Backfires

I got a center mount, 66/80CC ( from BGF), slant head, RT high performance carburetor (from Dax), expansion pipe ( from SBP), single bottle boost.

Problem--won't start. It tries to start but heavily bogs down and then dies at low throttle and immediately backfires and dies at high throttle. I adjusted the carburetor leaner but it didn't solve the problem.

The bike was running good until the CDI wire that goes to the spark plug vibrated loose and started arcing across to the cap which melted the plastic on the spark plug cap and the thing started backfiring and then died. It hasn't started since. I replaced the spark plug and the CDI but no change.

Note: Before this happened I had a problem with a head/cylinder stud that broke. I put lock nuts on the head studs and when I tightened one of them down the stud broke (the threads were really poorly made). I replaced the stud with another one from an old engine and used lock-tight but the head gasket has trouble fitting over the studs, it just doesn't line up perfect anymore. It's very difficult to get a the head gasket on over the studs without bending it. I think I have to drill out one of the stud holes on the gasket just a little bit to get it to go on straight. Could my problem be an air leak from the head gasket? Would an air leak cause backfires? I could try to replace the RT carburetor with the stock NT carburetor, is that a good idea? Do I need a new engine?

Any help is much appreciated!

Kioshk said:

Are you sure it's an actual backfire (out of the muffler) and not a head-leak? I once had a similar condition, and it was due to the head-nuts needing tightening. From your description of the stud replacement, head-leak seems likely. The head-gasket should be free of stress and strain from the head-bolts. Try dremelling out its bolt holes to accommodate this. I suggest you also lap the head and cylinder. Get a pane of glass or some other good-n-flat hard surface, lay down some 60 grain (at least) sandpaper, and gently sand down their mating-surfaces in a circular motion. You don't need to sand off much, just get a decent, uniform clean flat surface on both. You should also make sure you replace the gasket in its original orientation to make sure its compressed surface fits the cylinder properly. Better yet, my GT5 kit came with an extra head-gasket; if you have an extra, use that one instead to assure a better seal.

Click to expand...

I think you may be right about this. I have had nothing but trouble with the head seal since I got this motor. After I got it I replaced the acorn nuts on the head with lock nuts but one of the stud's threads was really poorly made and would only allow the nut to be screwed partially down the threads before it would stop and wouldn't turn anymore. It was far enough down that I thought it would be tight enough but it clearly wasn't. I tried putting on another washer to make it tight enough and it seemed to run fine for a short time however, as I was breaking it in it started leaking air again. This time I took off the faulty stud and used a die/cast tool in an attempt to fix the threads, also I replaced the head gasket with a new one too. After this it was holding fine until the spark problem when the wire from the CDI dislodged itself slightly from the spark plug cap and started arcing. I didn't notice it until it backfired and died. The plastic part of the spark plug cap had melted all over the spark plug and onto the head. I replaced the spark plug and the black CDI box with all new parts but it did nothing to fix it. I decided to check the head nut again and when I tried to loosen it the stud just broke off. I removed the broken head stud and tried to replace it with one I had from an old motor but the threads into the block were a different pitch. Again I used the die/cast tool to fix the threads which were very close but just the wrong pitch. After that the stud screwed right into the block very nicely but now the head gasket doesn't quite line up well with the studs. I'm not quite sure why this is but it's almost impossible to get the head gasket to slide over the studs without bending it. I think I'm going to try to do as you suggest and dremell the head gasket stud holes so that it will slide on without any pressure on the studs and get a decent, uniform clean flat surface and see if that will fix it. I will also gap the spark plug to .025" (thanks Venice Motor Bikes!!). If that don't fix it, I will try going back to the NT carb or maybe try changing the jet in the RT. If that fails I might try to adjust the timing but I've never even attempted that before. If that fails, well, I just don't know what else to try. I guess I will have to get another engine but it's kinda frustrating because I have only gotten maybe 400 miles off this engine and it's still pretty new.

Anyhow, I guess I have some stuff to try, thanks for your help!!

Hello friends!!

I've been meaning to update this thread but I got frustrated with the bike and decided to work on another project for awhile (my RV ). Anyhow, I finally got back to the bike today so I opened up the magnet plate to take a look at the magnet and coil. What I found in there was shredded pieces of metal. What happened was that the nut on the drive bolt had vibrated loose causing the magnet to shake which started to hit the sides of the horseshoe shaped part that holds the coil. This caused the medal to just break apart. The horseshoe shaped piece is made of pieces of metal all stacked on top of each other. I'm not sure what you call that kind of construction but it is held to the engine case with 4 screws. All 4 screws had all come loose from the impact of the spinning magnet and the medal pieces breaking apart. The entire coil part was totally shredded into many pieces and was trashed. The magnet was slightly scraped on the sides but might be still good. However, lucky for me I had another magnet and coil from an old motor that has been sitting around since the needle bearing broke off. I decided to put the coil and magnet from the old motor on this motor. Also, lucky, the key for the magnet/drive bolt connection had shaken loose but was stuck to the magnet and wasn't damaged.

