Strange sound on my scooter

I recently had a replacement cylinder with larger volume after a break down.

After break it in and I start to drive faster I hear the combustion sound change.
My brain think for a few seconds and my hand go down to the key and turn it off, but the engine continue to runn.

GREAT!! I have a diesel engine on my scooter now.

I wonder if it’s running too hot or if my octane rating is too low so I go to the place where I had the change and ask the owner.
He seams like he’s totaly unable to hear what I say and want to change the ignition lock, but in the end he gets it and tell me to change from 92 too 95 witch I do but I still have the sound so I move on to 98, but same shit.

I go back and he then figure out that 98 is too mutch and drain my gasoline tank and add 95 despite me telling him it wont work.

Afer he’s done I demonstrate by running high revs and then turning off the key that I was right and it did not work.

I ask him to do a compresion test, but who the hell has sutch advanced tool’s here. I know people have it at home for hobby use :raspberry:
He now want to change my piston to a lower model like that would help. I would like to know my compresion befour I let him do that.
The problem ocured at the same contidions on both 92,95 and 98. I think the same would hapend with octane boost too.

I belive it is the head gasket in steel that they cut up to fit the larger bore that cause the problem. The place where they cut is not looking nice and I think the sharpe edges is heated by the combustion on high rpm’s when the heat can not be transfered away fast enough and ignite the fuel mix like it’s done on smal RC car’s.

No listening :loco:

He has ben wrong 3 times, but has not taken my only sugestion yet. :fume:

If I only had a garadge and tool box :wink:

I think the problem would go away if I just threated the surface on the cylinder and head in sutch way that I would no longer nead the gasket.

I have tuned my own two stroke 50cc to reatch speed’s off 120km/h and did work for others too when I was 16-20 years old so I would asume I’m woth listening too when it come too smal 2-strokes.

If an engine runs after the ignition is turned off then this is usually a sign that the cylinder has developed a “hot spot”. A hot spot is a build up of carbon deposit inside the cylinder which heats up and glows red during engine run. WHen the spark is turned off with the ignition, the engine may still continue to run badly as the hot carbon deposite ignites the fuel/air mixture instead of the spark plug. Hot spots are usually caused by poor engine timing or dirty fuel/oil being burned within the cylinder. Switching to a higher octane gives you a better resistance to pre-igition, and so may help reduce the problem that you have, although will not illiminate the carbon deposits that are already within the cyclinder. Hot spots develop when the fuel/air mixture ignites before the piston reaches top-dead-center (Bad), causing some fuel to go unburned, linger in the chamber and then turn carbonous as well as piston “knock” which is probably the sound you are hearing (a tinkly diesel sound).

My advice "If you have been running with this problem for a while, then strip the engine again, clean it, check that the valves and ports are large enough to deliver adequate air and fuel. Rebuild the engine, check the compression, using a guage. If everything checks out, then check or replace the spark plug and spark plug lead. Check the ignition timing and adjust if neccesary or even capable (I’m not sure about scooters in this case.) Replace the engine oil (It may be watered down with petrol and may be slipping past the piston rings to easily.) Try it again, and you should be alright.

I wouldn’t ever advise anyone on boring out a scooter engine much however as there are a multitude of nightmares that this can cause. You are lucky that so far, all you have are hot spots and poor timing I think.

First off, you need to upgrade your mechanic. He’s a moron.

It would help if you’d tell us if your scooter is a 2 or 4-stroke, what make and model it is.

If he has bored the cylinder oversize then he should have fitted an appropriate gasket. It is indeed possible that using the wrong one, with some of the material sticking into the combustion chamber can be causing auto ignition as you describe. However, a thin strip of steel doesn’t take very long to burn away so if this were the case I wouldn’t expect it to last very long. The edge of the gasket should have all burned away within a few hundred kilometers and if that were the only problem then it would stop.

Have you inspected the sparkplug? You could just have a dollop of carbon on the plug that is causing this, or the plug could be overheating and hot earth electrode. For example, if you’re running very lean this could have happened.

