Newbie Rebuild

B

Bilbygilby

New Member
Hello! I'm new to all this, and I'm hoping you can help me out. Recently, I bought a mountain bike with a blown 80cc kit already on it. Apparently, a screw or something had gotten sucked into the combustion chamber, deeply scoring the cylinder and piston. I installed a new cylinder, piston, and rings. After some messing around, I did get it running. Currently, I'm running 16:1 fuel: oil, and an NGK B-6L gapped to about .025". It idles nicely, and runs smooth up to about 15mph. Then it starts 4 stroking the rest of the way up to 25. I have the carb needle clip on the top notch. I just hit 70 miles on it today. Will the four stroking go away once the new parts get broken in? also, the book says 16:1 for first 500 km, could the rich mix be causing the four stroking, and if so, is it safe to go to 24:1 for the break in period? Thanks for any advice!
 
Alright. I switched over to 24:1. Immediately it didn't want to run. Apparently the 16:1 had fouled the plug. ran, but hesitated. Pulled the plug boot of the plug and the brass insert that the plug tip clips into stayed on top of the plug. I pulled it off and screwed it back into the boot on the end of the wire. It ran better, but started 4 stroking around 15 mph. Did that til it hit top end, around 25 mph. Then it started surging, like it was losing and gaining power. Or like the spark was cutting out and coming back. I ordered a B-4L, I'm hoping that might help?
 
you need to adjust the main jet and needle for your elevation.
The books on these engines are off base on the fuel mix.
32 to1 is fine, I run Polan 2 stroke syn. oil at 40 to 1 and haven't had any problems.
 
Today I got a B-4L and put in it. Didn't help the four stroking, but stayed clean and didn't foul. Made a big difference. Allowed me to get ten minutes around 20-25mph, and got it warm. Then it did start smoothing out, only after it was super warm. The brass insert from inside the plug write boot came out again, it's stripped out and I think that may be a big part of my problem. The brass is covered in black stuff, like it's been arcing. I'll see if I can find a generic plug wire and see if that helps. I have a 41 tooth sprocket, what should my top speed be?
 
Today I got a new plug wire ($3.22 at my local lawnmower repair shop) and put it on. The old faulty one was apparently 95% of my problem. Immediately the surging stopped. When it's at WOT I have constant power. The four stroking gets better as the motor gets hotter. When it's fully warm, the four stroking stops except for one band from 18-22 mph. The plug insulator is now a nice rich golden brown color. Would it be best to wait out the break-in period before changing the main jet?
 
If you know the size of the main jet? You could then go one size smaller and maybe get the 4 strokes completely eliminated.

I have a sticky on how to take apart and put back together the stock spark plug boot.
 
alright, I see that the stock jet is .7mm- and they come in.02mm increments. How much smaller should I go? The tip of the insulator on my plug is a rich dark brown.
 
I use a set of micro drills solder the main jet shut and redrill it. I show that in one of the stickys. For a drill I use a thing called a pin vice you spin it with your fingers.
I got the micro drill bits at a gun show. they are very small drill bits.
http://motorbicycling.com/attachmen...le-carburetor-install-rebuild-carbpict005.jpg
this is on carb tear down and rebuild.sticky. the pin vice is laying on the drill bit set.
my guess is only one size smaller. 0.65 mm I think? I don't mess with them much anymore, I'm playing with other projects and not messing with the mb's.
set the needle at the second clip notch I think it from the bottom. hey look at the sticky's and read them I even cover the spark plug boot. and most of your questions about the bike engines. I tried to make them interesting to read.
 
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I checked out the sticky's, very helpful. But buying another jet was cheaper. I got a .68 mm, they say factory is .70 mm. It helped, runs real smooth up to 25mph, holds smooth for about 3 seconds, then starts cutting out. I have the needle clip on the top notch, where it runs best. It acts like at full speed, WOT, it gets starved of something. Plug is an NGK B-6L, gapped to .023". I added a fuel filter, maybe it's starving it of enough fuel to run smoothly? It has about 170 miles on it since the rebuild, maybe it still needs a little more breaking in?
 
I'd go with a 0.65 jet. that's why I solder and drill mine. no need to find a hard to find jet. With a set of micro drills a pin vice, and some solder I can make any size jet I need. Try that size jet and set the needle clip at second clip from bottom.
 
Thank you so much for all of your help. But I think the motor may now be beyond help. It's kind of a long story, but I'm gonna try to not leave out any details which may be relevant. Unable to locate the correct size still bit, I decided to try lowering the float level to lean it out. I lowered out about 4mm, 1mm at a time, and it ran better. Plug still brown and still four stroking so I figured I had not gone too lean. Ran a little rich for 100 miles or so, and when break in complete I switched from 32:1 to 40:1 fuel: oil mix. Then I found a cruiser frame and switched the kit over to that. Rode so much nicer. About 10 miles after that, it started making a smacking sound. I pulled the head off and found that something had gotten into the combustion chamber and had made about 10 sets of matching dings on the piston and head. Whatever it was feel out and got lost on my dirty floor. I searched the carb, head, and muffler and could find nothing missing. So, I thought while switching the motor over something could have gotten in then. I put it back together and it ran great for 10 miles. Then it started making that horrible smacking sound again, this time I was at about 25 mph, and whatever it was did alot more ramage. Big score inside the cylinder, and it's pretty deep. I found the pieces that did it this time, tiny little chunks of metal about 1-2mm across. About 3 of them. But I can't find where they came from.
 
My best guess is that they came from down inside the case somewhere. The large scratch passes right through one of the ports in the cylinder that lead down into the case. I have no idea how many miles were on the kit before I got it. I guess it was alot.
 
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