Shortstroke 500 Scrambler, Slow Build

[johnmain]

So decided to build a pr60 motocross bike for some classic racing. And decided I could build a very competitive engine out of the Ariel, and it's not a marque I have ever seen
. I currently ride Twinshock with a 1977 RM370, but always fancied a go at building a classic racer. I will post my progress here, and any questions on things I am stuck with

Found this Video for some inspiration

http://s414.photobucket.com/user/jimg1a ... c.mp4.html

Goldstar inlet valve. Has anyone done this mod on the 500, is it just a case of new seat and valve, which I can get done locally. Or is it more involved than that?
John

[brenton.roy]

Hi John,
You'll need a new guide as well. The valve stem is narrower.
Otherwise, it's a pretty straight forward exercise.
It's also a good opportunity to do a little porting.
It's a nice looking valve - lighter and clearly better airflow.

[brenton.roy]

Here's the thread. http://forum.arielownersmcc.com/viewt ... tar#p19541

[johnmain]

Thanks that's very helpfull
John

[david.anderson]

Brenton
Did you need to machine the valve for the top collar to set the correct spring length.
It is also possible to use a chev inlet valve. One of my bikes had a deep set inlet valve. I went to my local engine reconditioner and asked about bigger valves to cure my seat problem. The second valve I picked up was the correct stem dia for the standard VH guide. It was longer than the VH valve so a new top collar retaining groove was machined and the stem shortened to length. I cannot remember what chev the valve was from other than it was a V8.
But like Brenton said the thinner stem of the gold star valve does aid flow.
David

[brenton.roy]

Hi David,
The length was Ok, but I did need to machine the top of the stem to fit the collets that came with the titanium keeper.
The BSA valve was the closest to Ariel on the valve chart (with larger head diameter).

[johnmain]

I have secured a rough cylinder head which I am sending to Bowdens in Bristol. He has gold star inlets in stock, so I will discuss with him when he gets it. I'm using this bike for mx as you know David, any thoughts on carb size and setup or any other worthwhile mods ?

[david.anderson]

John
I am running a 32mm amal concentric on my bikes and am happy with the performance. 32mm is the largest flange mounted carb that you can fit unless you fit a manifold. If you do fit a manifold/ adapter and a larger carb I would taper the manifold from the carb size down to the port in the head. I would not enlarge the port over 32mm.
From what I hear the flat slide carburettors may be more responsive but I do not have any experience with them. I believe that Brenton has a 30mm PWK on his bike so he can probably provide some feedback.
I am running 9 to 1 JP piston which is the highest com available without a special order. There is plenty of piston to valve clearance with the 56mk1 cam and this piston, although with the larger valve you would have to check if the valve diameter cut out is large enough.
I would also make sure that the exhaust valve guide has not come loose in the head as they have a reputation for it. I have oversize exhaust guides in 3 of my 4 alloy heads.
David

[brenton.roy]

The PWK has been relatively easy to set up and it works well. it's reliable, the bike starts well on choke and it runs cleanly right through the rev range.
I did clean up an edge of the cutaway for clearance, but this was not really necessary. It's obviously something to check.

[johnmain]

I'm going to run the bike in the pre60 class so the Amal is a must, Bowdens are going to re do the whole head, so no worries there. So far
I'm running an XT500 87mm piston at 9:1, which with the 350 conrod (85mm ) should give me a nice short stroke motor. i will probably loo at running Meth in the future, but going to get her up and running and upgrade as I go along.

I will change the title of this thread if I can and keep you updated, of the bikes progress, as things happen, like conrod snapping and castrating me etc LOL