In any case I’d say that dieseling like this is a bad enough symptom you probably want to have a look into the cylinder and see what the damage is. If you run it like this for very long the spikes in cylinder pressure from detonation are going to destroy the crank and/or big-end bearings, so the sooner you get this sorted the better.

It’s a two stroke.

I did maybe 60km after the change.

I wish they had some do it yourself garadges so I would not nead to go to mecanics :fume:

I adjusted the carb once to get a less lean mix, but it did not help mutch. Now it’s only harder to start.

The spark plug look ok.

I went to other mecanic and he did not speak anny english witch is ok by me. I just ask for a penn and draw up a cylinder unit and he understand it’s a piston and a cylinder. Then I draw a manometer where the spark plug is suposed to be and he lightens up and runn away too fill air in my tire :loco: :wall:

Two strokes are hard to screw up really, and that’s why I assumed it was a four stroke. Anyway your mechanic IS a moron, and you might end up having to take it to a proper garage…in…erm…Germany!?..Maybe England…At least not in Taiwan, that much I’m sure of.

I’m going to atend a f1 race in japan in october, do you think I could bring it with me so I could get it to a real mecanic witch actualy have a education?

Or can someone tell me a place in Ylan, Loudong or So-Au that can provide me with a co,presion tester.

I’m willing to buy one myself if it’s necesary just to show him how wron he is. A trained ear can hear if the compresion is high and I can hear mine is not.

I found octane booster in Loudong so I asumed they also had copresiontesters, but NO.

Moust off my friends at home have a compresiontester in there garadge and here “profesional” repair shops do not have.

[quote=“Stian”]It’s a two stroke.

I did maybe 60km after the change.
[/quote]60km is nothing like enough time for break-in given the quality of parts and machining work here.

[quote=“Stian”]
I adjusted the carb once to get a less lean mix, but it did not help mutch. Now it’s only harder to start.[/quote]If you mean you adjusted the carburettor via the idle mixture screw, it has no effect beyond about 10% throttle.

[quote=“Stian”]I went to other mecanic and he did not speak anny English witch is ok by me. I just ask for a penn and draw up a cylinder unit and he understand it’s a piston and a cylinder. Then I draw a manometer where the spark plug is suposed to be and he lightens up and runn away too fill air in my tire :loco: :wall:[/quote]Actually, a compression test on a two-stroke tells you almost nothing. What you would need to do is a leak-down test… much more difficult, not really possible without taking the whole engine out and making some special tools and adapters.

Either force your mechanic to strip the top end for inspection or just get yourself a socket set and get the cylinder head off. I used to do this outside my apartment or in the basement. It’s a couple of bolts at most once the cooling shroud is off, then check the following:

The cylinder head gasket doesn’t cover the cylinder bore. ie, the hole is bigger than the bore diameter. Find the correct gasket by all possible means.

The combustion chamber itself should have no deep marks of any kind. If you had an explosion in there before, like a broken piston or ring, then it’s likely the head has deep pits and gouges in it. Those will get hot enough to ignite the mixture as sulavaca explained.

The cylinder walls should be smooth and show no scuffing or vertical gouges. It sounds to me like you have been riding it too hard, too soon after the rebuild. It’s super easy to seize (even momentarily) a fresh top end, and one scrape is all it takes to ruin the seal between the cylinder and rings.

Check for oily films around the intake gaskets, probably a reed block, that show you have an air leak there. Same at the cylinder base gasket.

HTH

Sorry. I mean 60miles(metric that is)

I also raised the nidle to max.

[quote=“redwagon”]
Actually, a compression test on a two-stroke tells you almost nothing. [/quote]

I would prove him wrong.I have runn two stroke on 12:1 in compresion befour without anny problem and I have told that macinic tat and he say that is ok, but he claim I have more than that now.

I have mentioned gasket and carbon resedues too him a 100 times, but the jerk refuse to listen. Maybe I have to make him it was he’s idea and not mine.

60 miles is nothing like enough either. The oversize pistons they make here are (for the most part) garbage. They do not do proper cam grinding on the pistons to compensate for expansion, so they are not actually round when hot. It means that you get tight spots when the pistons are new and you have to run them in very carefully. I usually put at least 500km on a piston before running it hard and 1,000km before running it hard for extended periods.

