Hey Everyone:

I've got a '72 CT2 which I've returned to now that I'm wrapping up with school. About 2 years ago I had the cylinder bored to the next size up and got a new Wiseco piston, since then I've barely ridden the bike. Now that I've returned to the bike, I've started it up and rode it around a bit; however, it sounds like a marble in a coffee can!! The bike will start up in one kick when cold which is awesome, but with the ignition off I can hear some piston slap while kicking it over. Additionally, it sounds like the bike is detonating like a madman when revving it in neutral and when rolling off of the throttle from accelerating... I thought detonation was primarily during acceleration/ under load! (Edit: after writing this and thinking, maybe I should play with the timing again) 

In the past I have:
• Replaced the big and small end bearing on the crank
• Replaced the bottom end bearings on the crankshaft
• Replaced the reed valves

Oh! And I'm running premix, ~ 30:1 as the oil pump was taken out by some previous owner.

 My questions to you fella's:
1. In your experience, do some machine shops bore just a teeeenny tiiiny bit over to "be safe" and avoid an angry customer having a stuck piston?
2. After a rebore to the next size up, is it common to have detonation if the jets in the carb have not been sized up as well? 
3. From a preventive maintenance stand point, what are the primary concerns of piston slap when it comes to cylinder/piston health? I figure the skirt could break apart if its really bad.

Sorry for the long post! I look forward to any input y'all may have, this site has been my bike-bible as of late  .

Thank you - M
 

Just a guess but old style wiseco's like i've used once on my 360 run extra clearance so i wonder if they gave it extra extra or simply made a mistake in measuring. Especially if you can hear it kicking over. Best lift the cylinder & piston to do a simple check for a start & see what feeler blade you can get between lower skirt & cylinder. Pop rings off & insert it as it would be when running, arrow forward--if has an arrow. Probably what ya hearing on a closed throttle from revs is still piston rattle. Be sure the piston isn't contacting the head too. Could reach in with reed valve removed & piston up high in the inlet port & see if clearance can be felt. Then again maybe not as don't want the flywheel magnets moving piston & catch ya finger--was just a random thought.

I don’t like to quote people especially when I am not certain as age / memory set in. 

Doesn’t ??????????  call them; SeizeCo pistons?

None the less a CT2 is not that difficult to find in Yamaha. And detonation may not be directly related to Weisco. 3.5 thousandths might be loose enough for expansion of that specific piston.

Rule of thumb on jetting is go up one or two sizes on the main jet on auto pump delete. 

My guess you’re running lean two sizes too small on the main… easy fix if you catch it soon enough.

asco

If you haven’t taken it apart take a video of it running and post it

Nothing wrong with Wiseco.  I've used them for decades under extreme conditions. 

Hard to find a machine shop these days that sets the clearance properly to Yamaha... or in this case Wiseco specs...  without adding a little extra. 

You won't break a skirt on a Wiseco at least...  stock cast piston could break with excessive clearance. 

Forged pistons need more bore clearance, and are not a good idea for DT motors, as cast pistons are easily available.

When the 2T 500GP class was a thing, all the factories used cast pistons, except for Kawasaki, who used forged pistons for one season before returning to cast.

Remove top end of your motor, and check the piston to cylinder clearance with feeler gauges. If it's excessive, that will be the cause of the noise.

Wow. Thank you guys for the detailed thoughts and opinions! This helps me out so much, I'm so grateful for your help and this site as a research resource.

I absolutely love working on bikes and have taken a fair share of heads and cylinders off so I don't think any of the suggestions are too much! Also, after reading some more I figure its not detonation or pre-ignition going on, more than likely some slappy piston. Regardless taking the head off will certainly help me diagnose some more. I am relived to hear that Wiseco pistons aren't necessarily a bad piston!

- Quick side note of pure Yamaha enduro luck... my friend - who is also a Yamaha enduro fanatic like me - won an auction where we live on a 1978 IT175 for $200 CAD a few weeks ago. Listed as Inoperable (with all plastics and stock parts on the bike) we got it into my garage and its got 130psi compression even with the air filter delete package!! We cleaned the carb and she fired right on up, always a glorious feeling!

Thanks again for the help everyone, y'alls opinions are highly valued!

-M

When those cylinders have excessive clearance, they are pretty loud. If your cylinder does have 2 much clearance, you will need to either live with the noise, or get it bored and a bigger piston fitted.

Bear in mind, that if you have a forged piston fitted, its a good idea to warm the motor up, before using wider throttle openings, which will give the piston the chance to expand as much as it going to.

You can only see pitting on the inside of the head after removing it if it’s truly detonation. Moon craters is a good visual description. My eBay listing for a DT1 GYT CYLINDER HEAD at $195.00 shows previous detonation.

asco

I don’t see in the post if machine shop had possession of piston when it was bored … final hone and fitment of piston is required.