I recently installed a 4 stroke engine on my bicycle. It ran at first. But then after I let it idle for like 15 minutes in an attempt to break it in it refused to start afterward. Except once.. I got it to start briefly but it ran rough and now no matter how I try it just refuses. The oil is new. The gas is good. It has compression. It has spark. The airways are clear and I've did many combinations of adjustment to the idle screw and whatnot... Is there anything I could be missing? I'm not pro at engines... This is a Chinese engine... Could that in of itself be the issue?

Originally the engine either wasn't running at high enough rpms to trigger the clutch or the clutch springs weren't adjusted properly so it never really functioned correctly in the first place.


What could I look for to possibly make it work right?

I'm using regular unleaded fuel from the pump. The cheaper stuff.

I think the best would be to let it diagnose someone who knows what these engines are about. Hard to do over such forum, but I will still try...

In order to run, it needs three things:
- Something to burn (fuel and air, in a correct ratio)
- something to ignite the fuel (so sufficient HV, good and clean spark plug)
- Do that at exactly the correct time (so the maximum pressure after combustion is reached just after top dead center)

Plus it should not have any excessive leak, so the pressurized gasses have no other way to go than to pressurize during compression and turn the crank after they were ignited.


Try to remove the plug, inspect it (if not carbonized, excessively wet,...), disable the ignition (by the kill switch,...), cover hole by a finger, set the carburetor to start (fill it, choke on) and try to crank (by the hand pull starter rope, never use electrical starter when poking so close around moving parts like the piston here; you need the cranking to be reasonably slow, so you would be able to follow the intake-compression-power-exhaust phases of the engine).
There should be slight suction during intake stroke (intake passage open by the intake valve, slight resistance by the carbueretter),
quite distinct blow from the cylinder during compression stroke (cylinder sealed by both valves, so the air has no way to go except past your finger, plus you should see the finger getting wet from the gas that should be contained in the air in the cylinder),
strong suction during power stroke
and no pressure during exhaust (the cylinder should be open to the outside by the exhaust valve)..
If you feel the strong suction for each rotation (so both for intake, as well as power stroke phase), you have blocked intake somewhere, or the intake valve is not opening The second may mean quite serious internal failure.
Same, if you feel the pressure buildup each revolution (so both during compression, as well as exhaust stroke), the exhaust passage may be blocked or the exhaust valve cam collapsed, so again likely beyond repair.
If no pressures/suction are build during supposed compression/power satrokes, the chamber sealing is extremely bad, either piston rings or valves jammed open. Need disassembly to fix it, there is a possibility for the engine to be really busted inside as well.
At the same time (mainly without the finger) the engine should turn really lightly, without any resistance at all Otherwise you have seizing problem, very likely accompanied by lackl of compression ability (not sealing).

Then put the plug back but do not connect it yet, and try to crank again. You should feel (or hear, once spinning faster) very significant compression resistance (every 2'nd crank rotation). If you have a compression tester, you may use that instead of the spark plug in this step and read the result. The compression should be in the 5:1..8:1 range (consult the tester gauge manual on gow to read it; generally it should develop at least 6 bar), but because it is just a single cylinder engine, the tester usage is limited (normally what is sought after are the differences between cylinders, not the absolute value itself)
Then try to block the intake by your finger and try to crank again. If there is strong vacuum buildup, the intake manifold is OK. If you feel no significant suction, there is likely a leak in the intake, very likely you did not have much or any gas on your finger in the first test.

If the ignition has contact points, check when they are opening the circuit. If it is contactless ignition (hard ferromagnetic, electronic,...), you have to determine the firing point by trying to rotate the crank and looking when it fires. It should be at or just a bit before compression TDC. There could be one extra spark at the exhaust TDC (the other revolution), this does not matter (it uses to be imlemented that way to have the ignition controlled in the simplest way directly by the crank, so it then fires each revolution, the spark at the end of exhaust has no effect at all).
Very frequent problem (mainly after disassembly, but it could be not paying enough attention during assembly) is the magneto rortor being misaligned with the crank (missing or snapped key, misaligned with designs not using the key there, the bolt/nut holding it ion the crank getting loose,...), resulting in bad phasing of the ignition vs the crank.

