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Could the updated blue dot tensioner be defective?

andrey2004

Newbie
Joined
Dec 14, 2022
Messages
7
Age
38
Location
Shakopee, MN
Country
USA
Snowmobile
2007 Phazer,
2004 RX1
2005 Venture RS
This is very puzzling...

A year ago I purchased 2005 Venture RS sled with ~4,500k miles with a bad tensioner (orange dot), and a jumped exhaust cam (by one tooth). It was ticking.

After replacing the tensioner with the blue dot, (purchased OEM from Partzilla), compression was around 245psi on all three cylinders, I ran the sled for about 50 miles, and exhaust timing jumped during riding & steady speed of about 20-30mph.

Next thing I thought was maybe the timing chain was worn out, so I now have the sled disassembled and motor completely out. Compared old and new chain, and they are pretty much identical in length. Holding them on the side, hanging, the original chain is bending down just a bit. Tensioner side chain guide doesn't show any signs of wear, and the second chain guide has less than 1mm of wear. Everything else - gears, oil, motor's internals look good. I could swap the chain guides, but will need to wait for them to get shipped from Japan... but would only gain less than 1mm. With the new chain installed, the tensioner extends by about 40% of its length.

It bothers me that I will put it back together with only the timing chain replaced, (and it being pretty much identical to original chain). Meaning I haven't really 'fixed' anything. This points me back to the tensioner... could the new tensioner be defective, and should I buy a 2nd new tensioner? Price is up by 35% from a year ago, as they now sell them for $156.00 on Partszilla, and $209.00 at a dealer :(

Any suggestions of why timing would jump with the new tensioner installed?
 

How many times did you rotate the engine by hand after installing the new tensioner? It can take several revolutions to adjust out all of the chain slack so it doesn’t jump a tooth when you fire it up for the first time.
 
I followed instructions listed in the forum here, and so being aware of that possibility, we made about 3-4 rotations. It took multiple tries to make it proper, as we eventually had to 'counter' exhaust cam a little until it settled properly. We checked compression right after. Did it with my brother-in-law, who is a car mechanic.

It was after about 2 hours of riding it, when it suddenly began ticking loudly, and I could not turn engine by hand without 'jumping' a resistance point... it for sure was making contact with the exhaust valve... same as when I bought it. We had it towed back to cabin.

I have rebuilt many engines before, but all were 2 stroke. This is my first 4 stroke troubleshooting/repair.
 
Here is the condition of chain dampers, which seem okay to me, considering remaining potential travel in tensioner. I figure the damper wear is less than 2mm in total.

Tiny wear groove in this one, less than 1mm

IMG-1519.JPG
img-1521-jpg.170402


Almost no wear in this one, which the tensioner pushes:
IMG-1525.JPG


Slight bulge on sides where tensioner pushes:
IMG-1524.JPG
 

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I've never heard of the new one failing but anything is possible. I had a 2019 Honda SXS here this year that all of a sudden became very noisy, and it was a failed tensioner. However, it was still running and didn't skip time.
There are situations where a tensioner can fail and not skip time, more so if the chain is new. And a bit of luck on your side.
Engines with tensioners that have failed are quite noisy at idle. Was it noisy?
It seems very odd to me that it would jump a tooth while running if it wasn't mechanically noisy while idling.
 
I installed the chain, timing marks dead on. Motor wouldn't turn 360, stopping with cyl-3 @ TDC. At that point didn't need a scope :(
Removed the head and.....

This is why I stayed away from 4-strokes for so long... $$$$


valves1.JPG

piston1.JPG
 
In my friends/family group of Yamaha 4 stroke sled owners/riders we literally have well over 100 000 km collectively on these three holers and I’ve never seen anything like this. Come to think of it we might be over 200 000 km now, and I’ve had my wrenches on most of them for ongoing maintenance at some point. This is in no way typical.
 
Funny that a similar thing happened on our 2nd Arctic Cat Z120 sled a few years ago. My kids were riding it and the valve broke in exact same manner, and was driven into the piston sideways. On a single cylinder 120cc engine. I have a picture of it somewhere.

Anyhow with this adventure now. Since the head's valve seat is destroyed, will now need to replace the whole thing.
 
Funny that a similar thing happened on our 2nd Arctic Cat Z120 sled a few years ago. My kids were riding it and the valve broke in exact same manner, and was driven into the piston sideways. On a single cylinder 120cc engine. I have a picture of it somewhere.

Anyhow with this adventure now. Since the head's valve seat is destroyed, will now need to replace the whole thing.
You will never know exactly what happened first but its likely it jumped time at some point before the valve broke. Even at a idle all bets are off for total engine destruction at that point. It looks like you will get away with just head. I would be confident in buying used off a running engine. Shouldnt be too bad and you will soon be rewarded with many years of reliable use. Its very discouraging. Been there. The rewards when out riding do make it worth it.
 


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