# Small engine runs only in full throttle



## ukrkoz (Dec 31, 2010)

I have one of those small China made motorcycles with gas engine. Don't ask, it's sold for $ 400 something in many stores. I got it for free, neighbor set two of them outside his property for grab.
Surprisingly, it worked for few rides. 
Few days ago, my son ran it and it quit. Just stopped running
I took carb out, it's perfectly clean. Entire thing looks like it was barely used, to be honest.
Anyhow. Took carb out, sprayed it with cleaner, removed bowl, cleaned all that. Removed and cleaned exhaust muffler. That was full of soot.
Got it running, but it runs ONLY in full throttle. The very moment I let go of the throttle, it stops. 
Gas tank cap loose or tight. Air filter on or off. Let go of throttle, it quits.
I am presuming, it is running super rich and just kills the spark, as plug is indeed fouled with gas.
Now, it's a free toy and I have little intention to start tossing money at it.
Any ideas for some quick and dirty fixes? Before it goes outside MY property this time? 
I even filled part tank with Seafoam and it did improve how it runs. But. No idle, let go of throttle - done.


----------



## User02 (Sep 17, 2015)

When an engine won't idle, its usually because the idle port in the carb is plugged.
If you have fuel and spark, then the only thing left is air.. If the spark plug is wet, then its probably either spark or air.. and since it runs at full throttle and starts, it sounds like spark is fine, so that leaves air flow.

Check the idle adjustment to make sure the throttle plate isn't completely closing when you let go of the throttle.. Does it require starting fluid to get it to run? If it does, that could be a weak spark, but if it starts with a pull on its own, then spark is fine.. which takes us back to the air flow.

If it has a vacuum operated fuel pump, it could be either a weak fuel pump or a leak in the vacuum line.. both could create a problem running at low speed.

One other bit.. I bought a mantis tiller years ago and it had a California CARB compliant carb.. the thing wouldn't run on regular gas, it demanded ethanol free gas.. Strangest thing.. it did the same, wouldn't idle at low RPM.. switched to the non-ethanol fuel and the problem went away on the first tank of gas. I still can't figure that one out.


----------



## ukrkoz (Dec 31, 2010)

It's real basic basic engine. No pumps etc.
I'll check throttle plate. I should also check, if it has idle screw. That would have been the easiest, just to set it to higher rpm by keeping throttle plate slightly open. Not sure, but I believe, SP has a hole drilled in it... I'll check tomorrow, it's not hard to get to.
I am in agreement that it either floods badly or does not somehow has enough air going to make proper mix. 
No, it does not need starter fluid. Starts on about 2nd pull. But I have to throttle it right away, what is a bit awkward to do. Plus, it then wants to run away from you, as clutch engages.


----------



## quatsch (Feb 4, 2021)

Some carbs have a low speed adj & a hi speed adj. 
Draw their present positions B4 tweaking them, maybe a half turn in both directions.


----------



## Steve2444 (Sep 28, 2020)

I am not familiar with those cycles,
running rich... My guesses in order of possibilities based on your info.

1. Carb float leaky
2. Needle and seat bad/dirty
3. Carb jets fouled
4. Needle/seat/float getting hung up.
5. Fouled plug

Could you post the brand and model # info
Or carb brand and model #









How To Fix A Motorcycle Running Rich: 6 Easy Ways | Motorcycle Habit


If your motorcycle is running rich, you’ll usually be able to smell a difference in the exhaust if you pay attention. Don’t stick your face…




motorcyclehabit.com


----------



## ukrkoz (Dec 31, 2010)

Got it


----------



## Fix'n it (Mar 12, 2012)

those little carbs have tiny little ports in them, basicly, your not getting them clean. buy a new one on ebay/amazon/whatever, $20 or so.


----------



## pumpkin11 (Oct 31, 2020)

Fix'n it said:


> those little carbs have tiny little ports in them, basicly, your not getting them clean. buy a new one on ebay/amazon/whatever, $20 or so.


