A forum for owners of Yamaha TTR250 trail and enduro bikes!

dlee22 · Senior Member

Front cam chain guide snapped AGAIN

dlee22 · Senior Member

And now like before the cam shafts aren't tdc again, hope the chain hasn't been battered and need replacing again!

Fingers crossed they are both out by the same but minor amount.

JOHNYBOY · Super Guru

has the guide gone brittal ive known this on 750 4 cylinder hondas

dlee22 · Senior Member

Maybe that could have been the case if a single one broke.

But there has been two different 27/28 year old ones to break on the same person within the space of a few months. I think the likelihood for two being too brittle would be very low - certainly not impossible though.

Cheers for the suggestion though

ttr steve · Super Guru

OMG, did the other one snap in the same place?

It looks like it has been crushed downwards

dlee22 · Senior Member

Yes it did Steve, absolutely gutted.

I'm partly glad that it wasn't me the first time but it means there are more questions and wasted time and money.

Thankfully Simon is a top bloke and taking a look into it!:)

Christos · Veteran Member

That is very disturbing. I have read about this once but the owner had put an aftermarket cam chain, probably Chinese. Different chain tolerance resulted in front slider snap.
Is your cam chain OEM and not stretched, that is fairly new?

dlee22 · Senior Member

After fitting the 350 cc the first time the guidebsplit the chain was genuine Yamaha and between 1000-2000 miles old, the chain was also ruined, most likely by this.

I replaced the chain guide and chain the second 350 rebuild with genuine parts.

Its alarming its split again, I hope the chain is okay.

Simon has been good enough to take a look into this for me, top bloke!

Can't wait to know the problem, I'm gutted with the waste of money and time so far, bikes ehh? 😅

TTRfan · Super Guru

Going through some old photos I found this one from 2013. It's a cam chain guide from a standard 250cc engine split at exactly the same place as yours Dan.

Split slipper.jpg

And in the last few days Steve has been talking to an owner with a standard 250cc engine whose guide has suffered the same way.

Damaged camchain guide.jpeg

This doesn't help explain why it happens but I feel more comfortable knowing that it is almost certainly not a fault in the 350 barrels.

The cam chain is running upwards from where the split occurs. I confess that I am at a loss as to why these guides split in this way. Is it possibly to do with the cam chain whipping it perhaps confuse

Brian

ttr steve · Super Guru

Dans problems are definitely nothing to do with his 350. It seems he has been very unlucky to have 2 guides fail 😕 interesting to see others go in the same place, what are the symptoms when this happens?

I have only seen when they snap, where the tensioner has been and then nearly always, when a manual tensioner has been fitted.

dlee22 · Senior Member

I rode that ttr for 17,000miles and all was well with the guide.

After getting the 350 kit, within a few hundred miles I notice something wasn't right, and took the bike off the road.

Sure enough after lengthy investigation I found it had split.

Nothing else changed in between fitting the 350 to the 250.

Both times after the rebuilds every thing was in order in terms of the cam positions/timings. The only thing that I can say is I fitted a new cam chain and it had stretched after a few 1000 miles on the first 350 rebuild but was replaced for the second. Luckily the timing lines are still nice and parallel so I don't its stretched.

For me It just feels like there is a bit of a coincidence that my problems started after the 350 rebuild, that does not to mean to cast blame as such it is just how it has happened sadly, hopefully we will know more soon:)

JOHNYBOY · Super Guru

you are not putting any more pressure on the chain from going from 250 to 350 possible chain tensioner slider gone hard due to shelf life on someones parts bin had this on cb 750/s

ttr steve · Super Guru

What wasn't right, what were the symptoms

dlee22 · Senior Member

I understand that there is no extra pressure for the increase in power etc.

The tensioner works slides in and out fine, and is self tensioning right?

If there was incorrect tensioner would the cams be level when "timed"? Mine was fine before the guide snapped.

ttr steve · Super Guru

How did you know the guide had snapped?

JOHNYBOY · Super Guru

if the chain was slack you would get backlash from crank to cam & timing marks would not line up as should do

dlee22 · Senior Member

Steve - the first time the 350cc bike wouldn't run quite right, worked out the timing was off, the bike had a 1-2000mile cam chain on it so I wasn't expecting it to be stretched.

I cant remember why but I took the clutch casing off and found it snapped. Also turned out my chain had stretch in that short period of time.

dlee22 · Senior Member

Johny- the timing marks were fine when the bike was a 250, after the 350 was fitted it was fine until a while later when I investigated running issues/tweaks at this point the timing was off, I then spent ages trying to find the issue which I then I identified the first split guide. After the 350 was rebuilt (based on the suggestion I had not fitted the guide right) I had to replace the cam chain as the cams were off, this solved the timing.

