Yamaha g9 backfire through carb

[Bryans200583]

Ok, so I got a cart and it would run when I got it but smoke like crazy. I took it apart and installed new rings, bearings, seals and of course gaskets. Now it won’t run. All it’ll do is backfire through the carb. Has factory airbox, I’ve double checked my valves and took it back apart to make sure I had my timing marks right. Anything else could it possibly be? I’ve also checked the spark and I got plenty of spark. I’m about ready to drive it into the river and say forget about it. Thanks for the help

[cgtech]

What's the compression at?

[slonomo]

What did you set the valve to? Did you set the intake valve lash when the exhaust was closed and vice versa? Or at TDC? I've never had luck setting valve lash at TDC, I prefer the other method.

If it's backfiring through the intake valves, the timing is not right. It means the intake valve is open during combustion. The intake valve should be fully closed and seated during the compression stroke, and remain closed until right after the exhaust valve opens.

So take the valve cover off and remove the spark plug. Insert a plastic drinking straw in the spark plug hole so it will move up and down with the piston, you can also use a pencil, just be careful nothing breaks off. Turn the engine by hand until you see the intake valve open. This is the beginning of the 4 strokes. Continue spinning the engine and watch the straw/piston move up and down. Watch the operation through all 4 strokes and make sure the intake valve opens, then closes, then the exhaust valve opens then closes.

It's either the intake valve lash is not right, or the timing is not right.

[Bryans200583]

I have between 135 to 140 psi compression. I know my timing is right I double checked it already to make sure. I set the valves to .004. I’ll do what you said with the straw and check that out. I’m taking me a couple days away from it cause I was Bout ready to drive it in the river and forget about it. Thanks for the help.

[Bryans200583]

So, I got my valve cover off and the spark plug out. If looks like the exhaust valve starts opening a split second before the intake valve is all the way closed. That would mean my timing off correct. I know I got my timing marks lined up. But, I guess I’ll take the cover back off and check it again.

[cgtech]

That's called "overlap", and that can't be changed by cam installation. That is just how the cam is made. Not your error for I/E overlap.

Not the best pic in the world, but it shows what your seeing. Totally normal & expected. Almost every cam ever made would be expected to have some "overlap". (I've never heard of one that didnt, but you know, "theres always a first"). The reason why overlap exists is to try to get the most "combustible air/fuel charge" into the cylinder as possible for the next ignition cycle as possible. It's always a war to "get used gasses out" & "get the next cubustable charge in" for a gas engine. There is also the concept of "scavenging" to consider, if the outflow can help "draw in" the next charge, that could be beneficial when tuned to the rpm range of the engine. Engineers arent blunt to these ideas, and look up "resonant tuning" if you feel interested in the concepts.

They didnt make that to look cool, it did what it was supposed to. Cam building is a science, so are intake manifolds. Most people dont even know that many vehicles effectively have "2 intake manifolds". One with "long runners" for low & mid torque. And a "flap" that changes it to "short runners" for top end.(not this Chrysler example, it just had "really long runners" that made "really good torque") single camshafts dont have that luxury, they are "just as their cut". A single cam can be "advanced" or "retarded" if your engine supports "variable valve timing", but the profile of the cam remains the same. It can be "shifted forward" or "backward", but its cut still remained unchanged.. I suppose that on those "variable valve timing" engines with multiple camshafts, the overlap can change, but that's not a concept implemented on that small of an engine yet (and I hope it isnt, only due to complexity & reliability, not against its merits as efficient advancements). I wish I could love the OHC Robin, but it wasnt a "itll run forever" motor. Adding VVT to that would be a disaster. These are small engines, and .....oh wait, nevermind, let's make them last no longer that the 4 year warranty, while adding more "whiz-bang".

[slonomo]

It's very possible you may have a damaged intake valve, valve seat, or both. No way to know until you take the head off. If you have checked timing and valve lash.....that's all that's left.

If it's a carburetor issue, it will pop a little bit from the intake, but not backfire. Is it possible it's not really a backfire? A lean running carb will "pop" and "spit and sputter". Could it be that?

To me a "backfire" sounds like a shotgun. That is actually the combustion/explosion in the cylinder escaping through an open valve. If it's an intake valve that is staying open, it will backfire through the carb. If it's an exhaust valve that's staying open, it will backfire through the exhaust. When I say "sounds like a shotgun", I'm not kidding. Does it sound like a shotgun?

[Bryans200583]

Thanks both of y’all for the info. It really don’t sound like a shotgun. The engine won’t even run it just turns over and pops out the carb and if I take the airbox off right afterwards there’s a lot of smoke. I have tried another carb and it does the same thing. I have another engine and I took the head off it and used it and it does the same. I had a club car do this to me once and all I done was clean up everything and then it ran great. I guess I’ll get new valves and try that. Thanks.

[slonomo]

A new carb? Meaning.....a new "Yamaha" brand carb?

If you are using an aftermarket carb at all.....throw it in the garbage and put the original "Mikuni" carb on there.

Also, while you had the head off, what did the valves look like? Did you check them to see if they leak? Set the head upside down on the benchtop, level it, then take some gasoline and fill up the combustion chamber. Wait a few minutes. Look to see if the gasoline is leaking past the valves by shining a flashlight in the ports. If you see gasoline leaking, you need a valvejob.

Also, did you check the fuel pump and make sure you're getting fuel to the carb? Take the hose off the carb and crank the engine. It should squirt out pretty good. If not, time for new hoses and a fuel pump.

While you're at it, did you pull off the fuel filter? Those have a cap at the bottom of the fuel tank that has a built in filter. Take the cap off and pull that old filter off and toss it in the garbage, make sure you get all the little bits of paper filter off the cap, then reinstall it. Get a 1/4" in line fuel filter from Autozone for $5 and install it in the fuel line someplace safe.

[Bryans200583]

The extra carburetor I have is a mikuni. I have two Yamaha g9s, trying to make one good one out of two. One needed a body and the rear end was messed up.
Ok, When I had the head off I did check the valves for leakage and they didn’t leak at all. I got everything set back and put it back together. I took the carburetor and cleaned it and all the jets and all with a torch top cleaner. It will turn over like it wants to start and will fire every once in a while but won’t run at all. I tried both exhaust I have just in case one was clogged. I took the engine back out also and rechecked the timing again and made sure everything looked good internally. I had another g9 that I got from my grandpa that was doing the same thing and it ended being the carburetor spacer was cracked and I didn’t realize it but that’s not the issue with this one.. I checked that. I’m getting fuel to the carburetor and I don’t what you said with the filter on the bottom of the tank before because I drained all the old gas out and put new gas in tank. I also had already replaced the fuel line and inline fuel filter. I have actually tried not heads and both sets of valves that I have.
Sorry for being so long. It’s just about drive me crazy. Thanks a lot again for your help. On more question I’m sure I’m right but the rotation of the engine is ccw looking at the clutch, right? I just wanna make sure I got my wires right on the starter. Just trying to cover all I can thing of.