This is what I did:

(1) I had to use a dye/cast kit to fix the drive bolt threads that were too thrashed to screw the nut back on.

(2) I put the key in the slot on the drive bolt, put the split wire washer on on the drive bolt and then slid the magnet (from the other motor) on the bolt and screwed the nut back on.

(3) I soldered the wires to the coil (from the other engine) and tested with the Ohm meter. I cut the unsued white wire off. I screwed the 4 screws back on to attach the horseshoe shaped coil part to the engine case.

I also thought that it is possible that I had an air leak at the head so:

(4) I replaced the head gasket with a brand new one. I drilled one hole of it out a little bit with a drill to make it go over the studs easily without bending it all up.

(5) I replaced the lock nuts with regular nuts and put one flat washer and one lock washer on each head bolt.

Also:

(6) I replaced the CDI unit again with another new one which I had laying around just to make sure it didn't get fried while I have been trying to start it.

(7) I took the RT carb off to check it for any clogs or anything, I blew air through it and everything seemed fine. I put it back on the bike.

(8) I replaced the fuel filter with a new one.

I tried to start the bike and nothing has changed. When I pop the clutch it tries to start but only backfires once and stops. I'm clearly getting gas (spark plug is wet) and I'm getting a spark (it backfires).

Questions

(1) Did I do anything wrong replacing the coil and magnet?? Did I miss anything with this??

(2) Does the coil/magnet outside case have to be screwed back on to test it? I have left it off to test but I noticed that it has a gasket. Does the plate need to be sealed just to test it?

(3) What would you try next if you were me? What else could it be? I'm thinking of trying to replace the RT carb with the stock NT one. If for no other reason than to be able to rule out the carb as the problem but it is definately getting gas with the RT. I had it dialed in with the 65 jet and it was running great before this happened.

I'm thinking of buying another crappy time china girl engine, ugggg. I wouldn't mind spending more for a better built engine that might not fall apart but I haven't shopped them and don't know enough about what is really available and legal here in CA. Just to remind you, my setup is: 66/80cc 2 stroke slant head from BGF, center mount, direct chain drive, RT carb from Dax, expansion chamber pipe from SBP, single bottle booster, everything else is stock.

All advice is welcome and appreciated!!

Update:

I have good news. My engine is running again!! Sweet!! I thought that maybe I had put the magnet on upside down so I took off the nut to remove it and I noticed that it wasn't down very far. I also noticed that it slipped over the bolt very easily but I had to wedge it off with a screwdriver to get it off. I decided to use the key from the other engine. The 2 keys were slightly different in size. I had switched the magnets out but I used the same key that came with the newer engine. After I exchanged the keys I noticed that it was hard to put the magnet down tight. I had to tap it on with a hammer and a piece of wood until it went all the way down. I then put the nut back on and it started right up!! However, I think that the drive bolt is now slightly bent. It has a very slight wobble. It's not enough to cause the magnet to hit the coil part but it is something I must put on my pre-ride check list and keep an eye on.

Thanks to everyone for taking the time to respond to this thread. Lessons I have learned are: (1) make sure the magnet is down all the way on the drive bolt, it should be close to flush against the coil part. (2) Make sure the magnet is not on upside down, (3) Take off the magneto plate and tighten nut and screws regularly. I know I'm still a rookie but I'm learning.

Anyhow, I'm on the road again!!

kevyleven007 said:

good job!You kept at it and fixed it

Click to expand...

Well I kinda have to try to fix it myself. I live in CA and it is against the law to ship a 2 stroke engine into the state. It is not against the law to have one or to ship one within the state. There are very few places that will send one to me and those few places that will jack up the price. If I want to get a new engine I usually have it shipped to a friend in another state and have them ship it to me.

2door said:

You might have bent the crankshaft when you were prying on the rotor (magnet) to get it off. The crank is only about 5/16" diameter where the rotor sits and any heavy handed prying can distort the end of the crank, hence the wobble you see.
We always suggest using an appropriate puller when removing the rotor.
Also I concur with the earlier advice by Crassius that the seal behind the rotor could be damaged from the metal particles. That will create a vacuum (air) leak that will cause a high idle condition and poor performance generally.

Tom

Click to expand...

Thanks Tom:
Yes, it is possible that I slightly bent the crankshaft when I was prying the rotor off or maybe when I was taping the replacement rotor back on, I didn't have the correct puller for getting it off and I had to tap the replacement rotor pretty hard to get it to slide over the crankshaft. I did the best I could but I do have a very slight wobble now. Also, Crassisu was correct that the seal behind the rotor was damaged in fact, the rubber part of the seal was completely gone or maybe my 2 engines had slightly different types of seals but it was impossible to seal again. However, I had already completely disassembled and had split the 'parts engine' so it was very easy to remove the rubber seal from that and slide it over the crank and pop it in the hole. The rubber had a slot in it that made it fit snug on the engine case. I don't think I have a leak, it doesn't seem to be racing or having any idle problems. It idles at low RPMs nicely, kinda throaty sounding. I will keep an eye out for a leak there though, I haven't driven it around much since I got it started (the sun was going down) but it sure was nice to hear that thing fire up.