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Makotosun

1972 Yamaha RT2 360 Does this sound like detonation?

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You're really not trying to keep the "wet" in or out.

For this design of two stroke engine the crankcase must be sealed well for both vacuum and pressure. 

If mag seal doesn't hold vacuum, when piston goes up, it will draw in fresh air through the seal instead the of proper ratio air/fuel mixture through the carb.  If it runs, it will be lean.  "Runaway" is a common and very scary result...  engine destruction is also common.  Meltdown. 
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 Jun 2022 19:51 #31

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  • Tom P
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In all the years I rode my CT1 back in the 70s, with the original seals that did not have a lip on the outside of the magneto seal, I never had runaway condition, and others I knew that had Enduros back then never had that problem.

It could be if you're racing on a regular basis and exposing the seals to a lot of heat and abuse, maybe the seals just wear out quicker, and need to be replaced more often, but I just can't see how having that fragile spring lip on the dry side could be a good thing. Without being lubricated, I just can't see that fragile outer mag-side spring lip lasting that long. The only way it could be lubricated, is if the inner lip leaks.

We can agree to disagree, but I think the Yamaha engineers got it right back then.
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Last edit: 24 Jun 2022 20:36 by Tom P.
24 Jun 2022 20:07 #32

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  • The3ngineer
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I couldn't find the OEM crank seals and the ones from ebay didn't have the spring so I ordered a set from Australia that had the inner and outer spring. Shipping took 3-4 weeks but I feel better knowing I did all I could. 

Anyone know where the shift lever spring lives? 
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24 Jun 2022 20:52 #33

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Yamaha Enduros larger than 175 had dual lip seals on the mag side.  Not "fragile" at all.  It's more robust.

The smaller engines have less "pulse"...  much smaller piston...  and can get away with a more marginal seal most of the time.  175's are pushing the limit a little harder than a smaller engine the way I look at it.

125 and 175 YZ's and IT's came with a dual lip mag seal that's readily available...  fits the Enduro 125 and 175 and almost the same price.  So why not use the better seal?

I benefited greatly from a mag side crank seal issue on the very first Enduro I bought when I got back into riding a decade or two ago.  Second owner 1979 DT175 that had been garage kept it's whole life.  Like new, low miles.  Owner wanted to teach his kids to ride.  It ran ran lean and had a runaway condition as soon as it warmed up.  Classic crank seal air leak symptom.  He had spent well over what the bike was worth at the Yamaha Dealer and a local independent bike shop.  They could not fix it.  They told him it was probably the CDI. I got it super cheap. He just wanted it gone.

He had receipts...  both shops had pressure tested the engine.  No leaks.  I figured they screwed up or charged him and didn't test it or something.  My heart sank when I pressure tested and engine held 5 psi overnight.  

Then I vacuum tested.  I could not get my miyvac to pull a vacuum.  Hose off, golf tee in hose... mityvac worked fine.  I packed thick grease around the crank seal and it held vacuum.  Replaced the seal with a dual lip IT175 seal and bike ran like new.  $6.00 in parts..  

I learned to vacuum test after losing a race and an engine decades ago.  Rebuilt the night before the race, new seals, pressure tested fine.  Melted down in practice.  One of the older, wiser racers suggested we pressure test before tearing down.  Still pressure tested fine.  Leaked vacuum like a sieve on the ignition side.  It was a dual lip seal but had a defect.  I got a new seal for free from the dealer.  But that was the only race I didn't finish top 3 all season and cost me the championship. 

That said, there are many 175's running with the original single lip seal.  I think I have two myself.  If I change the seals, I'm always going to get a dual lip if I can.  The cost difference is insignificant and...  knock wood...  dual lip is readily available.  

Earlier 250/360/400 models are tough to find dual lip seals for.  So sometimes someone might have to go with the single lip.  And again, single lip can work fine even on the bigger engines that never had them.  Until they don't.  That's a much harder choice due to cost and availability than upgrading the seal on a 125/175 to dual lip in my book.

