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Camchain skipped a tooth!

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Basil Moss
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Camchain skipped a tooth!

Post by Basil Moss on Fri 06 Apr 2018, 7:10 am

Did my valve clearances 300 miles ago, it's been running sweet as a nut since then. Yesterday rode to work, fine, then when I came to leave, could barely get it started, needed throttle to idle, and no power at all. Limped it home, checked spark, checked compression: ~100psi on each cylinder. Valve cover off, and there's the problem: camchain has skipped a tooth on the inlet valves.

I pulled the holders and realigned it, put it back together with a spare CCT from my old bike, and started it - same issue. It was late then so I called it a day. This evening I'll get back in there and see if it skipped again.

My thoughts:

1) Has it bent the valves? If so, bugger...
2) Is the CCT the cause of the fault? Neither of the two I've got have anything visibly amiss, and I've seen enough threads on here about their bikes rattling with duff CCTs and nothing as bad as it skipping a tooth
3) Is the camchain worn and hence able to skip?

I've ordered a mechanical CCT as a stopgap measure, I can be sure that when I've installed and tightened it, the chain will be tight.
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ceejay
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Re: Camchain skipped a tooth!

Post by ceejay on Fri 06 Apr 2018, 10:12 am

I can't offer much than to say that when you put the cam chain back on after doing your valves did you ensure any slack in the chain was on the tensioner side (rear) and none on the at the front and ensure the correct number of links are between the two camshafts?

Hopefully you won't have damaged valves with it being just a single tooth out
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Jameshambleton
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Re: Camchain skipped a tooth!

Post by Jameshambleton on Fri 06 Apr 2018, 7:36 pm

Been a single tooth out your valve will be fine.
I would imagine that it would most likely be the exhaust that might be out rather than the intake but remember that the lines on the cams do NOT line up parallel with each other
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Basil Moss
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Re: Camchain skipped a tooth!

Post by Basil Moss on Mon 09 Apr 2018, 8:28 am

So, I put the timing right, but it didn't run well. Compression is 100psi in both cylinders. Wet test didn't increase it. My provisional diagnosis is burnt intake valves, so I'm going to get the head off and have a look at them. It was intake camshaft that skipped, probably because its on the baggier side of the chain.
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Jameshambleton
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Re: Camchain skipped a tooth!

Post by Jameshambleton on Mon 09 Apr 2018, 11:27 am

Just been reading up and your engine will run (min pressure 75psi) but given that your oil test didn't change the reading it will be indeed the valves that have had it rather than the piston rings.

Most of the engines the racing lads are using are about 110-120psi
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Basil Moss
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Re: Camchain skipped a tooth!

Post by Basil Moss on Mon 09 Apr 2018, 8:50 pm

I can't believe the racers are on such low compression, it wasn't running well at all at 100, wouldn't idle without the choke and kept firing out of the exhaust BANG! every minute or so. My understanding is that any bike less than 100psi can't be expected to run, and 100 isn't exactly healthy compression. The CG125s I've worked on have had 130 - 150, and they aren't much more than a bean tin thumping up and down inside another bean tin! Textbook is 200 PSI for this bike, and after valve clearances every time I've owned this one it's been a smidgen over that, probably from coke buildup minimising the space. Anyway I'll let you all know after I've taken the head off.

In other news I've got another bike to keep me mobile while I dismantle the CB at my leisure... A CBR600 F3! Dead excited!
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ANDYC
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Re: Camchain skipped a tooth!

Post by ANDYC on Tue 10 Apr 2018, 8:18 am

Being just one tooth out is unfortunately enough to leave the inlet valves at the mercy of the piston crown. I've had this experience with a 929 fireblade engine last summer that I sold. 
It ran perfectly when I removed it from the bike, when the guy fitted it it wouldn't run properly. When he bought the bike over to me me the inlet valves were slightly bent and the camshaft out by one tooth. Only then did he confess that he had removed the tensioner to fit a new one and it must have jumped on the sprocket then.
I did change the head as I had a spare one available but now I will only guarantee an engine if I fit it myself. 
You should enjoy the f3, after all they're nearly as good as a CB500 Very Happy
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wornsprokets
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Re: Camchain skipped a tooth!

Post by wornsprokets on Tue 10 Apr 2018, 10:09 am

Cbr 600f3 a great bike you love it...comfy fast looks great...tt legend.... there going to be a collecter bike if there mint... had cbr 600fp (93)and 92... engine just as reliable as cb if not more
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ceejay
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Re: Camchain skipped a tooth!

Post by ceejay on Tue 10 Apr 2018, 11:02 am

@ANDYC wrote:Being just one tooth out is unfortunately enough to leave the inlet valves at the mercy of the piston crown. I've had this experience with a 929 fireblade engine last summer that I sold. 
It ran perfectly when I removed it from the bike, when the guy fitted it it wouldn't run properly. When he bought the bike over to me me the inlet valves were slightly bent and the camshaft out by one tooth. Only then did he confess that he had removed the tensioner to fit a new one and it must have jumped on the sprocket then.
I did change the head as I had a spare one available but now I will only guarantee an engine if I fit it myself. 
You should enjoy the f3, after all they're nearly as good as a CB500 Very Happy



Wow, I never would have thought that! Makes it an even more high stakes job. I marked my chain so I could ensure it went back on the cam shafts the exact place it came off... however when the cam shafts are removed completely and the chain is slack I wonder if it's possible for the chain to come off the crank at the bottom and reattach down there a tooth out? Obviously the timing marks provide a guide but mine never seem to be 100% precise anway!
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ANDYC
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Re: Camchain skipped a tooth!