Agreed, buy a new carb, they are cheap, got one for my snowblower on Amazon for $15 works perfectly


----------



## SW Dweller (Jan 6, 2021)

Do the research on the China motor. I probably had the same bike. Found that the China motors were an exact clone of Honda. All the Honda parts worked. I put a new carb and exhaust and got added performance. Then I replaced the clutch with a special one that locked up and the little machine became monster fun. I ran the snot out of it for two summers and then sold it to a guy that needed on one more for his family. 
Change the oil an pay attention to the tires they wear fast, or did under my 250 pounds.


----------



## ukrkoz (Dec 31, 2010)

So it's MonsterMoto80 with 3hp 80cc engine.
I have carb ordered, from manufacturer, it's $5 less than Amazon, actually. 





Monster Moto / Mega Moto Classic 80cc (MM-B80) Mini Bike Parts - Monster Moto / Mega Moto Models - Monster Moto / Mega Moto Mini Bike Parts - Vehicle Brands - Monster Scooter Parts







www.monsterscooterparts.com




SW, what was the clutch you ordered? Bike I have won't even pull me up the hill. About 15% grade.


----------



## ukrkoz (Dec 31, 2010)

SW Dweller said:


> Do the research on the China motor. I probably had the same bike. Found that the China motors were an exact clone of Honda. All the Honda parts worked. I put a new carb and exhaust and got added performance. Then I replaced the clutch with a special one that locked up and the little machine became monster fun. I ran the snot out of it for two summers and then sold it to a guy that needed on one more for his family.
> Change the oil an pay attention to the tires they wear fast, or did under my 250 pounds.


What exactly was that clutch? 
Yes, I know, those Chinese engines are pretty much made at the same factory Honda engines are.


----------



## ukrkoz (Dec 31, 2010)

OK, so I have a mystery here.
I received new carb. Just like one posted above.
Thing is, throttle shaft does not have return spring on it. As the result, when installed, throttle plate is always in full open position and does not return to idle position, when throttle is released.
It's a simple coil spring wrapped around the throttle shaft lever base on the existing carb. Unfortunately, it's lever end appears to be permenantly installed into the plastic, as I did think about transferring it onto the new carb. Otherwise, carbs look identical.
I just spent some time online, looking at those carbs and NONE of them has that spring on it. 
So what am I missing? It is a 19mm intake carb for 80cc Monster, direct order from manufacturer.
How do I transfer that spring then? Or else?


----------



## Mike Milam (Mar 3, 2017)

ukrkoz said:


> OK, so I have a mystery here.
> I received new carb. Just like one posted above.
> Thing is, throttle shaft does not have return spring on it. As the result, when installed, throttle plate is always in full open position and does not return to idle position, when throttle is released.
> It's a simple coil spring wrapped around the throttle shaft lever base on the existing carb. Unfortunately, it's lever end appears to be permenantly installed into the plastic, as I did think about transferring it onto the new carb. Otherwise, carbs look identical.
> ...


How about posting a picture of the old carb and the new one. New one can be from the website you ordered from.


----------



## Steve2444 (Sep 28, 2020)

They sell throttle springs.








Throttle Return Spring for Monster Moto / Mega Moto MM-B80 80cc & MM-B105 105cc Mini Bikes


If the throttle return spring on your Mega Moto breaks on you, things are going to get dangerous fast. Or very exciting depending on how you look at it. Either way, it must be fixed no matter how big of a thrill seeker you are. This throttle return sp




www.monsterscooterparts.com





Is the problem with your existing throttle cable or the spring?
Confused...


----------



## ukrkoz (Dec 31, 2010)

Here's the pics.


----------



## ukrkoz (Dec 31, 2010)

To add insult to injury, they charge $12 for return label. What is almost half of the carb cost. 
Basically, I need a return spring that will pull the throttle lever back into idle position. That lever is long silver "stick" attached to the throttle on carb with rod/spring.


----------



## ukrkoz (Dec 31, 2010)

I looked up Huayi 19mm bore carb and NONE of them has that spring on them



huayi 19mm bore carb - Google Search


----------



## Mike Milam (Mar 3, 2017)

Hillman 2-Pack 4-in Zinc-Plated Steel Window Screen Tension Springs in the Window Screen Tension Springs department at Lowes.com

Can easily be modified to fit.


