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YBR125 Turns over & Fires but won't run


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Hi all and apologies for the lack of introduction from a new subscriber


A bit of background.. I am an unqualified but enthusiastic tinkerer of motorbikes and have done a couple of nut and Bolt rebuilds in the last few years (ST1100 & Intruder 125) with great success. My son has a YBR 125 (2011 fuel injection) which he ran with no oil and completely knackered the crankshaft. Long story short.... I dismantled (cleaned out all the metal filings) and rebuilt the engine replacing with new parts where required but now it won't run. I have replaced the crankshaft, piston and rings, checked and double checked my valve clearances whilst at TDC. I have a good spark, fuel is getting through and good compression. But when I try to start it, it fires but it won't run at all.


The only problem I can find is the starter motor is noisy when I turn it over (replacement required) but now at a loss on what to look for. There are no fault codes showing so any suggestions would be very welcome.

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Hi all and thank you for the welcome. I have tested my compression tonight and I have a reading of 125psi. According to the Haynes manual it should have a minimum of 148psi but not sure this would prevent the engine from running :|

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Hi all and thank you for the welcome. I have tested my compression tonight and I have a reading of 125psi. According to the Haynes manual it should have a minimum of 148psi but not sure this would prevent the engine from running :|

 

Tested wet or dry ?

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Hi all and apologies for the lack of introduction from a new subscriber


A bit of background.. I am an unqualified but enthusiastic tinkerer of motorbikes and have done a couple of nut and Bolt rebuilds in the last few years (ST1100 & Intruder 125) with great success. My son has a YBR 125 (2011 fuel injection) which he ran with no oil and completely knackered the crankshaft. Long story short.... I dismantled (cleaned out all the metal filings) and rebuilt the engine replacing with new parts where required but now it won't run. I have replaced the crankshaft, piston and rings, checked and double checked my valve clearances whilst at TDC. I have a good spark, fuel is getting through and good compression. But when I try to start it, it fires but it won't run at all.


The only problem I can find is the starter motor is noisy when I turn it over (replacement required) but now at a loss on what to look for. There are no fault codes showing so any suggestions would be very welcome.

 

Did you set the rings opposed ?

Did you hone the bores ?


All the parts fitted will need to bed in before giving optimum performance so a drop in compression may be expected to begin with


Are you sure you have timing correct. .

There are two TDC on a four stroke engine

One after compression and one after exhaust

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Yes I did set the rings opposed using the diagram in the official workshop manual. However, to the question did I test the compression wet or dry is where my knowledge is limited. Not sure what that means but I just followed the Haynes guidance of attaching the gauge and cranking the engine over until the needle stopped moving up.


In regards to there being two TDC's (I was not aware) although I thought I had pretty much followed the instructions to the letter I think it will be worth revisiting this aspect.


Many thanks for your help so far :D

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Yes I did set the rings opposed using the diagram in the official workshop manual. However, to the question did I test the compression wet or dry is where my knowledge is limited. Not sure what that means but I just followed the Haynes guidance of attaching the gauge and cranking the engine over until the needle stopped moving up.


In regards to there being two TDC's (I was not aware) although I thought I had pretty much followed the instructions to the letter I think it will be worth revisiting this aspect.


Many thanks for your help so far :D

 

A 'wet' test is done by placing a small amount of oil into the cylinder before testing the compression. The oil forms a temporary seal if the bore/rings are leaking. So if a cylinder is down on compression doing a dry then wet test will help determine where the leak is. If the wet result is higher than the dry then it's bore or rings that are leaking. If the wet result is no different then the leak is above the rings - ie valves, cylinder head.


I went to a guy a couple of years back who had set up the timing on the wrong cycle - a spark on the exhaust stroke don't go bang very well - or at least not the bang you want!

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In regards to there being two TDC's (I was not aware) although I thought I had pretty much followed the instructions to the letter I think it will be worth revisiting this aspect.


Many thanks for your help so far :D

 

There's only one true TDC, but it can appear to be at the exhaust side or intake. It's been a while since I looked at one so do read up on it as it sounds like you weren't aware and may have set it wrong.

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In regards to there being two TDC's (I was not aware) although I thought I had pretty much followed the instructions to the letter I think it will be worth revisiting this aspect.


Many thanks for your help so far :D

 

There's only one true TDC, but it can appear to be at the exhaust side or intake. It's been a while since I looked at one so do read up on it as it sounds like you weren't aware and may have set it wrong.

No incorrect .

Both are true top dead centre ...all that is different between them one if them is the aligned with timing requirements for ignition and valve timing

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In regards to there being two TDC's (I was not aware) although I thought I had pretty much followed the instructions to the letter I think it will be worth revisiting this aspect.


Many thanks for your help so far :D

 

There's only one true TDC, but it can appear to be at the exhaust side or intake. It's been a while since I looked at one so do read up on it as it sounds like you weren't aware and may have set it wrong.

No incorrect .

Both are true top dead centre ...all that is different between them one if them is the aligned with timing requirements for ignition and valve timing

 

180 degrees out springs to mind :)


As Tim says really TDC is TDC its whether the cam is timed correctly or not is when it becomes a problem

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I agree with Stu. The timing could be out because the cams weren't put in right or the cams could have skipped some teeth on the cam chain, or the vavles were adjusted wrong. You can sometimes tell that's been done because one of the adjusters is sticking up a good bit further than the other one because it's been turned in or out a good bit more than the other one. I don't think it's a compression problem because it had enough compression to fire once so it has enough compression to run and if it fired once it must be getting fuel and spark. I'd check the timing and the valve clearance first and go from there.

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  • 2 weeks later...

Hi all and thank you once again for your responses. Probably not necessary, but to make sure everything was as it should be I stripped the top end down, reassembled with new gaskets, checked and double checked the timing but still the flipping thing won't start. It doesn't even fire now (but I have a good spark) and to make things even more frustrating I now have no compression. It is absolutely zero!


The only thing I haven't done is strip the valves down to check if there is a problem there. Once again I am looking for help and inspiration :?

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I believe I have found my problem. Convinced it is a top end problem I stripped the head off and following the latest advice from you guys I inspected the valves. Neither valve is seating properly with the exhaust valve being the worst. No need to do a leak test I can see clear daylight around 75% of it and fluid pours through. The intake valve is marginally better but again there is a small area where daylight can be seen through one side and again it doesn't hold any fluid.

I considered stripping it down and replacing, but the cost of parts doesn't make sense over purchasing a complete new aftermarket top end which I have gone for. I will let you know how I get on :lol:


Excuse the delay in responding to your advice but I don't have a garage and I am dependent on good weather to work on the bike and evaluate your suggestions to the problem.


Cheers Jon

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This appears to be bent valves.

There are 3 ways they could have got bent.

1. They were damaged when the engine was run out of oil.

2. During assembly or disassembly, you rotated the crankshaft bringing the piston up to TDC while the valves were open.

You need to make sure the cam is positioned so the valves are all closed. Don't rotate the crank untill the vlaves are closed.

or

2. The timing was set wrong so the piston contacted the valves when the engine was rotated.


When you fit the new top end you need to make sure you dont make the same mistake again.

Most engines are timed with the piston at TDC. While the head is off, check the timing mark for the crank you are using puts the piston at TDC. Then check the timing mark for the camshaft positions the camshaft so both valves are closed.

After asembly, leave the spark plug out. Rotate the engine at least 2 full revolutions, feel for any resistance, if you feel any STOP, then recheck to find out what is wrong.

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  • 3 weeks later...

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