I have a friend who buys whatever the cheapest no-name brand oil and filter he can get on sale.  Almost never uses the same brand twice. "oil is oil".   I tend to try to get the best price on quality filters and oil...  and rarely change brands.  That's what my dad taught me.  He gets along fine the way he does it.  I sleep better at night the way I do it.  That's what makes the world such a great place.  We are free to choose what we prefer.  

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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
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24 Jun 2022 21:08 #34

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  • Tom P
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MarkT, I have a ton of respect for you, and I really appreciate all the advice you've given me on this website. It's helped me to get my old CT1 running, and I enjoyed a nice ride around town earlier today.

However I'm going to have to clink my beer to yours and disagree on this one. I think your post actually Illustrated what I was thinking. That the double lip seal on the mag side was sort of a racers quick fix for an issue that occurred because of a lot of engine abuse during racing.

It's like any type of racing, everything mechanical is expendable. You just want it to get you through the race, so you can win.

But for the average Joe just riding his old Enduro on trails, and around town... do you need a double spring lipped seal running dry on the magneto side? I don't think so. I think in the long run, for an average rider, you'd be better off not to have that fragile spring lip on the dry side of the magneto seal. I just don't think that fragile spring lip is meant to be run dry for extended periods of time

Just my humble opinion, and I'm not trying to start a fight, just being honest.
24 Jun 2022 21:29 #35

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No problem at all Tom...  Just remember if you get a bigger bike...  an Enduro...  not a race bike...  the Yamaha engineers used a dual lip seal on both sides.  Those engines don't rev as high or work as hard as a 125 or 175.

And the Honda engine I was racing...  a 350...  was a modified recreational engine and dual lip seals were also standard. 

My buddy will argue all day "oil is oil"...  and he has never had an oil-related engine failure to back up his claim that special manufacturer oil specs and weights are just a way to charge you more. 

You're not doing that... you're using standard spec parts in your CT.  I don't see a problem with that.  I just prefer that extra cushion of safety especially since it doesn't cost much. 

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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
24 Jun 2022 22:13 #36

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  • Tom P
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Yeah I am one that tends to stick to stock spec parts, and for the record, I don't skinp on oil, I run good oil and OEM filters in all my vehicles, and I have Yamalube in the CT1.

The double lip IT seals on the magneto side... are they exactly the same as the one on the clutch side? If I were designing them, I think I would make them a bit more robust for the dry side.
25 Jun 2022 07:07 #37

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Petrol on the fire time.
I have six CT engines. Some have been split prior to my ownership. All those have double lip single garter spring seals in place .
The ones that have never been apart have both . The one with the lowest miles on it has double garter double lip on the mag side and started once the carb had been sorted after standing for donkeys years. That's the CT1B.
The one ive just stripped had a double lip double garter on the primary side and a double lip single garter on the mag side .
Sorry for that input but i'm just saying what i've found on these. All the others are double lip single garter ( HT1B,RT1B, DT3,RT2 MX). Crank seals of course.
Last edit: 25 Jun 2022 08:27 by shyted.
25 Jun 2022 08:25 #38

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  • Tom P
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This is a picture of both sides of my original mag side seal. It actually does have two lips, but only the internal lip has a spring. The new one I purchased looks just like this one, which is how automotive seals look. The outside (dry side) of the seal has a lip but it doesn't have the spring and is solid. So technically, I do have a double lip seal 

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25 Jun 2022 08:58 #39

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Yes, a dust lip.  Not a true SW seal. 

And no, the seals are not the same size.  Yamaha crank seals for crank are made of special materials and are very high quality..  Some type of coating on the seal lips.  Not just a plain automotive shaft seal. 

You're fine either way.  For the tiny added cost I'd simply rather go with the more robust choice. 
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
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25 Jun 2022 12:38 #40

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