----------



## ukrkoz (Dec 31, 2010)

Mike Milam said:


> Hillman 2-Pack 4-in Zinc-Plated Steel Window Screen Tension Springs in the Window Screen Tension Springs department at Lowes.com
> 
> Can easily be modified to fit.



No. That's not the one. One you posted is connected down to the throttle lever rod and throttle cable bracket down on the engine. It pulls throttle rod to open throttle plate on carburetor. No problem there.


----------



## ukrkoz (Dec 31, 2010)

Mike Milam said:


> Hillman 2-Pack 4-in Zinc-Plated Steel Window Screen Tension Springs in the Window Screen Tension Springs department at Lowes.com
> 
> Can easily be modified to fit.


Then to think about it... maybe ai can ADD that spring to the throttle rod so that it PULLS rod BACK to throttle idle position, when gas handle is released. Hook it up to frame somewhere.... Rig it, basically.


----------



## Mike Milam (Mar 3, 2017)

ukrkoz said:


> No. That's not the one. One you posted is connected down to the throttle lever rod and throttle cable bracket down on the engine. It pulls throttle rod to open throttle plate on carburetor. No problem there.


Oh, sorry. It's hard to see exactly what's going on from here.


----------



## Mike Milam (Mar 3, 2017)

ukrkoz said:


> Then to think about it... maybe ai can ADD that spring to the throttle rod so that it PULLS rod BACK to throttle idle position, when gas handle is released. Hook it up to frame somewhere.... Rig it, basically.


That's what I was thinking but couldn't say that for sure.


----------



## Steve2444 (Sep 28, 2020)

ukrkoz said:


> No. That's not the one. One you posted is connected down to the throttle lever rod and throttle cable bracket down on the engine. It pulls throttle rod to open throttle plate on carburetor. No problem there.


So this is the spring you are looking for?
If so, I have no idea, perhaps call the vendor?


----------



## ukrkoz (Dec 31, 2010)

You know what.. I'll hook up rubber bands to that l can see, how it will work. If it does, I'll get that spring from Lowe's. It's 3 miles away. HD does not have them.
Thanks, Mike.


----------



## ukrkoz (Dec 31, 2010)

Rigged it.
It had throttle cable coming down to L shaped bracket on the engine. Short bracket end was pulling on the carb throttle lever via a spring. I simply replaced that spring with metal wire connector. As L bracket has return spring on it, same connector now returns throttle plate to idle position. Now it both pulls and pushes long carb throttle lever per L bracket position.
Connector will hold for some time as is, I'll eventually will get something made out of steel for it.


----------



## Steve2444 (Sep 28, 2020)

ukrkoz said:


> Rigged it.
> It had throttle cable coming down to L shaped bracket on the engine. Short bracket end was pulling on the carb throttle lever via a spring. I simply replaced that spring with metal wire connector. As L bracket has return spring on it, same connector now returns throttle plate to idle position. Now it both pulls and pushes long carb throttle lever per L bracket position.
> Connector will hold for some time as is, I'll eventually will get something made out of steel for it.


 Cool  The question now is... does the bike function well given it was a freebee?


----------



## ukrkoz (Dec 31, 2010)

Still won't get me up the hill, but that me being 240lb and it rated for about a 100.
But ah tell yah what... as I had it worked on and run up on ramp, so that it freewheels... it clearly goes into MUCH higher RPMs than before. To be honest, I'll be afraid to let my 6 yo gson run it next year, when he grows enough to reach foot rests. I'll get my son on it. As it WAS, he had fun. I want to see, how he'll like it now.
Definitely works better.


----------



## Steve2444 (Sep 28, 2020)

ukrkoz said:


> Still won't get me up the hill, but that me being 240lb and it rated for about a 100.
> But ah tell yah what... as I had it worked on and run up on ramp, so that it freewheels... it clearly goes into MUCH higher RPMs than before. To be honest, I'll be afraid to let my 6 yo gson run it next year, when he grows enough to reach foot rests. I'll get my son on it. As it WAS, he had fun. I want to see, how he'll like it now.
> Definitely works better.


Great, Perhaps sell it as is, and use the profit for new one?


----------

