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hardleydavidson
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Hi,


I recently started riding and being very naive, bought myself a second hand chinese 50cc chopper style moped. Now a lot of people say very derogatory things about these chinese bikes, putting them down and saying how rubbish they are. Personally I think they are wrong, these things are far worse than anyone makes out.


With a little bit of work I managed to get it through it's MOT but since then the exhaust manifold bolt has fallen out, it won't go up hill at all, there's only about a quarter throttle, the ignition cuts out randomly and I've had to push it home twice. Not to mention the previous owner wiring the positive to the earth which produced some comical lighting issues. Last night it felt like I was pushing a squeeky wheelie bin full of tin cans. The problem I now have is that I'm not flush when it comes to cash so I'm stuck with it and I'm determined to get this thing up to scratch. I am toying with the idea of slowly replacing parts with more reliable ones from a different manufacturer.


When it does go, which is rare at the moment, I'm really enjoying riding.


I do have one question if anyone knows the answer, if there is little to no back pressure on a 4 stroke will that affect power in higher gears or up hill?

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Yep, you need a bit of back pressure or you will have a drop off in power.


To say all Chinese bikes are shit is a little short sighted, I spent a few months on Asia last year and there are loads of them - pretty much all kept going with back street garages with few quality tools!


Spend a bit of time going over your bike and I'm sure you'll be able to nurse it back to health. :-)


Cheers,

Fro

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I'm confused by the need for back pressure on a 4 stroke......until the need to quiet them down all racing 4 strokes ran straight through pipes but the impostant bit was for them to be smooth curved and the same length.


It was two strokes that had to have back pressure in order for the exchange porting to work...

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All I know is that since the bolt fell out the whole thing lurches and splutters along at walking speed.


I'm going to try and fit a new bolt in the exhaust manifold this evening. This should solve any back pressure issue if there is one. Next I'll take out the spark plug and check the compression as it's quite possible the piston rings or gaskets might be shot.


So far..........


Before the exhaust bolt incident it would start and run ok but very slowly. Pulls away fine in first and gets up to revs but 2nd is slower and third is a crawl.


Bike slows and dies on the slightest incline.


Only seem to have a quarter throttle and full throttle kills the engine.


There is a sweet spot on the throttle but any less or more and I lose power. Yet when stationary the throttle works.


Could be coincidence but the only time I lose ignition is during wet weather.


Once I've got these bits sorted and been round the whole bike tightening anything loose it should be ok.

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All I know is that since the bolt fell out the whole thing lurches and splutters along at walking speed.


I'm going to try and fit a new bolt in the exhaust manifold this evening. This should solve any back pressure issue if there is one. Next I'll take out the spark plug and check the compression as it's quite possible the piston rings or gaskets might be shot.


So far..........


Before the exhaust bolt incident it would start and run ok but very slowly. Pulls away fine in first and gets up to revs but 2nd is slower and third is a crawl.


Bike slows and dies on the slightest incline.


Only seem to have a quarter throttle and full throttle kills the engine.


There is a sweet spot on the throttle but any less or more and I lose power. Yet when stationary the throttle works.


Could be coincidence but the only time I lose ignition is during wet weather.


Once I've got these bits sorted and been round the whole bike tightening anything loose it should be ok.

 

Are you sure that your moped is a 4 stroke?....because most that I see aren't...just a question

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Lack of backpressure in a 4-stroke is normally compensated for by upping the main jets a bit......it'll run a bit lean if not......but as Derek says, on a 2-stroke it can make a huge difference.

If the exhaust is leaking around the manifold due to the missing bolt it could really upset the engine with the resulting lack of power......that you've noticed...... :wink:

Good luck with sorting it........and when you tighten stuff up on the bike, don't overtighten as a lot of these bikes use cheese instead of alloy and you'll strip threads out if you're not careful...... :wink:

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Lack of backpressure in a 4-stroke is normally compensated for by upping the main jets a bit......it'll run a bit lean if not......but as Derek says, on a 2-stroke it can make a huge difference.

