Scooter misfiring after valve clearance

[robwyatt]

Hi all. Strange thing has started happening to my BT50-QT11

Background:-

Valves done to 0.05mm in last 2 weeks
Mixture screw has been screwed in and back out between 2.5 + 3turns
New spark plug less than 500Km ago
New rollers (tried 4.5, then went for 5.1)
Oil changed on the weekend

All been working REALLY well till a sound that reminds me of the first ever sound you hear from a car exhaust alerting you that in the future, its gonna go i.e only noticeable by me as I live day in/out with the normal noise.

This in the last few days has got worse anf to the stage where the bike is misfiring and for want of a better word is 'farting along'!

Slight reduction in power but not a massive amount. Sound does not seem to be coming from the exhaust but seems to be coming from the center of the engine kind of near to the cylinder head.

Even weirder was that i went over a bumpo a few days ago and it went back to normal, but went over another bump and its back again and no amount of other bumps will make it go away again!!

Perfectly rideable but a bit embarrasing as the volume is more akin to our distant 2 stroke relatives.

Over to the experts???

What do you think??

Many thanks

Rob

[robwyatt]

think I may have found the solution! Just took a good look under my scooter and it seems that the extra vibrations caused by my new rollers have caused the 2 nuts that hold my exhaust on to come away!

I know that sounds a bit of a freak occurence as these are tightned very hard, but recently i tried to take the exhaust off to paint it so I must not have tightened it sufficiently when I put it back now as the 2 bolts are missing! (the bolts are number 8 on the following link

www.funracer.nl/FunShop/parts_list_category_2.php?database=p_baotian&category=34

Can someone confirm that if the exhaust was loose that it would display the symptoms I have been experiencing?

Anyone have any idea what diameter bolt I need to get and what you call the design of them as I am not moving my bike until I get this sorted! Cheers Rob

[BillyGreen1973]

Yeah, those missing bolts would cause the exhaust to become loose, that would mean the seal between the cylinder head and the exhaust downpipe would not be good. You would then hear the blowing or popping noises coming from the cylinder head.

Just replace those bolts and you should find things will be OK again.

[robwyatt]

I have just replaced the bolts and it has made no difference at all!

Any other ideas?

Cheers

[tls2571]

Remove entire exhaust and shake violently, anything loose or rattling? Was there a gasket where the exhaust mounted to the engine? You said you adjusted your valves a couple of weeks ago, your lock nuts could have come loose. Clogged jet, bad air/fuel mixture, clogged air box. These could all cause that popping sound. Just check one at a time until you find the bugger.

Troy

[robwyatt]

Thanks for advice Troy,but cant get the exhaust off as I have threraded one of the screws and aprt from checking the valves again, I dont understand how to check the rest as my knowledge is via walkthroughs on this site! any pointers would be happily received

[r08813]

2 things to check.

1. Spark Plug.

Have you replaced the chinese sparkplug with a proper make? if so, the spark plug lead doesn't allways connect as well on the proper makes and can move or lose contact at speed. This will produce a missfire and popping. (With the new spark plug you often need to change the lead too). Also, is the spark plug screwed in correctly, they need to be hand tightened until they are as far as they go, then turned a further 1/16th turn or so (check spark plug manufacturer to make sure) with a spanner. Too loose or too tight doesn't compress the spark plug washer to give the correct gas seal.

2. Do you have an emmission valve on the exhaust (usually a rubber pipe fitted on to a small box attached to the exhaust), these are very waek parts (and not really essential). Open it up (2 screws - no exhaust to remove) and see if the reed inside is damage (probably is) if so, exhaust gas is forced back into the airbox and the engine will missfire at speed. Rectify this by blocking the pipe and removing or sealing the valve.

[robwyatt]

I have got to the bottom of everything now.

It is definately the nuts that hold the exhaust on under the engine. I used some different nuts and they are not quite making a seal.

Also the black ring/seal is all bent and messed up so there is air esacping the exhaust that I guess is making all the noise!

Does anyone know of the stats of the bolts so I can get new ones and also anyone know the size of the seal or if I can use anything else to do the job (or if anyone supplies these, as parts lists dont have the nuts and seal itemised)

Cheers all,

Rob

[enu06]

If your engine is a GY6 (Honda) you will find 'GY6' on the engine castings somewhere, I would guess you should get one from a Honda dealer, motorcycles or even garden tool suppliers, i.e. lawn mowers. Take the knackered bits with you as a pattern. If you can find a smaller motorcycle dealer they may be more willing to help than the big boys, we had boxes of odd gaskets in the small shop I worked in, Hope it helps.

[robwyatt]

Phew, its warm and another fustrating day under my bike! I have fixed the exhaust sound/problem, by getting a new gasket and bolts, but when I cranked the bike up it only creeps forward at less than 1 mph! barely moving arggggggg.

I have monkeyed about with the valves again and set them to 0.1 as opposed to 0.05. Could this be my problem as its such a power difference I am not not sure just this could be the factor.

background - before I took the exhaust off, it wa running like a dream except for the noise!

help! I want to ride my bike in the sun!!

[ally1756]

check and see wether the little valve thigny on the exhuast is duff

[robwyatt]

little valve thingy? explain??

[ally1756]

lol, theres a little valve behind the can on the ehxhaust, its silver and ahs a pipe going from it to the airinlet, get a pair of vice grips and stick them on to "close the tube" and then try it


ally

[robwyatt]

I dont have any vice grips unfortunately and it sounds a bit odd that this would be the fault? It was working pefectly before, albeit with a loose exhaust that made a loud noise.

Noise has now gone, but so has the power!

[ally1756]

that valve tends to melt inside

[tls2571]

It sounds like your gasket opening is to small, thus creating to much back pressure. if you use a gasket make sure the the inlet is a least 1mm larger than the actual engine opening

Troy

[mick]

ihad the same problem my bike was miss fiering i found that it was a lose earth cable so the power wernt compleat hope this helps cheak your earth cables mine had gone on the starter motor

[robwyatt]

Hey Troy, now that sounds like a winner, as the gasket I fitted was not a standard one for the bike as I picked up a polini one on ebay to see if it would do the job. (23mm internal diameter)

It fitted like a glove, so what you are saying is that the diameter needs to be 1 or 2 mm larger that the hole it goes around? by the hole, I mean the hole at the end of the exhaust which is the point of contact with the engine under the bike (see pic)


Is that what you mean?

What do you think I should do? I have some silicone gasket maker, could I use this and leave a few mm gap away from the hole?