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TOPIC: Rebuilding my friend's At1should I advise going to 175cc?

Rebuilding my friend's At1should I advise going to 175cc? 06 Aug 2019 21:09 #1

  • Titan Joe
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I've owned a few bikes, but never any 2 strokes. I've read enough one here to get me into a major project.

Here is the deal: my friend rescued an AT3 from the junk yard with his grandfather many years ago by putting two identical bikes together that were next to each other. Now he wants me to fix it up so he can ride it. Using the resources here i was able to get it running again, but the question is where should I advise him to go from here?

I like more power so I think the 175cc top-end swap sounds great. I think I found a few possible core engines at a nearby boneyard, but I wanted to make sure that I got the right one. I wanted to know externally how I can tell if I have a reed engine or a piston-port engine? I tried to find some pictures and I assume that there is something that makes the reed apparent like a squareness to the intake port, but I don't have much to compare with.

I assume that I should also find a carb from a 175 to keep it simple. These core engines will probably need to be bored out from years sitting in the junkyard and I sounded like the pistons are the same between reed engines and piston-port engines. is that correct?

The bike is also devoid of an airfilter element and the intake boot into the carburetor. I've seen some of those aftermarket ones used on the early AT1's on this site. I would like to keep the re-tuning of this bike simple since it is not going to be under my watchful eye for very long. So I was thinking that it would be best just to turn it into a 175 in mostly stock form.

The biggest reason for considering this swap is we were planning on rebuilding the engine to get compression back and to make it cease it's leaky behavior. At this point is it rideable and seems to be oiling just fine.

Suggestions are welcome.

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I like burning petrol.

Rebuilding my friend's At1should I advise going to 175cc? 06 Aug 2019 23:59 #2

  • Gr8uncleal
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All sounds quite reasonable to me.

It looks like you've already sussed it, but just to confirm, the reed valve sits between the carb and the barrel, is retained by 4 bolts and is square in shape.

Good luck!

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Rebuilding my friend's At1should I advise going to 175cc? 07 Aug 2019 00:19 #3

  • Devin
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if you do the 175 swap, An AT3 would use the same carb as a 175 CT3.

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Rebuilding my friend's At1should I advise going to 175cc? 07 Aug 2019 06:10 #4

  • MarkT
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And no, piston-port engine and reed valve engine pistons are not the same.

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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

Rebuilding my friend's At1should I advise going to 175cc? 15 Aug 2019 10:22 #5

  • jet100
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I just did the 175 conversion on my AT3 and highly recommend it. Barrels are a little hard to find but a CT3 one turned up on e-bay for $35. Heads are plentiful as are genuine Yamaha piston/ring sets. Prices vary wildly on e-bay so shop carefully. You need a 175 piston pin too and clips. I had my barrel bored .25 over and splurged on vapor blasting the barrel and cylinder head. Looks great. Carbs are identical. Downside is the electric start struggles now. A fair trade in my opinion. Just upgraded my clutch too with new springs and plates. Good luck.
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Rebuilding my friend's At1should I advise going to 175cc? 15 Aug 2019 22:20 #6

  • KJB
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How about using a "chain saw type" decompressor and adapter into the spare plug hole ?
Anyone done this on an electric start AT to CT conversion?

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KB ( Australia)
'69 Yamaha DT1 S
'71 Yamaha DT1 .................................. '70 Suzuki T350
'72 Yamaha AT3 . ................................... '73 Suzuki T500
'86 Yamaha DT175 ..................................'68 Fuji Rabbit Hi Super 90
'06 Yamaha PW50





Rebuilding my friend's At1should I advise going to 175cc? 15 Aug 2019 23:51 #7

  • Rick C.
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KJB wrote: How about using a "chain saw type" decompressor and adapter into the spare plug hole ?
Anyone done this on an electric start AT to CT conversion?


I try to stay away from those that go into any plug hole, even though I have maybe 3 in stock.
Those tend to let the crud into the cylinder/ piston area , which is always a bad thing for these bikes...or any bike.

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