If the exhaust is leaking around the manifold due to the missing bolt it could really upset the engine with the resulting lack of power......that you've noticed...... :wink:

Good luck with sorting it........and when you tighten stuff up on the bike, don't overtighten as a lot of these bikes use cheese instead of alloy and you'll strip threads out if you're not careful...... :wink:

 

good tip, thanks. Yeah it's definitely a four stroke.


On the subject of main jets, I heard that the pilot jet is for slow speed and is about a quarter of the throttle, if I'm not getting anything from the main jet then that would account for not having anything from about quarter throttle up. Plus it means I would be riding solely on the pilot jet. I did strip the carb when I got it as the bike had been sitting for months and the jets were gummed up. At the moment the main jet is screwed all the way in, do I adjust it but unscrewing it a bit? I've already shortened the pin by one notch??

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Well, overfuelling can be as bad as underfuelling......so make sure the choke isn't sticking on....... :wink:

If you've got the pin on the wrong slot on the throttle slider you can have all sorts of problems.......so you may need to try to find out which notch on the pin the circlip is meant to go.....otherwise you'll have to strip the carb and try different notches until you find the right one..... :shock:

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'Google' has a plethora of 'carb setting' info, including vids.


Your Carb should have an 'idle' screw (set to just under 1000 rpm) and a 'mixture' screw - set fully in and when running with a slightly open throttle, adjust for 'peak' revs. Then look for 'flat spots' as you open the throttle to full.


BUT... before setting up the carb, you need to ensure that your air filter is clean and servicable and your ignition timing correct (including a functional 'ignition advance').


Have you got the Workshop Manual or are you working from the generic Haynes manual?


:cheers:

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There's no rev counter so I don't know how many revs it's doing apart from listening to the engine noise.


Thanks for the mixture tip that will help no end as I don't really know what I'm doing with it. As for the manual, it's pretty much, "sit on bike, turn bike on, press button you have lots of fun riding, you not fall off"


I did manage to get hold of the honda c50 shop manual which is a help as the engine is practically the same.

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Is there any fuel in the tank? If so, wheel the bike onto some wasteland, empty tank contents onto bike, retire to a safe distance and throw a lighted match into it. Cheapest way of sorting Chinese bikes!!!

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I did manage to get hold of the honda c50 shop manual which is a help as the engine is practically the same.

 

The C50 engine will run without any problems without an exhaust fitted - I know, I had one!


This then would lead me to check that the timing is correct and fuel / air is also OK. Basically, that is all you need to get a C50 engine running.


The C50 Haynes manual decribes how to set up the carb as well as ignition timing, valve clearances (& timing) and checking the 'spark'.


:cheers:

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Don't knock chinese bikes, I got a 125 to learn on, came crated for £700 brand new, 2 evenings to assemble. Only time I pushed it home was when the rear tyre blew. Sold it for £650 after I passed my Direct Access (old rules)

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Ok, so I've fixed the exhaust, cleaned the carb and the jets, reset the air screw and the idle screw, checked the new air filter and I still don't have any power.


The bike starts and idles ok, if you open the throttle it will respond all the way to full throttle, however opening the throttle a second time will result in no revs after a quarter throttle and full throttle kills the engine.


The bike will pull away but just has no power at all, first is slow, second a joke and third

is pointless.


Any ideas?

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It really sounds like it's flooding......have you set the main jet needle position on the throttle slider?.......may take a bit of experimenting to find the right position for it...... :wink:

Pull the spark plug out and check the colour......even though you can't run it far, the colour will give an indication on whether the bike is running too rich or not...... 8-)

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Ok, really getting hacked off with it now.


won't run unless the choke is on, with choke off the throttle kills the engine which would suggest too much air getting in somewhere, check the spark and it's black suggesting too much fuel.


Makes absolutely no sense to me, unless it's too much mixture in general? is that possible? too much air and fuel?

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