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Author Topic: I stepped in it - XS750/896 build  (Read 2461 times)

Offline Frenchtom

  • Posts: 15
Re: I stepped in it - XS750/896 build
« Reply #30 on: Jun 04, 2018, 07:24:03 »
Hi Nanno - yup, this would be my first time.  But luckily I have two heads in case I screw one up so I would like to try some king of porting although I am not seeing what you see as far as opportunity goes.  These ports look pretty clean to me but then again, I am no expert.
Tom
1963 Honda Sport Cub
1977 Yamaha XS896

Billet: about as creative as duct tape

Offline der_nanno

  • Posts: 358
  • Faster!
    • Greasy Greg
Re: I stepped in it - XS750/896 build
« Reply #31 on: Jun 04, 2018, 08:24:43 »
No problem.

There's basically two schools of thought on how to port an engine.
1) increase port speed
2) increase amount of flow through the port (volume basically)

Unfortunately 1 and 2 contradict each other. Because the bigger the port gets, the lower the speed. Which incidentially is also the reason why installing a HUGE carb on a small port won't work. (Because the vacuum created in the jet's orifice will be weaker...)

Luckily the above only partially applies to your problem, as you don't plan to go berserk on the poor port, but just improve what's there. In this case this means, to knife-edge the bosses around the valve guides, so the air flows nicer around them (increasing speed and decreasing turbulence in the port. The second part will be clear up the area BEHIND the valve stem as you want to slow down the air-fuel mixture, but leave a knife-edge separator to help aim the gasflow towards the combustion chamber. What you want to end up with is a teardrop-shaped boss around the valve stem, with a small knife-edged ridge going to the back. (on the inlet) The walls on the inlet-port can do with a gentle touch up with a mini-flap-wheel on a dremel, but DON'T polish it as that will actually be counter productive.

On the outlet all you want to do is to knife-edge the boss and blend it in towards the valve guide. The exhaust port can be polished, but it's more to give you that warm-fuzzy feeling of having done something to the exhaust port as well.
Real freedom starts with the freedom of thought.

My Blog:
http://greasygreg.blogspot.co.at

My Mule TR1.1 build here on DTT:
http://www.dotheton.com/forum/index.php?topic=75458.0

My Turbo TR1.1 (XV1000) build here on DTT:
http://www.dotheton.com/forum/index.php?topic=72571

My XS Triple Sidecar build here on DTT:
http://www.dotheton.com/forum/index.php?topic=74211

Offline Frenchtom

  • Posts: 15
Re: I stepped in it - XS750/896 build
« Reply #32 on: Jun 05, 2018, 07:20:55 »
Hi Nanno and thanks.  I understand everything you described although "knife-edge" is a bit interpretive.  I found the following image and I am thinking that this is what you mean?

Also, do I remove the valve guide or leave it in when I do the machining work?  I have an air tool set and plan to use flapper wheels and maybe a grinding stone if I am feeling confident.  More than likely I'll stick with some kind of sandpaper based bit.

Tom
1963 Honda Sport Cub
1977 Yamaha XS896

Billet: about as creative as duct tape

Offline der_nanno

  • Posts: 358
  • Faster!
    • Greasy Greg
Re: I stepped in it - XS750/896 build
« Reply #33 on: Jun 06, 2018, 03:35:16 »
Hi Tom,

yep, that gets pretty close. The line on the front is called a knife edge and depending on port design, it can be more or less prominent. Keep in mind, that the port you're showing here has the valve-boss sitting almost perpendicular to the port, whereas it is more countersunk on the XS. You leave the guide in and at best (for a first try) don't touch it. Remember you have to do this in an equal way three times, so best keep it (relatively) simple at first. Also in the port in the picture, they decided to go with lots more volume (and thus a lot lower port speed), as they could then directly reach the valve, shortenning the distance dramatically the gas has to travel through the port.
Real freedom starts with the freedom of thought.

My Blog:
http://greasygreg.blogspot.co.at

My Mule TR1.1 build here on DTT:
http://www.dotheton.com/forum/index.php?topic=75458.0

My Turbo TR1.1 (XV1000) build here on DTT:
http://www.dotheton.com/forum/index.php?topic=72571

My XS Triple Sidecar build here on DTT:
http://www.dotheton.com/forum/index.php?topic=74211

Offline XS750AU

  • DTT BOTM WINNER
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  • Posts: 406
Re: I stepped in it - XS750/896 build
« Reply #34 on: Jun 06, 2018, 06:17:50 »
I don't want to dampen your enthusiasm, and it is great that you have 2 heads and can afford to experiment with one and not worry about losing a head.
I started having this debate when I was building the 896. The debate about gas speed vs volume worried me as I was not experienced with porting. I was all for opening up the exhaust port but was warned about reduced exhaust gas speed affect on ultimate power. In the end I just cleaned up the ports and polished up the exhaust side.
The gas flow on my XS896 has been improved by Mikuni BST36SS cabies and using triumph 1050 header with a shorty can.
It pulls hard to redline in all gears and does not run out of puff, it runs out of gears! So I am not sure that you will actually get any measurable/felt improvement by porting alone?
Both your heads are the later design which is good and provides a higher compression ratio, and more aggressive cam timing.
Sure give it a go, but without carbies and header work you may be a little disappointed.
“Engineering is the art of being approximately right rather than exactly wrong.”

Yamaha XS750-2D
Yamaha XS896
Husaberg FE550
Yamaha TT250
Yamaha IT200N

Offline datadavid

  • Posts: 1318
Re: I stepped in it - XS750/896 build
« Reply #35 on: Jun 06, 2018, 06:43:44 »
I just sanded off some casting imperfections in my intake ports to decrease turbulence and it wheelies in first, no clutch and body weight forward, so it must have worked!

Offline Frenchtom

  • Posts: 15
Re: I stepped in it - XS750/896 build
« Reply #36 on: Jun 06, 2018, 21:04:03 »
Hi guys and thanks for the input.  For some background, I just want to try my hand at machining the ports and make some marginal improvements since I probably won't have the interest once the bike is reassembled.  I am going to do some other speed shop stuff like getting the pistons and rods all the same weight and try to balance the crank.  My goal is an above average, smooth running bike although pulling wheelies in first gear sounds like a lot of fun!
Tom
1963 Honda Sport Cub
1977 Yamaha XS896

Billet: about as creative as duct tape

Offline datadavid

  • Posts: 1318
Re: I stepped in it - XS750/896 build
« Reply #37 on: Jun 07, 2018, 08:56:03 »
Since it does that basically untouched, who knows what a mild port job can do? I just got the advice from several people that it looked quite good and just needed some light smoothing on the casting surface, but it can definitely be improved.