sgtp
Junior Dawg
Posts: 14
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Post by sgtp on Sept 21, 2012 23:14:48 GMT -5
Hello all... here is the back story: I have a sachs sx1 with a 40qmb 2 stroke with 10,500km. It experienced a few soft-seizes then a hard seize eventually killed it. It was an interesting death. The piston seized and grenaded the wrist pin bearing, sending it and the circlips between the piston and bore, destroying the piston/bore and left a nice impression of the needle bearings on the wrist pin. So, I ordered a 70cc kit off ebay with a new head as mine was dinged up from metal bits bouncing around the cylinder. I decided to completely remove the engine and rebuild it inside the house as I dont have a garage. Everything went well ( followed this writeup 49ccscoot.proboards.com/index.cgi?action=display&board=yamajog&thread=556&page=1) and while braking down and cleaning the carb, I believe I found the culprit. The oil injection port seemed clogged and eventually I got a clump of dirt/crap to pass thru it while spraying carb cleaner thru it. During the rebuild, I converted my broken auto-choke to a manual one. Its worth mentioning, i guess, but I don't suspect its contributing to the starting problem. The scoot came with a 60 main jet, and I put a 85 main jet in. I put in a brand new spark plug. As I want to confirm the oil pump is working before I rely on it, I put some oil in the gas tank before trying to start it up.....My battery is nearly dead, so I am kick starting it, and of course, its not starting. I might not have the tool to know how much, but I know I have compression. I know I have fuel being delivered... pulling the plug shows fuel all over it. Spark? yeah, I don't know. Can you test for spark like this video by kick starting it? Remember, battery is too dead to turn the engine over. www.youtube.com/watch?v=LogSfY7NhkQMy other suspicion was that my spark plug cable is to blame as one end felt quite loose, so I cut 1 cm off each end and re-terminated the cable, no luck there. Could I just have way too much fuel with the 85 main jet? I had to disconnect the wire harness to completely remove the engine from the frame. There were 3 wires without keyed ends to know what wire connected to which, so i matched up the wire colors, but wonder if Chinese factories even follow that convention   I would like to believe the cdi and stator are not to blame (ie: scoot ran fine before the hard seize, why would they fail after a bbk upgrade?)...any ideas? ?? |
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Post by paemt1 on Sept 22, 2012 17:55:08 GMT -5
Checking the spark via the kick is going to be difficult but possible. The change in jets does seem to be a large jump. Did you change anything else like carb/air filter or exhaust? If not, that seems to be too big of a step.
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Post by dude on Sept 22, 2012 21:49:33 GMT -5
When it grenaded the metal parts may of went into the crank case, into the crank bearings, crank seals. The seals may be leaking. Stuff stuck in the reed cage.
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sgtp
Junior Dawg
Posts: 14
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Post by sgtp on Sept 22, 2012 21:58:43 GMT -5
Well, my instincts were correct because I had two cables connected to the wrong colored wires on the wiring harness...swapped them around and it fired up on the first kick, but any throttle would bog it down on with the 85 jet in there.
I put in a 80 main jet and it will idle just fine, rev up, but will stumble holding it at 1/4 throttle, and the spark plug is covered in fuel, so I guess I am way too rich still. I don't have any lower jets (why oh why did i buy a set of jets 80-110???) so would porting the stock air box be a good option? or should I get a 70 or 75 jet first? If anything, I am tempted to at least remove the snorkel from the air box and try that. Ive read others have done that before. Any suggestions? BTW, the stock jet was a 60
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Post by stiv625 on Sept 23, 2012 9:01:17 GMT -5
Well, my instincts were correct because I had two cables connected to the wrong colored wires on the wiring harness...swapped them around and it fired up on the first kick, but any throttle would bog it down on with the 85 jet in there. I put in a 80 main jet and it will idle just fine, rev up, but will stumble holding it at 1/4 throttle, and the spark plug is covered in fuel, so I guess I am way too rich still. I don't have any lower jets (why oh why did i buy a set of jets 80-110???) so would porting the stock air box be a good option? or should I get a 70 or 75 jet first? If anything, I am tempted to at least remove the snorkel from the air box and try that. Ive read others have done that before. Any suggestions? BTW, the stock jet was a 60 With a BBK I'd be willing to bet you could coax some more air thru the intake to keep up with an 80 main jet. I upjetted (my 4T) from 75 factory to an 85 with no mods other than intake. |
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Post by gregw on Sept 23, 2012 9:29:15 GMT -5
just because it stumbles when it's at a quarter throttle doesn't mean your main jet is too large. Does it run well at WOT? If the answer is yes, your main jet is fine. If it runs well at WOT and stumbles at a quarter throttle, it has nothing to do with your main jet. Try lowering your needle jet (raise the clip position on the needle). If it's running rich at an idle, and at a quarter throttle, you may need to lean out your idle jet (low speed jet) and lean out (lower) your needle jet.
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Post by gregw on Sept 23, 2012 9:34:10 GMT -5
When I upjetted to a 100 main, I had to buy a fatter needle jet because even when I lowered my stock needle jet all the way, it was still too rich. Whenever you change a main jet, it may require changing all other carberation adjustments too. Always adjust jetting from the top down (main jet, needle jet, idle mixture).
