Yanmar l100ae no compression

[emyrhunnam]

I've recently been restoring an old Polaris D-Bat 400 quad bike that uses a yanmar l100ae. After fitting the necessary parts I was eager to start it up without an air filter, bad idea. A piece of square cloth about 5cm got sucked up and I immediately turned off the engine. Since this I have had no compession at all. I've removed the head and the valves and piston rings seem to be working fine. There are no obvious cracks on the engine and I fitted a new head gasket whilst it was off. I'm only 17 so I really have that much knowlage at all so any help to troubleshoot this will be much appreciated.

 

[henry Kadzielski]

Could also be a partially bent valve, thus not sealing

 

[Spark plug]

Could also be a partially bent valve, thus not sealing

^^ this would be my guess, thake the head back off, sit it upside down on the bench and pour some petrol over the valves, if a valve is bent the petrol will run through.

Oh just another Thort, before you take the head back off check the valve clearance just in case.

 

[dobbslc]

If you've found the piece of cloth I'd say it's a bend valve. Should be easy enough to get a new one.

 

[normspanners]

Yeah chaps I think it could be a bent valve but If he took the valves out you would think it was obvious by the valve clearance a bent valve usually has large clearance compared to what it did have and also there would be witness marks on the piston head where it was clipped.
Hang on----- a yanmar has a decom lever are you sure that has gone 'in' correctly ? and not suffered for some reason
HANGON AGAIN that engine I am sure is a all square job so unlikely to have bent a valve more like it has broken/cracked a rocker, or bent a pushrod, torn a rocker bolt out?? and is stopping the inlet valve opening you need to take the rocker cover off and crank it over and watch the valve operate BEFORE spending on another H/gasket

 

[emyrhunnam]

I was hoping that I would find a piece of cloth when I touch the cover off but found nothin. I slowly cranked the engine over when the rocker cover was off and both valves seem to be operating fine
Thanks for the help

 

[normspanners]

cloth when I touch the cover off

The cover off?? you wouldnt find the rag under the rocker cover

Have you checked the clearance and the decom ?

 

[emyrhunnam]

The cover off?? you wouldnt find the rag under the rocker cover

Have you checked the clearance and the decom ?

No the air filter cover on the quad not the engine, I'll be more sepecific next time. I've made slight adjustment to the rocker arm under instruction of my grandfather. He has lots of experience with engines and has helped a lot with the rebuild, but he is slowly loosing his eyesight which is why he is unable to do it himself.

 

[emyrhunnam]

Yeah chaps I think it could be a bent valve but If he took the valves out you would think it was obvious by the valve clearance a bent valve usually has large clearance compared to what it did have and also there would be witness marks on the piston head where it was clipped.
Hang on----- a yanmar has a decom lever are you sure that has gone 'in' correctly ? and not suffered for some reason
HANGON AGAIN that engine I am sure is a all square job so unlikely to have bent a valve more like it has broken/cracked a rocker, or bent a pushrod, torn a rocker bolt out?? and is stopping the inlet valve opening you need to take the rocker cover off and crank it over and watch the valve operate BEFORE spending on another H/gasket

Thanks for the advice. I have done what you said with cranking the engine over when the rocker cover is off to watch the process, and everything seems fine. I know that the pushrods are straight because we took them out and rolled them down a flat table and they were faultless. I think that I've tried most of the troubleshooting causes for no compression, I was just hoping that someone would give an obvious suggestion that I've missed out.

 

[armalites]

One thing that hasn't been asked, was this running before you started tinkering?

 

[emyrhunnam]

One thing that hasn't been asked, was this running before you started tinkering?

Yes, before this happened there was one problem with a fuel pump but that was replaced and it started up and was running fine

 

[addjunkie]

Yes, before this happened there was one problem with a fuel pump but that was replaced and it started up and was running fine

So did you learn that " if its not broke, dont fix it" or "many thinks get broke when being fixed"

Id be looking at de compression mechanism first then valves

 

[Justme]

Rag in engine will be valves / rings / holed piston.

The L100 is am easy engine to take the head off & you can re use the head gasket as its a copper ring.

Just anneal it.

Cant see the decompressor being the issue as it works in the rocker space so the rag would not get near it.

My money is on a valve being bent just enough to stop a seal.

The L100 is good for about 3500 hours of hard work before it needs a full rebuild that is not cost effective. You can give them another 1000+ hours with a cheaper rebuild of bearings, rings, seals, injector nozzle & injector pump. Make sure you get the right injector pump / governor as the genny one is different to the rev able engine one.

 

[Justme]

Id be looking at de compression mechanism first then valves

It just holds down a rocker so wont be that from a rag in the inlet.

 

[Spark plug]

It just holds down a rocker so wont be that from a rag in the inlet.

Also he says it has no compression with the rocker cover off, the de-compressor is mounted in the cover.

 

[normspanners]

A single cylinder engine is easy to diagnose a bent valve as you crank it, it will hiss / chuff out of the port with the bent valve,and I would think it would be the inlet on this occasion. you just need to take the air filter off and listen.
if it has the auto decom and it has been rebuilt sightly incorrectly there maybe a chance it has not seated properly also just thought make sure the rockers have seated in the end of the pushrods correctly.

 

[normspanners]

I've made slight adjustment to the rocker arm under instruction of my grandfather.

what adjustment ? the clearance ?