Post by ANDYC on Tue 10 Apr 2018, 11:07 am

In a word yes it can come off the crankshaft.
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Basil Moss
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Re: Camchain skipped a tooth!

Post by Basil Moss on Mon 23 Apr 2018, 8:51 pm

So I got the head off. There are marks on the crown of the piston from contact with valves, and although they seem to be closed properly they fail the carb cleaner leak test. I think I'm going to send the head to BenCoyleRacing https://www.bencoyleracing.co.uk/cb-500-engine-tuning for service, might treat it to a cheeky port job while I'm at it!

Looking in the cylinders, they look a bit polished. Still have some hone marks, but not like a newly honed cylinder. I'm wondering if I should say in for a penny in for a pound and get a rebore while I'm in there. Anyone got any experience with this? Can I get the cylinders off with the motor in the frame?
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ANDYC
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Re: Camchain skipped a tooth!

Post by ANDYC on Mon 23 Apr 2018, 10:29 pm

No, the barrels are cast into the upper crankcase. I do have a pair of 0.50mm oversize pistons if you want to rebore the cylinders whilst you're at it.
They are from one of my bikes, before I decided to switch to forged high compression pistons.
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ANDYC
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Re: Camchain skipped a tooth!

Post by ANDYC on Mon 23 Apr 2018, 10:35 pm

In my experience porting the head won't give much by way of improvement as the main restrictions are in the air intake side of things.
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Basil Moss
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Re: Camchain skipped a tooth!

Post by Basil Moss on Wed 09 May 2018, 9:34 am

Hi Andy,

I've decided to leave the pistons as they are, the cylinders look good and the wet compression test didn't point to a problem there. Do you recommend any modifications to the airbox to improve flow? My understanding is that pod filters often harm performance because they gather hotter air that's passed through the radiator and past the engine, as well as missing out on the benefits of acoustics that the airbox offers.
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ANDYC
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Re: Camchain skipped a tooth!

Post by ANDYC on Wed 09 May 2018, 1:19 pm

The pod type filters would alter the characteristics of the engine of course. But that's what people want to do to get more power. 
Hot air from the engine and radiator would mean that you're not going fast enough to need more power anyway.
Especially on a bike as open as ours, fully faired bikes may have a point but ours even the S model is surrounded by fresh air.
Carbs nowadays are much harder to set up to run anything other than a standard airbox,  I think this is by design. It can be done, I ran my bike with larger 38mm cv carbs and a modified airbox. It's all depending on the amount of effort you're prepared to put into it really, and whether you would really use the extra power if you did have it.
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Basil Moss
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Re: Camchain skipped a tooth!

Post by Basil Moss on Thu 10 May 2018, 7:52 am

Well, I do tend to rag the CB500 just abut everywhere I ride it, so probably would use the extra beans... That may or may not be a good thing!

What did you change about the airbox, and how much difference did it make? Also, what carbs did you fit?

I think I recall another issue with pod filters is that they can create a lot of turbulent airflow with vacuums in the area of the carbs, kind of like when you blow across the neck of a bottle to make it whistle. Conversely, the airbox creates resonance at certain RPM which hammers the charges into the cylinders. I read a report ages ago on a heavily modified CB with pod fiters, which after dozens of dyno runs had a marginally higher peak power, but had lost power everywhere else and apparently wasn't very rideable.
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Jameshambleton
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Re: Camchain skipped a tooth!

Post by Jameshambleton on Thu 10 May 2018, 8:14 am

Change the gearing if you haven't already, it'll make tonnes more difference to the performance than engine modification and is a cheaper and easier option. Wink
Unless you start messing about getting the head ported ect you'll be spending mig money for very little performance.


I know the bike you're talking about - someone on here bought it - max bhp yes, ride-ability on it sucked and the jetting in the carbs were massive. IIRC he put it back to a stock/standard setup.
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Basil Moss
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Re: Camchain skipped a tooth!

Post by Basil Moss on Thu 10 May 2018, 8:57 am

Andy, I just found your thread about the bike:

http://www.cb500club.net/t3915p100-my-track-bike-project

Sounds awesome! How is it running?
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ANDYC
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Re: Camchain skipped a tooth!

Post by ANDYC on Thu 10 May 2018, 9:21 am

It's not, much to my dismay other obligations have kept me away from it.
I am however in the process of building the engine at present and I have to admit that I'm very pleased with it so far.
I'm a bit delayed because I have decided to replace the main crankshaft bearing bolts with arp stainless ones as I have had one shear on me in the past. Unfortunately these have to come from the USA which takes time. I'll update my thread with a few pictures today if I get time.
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Basil Moss
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Re: Camchain skipped a tooth!

Post by Basil Moss on Sun 03 Jun 2018, 8:00 am

So the engine is totally dismantled... affraid I'm going to rebore, replace all the bearing shells, new camchain etc. Give it a general tidy up while I'm in there. Hope I remember where everthing does back, it's got a lot more parts than the CG125! The head has been sent off to Ben Coyle for valve job and a tidy up of the ports. I'll update when I start putting it all back together.
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eternally_troubled
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Re: Camchain skipped a tooth!

Post by eternally_troubled on Tue 12 Jun 2018, 8:26 pm

@Basil Moss wrote:So the engine is totally dismantled... :affraid:

Good man! Brave. I'm sure it will all go back together :)

There are a few more bits than a CG125... at least double ;)

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