post by vaporz on Nov 24, 2013 21:44:09 GMT -5
+1 to GT's reply
Nice thorough analysis you wrote up.
This definitely sounds like a carb jetting problem. One thing your analysis left off was, type of carb, the size of the carb and jet setup pilot and main. Also if you are running an open air filter or stock box.
If your idle is spot on it's obviously not your pilot jet. Your main jet is either to big or to small. I am leaning to the BIG side of that.
Idle is great....hit gas and you flood the engine causing instant death.
Also 2 stroke jetting does not run in HALF SIZES to the best of my knowledge. 4 strokes do. You need to make sure you are using the proper jets. Since it possibly has a 4 stroke jet in there the carb maybe setup for a 4 stroke engine.
Lastly, 120 compression is ok. Not great. Not the worst. If it read 90psi I would tell you to inspect your cylinder, piston and ring(s). Something I also just learned was that, if your gauge on your compression tester is on a hose more than a few inches it will give you a lower than actual reading. An 18" hose can be off as far as 20-30psi. Does the bike start first or second kick? Than I wouldn't worry to much. Without proper compression the engine wouldn't start.
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Post by 90GTVert on Nov 25, 2013 7:39:13 GMT -5
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Post by tansen on Nov 25, 2013 13:53:20 GMT -5
Thanks to both you guys! Glad to be a part of this scene . I can clean the plug, then run the bike for a couple minutes and it comes out black. When revving, it does smoke a lot as it's bogging. Hard to argue that it's not rich. Oil supply is off the list as I ran it with an external tank. The choke didn't test well originally (passage didn't open up with battery voltage even after several minutes of battery power) so I swapped it with the one from the other newer looking carb. It bench tested fine but no improvement in running. My friend left 2 new carbs under the seat. The one in the bike must be the original, it's a Mikuni with an 18.8mm bore. Sadly the other main jets are not numbered but the orifices are both bigger than the 47.5. One by a bit, the other by even more. But like you said, smaller than 47.5 ?? Pilot mixture is at 1 1/4, starts fine like this. Seems to idle a bit better at 3/4 but on a plug chop it comes out too rich. Removing the air box doesn't help. Doesn't want to start at 1 1/2, or leaner. Is there anything other than jetting on a 2 stroke that would cause a rich bogging? Are size 45 main jets obtainable for these carbs? This carb is supposed to be for this bike. Man, what a head scratcher!!  Attachments:
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vaporz
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Post by vaporz on Nov 25, 2013 14:10:41 GMT -5
The numbers your putting out there all seem to be pilot jet sizes in my experience. Also I just learned that according to GT that half sizes are available for 2t carbs. Never knew that. 1st person and time I have heard anyone say that. Your black "sooty" plug is definitely a "rich" condition sign (excess carbon not fully burned up) but with no numbers on your jets you can and probably are chasing your tail with this. Is it a "stock" minarelli carb with the push/pull throttle assembly? Read my previous post for the answers to your compression question. I think this is a matter of A/F screw settings along with your jet selection. It sounds like your close I will add. Plug isn't dripping so It's not overly rich. It's not white which is overly lean which most likely would cause your engine rpms to run away on you. Have you done a leakdown test? Carb cleaner spraying around your main spots to check, intake manifold, reed block, case halves, cylinder base and head and crank oil seals will drop your rpms if there is a leak. But t I think a leakdown test before you waste anymore time figuring out jet sizes is needed.....
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vaporz
Scoot Enthusiast
IF YA AINT FIRST..... YOUR LAST SUCKA!!!!!!!!