Based on the fact I have had the timing spot on twice since the 350 kit I imagine the tensioner has the ability to push the rear guard with the correct force.

mossproof · Super Guru

Having had Dan's engine apart, I have inspected cam chain, sprockets, tensioner, rear chain slider, dimensions of cyl. head (in case it was skimmed at some point in it's life) and dimensions of the barrel. Anything else I've missed?
The only thing I can find amiss is the casting of the void at the bottom of the cam chain gallery is rough and slightly misaligned. The chain slider in question (front one) fits fine, as Dan found when he did the rebuild, but it MIGHT be putting slightly more pressure on the slider than normal and when the engine warms up, the expansion causes too much pressure, especially for an elderly piece of plastic exposed to oil and heat for 15 years!
This misalignment will have happened at the foundry when the mould was put together, and I stopped using them after the first batch 'cause I wasn't happy with their workmanship.
New sliders are on back order, but I don't want to use another pre-loved one with unknown history! Waiting game again...

dlee22 · Senior Member

Thanks for keeping me in the loop and taking a look, glad to hear that you have found an issue, good to know you have found a better founder for the kits!

mossproof · Super Guru

Johnnyboy you are correct that there will be backlash if the tensioner isn't working, (and I checked it 'cause I thought that backlash might be fracturing the slider, nope!) but the timing marks should still line up if you rotate the cams anticlockwise to take up the slack manually.

This is an odd one, and I can barely believe that barely more than 1mm is making the difference, but as Sherlock says, when you've eliminated all the other possibilities, the only one left has to be the one.

PS If anyone has a left hand carb for a Honda CL160, that's my other current headache...

mossproof · Super Guru

My machinist doesn't think a break like that would be caused by the slight side pressure exerted by the rough casting, but like the rest of us he is at a loss to explain why it has broken. Ho hum! Anyway, "blueprint" the barrel and stick it back together is all I can do...

Jeff B · Veteran Member

Daniel,

Was this the cam chain guide on the exhaust side (engine front) opposite of the tensioner? That one is a bit tricky to seat properly when installing the cylinder. Is there a possibility it was not properly seated during assembly?

dlee22 · Senior Member

Hi Jeff,

Yes it was, I have rebuilt 2 ttr engines prior to the 350 rebuild, I have never found it difficult to seat the guide.

On the first 350 build I cant remember struggling to fit the guide and and the second time on the 350 it was also fine.

Ttr tony messaged me to say that there are two types of front guides, see attached. I used the same type both times which matched the 1993 engine.

For the issue just to spring up after the build could be a massive coincidence but like Simon has said he has checked the original components.

Its gutting as this was not cheap and neither was the second rebuild.

I am grateful for the help guys, especially Simon!

SussexTony · Senior Member

Ha ya Dan , it was just I received the new cam guide from Steve at totally ttrs it was a. Used one ,but like brand new! It just would not fit properly! So that made me think there must be different ones! But no , turned out that motorcycle shop just cut 0:003 off of each side of the lugs on the guide so it was a properly snugg fit in the barrel doesn’t sound a lot to cut off the lugs but enough to cause a problem all sorted now, many thanks Tony

ttr steve · Super Guru

SussexTony wrote:
Ha ya Dan , it was just I received the new cam guide from Steve at totally ttrs it was a. Used one ,but like brand new! It just would not fit properly! So that made me think there must be different ones! But no , turned out that motorcycle shop just cut 0:003 off of each side of the lugs on the guide so it was a properly snugg fit in the barrel doesn’t sound a lot to cut off the lugs but enough to cause a problem all sorted now, many thanks Tony

Weird why your mechanic has had to cut it?? all the guides are the same for all years and models, the part numbers are exactly the same.

Very strange

dlee22 · Senior Member

This is a better image of the two guides Tony's, both have 4gy on them. One has a distinct curve, one is slightly flatter.

SussexTony · Senior Member

Ya ha ya Steve I understand that they are all the same part number and are the same and my one was split at the bottom! It was just I could not get it to fit so I took engine to Sam at boretec racing in East Sussex and he said it wasn’t me , it was cam guide slightly to wide at the lugs witch fit into the barrel, very weird!

SussexTony · Senior Member

but all done now , running fine ! Here’s a picture of the engine that it would not fit very weird ! and my boy has just got a spare engine as well , not su what he’s going to do with that ! Probably cost me a few quid ! Kids, just got my road bike back so can’t wait until lock down over so I can blow some cobwebs out of the bike and me!

SussexTony · Senior Member

Thanks to totally ttrs for the parts over Christmas period they came within a couple of days and the used parts were like new!

dlee22 · Senior Member

SussexTony wrote:
Ya ha ya Steve I understand that they are all the same part number and are the same and my one was split at the bottom! It was just I could not get it to fit so I took engine to Sam at boretec racing in East Sussex and he said it wasn’t me , it was cam guide slightly to wide at the lugs witch fit into the barrel, very weird!

Hi Tony,

Just to be clear and to avoid any confusion, did you find two different types stamped 4gy? This is what you said in the message but I agree with what Steve said about part numbers as I checked this straight away too. I take it you are fitting it to a 250 barrel?

In the clearer photo above there are two cam guides, has the bottom one split? If so that would explain the different lower mounts to the left of the image.

Cheers!

-- Edited by dlee22 on Wednesday 20th of January 2021 07:07:19 AM