Hello to all ,
I need a bit of expert advice , i have my 1974 DT360A up and running and its going really well with a nice bit of power , good slow tickover and easy starting , however it has blown out the drive/clutch side crank seal for the second time , i thought the first time it was because the ignition timing was a bit off ..... but this time around it was going well ...
Thanks in advance for the help
Regards
Bill

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I'll toss out two comments . . .
1. I don't know what you're using, but the "Genuine Yamaha" seals may fit and hold better then some of the generic replacement seals that are available.
2. Are you using the seal retainer plate that helps to hold the seal in place?

The genuine Yamaha part number for the right side seal is . . . 93103-38086

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Pretty much unheard of on those old girls. Fit a new genuine seal & think you may be able to fit two retainers opposed to each other mounted in the 8mm threaded case puller holes, [it's been a while since looking in at one]. Shouldn't be necessary to do 'that' realy.

Ok so there are two retainers held by 8mm posi head screws , its as if when the bike does a bit of a kickback when starting it has twice ignited the mixture in the crankcase and blown the seal out when it does this it bends the retainers...

not a common story unless the flywheel key is broken. For that to happen i'd imagine the timing would have to be that far out that the it must be firing when the transfers are open which is hell of a long way from near tdc. In saying that i did have just that happen on the mag side of an old yg5t 80cc but think it was purely from a 50 year old seal loosing its tight fit and crankcase compression fired it out.

Wondering if when this happens it's after the bike sits a while? Does the oil in the trans smell like gas afterwards?

My thought is a leaky petcock might be filling the crankcase with gas and the hydraulic pressure forces the seal out?

So while kicking her over to start - BOOM - like a firecracker went off, and you find the crankshaft seal pushed past the retainers ?
If that's the case ...bad gas , old tired spark plug, weak coil, plug wire defective, something along those lines , she's loading the exhaust up with gasoline.
Compression OK? Cold start or a hot motor when it happens ?

Hello to all again , I did get to the bottom of this in the end ...... I had set the points clearance way too small (it may have slipped maybe) but there is a second "bump" on the cam that opens the points , this was causing a second spark about 180 degrees out , this spark was igniting the crankcase mixture and blowing the seal out...

Regards
Bill

Thanks for the update & good you got it sorted. I'll have a think about what you said, but good news.

Exact same problem.
Exact same bike.
Exact same first name.


Bill