Posts: 117
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Post by vaporz on Nov 25, 2013 14:48:03 GMT -5
In the pic of your carb you just posted what is the number on the big jet that is in the carb?  ?? Also....checking your plug from on the stand idling and giving it gas is not a proper plug chop nor does this reading tell you much as far as being an accurate depiction of what is going on. Your problem lies in pilot jet to needle to main transition....which from your description of excess smoking from the exhaust, black sooty plug and instant death when twisting the throtte all point to improper jet sizing. This doesn't mean that your pilot could not be to rich as well even though you say it idles great. Pilot can still be rich too.... |
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Post by tansen on Nov 25, 2013 14:56:41 GMT -5
Hey vaporz, sorry I was editing my post when your reply came in. Thanks again. So this is the carb in the bike and then the 2 extras:   The one in the bike is a Mikuni with CPI etched on it as well. The other two are Tungsheng and KeiKhink (funny Keihin knockoff??). The bore in both of the new carbs' main jets is bigger than the 47.5 one in the bike (easily seen by holding them up to a light source) but they're not numbered. So my friend says he tried using these 2 newer carbs with same results. Makes sense given the larger mains. I ran the bike with each of these 2 mains with the same outcome. I'm aware of non standardized numbering with jet manufacturers. Maybe this 47.5 is huge, but then why would these 2 other carbs come with even bigger mains? I'd love to lean out the main circuit but removing the air box isn't enough. Would leaks in the case cause this rich condition? I'll try spraying both ends of the crank while it's running and see what happens... |
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vaporz
Scoot Enthusiast
IF YA AINT FIRST..... YOUR LAST SUCKA!!!!!!!!
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Post by vaporz on Nov 25, 2013 15:07:21 GMT -5
I can 100 percent for sure say I have NEVER heard anyone use a number as low as 47.5 for a main!!! Most bikes like GT said (stock bikes) mains run from 70-80. Stock bikes usually come wit a 40 pilot if I remember correctly. Modded bikes use 86 mainsand up pending on the build.
Clone carbs have problems all in themselves. It's hit or miss if "clone" carbs will perform like there "brand name" brothers and sisters. Also carbs in general come with different setups for different applications. Why the jets are different all depends on who made it, where it was bought and the application listed for the carb on the site you bought it from.
Leakdown test/air leaks.....if your carb is super super rich and you have a leak it might be making you think you just have a slight over rich condition where in actuallity without the leak you WOULD be getting a dripping wet plug. Food for thought.....Leakdown tests are a necessity when diagnosing carb problems as you describe and cannot find a solution especially when someone like yourself knows what they are doing.
When it comes to mods I spend the $$$$ on tried and true top name parts so I can eliminate the possibility (99%) of an improperly functioning piece of gear.
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Post by tansen on Nov 25, 2013 15:13:10 GMT -5
Crap! Just read your last post right after posting mine. Again!! Ha ha. The pilot jet is a 15. I've tried bringing screw out to 1 1/2 while idling but the the bike still can't make it passed 1/4 throttle. I think if you believe there's no other explanation for the sooty plug than jetting, I can start by leaning out the needle all the way, starting the bike at 1 turn and then bringing it out lean as much as I can without stalling and then seeing if it makes it passed 1/4 throttle. All this without airbox, of course.
Thanks again!!
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vaporz
Scoot Enthusiast
IF YA AINT FIRST..... YOUR LAST SUCKA!!!!!!!!
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Post by vaporz on Nov 25, 2013 15:18:44 GMT -5
A 15 pilot LMAO!!!! Throw that thing out......
A) stop trying to tune your carb without the airbox on.......IT MUST BE ON!!!
B) Whoever gave you those carbs the jets are all twisted and were probably just put in there so they had jets in there.
C) Buy some Pilots 34-40
D) Buy some mains 74-80
Do not touch your needle. That is the last thing you adjust AFTER you determine proper pilot and main. The needle is all transition.....
Gotta go dude. I'll be back later.
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Post by tansen on Nov 25, 2013 15:21:28 GMT -5
I think you're on to something with that 47.5 main not belonging in there! Maybe it IS a 4 stroke jet...
I guess I should just order a few jets in the 70 to 80 range and see if when held up to a light their bore is actually smaller than this stupid 47.5:)
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Post by 90GTVert on Nov 25, 2013 15:24:38 GMT -5
The fact that all carbs do this as well as the 47.5 jet (which is tiny, unless maybe you're scooting atop Everest) makes me think you have something else going on. That appears to be a jet for that carb to me. To my knowledge there is no 2T or 4T jet. Orifice sizes may differ by manufacturer, but not by so much that a 47.5 would work like an 80 or whatever.
Is the exhaust clogged by any chance?
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Post by tansen on Nov 25, 2013 15:25:51 GMT -5
You got it!! Thanks again. This jetting is most likely all nutty. I'll get some pilots and mains and take it from there. Really appreciate your input. Here's a couple of plug pics. Never wet, really. Just black. Attachments:
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Post by tansen on Nov 25, 2013 15:27:29 GMT -5
Aaaaaaah! I have NOT checked the exhaust. Maybe just run it with the exhaust off for a few seconds?
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