Occasional backfire thru muffler

[HabHazard]

Hi all, got another prob I just cant seem to get a handle on. I have a 2002 Kymco cobra topbot 2t. Upgraded the jug and piston with the 70cc kit, new malossi carb, new coil, and a new stator just to be sure. Bike has like 29k on it. The problem is, now it will not start at all, it will occasionally backfire thru the muffler. Spark and fuel appear to be good. Ive chaged out pickups just in case but the prob is still the same, even with 2 different cdi's which is what i thought the orig. prob. was. any ideas would be greatly appreciated.

[zombie]

It may be flooding out, and igniting the spent fuel in the pipe. What jets did you use?

[Vivo]

Or ignition timing? or stuck choke?

[zombie]

Choke!

[HabHazard]

The main jet is a 100, not sure what the idle jet is, the new carb has a manual choke. The scoot itself was running fantastic. Idle was smooth as silk, acceleration was great, top end was about 50. I was riding one morning at wot when the bike just suddenly died on me. No pop or noise of any kind, just instantly dead. I haven't done a compression check yet, that's about the only thing I haven't done.

[HabHazard]

Think I found the problem! Don't know what the technical term is for the area between the piston rings, but long story short, theres a piece of it missing. So, looking at tearing the whole engine apart again to make sure the piece of missing piston is not down in the lower half. Found some rough spots on the inside of the jug, what appeared to be build up, not gouges. Took my dremmel and the wire brush attachment, lightly touched it and it was gone....., but no real damage to the jug, that i can see, thank god.... So, looks like I'll be tearing the whole thing apart to make sure there's no loose metal in the lower half. Ordered a new piston, will keep you guys updated on what happens.

[zombie]

You HAVE to chamfer all the port edges in the jug. Those edges are RAZOR sharp out of the box. My bet is the piston caught on one of them. Where is the bad spot? in line with a port?

[HabHazard]

Coincedently yes Zombie, the bad spot just happens to be inline with the exhaust port..... and yes i plan on doing exactly that before i put the new piston in...

[zombie]

Get some Muriatic acid (brick wash). USE GLOVES, AND DO THIS OUTSIDE!!! Pour some acid on a polyester rag, and sit it inside the jug where the injury happened. This will dissolve any aluminum left in the pores of the jug. You may not see it but it's there. Apply it 4-5 times. at about 45 seconds to one minute per time. It will not harm the jug if you rinse THOROUGHLY with water each dose. This is better than ball honing a new jug, and it will allow the rings to seat properly.

[HabHazard]

Well got the jug taken care of, got the new piston installed no prob. put back together and now i have no spark. Had spark last week, install piston, no spark this week. went over fuses, battery installed, switched out cdi's, plugs and coils. Have found some specs. on reading coils on the site, but not sure they apply to this scoot, good for ref. if nothing else though. Have 9 -10v at the coils. tried to read from the plug cap back to ground, its showing open, infin.

[zombie]

Yours is dc fired. There is no resistance from tip to ground. Only with some kill switches on.
Maybe re check the wiring. It will always do what you have. Fine one minute, and F'd the next.

[HabHazard]

By dc fired zombie, do you mean at the coil or the cdi? I ask because the cdi is ac, learned that the hard way. Also have a question about the pickup from the generator. If i suspect that a pickup is bad, can i cut a known good pickup from a stator and splice it in place of the suspected bad pickup without causing more problems? or will this affect the pickup to signal the cdi to release spark? Really appreciate all the input.

[zombie]

My Bad on the DC fired... You are correct. You can simply splice in a new pick up coil. My bet would be one of the bullet connectors near the battery hook up may be apart but still inside the plastic casing. Double check all the connections too.

[HabHazard]

Thanks Zombie, I will check all the connectors just to be sure, splicing the pickup in didn't work, so...onward n upward Thanks again

[HabHazard]

Ok Zombie, here's what i have so far.

Found that the cdi ground wire for the pickup from the coil had no ground.
Traced the wire itself back thru the harness to the kill switch. Pulled the kill switch and check the continuity thru the kill switch connector and it read open, even though the switch was in the on position.

After wasting some time searching for a needle to short the wires together b4 the switch to check continuity, I decided to just cut the wires about 2 inchs or so back from the switch, stripped them and twisted them together, and tested continuity, test was good.

Sooo, i connected the switch back to the harness and checked the ground for the pickup at the cdi, and i now have a ground at the cdi for the pickup, but still have no spark. Swapped out cdi, both are new, no spark, swapped out the coil, swapped out the stator, no spark. To be continued........