2 Strokes Down Under

Hi all. My name is John, I've been playing with 2 strokes on and off for 30 odd years now. Here are my bikes; the blue Sherpa T I've had for a couple of years now. The red Metralla GT 370 I've owned for over 35 years now. I love trials riding more than anything but the road bike is probably the one you blokes would be interested in - with around 50 -55hp and 110kg it's a lot of fun. I might put up a post later detailing the engine work on the Metralla or you can find more about it here: http://www.oldjohnno.id.au/bul.html

Very nice bikes mate. Welcome to the forum!

to both, very nice.

Especially the Shepra

But Metralla may not have the bling but I would bet it would be a hoot on the right road - like up, or down, the Oxley. A Matralla 360 won a 'big deal' (24hr ?) public road race around Barcelona many years ago IIRC.

I have mucked about on trials bikes enough to know it is physically hard work, ya gotta be coordinated and gotta have gonads - respect. I prefer speed and let spinning gyros do the work .

I've peek at your link - it might take some time to read and digest but it looks interesting

Welcome! I'm sure you'll be a good addition round here

OzzyElsie wrote: to both, very nice.

But Metralla may not have the bling but I would bet it would be a hoot on the right road - like up, or down, the Oxley. A Matralla 360 won a 'big deal' (24hr ?) public road race around Barcelona many years ago IIRC.

That was the 24hr Montjuic endurance race in 72, won by Benjamin Grau pictured below. It was a big deal for Bul because it was at home and also because the little bike beat the japanese factory 750s as well as the 750 Ducatis.


The factory planned to release a replica named Montjuic but it was never released and only one prototype was built.

Welcome aboard Johnno. Excellent blog article there! What a journey. Your a man after Jennings' own heart. I guess that a stretch of good old country road will tell you everything that a dyno would, without the numbers! You obviously have finely calibrated 'pants seat'.

Interesting that those Chinese CDI units keep turning up here. I guess if you buy at least 2, you should be ok....

I've got a bit of a soft spot for Bultacos. I had to fight myself off a few months ago when a shed full of Metralla bits (4 or 5 bikes) turned up on Gumtree but I figured that was really jumping in at the deep end and ended up doing an RD project instead.

I imagine that the single cylinder configuration would provide the ultimate development limit when compared to a twin?

Some great stuff there mate. Brilliant.

What a great write up, I was nodding away while reading it and enjoying the journey, thanks!

One question, that decomp valve, is it auto or do you pull the end out or something?

Thanks for all the kind words! I don't trust my arse-dyno at all (it's tricked me too many times already), but I can get reasonably accurate "Moroso HP" numbers from a G-Tech and terminal speed runs.

I've been working with EngMod2T lately, and hoping it'll help show the way to a few more horses. Will know more over the next few weeks.

The decomp valve is semi-automatic - you push the button in before you kick and as soon as it fires the button pops out and the valve closes.

nice, I like the sound of that, cheers!

The decompression valve looks like a winner. Husky you say? I've got a bigassed Husky chainsaw that I rarely use because it has so much compression that it's a complete bitch to start. If it doesn't start by the third pull, you're stuffed. Your hand is too sore for 4! It needs a decomp valve.... I liked your 2mm bleed hole installation too. Nice.

Johno. its interesting reading your tech page. " The traditional stinger entry at the rearmost point is almost impossibly loud but it can be improved quite a bit simply by sliding it forwards so the entrance is in the belly of the pipe. A further improvement in noise control can be had by using the "belly bleed" design where the rear of the baffle cone is blanked and the stinger exits from the side of the belly section. Running the stinger into a muffler mounted in the usual position behind the baffle cone can produce a relatively quiet (well, not too raucous) pipe that looks reasonably normal. This is what I run. "

I like quite'ish bikes for all sorts of reasons and I thought of doing this sometime ago (my thoughts were that releasing low pressure rather than high pressure and low speed rather than high speed gas movement would be easier to quieten without being resistive). I have never had bespoke built pipes so I have never done it. It is nice to know that it works in practice. I also suss it would change the tone of the exhaust giving it a deeper pitch. I think 'in theory' the reflected pressure wave should be better and hence the performance should improve. I suss that would be true but probably insignificant (otherwise it would be common practice).



Here's Johno's pipe from his tech page - for the benefit of form members to see what the hell we are talking about .

Ozzy, I did this mod a while ago to my LC pipes.
I made new stingers with a minor bell (expansion) on the inside end and slid them in to the start of the rear cone.
I angled them down towards the bottom of the pipe, drilled a hole in the belly and welded the stinger to the belly.

The result is nice. The pitch of the sound itself hasn't changed much, but it's definitely lowered the volume just enough.
If I wasn't hoping one day to have custom pipes made for the 500 I would definitely do the same to its pipes. Definitely no loss of power.. maybe an increase, but I have nothing but butt dyno to quantify.

This is the important bit:

Something to keep in mind with these alternative stinger designs is that they flow a little less than the traditional style and therefore have to be made slightly bigger in diameter to compensate. I underestimated this effect, and the 32mm OD outlet I used severely limited the high-rpm power as well as contributing to the det. problem, and it took me longer than it should have to recognize what was happening.

Also keep in mind that an internal stinger makes the baffle cone behave as if its included angle is just a bit wider. The belly bleed and internal stingers certainly are less noisy but you need to be prepared to do some trial-and-error work to find the optimum dimensions. I'll be starting on a new pipe soon and I'm seriously considering reverting to a "normal" stinger. Yes, it'll be hideously loud and difficult to quieten with any reasonably sized silencer but the conventional stinger just seems a bit more predictable.

Hi John
Great you read the Pit Lane forum where I manage to squeeze some great info from Frits and Jan I have been putting that into practice for the last 6 or 7 years and it works. One area you haven't addressed is blowdown time/area which defines the output of the engine. Not inlet transfer or exhaust. It's what Jennings Bell and Blair all fucked up. Jan Thiel does not have a kind assessment of Blairs abilities to develop a GP winning engine.

Yes MX tuning theories out of the US are pretty much shit, I have yet to dyno 1 aftermarket US made mx pipe that betters the standard pipe. Most standard pipes outperform the brand name pipes by 2-3hp across the range. Pure snake oil.

If you want and ignition there are very good aftermarket programmables today that put those old shitters your been using in the bin.

Internal stingers cost hp an effective silencer is much much better.

Engmod can get you very close to the money provided you input the correct data.

Cheers
Lozza

Thanks for your reply Lozza; it'll be interesting to see how EngMod2T assesses the current port layout (which is basically all Blair) and also what is left to be gained. I may start a thread showing its assessment of the current and proposed layouts.

Internal stingers cost hp an effective silencer is much much better.

You've just convinced me to revert to a conventional stinger. But what's an effective silencer? Keeping in mind that this is a streetbike? Noise control is important enough to me that if I have to sacrifice a HP or two then that's just what I have to do. I just don't need any unwanted attention from the po-po..