G9 airflow problem

[dtjim]

Hi I have a G9 I just bought with a weird issue. I'm an auto mechanic but not much experience with these things and its driving me crazy. I've read on here many times that these wont run well without a tight sealed airbox but the issue I'm having seems backwards to everything I know about engines. It had a bad rod bearing when I bought it fixed that and inspected everything else.

When I went to start it it wouldnt run right unless the choke was on. Then I noticed that with the airbox disconnected from the carb it ran pretty good. If I put the airbox hose/snorkel on the carburetor with the other end disconnected from the air box (just the hose on the carb) the engine dies unless I choke it. I also get a popping from the exhaust now and then.

Running better with hose attached and choke tells me the engine is running lean and getting too much air but with an open carb you would think it is getting more air than with a hose attached I feel like I'm in an upside down universe. It seems to me that attaching that hose to the carb would slow down the volume of air coming through or am I wrong does that hose being on create a venturi/velocity like effect and increase the air volume?

Heres some history and data
Bought with a knock in lower end. Put in new rod bearing crank seals and gaskets.

Cylinder and piston within spec. Rings within spec. Didnt touch the valves doing this on a budget and didnt want to wait for parts.

Compression 160
Valves properly adjusted
Went through carb it was already very clean when I took it apart
Hooked up a temporary gravity fed fuel supply to eliminate the pump
Replaced carb spacer
Checked for air leaks with propane and carb cleaner
New spark plug
Removed valve cover and double checked valve clearance
Verified choke plate is not loose or flopping around

[slonomo]

First thing that comes to mind is a plugged passage in the carb. When you say you "went through carb", did you just spray it out with carb cleaner? If I think the carb is suspect, I take the carb fully apart and soak the parts in a bucket of hot water with Simple Green. It's always worked for me. I also take torch tip cleaners and poke them into all the passageways, particularly the pilot jet.

If it runs with the choke on, yes it's running lean. If your carb is plugged somewhere it will limit the amount of fuel in the air. Less fuel means lean condition. Golf carts are more suspect to this problem because they tend to sit a lot and the fuel gets old quite often. Cars on the other hand usually get used several times per week if not daily so the fuel doesn't have time to form varnish.

You could get a can of "seafoam" and run it in the fuel and the oil per the directions. In the fuel it will work its way into the carb passages and help clean them out. In the oil it will help clean off the sticky varnish on the critical engine parts as well.

[dtjim]

Thanks for the reply Ya when I say went through the carb I meant thoroughly cleaned all passages with torch cleaners and bottle brushes and compressed air check float level etc but the carb was also extremely clean when I took it apart so didnt take much.

I dont understand it but I siliconed everything on the air box sealed it up nice and tight and now it runs great. Apparently having the airbox hooked up allows more air into the engine/lets it breathe better than having an open carb. This is really strange.

I've been a mechanic for 25 years and my dad for probably 50 we were both baffled by this. I've seen where an engine might run a little better with an air filter hooked up but have never seen an engine that didnt get enough air without the filter to the point it had to be choked to run at all. Either that airbox lets the engine breathe better or maybe without it the air moves too fast through the carb and the air isn't getting pulled through the jets properly. I dont have a ton of experience with small engines but I do have a good bit and this is weird. I really think there should be a sticky explaining that these engines must have a tightly sealed box and what happens if they dont. Through searches on here I have seen people with the same headache but none have them had followed up to say what cured the problem

[slonomo]

Having the stock air filter setup connected and sealed properly allows the carb to get the correct a/f ratio. If you allow more air in, it throws the mix off. Most bikes, atv's, and utv's are the same. Guys like to get a new ride and take the filter lid off without rejetting, same issue, it will run like crap. Tiny changes in fuel and air will throw these little carbs off. In a 350 Chevy, it's a little different story, most of them are over fueled lol.

Sounds like you got it fixed.