Bike starts and dies immediately, compression problem

[wadokai]

Hi there,

My bike is in a mechanic shop and they've tried fixing it and are running into problems. Initially the problem was the bike wasn't firing up; resolution was that the pick up coil was dead and so we replaced the whole stator assembly. Now there is spark being received by spark plugs (which are brand new).

Problem is that when the bike is started now, it fires up and then backfires a bit and then dies. The mechanic checked and found that the compression on cylinder 1 is very low; the other 3 cylinders are fine.

i was wondering if any of you have come across a problem like this and can maybe provide some feed back; i can forward this feedback to my mechanic so hopefully he can try to fix it.

Problem is that he can go ahead and investigate deeper and deeper in the bike but I'm very limited on my $$$ so I'd rather get some ideas first before I suggest him to just dig deeper. Thanks for your help!

[reelrazor]

The guy needs more experience.

Unfortunately, he is getting it at your expense.

He should have verified the fundamentals BEFORE proceeding to install pricey electrical items.

A simple 'leakdown' test will (would have) tell him everything needed about the compression state of the engine.

I suspect there is a sticky valve on that cylinder-but, the engine should continue to run on three cylinders-something more is amiss.

[haro504]

maybe the valve adjustments and its backfireing out of the no 1 what are the other compression numbers kinda need some more info on the bike history and how this came about

[thatkid]

If the mechanic doesn't have a leak down tester (good shops should), a squirt of oil in cylinder #1 (he should know this if he's worth his salt), if the compression shoots up then it points to piston rings.

If not it indicates as mentioned sticky valve/valves ,the valve clearances could need adjusting, burnt valves/seats, leaking head gasket.

Like haro said more info on the bikes history is needed.

1. Has the bike been stood for a long time? could be rusty valve seats on #1

2. Were you having misfires, running on 3 cylinders for a while? If cylinder #1 was misfiring the unburnt fuel could wash the oil off the piston/rings, causing the low compression.

3. Mileage.

Most importantly reread RR's post.

Ask the mechanic questions (you telling him what to do will only p#@s him off) as to why he cannot identify the cause of the compression loss, if you are not satisfied with his answers. Pay him what you owe, and get your bike out of there! Why should you pay for his experiments.

[_Will_]

It'll easily run on 3 cylinders, two probably.

[wadokai]

Initially problem started when I dropped the bike, Took me a good 20 min to get it back up. then it just wouldn't spark.

Now it sparks but just back fires and dies. Running on 3 cylinders is understandable but it just doesn't run really. It just fires up and then dies.

I've dropped it few times. Mileage on the bike is around 29000km

Bike always had start up problems and I would usually need to use choke on it for a while before it would run fine on its own. I think I'm gonna drop by his shop and write down all the things he might think it is, and report to you guys so maybe you can help out further. I agree with that I don't want to be paying for him to run his experiments. Thanks for your input

[yamaha_george]

Initially problem started when I dropped the bike, Took me a good 20 min to get it back up. then it just wouldn't spark.

Now it sparks but just back fires and dies. Running on 3 cylinders is understandable but it just doesn't run really. It just fires up and then dies.

I've dropped it few times. Mileage on the bike is around 29000km

Bike always had start up problems and I would usually need to use choke on it for a while before it would run fine on its own. I think I'm gonna drop by his shop and write down all the things he might think it is, and report to you guys so maybe you can help out further. I agree with that I don't want to be paying for him to run his experiments. Thanks for your input

Hi,
dropping the bike MAY have flooded it, 29K is a reasonably high mileage when "stuff" needs sorting, as some one else said checking the electrics before a leak down an experienced (without a service history of the bike) is a natural first thing to do (mostly from the garages point of view stops wasted time and can inflate the price if you are selling services & gives you the garage a starting reference as to where to start work.)

From experience stators do not stop working after a crash UNLESS a] unless the cover is chewed thro and b} the wires got eaten on that upper section of loom.


The fact you used the choke to start it does mean much as we have no idea where you are as you have not put your location in the personal profile. I use the choke every time too but I live in the "sunny" UK.

Get back to us and be prepared to bail on this guy and do the work yourself

[reelrazor]

Okay, gonna throw this out there...may have been overlooked/not thought of...

Is the exhaust clear of obstructions?

If air cannot get out, then compression readings will be skewed to the low side. #1 may be the most affected, but this will also cause the other symptoms you describe.

Upon firing up, pressure builds and will stop the engine from breathing-engines quits.

[thatkid]

Initially problem started when I dropped the bike, Took me a good 20 min to get it back up. then it just wouldn't spark.

Did you drop it on the left hand side?

[haro504]

would a pluged exhaust lower the compression reading i would think it deffinatly would suffocate the engine but cant see how it would lower the compression. there now that i played with fire i await to get burned (be gentle)

[thatkid]

I'm guessing he dropped it left hand side, Hence the "so we replaced the whole stator assembly" was it a YZF stator/pick up coil? which can cause problems.

Laying down for 20 mins left hand side (gravity). Which carb/cylinder is more likely to flood #1? washing the oil out of that cylinder?

[haro504]

good call with the flooding and oil in cylinder 1

[reelrazor]

would a pluged exhaust lower the compression reading i would think it deffinatly would suffocate the engine but cant see how it would lower the compression. there now that i played with fire i await to get burned (be gentle)

Yes, it will definately lower the compression reading. If air cannot get out the exhaust..it pressurizes..the exhaust valves open and that pressure 'backfeeds' the cylinder. The air coming in the intake is only being drawn by the vacuum (relative to the atmospheric pressure upstream of the carbs/airbox) created by the falling piston while the innnie valves are open. The pressure from the exhaust NOT moving air out translates into a lower differential (less vacuum). This decreases the effective volume drawn into the cylinder, thus lowering compression.

Next time you use a compression tester, try TWO things....

1) take a reading, then tape the exhaust and try again.

2) take a reading with throttles (butterfly AND CV slide) wide open, and then fully closed.

in either case I guarantee you that you will see a dramatically lower reading at the gauge

[thatkid]

RR I get were your coming from.

When performing a compression test, the butterfly should be wide open (throttle) and all 4 plugs removed, what do you use to open the CV slides?

Are you saying that all CV slides should be open? or only the cylinder being tested? If it's all 4, whatever is used to keep them open could be sucked into the engine. If its only the cylinder being tested then a finger will suffice.