Transfer Ports - 400 motor

[adrian_goold]

Guys,

Going over my '74 400 motor yesterday while it's in pieces.
Can anyone give me an explanation as to why the lower edge of transfer ports are considerable distance lower than the BDC setting of the piston top?
ie
If the stroke + deck height = 69 + 1.5 = 70.5mm.
The lower edge of the transfer is 73mm.

Just something i noticed as i was doing a shading of the port layout on paper, and some more knowledgeable guys might know the answer too.

p.s. Would someone have the carl cranke porting specs for a 400? if they were available?
I have the 250, but not the 400.

regards,
Adrian

Regards,
Adrian Goold #101
http://www.ozktm.com

'73 175 Rebuilt|'74.5 250 Rebuilt|'74.5 400 Restoring|'76 MC5 250 Restoring|'76 MC5 400 Restoring|'78 250 in pieces|'81 250 Now Racing| '81 495 Now Racing|'84 125 Selling|'07 KTM 300| 74 CZ 250|74 CR250M x2|

[JP Morgen]

I have seen many engines, some very high performance, with the bottom edge of the transfers and exhaust lower than the top of the piston at bdc. Why?, I have never read an explanation. The only 400 motors I have built were with reed valves, I can dig up those specs. if you want.
JP

[454MRW]

I have seen different engines that had the tops of the piston notched in the port area to expose the ports earlier and allow them to remain open longer. I suppose that would actually be easier to experiment with port timing than to have several differently modified cylinders, only requiring changing pistons. Mike

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
1975 Can Am 175 TNT & 77 250 Black Widow
1979 Husqvarna OR390
1976-78 RM & 77-79 PE Suzuki's
1974 CR250M 07 CR125R 79 CR250R

[adrian_goold]

JP,
that would be great to have some specs/info on that setup. What reed setup do you use? i do have Gem on a 250 top end i could use. But they look small compared to say an equivalent yamaha...

Mike, yep i understand the filing of pistons to change the transfer timing, i will post a photo of what i mean tonight (of my shading port shading). I would like to increase this timing, but using the dykes ringed piston to cant file it down to achieve this change. Unless JP knows of a way?

What i am trying to achieve is match port the setup, ie when the piston is at BDC (+ dim x), the exhaust port is matched to achieve maximum open time. This is where i discovered the transfers lower edge being 2.5mm lower than the exhaust port...hence not fully open at BDC.
Maybe it was done for better piston cooling with the charge of fresh fuel?

Adrian

[gooddirt]

I have the front forks you are looking for but shipping... hummmm?

[JP Morgen]

Adrian,
  Here are some old hop up specs. I saw printed: exhaust timing 188 degrees= 37.5mm from top of barrel, transfers 128 degrees=53.5mm
pipe 48-56.5X89
     56.5-56.5X232
     56.5-90X338
     90-100X50
     100-95X110
     95-26X340 all numbers are inside diameters in mm times(X) the length of the section. Kind of weird pipe, I never tried one, I have used 495 and 420 pipes with good luck, and as I said all the motors I built were with Yamaha six petal reed cages.

[adrian_goold]

Cheers JP! Will look into it.

Gooddirt, i have PM'd, but have heard this is not working. Could you send me a message on [email protected]

Cheers Adrian

[adrian_goold]

gooddirt, could you send me a message on [email protected]

Cheers,
Adrian

[gooddirt]

OK