1994 yamaha G9 backfire through carburetor

[mseg]

Hi, I have a cart a customer brought me that he bought at auction. The carb mounting block was broke so I replaced that and a new carb. It would start but backfires through carb at slight accel. and had no power after about 1/8-1/4 throttle. I diagnosed all I could think about, good compression, valve lash, remove fuel tank and cleaned out and replaced fuel lines and fuel pump. I figured a valve timing issue from worn cam so I sent the engine to Jakes Lift kits in PA. They rebuilt the engine, said they found a worn crank so replaced that also. Got the engine back and it is still doing the same thing. I called jakes and talked to tech support, told him after running or trying to run engine about 5-10 min. the plug was dry carbon black. He said to check what the fire looked like at the plug, I told him it was dim white in color and steady. I asked him if it could be the CDI / spark igniter (what ever it is called on these) or coil. He told me to replace the coil, I did. Still got the problem.

Any help would be such appreciated
Leo
My Small Engine Guy

[Spicyunicorn]

What's the air box look like it's very critical on these if they aren't sealed perfectly they will run like crap

[cgtech]

Also, a bad Ignitor can do this. (Black ignition box on side of gas tank). Just because it has spark, doesn't mean timing is correct. But, check the airbox for perfect seal, all 4 external hooks good, rubber "elbow" inside not folded over, lid not warped, rubber gasket around edge of lid good.

[mseg]

Most clips are broke off, small gap around top. So are you saying can't run with open carburetor

[Spicyunicorn]

No they don't like it I've used sip ties and gorilla tape on the ones that are warped bad

[YamiG2]

I have a 89-90 G2 and I removed the entire air box system and replaced with an open air filter that connects directly to the carb. Can also be removed and cleaned super easy and opens up the engine box for add'l air flow.

Yami Engine.jpg

[Spicyunicorn]

Originally Posted by YamiG2

I have a 89-90 G2 and I removed the entire air box system and replaced with an open air filter that connects directly to the carb. Can also be removed and cleaned super easy and opens up the engine box for add'l air flow.

Attachment 161364

I wouldn't recommend this at this point need to figure out why it's not running correctly before you add potential issues

[cfherrman]

Sounds like you have bad ignition timing. Get a test light, make a mark at tdc on clutch cover and bend a metal stuck to point at the mark.

If your timing light has advised degrees in it your can tell easy, if not every finger is about 2 degrees and you should see it advance before tdc and not after