Raising the needle is a good tactic. Be sure you don’t overdo it though since excess gas will wash the oil off the bore and then are still screwed. How does your sparkplug read?

It’s also possible that you have a squish clearance issue as well as the gasket being wrong. Some two-strokes with wide squish bands as stock will have too much squish area when overbored. The NSR engine will ping when overbored if you don’t open the dome a little. I never did figure out if it was just that the compression was too high or if it was the squish clearance, but it pinged like hell until I opened the chamber up. Not sure if this is your problem but it’s something to consider.

My oil pump do not work annymore(at least not mutch) so I do all mixing in the fuel.

And I stay away from ching chongs superway 2-T or whatever thay call the crap that is sold off cheap. I only use mobil or castrol.

I’m considering trying a coolder plug. I remeber one off my friends plugs melted during high loads so he neaded one with diferent heat value.

I’m quite shure the gesket is the one to blaim since it’s the original one and is just cut biger with some tool(maybe a knife or sisser or maybe even a spade)

I would like to just take the cylinder and head off and brind them down until they are 100% level and I can mount them back on with no gasket at all. I allways doed that myself and they never had a leak after that or caused anny other truble + I got more compresion.

Can’t they do annything right concerning viecles in this bloody country?

[quote=“Stian”]My oil pump do not work annymore(at least not mutch) so I do all mixing in the fuel.
[/quote]Did you increase the main jet size also to compensate? Remember that premix has less gas in it per unit volume…

[quote=“Stian”]And I stay away from ching chongs superway 2-T or whatever thay call the crap that is sold off cheap. I only use mobil or castrol.[/quote]I’ve said it before but I’ll say it again. The best value 2T I have found on the market here is KYMCO’s NSR blend. Excellent stuff, formulation by Honda Japan. :notworthy:

[quote=“Stian”]I’m considering trying a coolder plug. I remeber one off my friends plugs melted during high loads so he neaded one with diferent heat value.[/quote]That may help. How does your plug read?

[quote=“Stian”]I’m quite shure the gesket is the one to blaim since it’s the original one and is just cut biger with some tool(maybe a knife or sisser or maybe even a spade)[/quote]Oh dear. :astonished:

[quote=“Stian”]I would like to just take the cylinder and head off and brind them down until they are 100% level and I can mount them back on with no gasket at all. I allways doed that myself and they never had a leak after that or caused anny other truble + I got more compresion.[/quote]Hmm. Remember you have already increased the compression by increasing cylinder volume while retaining similar combustion chamber volume. I would expect the compression is already on the high side. Even if you don’t cause detonation by removing the gasket, excessively high compression ratio may prevent the engine from reaching peak revs. That may be less of a problem on a scooter than a full-on race bike. :wink: You also want to check the squish clearance before running without the gasket since engines with roller bearing cranks can easily see the deck height change by 0.2mm at high revs. :wink:

[quote=“Stian”]
Can’t they do annything right concerning viecles in this bloody country?[/quote]There are a few notable exceptions, but generally, no. Chabuduo rules.

My plug is clean. No carbons and no oil. It could be new was it not for the fact that it have signs off heat.

I don’t buy premix gas, but mix it myself.
Right now it’s some binglan oil on it after the macanic changed from 98 to 95 like that would help.

I told him it would not help and I say told you so after he had done it.

I’m going out off the office now and if I can fing a cheap set off tool’s on my way home and a pice off side walk without binlan stains I will probartly fix it myself.

On the other hand I don’t like scooters since the engine is su fucking packed inn and have asked my other half for promision to buy a 125 motorbike instead where all parts can be changed in less then a day.

[quote=“Stian”]My plug is clean. No carbons and no oil. It could be new was it not for the fact that it have signs off heat.[/quote]OK.

