The infamous Popping.

[Builds4Christ]

Hey all, I have a spider-box with a QMJ157, 150cc.

I'm getting popping on deceleration. Checked the spark plug and it looks to be burning a bit lean.

I checked to see if there were any vacuum leaks anywhere, spraying carb cleaner here and there at seams, but was unable to find anything.

The carb is brand new, and it's gravity fed. Was thinking rejetting the stock carb.

What do you guys think?

 

[Master Hack]

. Checked the spark plug and it looks to be burning a bit lean

Jetting

 

[Builds4Christ]

I was thinking the same thing. I wouldn't mind upgrading the carb at some point too, but am having trouble with picking the right one to fit.

 

[Master Hack]

wouldn't mind upgrading the carb at some point too, but am having trouble with picking the right one to fit.

I'm not qualified to speak on that subject. Everyone has a fav. depends on what works for ya. I like Mikuni so due to my bias I am disqualified from Carburetor conversations. Our helpful, friendly panel of experts will be available to add more confusion...ERRRAHHH I mean help you wade through all the misinformation available on the net, if you only axe....

 

[bob58o]

What does the exhaust / muffler look like?
Air filter?

Many times decel popping is caused by too lean of a pilot jet. It doesn’t really bother me too much.

I jet the pilot circuit how I think it performs best, sometimes it pops. Especially with open exhaust.

 

[bob58o]

I think this video does a good job at explaining idle circuit tuning

 

[panchothedog]

Are you running a straight pipe, or a muffler? A straight pipe will cause that much more than a muffled exhaust.

 

[Builds4Christ]

I'm running a muffler, but I read that the stock ring type gasket is prone to leaking. I'm going to replace it tonight with a different type.

I'll take a look at the carb adjustment video too! Thanks all!

 

[Builds4Christ]

Welp the exhaust had come loose. Put in a new gasket. Torqued. Ran a bit, retorqued. Still popping.

I bought a jet kit and it came in, but all the jets are incorrect. I'm having a hard time finding the jets for this carb. Its the stock CVK type, I believe. Got it off amazon for a small price.

Do you guys know where I could get jets? Or should I just get it over with and get another carb? If the latter, which one is the best bang for the buck and do any of them have autochokes?

 

[bob58o]

This set goes from 0.014” to 0.039”. I believe it should cover your pilot jet needs. Find which one fits, then drill out to next larger size. Then test again and repeat if needed.

https://a.co/d/5SDwBkP

 

[Builds4Christ]

Is the pilot jet the main jet? I believe that's where I need more fuel. I actually have one of those bit sets. I took the car bout and went up on the main jet about .0015. Didn't seem to help. Maybe I'll go another size up.

 

[bob58o]

Is the pilot jet the main jet? I believe that's where I need more fuel. I actually have one of those bit sets. I took the car bout and went up on the main jet about .0015. Didn't seem to help. Maybe I'll go another size up.

Pilot jet is not the main jet.
Pilot jet is sometimes called the idle jet.

See my first post.
Popping on deceleration is caused by an “after-burn” in the exhaust not to be confused with a backfire in the intake.

When letting off the throttle, the butterfly valve closes. The engine is turning higher than idle rpm speeds, but pulling fuel from pilot circuit. The mixture is too lean to burn properly in the combustion chamber. The mixture exits into the exhaust and mixes with air and sometimes ignites and goes pop.

If fattening up the pilot jet doesn’t help, maybe it’s possibly that the loose exhaust has damaged the exhaust valve or spring.

 

[Builds4Christ]

Oh wow. The exhaust was loose when I first got the cart too. I really hope nothing is damaged! It otherwise runs ok, but who knows?

Alright so go one size up on the pilot valve. I'll try it and see what happens.

I appreciate all of y'all chiming in!

 

[Builds4Christ]

Well dang, I went up .002 on the pilot jet and it still pops on decel. Seemingly no change. I'm not certain where to go from here.

I'm not sure how big I can go on the jet before its beyond saving, if you know what I mean.

I really appreciate you guys chiming in and bearing with me.

 

[Whitetrashrocker]

Learn how to read the spark plug.

 

[bob58o]

If performance is good otherwise, I’d say don’t worry about it. But because there is still stock exhaust with muffler, I wouldn’t expect popping.
I’ve never run a GY6 type engine so I’m not too familiar with them.

I’d suggest cleaning the carb, but you said it was brand new and it has a new gasket.

If it seems low on power, I might suggest checking for a leaky exhaust valve… But just spit balling here. I’ve filled a cylinder head’s combustion chamber with a fluid and blew compressed air from the port. If it bubbles, lap the valves with grinding compound. Again, just guessing now. Maybe a loose exhaust can mess up the valve / seat seal? How does one burn a valve? I don’t know. I know I’ve heard of burnt valves. A bad valve seal might cause popping.

 

[bob58o]

Maybe set valve lash.

 

[Builds4Christ]

Sorry for the late response everyone, I've been indisposed for past few days with other kid stuff haha.

I set the lash recently actually. I wasn't sure when the last time was so I went ahead and did it.

Here are some pictures of the carb.

I THINK the performance is good. I'm not certain what this engine is supposed to feel like when its new and tuned properly, but it otherwise feels pretty good. It moves, has good throttle response.

 

[bob58o]

You can also check for intake leaks behind the carb. Get it running and spray starter fluid around the seals / gaskets / spigots … listen for rpm change. If the rpm’s go up or down, there’s a leak.

what’s going on here in the photo?
Like I said I’m not familiar with these engines and the photo doesn’t show it great.

Did somebody nibble on the intake?
Is there a visible gap?

 

[Builds4Christ]

I'm not sure, but I'll check whenni get home
Checking for leaks was the first thing I did actually. With a can of brake cleaner. There was no change in rpm anywhere I sprayed behind the carb. Seemed air tight.