I recently took a closer look at my (RD 350) exhaust ports and I'm thinking that they may have been ported/enlarged. I also found this link on porting from Erlenbach racing (http://erlenbachracing.co/porting.htm ) to help guide me through exactly what is involved. I don't see the exhaust bleed port, although I'm not sure this applies to my bike. Also, my stock exhaust gaskets protrude into the exhaust outlet a bit (creating a small lip).
I have pictures attached if anyone is able to verify any of this:)
400's had the bleeds.
Those look stock.
It looks like the piston skirt has been cut, or non-RD pistons fitted. Or maybe the port floor has been lowered.
In any case, the exhaust port should never be open to crankcase!
Quote from: Dvsrd on July 15, 2019, 07:17:08 AM
It looks like the piston skirt has been cut, or non-RD pistons fitted. Or maybe the port floor has been lowered.
In any case, the exhaust port should never be open to crankcase!
I think what you are seeing is the aluminum removed so the liner is visible.
Quote from: Dvsrd on July 15, 2019, 07:17:08 AM
It looks like the piston skirt has been cut, or non-RD pistons fitted. Or maybe the port floor has been lowered.
In any case, the exhaust port should never be open to crankcase!
D,
Your seeing a casting of the sleeve, not a piston skirt.
That is very normal on RD350's
As stated the '76-78 U.S. RD400 used the bleed hole and so did other 400 models from countries. Not present on 350's for the shorter 54mm stroke engine.
The 400 used them as they are 62mm stroke, so the hole is a easy kick start bleed for starting and has very little effect ant higher rpm range.
Looks stock except some sanding. Yes, the 350 and 400 exhaust ports let the ring gasket hang into the port, 350's a little worse than the 400's.
Just use Yamaha Blaster 200 ATV exhaust ring gaskets. They have a bigger ID. About $7 from Yamaha ea.
There also is a Suzuki gasket that was used too, but that has been lost in my brain years ago, possibility that Russ "The Encyclopedia" may have it in his computer!
Chuck
Thanks again! This has been a big help for troubleshooting and narrowing down possible tuning and performance issues.
Chuck is exactly right. That dark part is the liner and that's how 350 exhaust ports look.
And his second point about using a Blaster exhaust ring was what I did on a recent build on his recommendation and he was spot on.
You have inadvertently stumbled onto one of the greatest mysteries in RD engine design!! If you look in the factory R5/DS7 RD250/350 repair manual they have a portmap comparing the RD porting to the R5/DS7 porting and in it they give a dimension to the "Valley" as I call it in the exhaust port floor of RD cylinders.. actually giving a dimension to it (7mm) means they designed it in!! But why? It's horrible from an efficient exhaust design standpoint. You don't see it before the R5/DS7 and then it's gone by the RD250 2R8 cylinder never to be seen again. I've asked some great minds and never gotten a good answer but some Yamaha engineer from back then somewhere knows!! What you see sometimes though as a student of Two-Stroke evolution is that sometimes the factories would find something that "worked" "did what they wanted it to do" maybe even if for reasons they didn't quite understand yet (they've been learning over the years too) they got the desired result and later realized hey we can do this a lot better this way and not have to have such a horrible exhaust port floor design ect. Tapered main transfers windows on DS7/RD250's is a good example, or the "hump' in RD250/350 main transfer tunnels, as they realized they could do a better job by directing the transfer streams more effectively ect these went away. Ed
Stumbled indeed! Man...I wasn't expecting to hear from the guy who created the webpage I was referencing :righteous: Lots of great information here that I wouldn't have previously considered. Thanks again!