Most frequent problems (assume it is able to run for some while, but not have any power):
Leak in the intake manifold, letting in "false air", so no gas in the mixture entering cylinder. Often loose or broken bolts holding the manifold to the head, bad sealing gasket between the manifold and the head, sometimes crack in the manifold.
Clogged fuel passage in the carburetor (mainly after long time with no use), cracked priming pump push button bulb (then sucking air instead of building vacuum for the safety valve)
Bad spark timing
Worn out or dirty spark plug
Flooding engine (incorrect choke use, the regulator valve in the carb stuck open; but this usually yield only idle problems; once it idles at least somehow, it is usually able to gain full power anyway once the throttle is opened, even when this problem is persistent)
Too low ignition voltage or energy (bad contracts, bad arc quenching capacitor for contact point based ignition; insulation breakdown in the HV side)
Worn out piston rings, so piston looses seals; usually accompanied by excessive oil consumption.
The engine was attempted to run without oil (sometimes happen - some people just do not understand they should do at least some service to the machine like putting in the oil when taken brand new from the box, yielding severe damage, usually beyond repair on such cheap devices)

@Homebrewlamps, after following Medved's advice, please give us an update on how things go.
I had a feeling that at some point you would install an engine on your bicycle. Sounds like a potentially
interesting idea.
Also, is it truly 4 stroke? Or is it 2 stroke? Big differences between the two. lol
If you've done everything right, from there it could be errors from the manufacturing company in China.

@Homebrewlamps, after following Medved's advice, please give us an update on how things go.
I had a feeling that at some point you would install an engine on your bicycle. Sounds like a potentially
interesting idea.
Also, is it truly 4 stroke? Or is it 2 stroke? Big differences between the two. lol
If you've done everything right, from there it could be errors from the manufacturing company in China.

4 stroke bike kits aren't very common but they do exist. Project farm on YT has a really good review of a Chinese one.

Bike motors were available in the 40's and 50's seen ads for them in my old Popular Mechanics magazines from those times.Gas motor---Fuel,air,ignition.You miss any of those the motor won't run.

You need 5 things to make a 4 stroke work:

• Air
• Fuel
• Compression
• Timing
• Ignition

Check the simplest thing before you take it to bits. Check if there is fuel getting to the carby if there is fuel then check if the air box is blocked. If you have air and fuel then hand crank the engine (good to see if it seized) take the HT off the spark plug and check for spark if there is spark then remove spark plug and make sure it is good. Use a compression tester to see if there is good compression finally have a look at the timing setup, have a look at the cams, timing belt / chain and have a look at the valve setup to make sure the springs are ok and the valves. If it still isn't working have a look at the crankshaft, piston and piston rings and the cylinder wall. After all that it should work.

Thanks for all the info guys.

My friend said that the cam shaft is possibly slipped...

I'm using regular unleaded fuel from the pump. The cheaper stuff.

Most cheap gas is going to be ethanol blend gasoline, which isn't really good to use in small carbureted engine.  Most modern ones are rated for E10 (10% ethonol), which simply means that the fuel system components are resistant to the corrosive effects of the fuel.  It's fine in approved, frequently used applications, like a daily driven motorcycle, but will cause problems if allowed to sit for any time.  The ethanol will absorb water from the atmosphere, especially during times of high humidity.  The ethanol/water mix will than separate out from the gasoline and settle in the bottom of the tank, where the fuel pickup is, and water/ethanol does not make a very good fuel.  This doesn't just happen in the fuel tank.  Water/ethanol separating in the float bowl of the carburetor will gum up the small passages and cause it to not work right.  For a use like this, you will need to seek out and buy ethanol free straight gasoline.

