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Questions & Answers
I have a 50cc moped qt jinlun i just bought anew
Hi and welcome to FixYa,
To my understanding, most of these China made scooters uses a vacuum controlled cut-off fuel valve. Vacuum is tapped either from the carb out or intake coupler to activate and allow fuel flow; no vacuum no fuel from tank. Possibilities are:
- no or little vacuum to allow fuel flow;
- vacuum controlled cut-off fuel valve faulty.
A workaround is to install a manual cut-off or bypass it entirely.
Incidentally, the 'revs like mad' is a result of slight overflow when fuel first flows to a starved/empty carb bowl.
Good luck and thank you for using FixYa.
on Mar 04, 2013
Fuel not going through carbs all jets and pipes
1. check if gaz flows from tank with the hose off.
if not then vacum operated on/off/reserve valve defective or not getting vaccum
ps check with gaz filler cap off, the vent may be blocked. the tank needs to pull in air to replace fuel volume, so it can flow
on Jul 19, 2009
Moped keeps cutting out
1.turn the petcock off at the bottom of the gas tank.
2.get something to put the gas in so you can see if its dirty or not.
3.take the hose off be careful not to splash the gas on you or in your eyes.
4.drain all of the gas.
5.if there is stuff in the bottom of the tank you need to take the tank off and rinse it with gas and get it all out.
6.in reverse put it all back together then put new gas in it then go over it and make sure its all back together correct then take off
on Jun 06, 2009
I have 1983 Yamaha Virago
Yamaha used a starter switch with 2 sets of contacts on many models.
One set opens to shut off the headlight (conserving battery power for cranking) the other closes to energize starter motor.
Problem is the switch gets sticky....so releasing it stops starter, but the headlight doesn't come back on after startiing.
key off, spray some WD40 around the button to get it inside, and work the button in and out...slide your thumb off to let it snap out.
Retry with key on to see if headlight now returns after cranking.
on May 05, 2011
My 2008 Yamaha fz6 has a rough idle and it sputters until about 4000 rpm then it runs like a dream only have problems with low rpm and only when at operating temp?
Try the basics first. When was it given a Full service? (Spark plugs, air/oil/fuel filters and oil change. Have you got any warning lights on the dash? Has it been standing unused for a long time as sometimes a bad batch of fuel or old fuel can cause problems. Old fuel can go "Off" and leave a sticky varnish like coating clogging injectors and sticking valves.
Try this. When your fuel tank is down to about quarter full and the engine is cold add a bottle of injector cleaner to the tank. Start the bike and let it idle until it is up to working temperature. Don't 'rev it up' because at idle your getting a stronger mix of cleaner going through the injectors. Once she's warmed up take her out for a good long steady ride then fuel up on the way BACK home.
If this doesn't help you'll need to get a Good bike mechanic to look at it as it could be a worn engine part or an electronics issue.
on Nov 03, 2020
What should the cylinder compression be on a 1966 Yamaha 100cc twin jet motorcycle?
I remember these lightweight steeds that had an almost cult following in the UK, resulting in a large number of tuning parts becoming available at reasonable prices.
The noise of a gaggle of them pulling away from the lights all at once could and did break the windows of nearby houses.
The compression pressure of an exhaust tuned 2-stroke engine is necessarily quite low - because the cylinder capacity and the compression ratio "grows" at the rpm the reflected exhaust pressure wave effectively blocks the exhaust port and the engine power suddenly increases. Yamaha engines of that time were notoriously "sudden"...
I don't know the exact compression pressure but I would be happy if I did a test and achieved around 100 psi. My Suzuki of a similar design produced 105 psi on the kick starter. Much more pressure than that and there would be a danger of the fuel detonating in the cylinder when the engine "was on the port", usually resulting in a hole melted in a piston.
on Oct 16, 2020
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