[quote=“Stian”]I don’t buy premix gas, but mix it myself.
Right now it’s some binglan oil on it after the macanic changed from 98 to 95 like that would help.[/quote]Whether you mix it yourself or buy it, the jets still have to compensate for it. You add 5% oil to gas and then end up with 95cc of gas and 5cc of oil instead of 100cc of gas. The jets pass a given volume of liquid per minute under a standard temperature and pressure whatever is in that liquid. Therefore you will run leaner on premix than straight gas with the same jets. Therefore you have to increase the jet sizes to flow the same amount of gas + oil. :wink:

[quote=“Stian”]
I told him it would not help and I say told you so after he had done it.

I’m going out off the office now and if I can fing a cheap set off tool’s on my way home and a pice off side walk without binlan stains I will probartly fix it myself.

On the other hand I don’t like scooters since the engine is su fucking packed inn and have asked my other half for promision to buy a 125 motorbike instead where all parts can be changed in less then a day.[/quote]
:laughing: That’s a classic, like dropping the fuel octane is going to help a detonation or preignition problem! :laughing: :noway:

Yeah, DIY FTW. I refuse to work on scooters because of all the stupid plastic parts in the way. Naked wins everytime. :wink: :howyoudoin:

My motto: It’s easier to beg for forgiveness than ask for permission. :wink:

Ohhh I see now.
I’m just not used to the english term jet’s. I thought it was called nozzles.

The cylinder is 55ccm compared to original 49, but the nidle is lifted max and I have adjusted for more gas so I think I will do ok. I hawe done fine on more exsreme bores befour.

I went to buy a key for the spark plug since the original one is gone a long time ago.

I was lucky because I picked the wrong size and had to go back and change it witch I did not mind doing since the chick there was tall and slim dressed in tight jeans. My t-shirt is still wet on the chest.

I tok out the plug after I found a pice off sidewalk with no binglang stains and walked to two repair stores to find a coolder plug.

I have no idea if it will work since I did not test it since it look like a thunderstorm is about to strike here so I wont risk my life driving scooter just to test a plug. I can do that while driving to work tomorow.

And yes reducing the octane will help if you have pre detonation :loco: If anny one has exsperienced annything bether than this one then it should be a Nobel price for it

And my GF allways wonder why I ask hear to buy me wiskey when she is abroad for buisnes :unamused:

I just nead something to handle this.

EDIT.

When I ride to work this moring I found out that changing too a cooler spark plug did not help at all, but atleast I tryed and did not really exspect it to work. I gues I will be going back to the repairshop and have him strip down the engine.

My GF think I’m not polite since I stay around while a strip down is done, but I consider this the only safe way.

The only reason I go to a repair shop is because I think it’s worth 500 to save me the truble off all than plastick.

I went back today and had him take it apart and I found som colour on the head gasket indicating it has ben hot and I found some carbon resedues aroubd the spark plug area.

I scarped it out and I grinded nown the top and cylinder with some nice paper I bought, but could not find annything that was flat enough to work on so I simply had to grind down the head against the cylinder and the cylinder against the head, witch I almoust screwed up a few times.

I asked the mecanic to put it back together and he seamed a litle confused when I refused him to put back that pice off shit gasket, but he did what he was told.

I asked if he had a other jet around and he picket out from a corner a old carb and pulled out the jet in it.

I read them and the one in my scooter was 82 and the one he had in he’s carb was 90 so I went for it.

After it all was together a local guru shoved up and adjusted the carb and I understod from there conversation in chinese that he had a few braincells stil working. he had a look at the head gasket and he had a conversation with the mecanic wher he simulated the sound pre egnition would cause and I look like my mecanic finnaly understod that pices off thin metal can get hot enough to ignite gas.

When I ride home I was not able to make it runn with the key switched off and I really don’t care what did the trix, but it’s ok now. Maybe a new jet helped the combustionproces to runn coolder or maybe it was the carbon or gasket. Now it’s fine and I’m happy.

I payed him 500 and I would be happy to pay him 1000 if it means I don’t have to find a engine somewhere deep inside a scooter.

If annyone out there trust there mecanic I will just tell you your stupid. Take charge over your motorbike and don’t let this guys screw them up too mutch. I solved the problem without changin to a lower piston like he wanted to do in the first place. :laughing: