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Makotosun

Air Coolled Two Stroke Paranoia.

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Air Coolled Two Stroke Paranoia. was created by Tinkicker

Inspired by the how is this running thread.

Started the DT and got it fully warmed up at the weekend, first time since around September.  I have fallen victim to the two stroke rattle paranoia.
The cause of my paranoid episode is that after about 150 gentle miles since rebore and 10 or 12 heat and coolling cycles, it was time for a full throttle run to check the carburation at wide open throttle.
About half way in my full throttle test, an artfully bodged exhaust repair by the previous owner became apparent as a large piece of carefully sculpted exhaust repair putty blew out of the downpipe, the carburation immediately went very lean and it lost a lot of power.
I found a replacement pipe and happily rode it maybe another 50 miles.  It starts second kick when cold and first kick when warm, idles at 1500rpm all day long and yet...

Started it in the back yard, warmed it through enough to let it idle and went back in the house for a minute.  Came out and from where I was stood looking, it sounded horrendous.  Plenty of oil getting to it because I am one of the old school who, despite autolube and fully synthetic Belray oil,  still adds a small splash of oil to the tank every fillup.  Even just to prevent fuel system corrosion.

And of course, having the bike sat drained and idle all winter, the paranoia kicks in...  Did it sound as rattly as that before the top speed run?  Have I blown the piston?  I checked the big end for radial play carefully, but was it corroded after being stood for 40 years and has given up the ghost?

Nothing for it.  Gonna have to pull the barrel, check the piston and bore, crank for play and while I am at it, replace the stator side crank seal with an IT175 item and redo the timing for peace of mind.

Does anyone else fall victim to the two stroke rattle after a long winter layup?  I was happy with it, although felt the performance was not quite as I remembered it from my youth when I laid it up for winter.

 
21 Apr 2023 02:35 #1

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Replied by Tinkicker on topic Air Coolled Two Stroke Paranoia.

Barrel off. Found light scoring of the surface above the ports, not too bad, it will clear with a couple of minutes with the hone. Worse near the bottom of the cylinder, below the exhaust port. Some of those may not come all the way out with a hone.
Not too concerned with those.

The piston itself is not scored which is odd. Just gone dull on the thrust faces.  I was expecting to see black marks of a partial seizure on it.

The rings look ok visually, but I would say the gap is around 25 thou. I have a new TKJR piston kit for it and I measured the gap using a new ring.. 12 thou, which is the minimum allowable.

My micrometers, hone and bore gauges are at work, so cannot do more to check for size, oval and taper at the moment. Besides I need a new top end gasket set.

I reckon its just bad luck. A tightly bored bore, perhaps a full throttle run carried out a bit too soon and a chunk of filler blown out the exhaust causing a massive lean out.

At least the crank still feels ok.

I will be more carefull next time.
Last edit: 21 Apr 2023 02:53 by Tinkicker.
21 Apr 2023 02:46 #2

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Replied by RT325 on topic Air Coolled Two Stroke Paranoia.

All interesting info. Waiting for the next episode. Cheers.
21 Apr 2023 03:00 #3

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Replied by Tinkicker on topic Air Coolled Two Stroke Paranoia.

Next installment is not good news. Cleaned up the jug and gave it a closer inspection.

There is a slightly raised "imprint" of both piston intake window upper halfs about half a window depth above the intake port.

I am guessing the engineering shop boring machine has shot bearings and bored the barrel like a dogs hind leg. Only thing I can think of.
Piston skirt is nice and tight at the bottom of the stroke and is very loose up around the ports.

Despite over ten years in the motorcycle industry I never saw wear, especially very short term wear of the like before.
Last edit: 21 Apr 2023 06:54 by Tinkicker.
21 Apr 2023 06:52 #4

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Replied by MarkT on topic Air Coolled Two Stroke Paranoia.

When I was reading your story, my first thought was about quality of the machining...

Here in the USA, it's hard to find someone that knows how to recondition a cast iron two stroke cylinder properly. I think many of us have experienced the fresh rebore getting noisy almost immediately.

It's likely nothing you've done... some machines/operators struggle with not ending up with a tapered bore due to the ports (from what I'm told, I'm not a machinist)... others bore off-center or at an angle...

By far the most common I've seen is a well-meaning shop doesn't follow factory specs and "adds a little extra clearance" to keep the air cooled two stroke from seizing... which is a huge mistake, doesn't work, can actually make the piston MORE prone to seizing, and makes for a engine that rattles and then needs a bore much sooner than if factory specs were followed.

A bit of trivia I read about years ago is Suzuki once had a racing two stroke where the pistons were interference fit into the bores when cold. They had to heat the engine before it would even turn over! The piston crown takes a lot of heat and has to shed that heat through the rings and by transfer from piston skirt to cylinder wall. Excessive piston clearance can make a piston MORE likely to overheat and seize when run hard than one that has the proper clearance.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
The following user(s) Liked this Post: AirborneSilva, Ht1kid
Last edit: 21 Apr 2023 07:56 by MarkT.
21 Apr 2023 07:55 #5

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Replied by Ht1kid on topic Air Coolled Two Stroke Paranoia.

Thank goodness I have a local Yamaha shop whose operator has been boring since these bikes were new and knows the correct specs he’s semi retired but comes in to do the boring on these older Yamaha’s 
   PS he knows the difference between Yamaha and Weisco piston clearance 
Last edit: 21 Apr 2023 15:01 by Ht1kid.
21 Apr 2023 14:58 #6

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Replied by msavitt on topic Air Coolled Two Stroke Paranoia.

mark is so right about certain "experts" trying to add a thou or two to the spec because they know better than a bunch of Yamaha engineers that likely tested these engines to the limit in controlled lab environments.  
21 Apr 2023 17:11 #7

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Replied by Tinkicker on topic Air Coolled Two Stroke Paranoia.

Well I had the bore gauge down the cylinder. It has been bored barrel shaped with 10 thou difference between bottom of cylinder and mid cylinder.

The piston skirt is nice n tight at BDC and rattles around at TDC, and that is where the imprints of the piston windows are.

What a mess. I have made an executive decision to scrap the liner and get a new one made and fitted. It costs about 3x the price of a bore job, but at least it will be standard again and will be done by a reputable motorcycle specific shop. To bore it may take it to 1mm over to clear.

So thanks to my friendly local car engine reconditioners machine shop, my unworn, std but rust pitted bore has now become 1mm over in one rebore attempt.

Question. Is the OEM 3J1 piston from the 1981 DT175mx the same as the 2N5 (1979) apart from the part number?
24 Apr 2023 08:48 #8

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Replied by MarkT on topic Air Coolled Two Stroke Paranoia.

Probably a slight difference in piston. Enduronut may know if he sees this.

Generally, it's the same piston and as long as you get rings to match piston it would work.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
24 Apr 2023 17:42 #9

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Replied by Tinkicker on topic Air Coolled Two Stroke Paranoia.

I have ended up buying a NOS genuine Yamaha 2A7 std piston and genuine std rings.

I need to strip the furniture off the barrel, parcel it up and send it to the specialists for re sleeving this weekend.

Stupid of me trying to cut corners with a local car engine reconditioners. I am betting the jig borer tool clattered around the ports and they tried their best to clean it up with the hone. One thing boring a four stroke solid cylinder, quite another doing a two stroke.
Of course, if the piston crown clearance at TDC feels fine, and the skirt clearance at BDC feels fine, one does not expect the center to be off and I never checked.  Maybe I should have.  All I checked was the ring gaps about 10 mm down the bore.

I have double checked everything else. Piston is not scored, nor showing signs of seizure, although there is a polished streak, maybe 8mm wide where it appears to have run tighter than the rest.  Approx 45 degrees to the pin.  There is a blowby mark directly opposite this.

It has been run exclusively on Belray SI7 oil, the min oil pump stroke has been double checked and is 20 thou, it is using around  1/4 tank of oil to a tank of fuel and always has a small "dash" of oil added to a full tank of fuel to help lube the carb slide and prevent fuel tank corrosion.  I doubt lube is a problem.  

Never seen anything like it before.

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Last edit: 28 Apr 2023 07:02 by Tinkicker.
28 Apr 2023 06:43 #10

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