qt50 top-end post mortem

Rob LaFreniere /

To any and all,

What do you think were the causes for the following piston/ring damage?

background:

1984 Qt50 with 3600 miles

New oem PW50 head

New oem PW50 cylinder with mildly chamfered ports.

New oem NOS QT50 - standard size

New oem NOS QT50 rings - standard size

New oem base and head gaskets.

New oem reed valve gaskets

Boysen reeds

standard plug, whatever the manual calls for.

rebuilt Mikuni carb with new gaskets and 70 main jet.

MLM Peoples pipe

Uni Pod air filter.

Oil pump still intact and connected, presumed working

Using 50-1 premix

Replaced both crankshaft seals

compression at 110 psi

a leak down test held 7 psi for 6-7 minutes.

While, playing with jets and needle settings, there were occasions where it was running lean, which I determined by noticing a surge in power when applying the choke.

I was going 35mph on a level road, 30 going up a slight incline. Starting from a stop on a incline was painfully slow.

Temps where usually between 340-360. I saw it once go up to 378. I might have a soft seize at that point, the engine died but I didn't feel anything lock up. not sure what a seize would feel like. The bike started and ran a minute later.

There are perhaps a total of one and a half hours running on this top-end rebuild.

My guess is that it was running lean partly because of running premix with the oil injection still working in combination with poor carb settings.

I would like to know if it is possible that previously existing debris inside the crankcase could get inside the cylinder and cause this kind of damage.

I was about to install a malossi kit this weekend until I saw all this.

More photos to come.

Thanks,

Rob

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Re: qt50 top-end post mortem

Rob LaFreniere /

more photos:

IMG_20190902_173513.jpg
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Re: qt50 top-end post mortem

Rob LaFreniere /

some more photos:

IMG_20190902_173453.jpg
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IMG_20190902_173443.jpg

Re: qt50 top-end post mortem

Rob LaFreniere /

last batch

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Re: qt50 top-end post mortem

Pre mix and oil injection?

Re: qt50 top-end post mortem

Rob LaFreniere /

I did remove the oil pump a month ago and have been just running on premix.

Re: qt50 top-end post mortem

> Rob LaFreniere Wrote:

> -------------------------------------------------------

> I did remove the oil pump a month ago and have been just running on

> premix.

So that is a stock piston that was subjected to who knows what while running on stock injector? Stock injectors on these are fine for stock application as long as they dont run out of oil or the injector hose does not come off the carb.(edited)

Re: qt50 top-end post mortem

Heat and lots of it. Too low a mix ratio can do this or a no oil mix.

Re: qt50 top-end post mortem

That's not debris damage, that's a four corner seizure. The underside of the piston crown is burnt so you know it was getting too hot.

Re: qt50 top-end post mortem

Rob LaFreniere /

I appreciate the feedback. This was my first foray into engine rebuilding and carb tuning. I read as much as I could find in regards to the qt50. I'll try to find a book about 2 strokes in general before trying again.

Re: qt50 top-end post mortem

Jack Rutherford /

70 main jet is too small with MLM pipe.

"My guess is that it was running lean partly because of running premix with the oil injection still working in combination with poor carb settings."

If you were running premix and oil injection, you were running even leaner because more oil is taking the place of gas. Gas not only powers the engine but cools it. Too little gas means not enough cooling.

More oil is not a good thing - especially too much oil.

Your temperature of 378F is not necessarily a deal breaker for a properly jetted bike but since you were way under jetted and running too much oil, that probably led to your soft seize. Qt's also tend to build up temperature as you go up even slight inclines. You are always fighting heat build up on those damn things.

You need around a 75-78 main jet in the stock carb for stock cylinder bike running an mlm pipe. Also some of those PW heads are high compression heads. Probably not the case with yours since your compression was only 110psi.

Here are my latest findings with the qt. If you open the exhaust port completely and jet it right, it cools down real fast at full throttle. Mine would tend to get hot at idle after coming to a stop. I went to a slightly bigger main jet which should help (SHA carb on this one so main jet affects idle).

Re: qt50 top-end post mortem

I see you have a new piston and new cylinder. Did you check the piston to cylinder clearance before assembling? What was your clearance?

Re: qt50 top-end post mortem

Rob LaFreniere /

I did not, I plead ignorance.

Re: qt50 top-end post mortem

> Saabsonettguy . Wrote:

> -------------------------------------------------------

> That's not debris damage, that's a four corner seizure. The underside of

> the piston crown is burnt so you know it was getting too hot.

That was the first thing I noticed. All four corners and beyond. Underside is burnt incoming charge from overly hot piston.

Mixing a qt50 piston & rings with pw50 cylinder would mandate measuring bore vs. piston.

Re: qt50 top-end post mortem

Jack Rutherford /

pw50 cylinder = qt50 cylinder. Same goes for rings, piston and head. Same engine. No mention of any piston chatter which might suggest clearance between piston and cylinder was too great. If piston to cylinder clearance was too little . . . I guess it's a possibility but these pistons and rings are interchangeable between the pw50 and qt.

failure to up-jet from stock main jet when running performance exhaust and higher flow air filter led to seizure.(edited)

Re: qt50 top-end post mortem

Rob LaFreniere /

This is what it looked like after some break-in miles.

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Re: qt50 top-end post mortem

Jack Rutherford /

ever so slight scoring on the piston suggesting what was to come when you put a significant load on the engine up the incline.

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