Camshaft cutting for more overlap

[Expert]

Decided to get the degree disk on the bucket today to find out if cam timing is stopping the motor from reving out and was suprised by what i found out.
At 40 thou lift there is no overlap at all on the cam and there is actually a five degree period when both valves are effectivly shut in the middle of overlap. I got Exhaust opens 54 BBDC, closes 18 BTDC, inlet opens 15 BTDC, closes 19 ABDC. I retarded the cam a bit to get it roughly symetrical about TDC but am now thinking of boreing down the center of the cam, finding a bit of tube to fit up the middle, cutting down between the lobes to seperate them, move them together to get a bit of overlap and brazing them onto the pipe, voila hot cam!
My dad reckons he did this successfully several times when he used to race old british bikes but i wondered if anyone on here had any experience, particularly on brazing chilled iron cams and how it affects the hardness?
The lift is adequate for the diameter of the valves, just need some overlap to help with the revs.

[scumdog]

Sounds like a recipe for disaster.

Your duration and lift haven't changed and very likely you'll get no more power.

[Motu]

Your exhaust valve closes before TDC? Even a Briggs & Stratton doesn't do that.Get another cam and check again.

[Ixion]

ATDC, I presume a typo?

It sounds a monstrous bodge up, but its just the sort of thing that might work. But as Mr Scumdog says, with no change to lift or duration is it going to help much. On some old Briddish shitters you could build up the cam followers to give more duration, but I doubt that would be practical with an OHC.

[Motu]

If it's a typo it has 33 deg of overlap,not exactly sizzling but respectable.

[Brian d marge]

What bike is this , you might find it port choking , rather than cam .. My ole 500 cc Enfield single actually has very respectable cam numbers ,,,, and produces all ( on a good day ) off 20 BHP ( well maybe not Bhp ..more sort of Hp~ )
the port is 33 mm dia ,, the obstruction , is the bowl area of the port ,,,

off the top of me head , it was found that a max of 30 hp was gained from a 8 to one comp and moving the ex cam slightly 4 degrees I think with carefully shaped seats .. oh and a freer flowing exhaust ,,NOT loud ,,just made to work like it should

But as I said before ,,the thing with buckets is that you can try ,,, Make a plan ... the more reseach the more questions asked the less chance of a FUBAR ...

But if you do FUBAR it ,,,so what ,, cost of a engine is ??? a lot cheaper than ROSSIS !!!

SO .. go for it I rekon !!!


ya never know it might work !

Stephen

[Expert]

No Typos,
The exhaust valve is effectively shut BTDC. All measurements taken at 40 thou lift.
I've retarded the cam to keep that period as short as possible.
The exhaust has a duration of 252 and the inlet 204 which aren't bad compared to other small singles and the lift and therefore anular port area are more than adequate.

Diameter of inlet port=32mm, r=16x16x3.142=804mm2.
Circumference of port times lift of valve, c=100.6x8mm lift gives 804mm2.
So lift is not a problem but the fact there is no overlap concerns me on such a small engine.
Maybe Mr. Scumdogs harley revving at 2000 rpm max. could work with such a mild cam but a 150 single needs revs to make power and get the gasses flowing, i don't want to plow a field with this thing i need it to rev and as anyone knows revs means overlap.
British bikes are agricultural but they still follow the same rules of gas flow and inertia/resonance which high overlap utilises for max power at high revs.

Anyway, anyone know the effects of brazing chilled cast iron?

[Motu]

What sort of motor are we talking about here? As I mentioned before even low powered low revving stationary motors have overlap,and I've never heard of an exhaust valve closing before TDC - your timing is way out or else someone has attacked this thing with a grinder.

[Expert]

I thought that i had made a gaff and after checking it several times took the head off to look at the action of the valves around overlap. They are indeed both effectively shut at overlap (remembering that the readings are taken at 40 thou lift).
The motor is a bitza, it started off as an xt200 yamaha motor but i stroked down the crank to make it a 150, the advantages of doing this, of course are, the large valves originally designed for the 200 engine (the inlet valve from an xr200 motor fits perfectly in my exhaust port) and now i have a short stroke motor which should rev like fark with lower piston speeds and stress(67mm bore x 42mm stroke.
The cam came from one of those yt200 farm trike things which would probably explain why it's so mild but LOOKS similar to a worn xt200 cam i have, i don't have a decent xt200 cam to try.
The motor works very well but I reckon it's got lots more potential because of the short stroke/large valve area.

[Motu]

An XT200 definatly has overlap,even a farm bike version.I have seen severely worn camshafts,but never seen one where the exhaust valve closes before TDC,and yet the inlet opens before TDC,you have got it timed wrong for sure.Try 20 thou for checking you timing,40 seems exessive as you are only trying to eliminate the quietning ramps.

[Ixion]

Bear in mind that , where the valve gear does not use bucket followers, cam timing is determined by both the camshaft AND the followers. The size and shape of the follower is as important as the size and shape of the cam itself. If you are using an orphan camshaft, are you using the cam followers that went with it? They could be quite different to XT200 ones.

But I agree with Mr Motu, somethings stuffed up. No engine would ever be designed with such timing. Even side valves didn't have that