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sgtp
Junior Dawg
Posts: 14
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Post by sgtp on Sept 24, 2012 20:57:25 GMT -5
When it grenaded the metal parts may of went into the crank case, into the crank bearings, crank seals. The seals may be leaking. Stuff stuck in the reed cage. I realized this during the rebuild, but wasn't prepared to disassemble the bottom end, to be honest. I did fish out one spec of metal I found inside the case, but all I can do now is hope nothing else is in there, unfortunately. Good info, greg. The needle is still in the stock location (one notch up from the bottom, so in other words, relatively rich). With the snorkel removed, the 80 jet, and 2.25 turns out on the a/f screw, its running well thru the entire throttle range, but my novice plug reading stills thinks I am lean...ground strap is dry with a whitish flim on it (yet the edge next to the threads is wet). I'll need to lower the needle a notch to turn out the a/f screw more as it bogs at 1/4 throttle, with the a/f screw 2.5 turns out... What is the maximum, acceptable number of turn out on the a/f screw before you are better off putting a bigger jet in? |
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Post by gregw on Sept 25, 2012 19:34:29 GMT -5
Well, hopefully someone will chime in here that is more knowledgable, but I believe the a/f screw adjusts the air allowed in, and turning it out leans the idle mixture.
You mentioning running well thru the entire throttle range and reading the plug as whitish on the ground strap indicates to you, lean. Odds are, most of your riding is at WOT. If you think it might be lean, go richer (85mj). Get your main jet selected right running at WOT, then worry about the other adjustments in the carb...
I'd put your a/f screw back to one and a half turns out and leave it until you have your main jet selected correctly. Good luck!
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Post by dude on Sept 25, 2012 22:59:50 GMT -5
Well, hopefully someone will chime in here that is more knowledgable, but I believe the a/f screw adjusts the air allowed in, and turning it out leans the idle mixture.
I am Dyslexic I may not help but if your A/F is on the motor side lean is one way, if the A/F air box side its the oppisite. Just screw it until you reach the highest RPM (turn it a little then wait then a little more.)
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sgtp
Junior Dawg
Posts: 14
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Post by sgtp on Sept 29, 2012 21:54:19 GMT -5
Wow, seems my understanding of the A/F screw has been backwards all this time...  sh!t, thats embarrassing !  Anyway, I have the 85 mainjet in there, the needle lowered to the center notch, and the a/f screw 1.25 turns out. It runs ok, but starting when the engine is cold is a bit of a challenge as Im learning what works best with this new manual choke I rigged together. Here is the plug after about 50km of riding. I guess Im rich, right? Attachments:
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Post by skuttadawg on Sept 29, 2012 22:02:28 GMT -5
What size idle jet ? My 2T came with a 40 IJ and 68MJ . I replaced the 68 with a 75 and it ran great . I put on a small Uni which was a vent cover and not the proper airfilter with a 90 MJ and it was awesome . The 90 seemed too small with a BBK , proper Uni and unrestricted 70cc exhaust and a 110 was too big so I will test out a 95, 100 and 105 MJ . BTW I have a 19mm TK where many have a 17.5mm carb
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Post by gregw on Sept 30, 2012 7:34:23 GMT -5
The burn on the plug looks good to me (maybe a little rich, but better rich than lean). Was it a new plug? It looks like it has more than 50km on it.
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sgtp
Junior Dawg
Posts: 14
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Post by sgtp on Oct 16, 2012 2:10:43 GMT -5
OK, I have been using the scoot daily for my 10 minute commute and checking the plug often. I haven't done a proper plug chop yet (saving my new plug for that, haha), but for my varied riding with many hills etc, things are looking ok for tuning the main jet. I will still play close attention to this, but now my attention is turning to a problem that I had before the BBK...
The engine does not like cold starts. Before the BBK, I found I started needing to put my finger over the snorkel intake on the airbox to get it to start when cold. I simply concluded this was due to my auto-choke that crapped out (it stayed extended when not attached to 12v) and that I was too lean for the engine to fire up, and restricting the air going into the engine solved this on cold starts.
Before the rebuild I suspected I just had a bad vac leak causing it to be too lean, and making cold starts very difficult. I added a threaded pipe clam between the airbox and carb inlet, and used RTV everywhere between the reedblock to the airbox, but the problem persisted, so I just continued restricting the air intake to get it started when cold.
While doing the rebuild, I cleaned the idle jet and saw there was some tiny particle or fiber that partially blocked it. Doing this and converting the autochoke to a manual, I thought I solved this issue once and for all, but cold starts are still a challenge. Nothing catches using the starter motor, but I found kick starts will get it going, idling very low at first, and I will need to wait 2 minutes before its warmed up to the point that I can set off....or I can try to reach down to the air box and do the old trick of restricting the intake, but thats hard now that I an not running the snorkel anymore.
So, I know I am lean when cold...shall I try to find a bigger idle jet or is there something else I should be looking at? Sorry for the novel, lol
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Post by teddy554 on Oct 16, 2012 7:05:12 GMT -5
yeah think you are rich on your main if you dont have a pipe i would drop down to a 75 jet your idle jet should be fine never had to up jet the idle jet before but with the hard starting has to be a issue with your choke maybe its not set up right or maybe to rich it will start easier lean then rich that just my opinion
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sh!t, thats embarrassing ! 