Chinese engine could be an issue.  My nephew recently got a new generator from his father-in-law that didn't run.  He took it apart and found that debris was left in the carburetor when it was assembled, and the jet was plugged.  He cleaned it out, put it back together and it ran fine after that.

Ethanol can be bad for motorcycles too. 87 unleaded is also definitely bad for certain bikes. I accidentally put 87 unleaded
in my older bike back when I owned it. It was hesitant at idle but revved perfectly... but with tons and tons of smoke shooting
out from the exhaust with each rev. When I put 93 octane back in it, it ran fine after that, and now that's all I use for
my newer bike. Ethanol was causing problems with my new bike though, I think. I went to Shell to fill it up most of the time.
During that time, the exhaust valve kept opening and closing during idle, instantly activating the check engine light.
While I was at Shell, I read somewhere on the pump that they use 10% ethanol. Ever since I switched over and got gas at the Meijer
gas station instead, I haven't had any more exhaust valve problems, and the check engine light no longer activates.

But anyways, try the more expensive stuff for a brief moment @Homebrewlamps, with no ethanol. Maybe that might be one of the issues
here, but I'm sure it's not the only issue.

Dont blame ethanol for everything.
Actually the water problem is worse without the ethanol: The water will condense anyway, but the ethanol allows for at least limited amount to become diluted in the gas, so not affecting the engine. Without, it immediately pools.
To prevent water problems, you should NEVER store any equipment with partially filled fuel tank. Always either completely drain it and let it dry, or really top up the tank completely full. With partially filled tank the air vouid "breathe in" the humid air, the water condenses, sinks to the bottom, where the gas covering it prevents its further evaporation. So the water just accumulates there.
Completely dry thing does not cover the condensate, co it is free to evaporate back, so not much accumulation.
And completely full tank has no large air pocket to breathe in the humid air in the first place.

But what the ethanol does, it makes an electrolyte with the water, so allows corrosion (condensated water alone is not much conductive, so the corrosion is way slower). Plus some old rubber parts may get damaged by that. But any post 2000 machine should be already made resilient, mainly in the US market.

Ans by the way no ethanol, nor water could cause any gunk to form. If that happens, it means there is another contamination in the fuel, which is then the culprit. I woul guess the "bio-diesel" is here to blame (the vegetable oils do contain a lot of precursors for resins) - mainly cheap fuel carriers tend to use their tanker trucks alternatively on every fuel, then causing cross contamination.

I admit, @Medved, I'm very guilty of leaving my tank half empty on mine. (Currently half empty at the moment until I retrieve
the bike on Monday) That's a very good point. I wonder if this applies to cars and trucks as well? Because I'm even more guilty
of this with my car.

Well, if you use it daily, the water gets washed out (actually the ethanol is you best friend here - it helps to keep the water diluted), so can not accumulate.
But if it is some seasonal equipment (the bike, snow blower or lawn mower are), it is really important to keep the tanks either full or the fuel system empty and dry when storing them off season.
The best recommended practice is then when reviving at the start of the new season, it is recommended to drain the fuel from the tank to a container, run first tank on fresh fuel, then with next fills add the old fuel back to use it (or usually the second fill could be with the last years fuel, if it was not visibly dirty or spouiled).

With diesel the thing is more difficult (in countries where is mandatory to add bio component to all fuels). There the added "bio cpomonent" is ester of some fat (ethyl ester of rapeseed oil is the common thing here) and this tends to spoil. so these fuels have "storage life" (that means all time since manufactured till actual burning) 3 months. The thing is further complicated because with most system it is quite complex to really dry them out and then to revuive them up back from dry state at the start of the next season. So the best thing is to buy the clean diesel and the bio component separated (in many countries it is illegal to sell the fuel without the bio component, but laws allow to have them separated), add the bio component to the last tank load you are gonna to consume before storing the equipment (doubling the bio component is no problem, when it is not there for too long) and use the clean diesel fill for the storage.