Removed Air Filter - Running Too Lean?

LWalker

New Member
Apr 19, 2020
5
0
1
21
Hey Everyone,

I am constantly tinkering with my first build - I can't seem to get it to run quite right. I am running a YD100 engine with an NT carb mounted to a Mongoose dirt bike. I've recently done some modifications but now my bike runs TERRIBLY. Can't even crack 5 mph on flat ground and the engine keeps spluttering and popping.

I previously got it working ok - my top speed was only around 20 mph and I had to peddle going uphill but other than that no issues. I wanted to give my bike a little more power so I changed the red plastic air filter for a "performance" filter like the one found at this link:
This actually made my bike have less power. At this point, I went back to the stock NT carb air filter but I didn't include the red cover. Wanting to give my back as much air as possible, I simply attached the spongey filter material using the supplied plastic bracket.

While I was working on my bike I made a few other changes. I had seen my rear sprocket wasn't aligned so I took the back tire off and realigned the sprocket. Additionally I was using an extra extra long offset intake but since I removed the bulky red plastic cover this wasn't necessary. I changed the intake to a standard offset intake. I had hoped this, combined with removing the red cover, would give my bike more power. However, as mentioned, it now runs terribly.

My theory is that somehow removing the red plastic cover has made my bike run way way too lean. Would adjusting the c clip in the carb to supply more fuel address the problem?

One other idea I have is this. I had to manually twist the little sprocket that connects to the clutch lever in order to remove the chain so I could realign my rear-wheel sprocket. This was EXTREMELY difficult. Could I have potentially damaged my clutch somehow? Would that cause a loss of power?
 

LWalker

New Member
Apr 19, 2020
5
0
1
21
I really need to get this bike working by Wednesday - I'm getting a privates pilots license and I have my first lesson at the local airport on Wednesday. No car so...
 

LWalker

New Member
Apr 19, 2020
5
0
1
21
I'm using 40:1 trufuel mix which was working really well. If its not the filter than its likely either an air leak from replacing the intake pipe or I somehow damaged the clutch when I was manually twisting the little sprocket to remove the chain?
 

carburetor al

New Member
Jun 14, 2020
7
1
3
72
I'm using 40:1 trufuel mix which was working really well. If its not the filter than its likely either an air leak from replacing the intake pipe or I somehow damaged the clutch when I was manually twisting the little sprocket to remove the chain?
I'm probably sticking my neck out, since I am far from being a motorized bicycle expert. Removing the air filter reduces resistance of the air entering the carb .Lengthening the intake leans the charge .So both contribute to a lean condition. Buy some jets. It sounds like this is a main jet situation from your posts. Increase 1 (one )jet size .R ide the bike for some time, but only if it runs better. If this is good, remove the jet and try the next size up. Now if it gets better, great .If worse, go back to previous jet. Of course. if this makes it worse, it is not a jetting problem. This may help.
 

mapbike

Active Member
Mar 14, 2010
5,398
11
38
Central Texas
LWalker


Something that stands out to me immediately is that you said at your bikes best, it would only do 20mph and you had to pedal it up hills.

I do not own or have any personal experience with the "YD100" engine, but when set up properly even the little 49cc china girl engines will pull a bike faster and up moderate hills better than you described the YD100 doing on your bike.

__________________________________________

What size rear sprocket are you running?

Have you torqued down head/jug bolts to make sure you aren't losing compression?

What is the idle speed like? Does it idle down very low if you back the idle screw out and respond the opposite by idling very high when you screw it in?

have you replaced the junk china spark plug that comes with the engine kit with a quality NGK plug?

I have found on my engines with NT carb. 2nd notch from the top works best on needle adjustment.

make sure carb is sealing off on intake tube, I put thin skim of RTV silicone all the way around intake tube, I slide carb onto tube with carb fuel bowl pointing out horizontal then after carb bottoms out on tube I twist carb into correct position and tighten it down, I've never had an issue with leaking in that area after using this simple technique I was told about by some old timers on this forum back when I joined in 2010.


__________________________________________
That air filter and cover are not gonna have any positive or negative affects on your engine performace.

I've watched LAHover YouTube channel which runs the YD100 on one of his bikes, he runs the NT speed carb. with red cover, his bike runs great and his performace is very good overall.

Its about correct carb. jetting, no air leaks, good compression, good spark, good fuel and correct gearing, good flowing exhaust and the china girl engines will do
25-30+mph just about every time on just about any bike in my experience.

The YD100 engine should do this easily from what I have seen on YouTube vids since it seems to give better torque than the typical 49cc or 66cc/ (80cc) happy time engines (HT)

The slowest and crappiest HT 66cc engine with zero mods I have ever had which I bought back in 2009 will do 28-30mph with a 41T rear sprocket all day long and is still running today, worst engine I ever had as far as bad vibration and lower MPH, but I have carb tuned perfectly for the little dinky loud banana pipe it has on it.

I said all of this to say, that in my opinion, you have one or more very simple to address issues that need to be fixed and your engine should easily pull you 30+mph and make you smile like a monkey with a new banana...LOL


double or triple check the questions I ask above and make sure all these things are top notch.
 
Last edited: