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Discussion in '2 smokers' started by German Trick, Oct 31, 2012.
My forum namesake
Hi Mark, Williams hills pass I see my Frontera on the trailer ...Missed you at this years ride! Alpthusiast
Here's a couple of Bul's-The blue 78 250 is a recent acquisition. Needs some love but runs and is pretty complete. It will be an AHRMA cross country bike.
The '76 360 is a bike I got at mid Ohio. Did some painting and sorting out. While I had the stock bazooka for it, I bought a Pursang pipe for riding.. Much lighter. This one went in trade for a 1970 CZ side piper yesterday.
My sweet little Sherpy Derp..
How lovely! Nickle plated frame?
Thank you. Sammy Miller Hi-Boy frame. And Falcon shocks with ultra-light damping that absolutely transformed the bike..
My final ride of 2019 was in the woods on my Alpina. They are just such a nice woods bike
I am planning to steal the Husqvarna head swap mod @snarlyjohn came up with. It's a concentric rather than trench combustion chamber shape. I just need to modify it for the Bultaco's slightly different stud spacing, and be very careful about the compression ratio. John, did you skip the head bolt locations furthest outboard in the Bultaco cylinder? Any other tricks I need to know about?
I use all the original studs. The six existing Husky holes are on a slightly smaller bolt circle than the Bul ones, so the new holes break into the old ones forming figure-8 slots. You need to be accurate with your work as the old holes encroach onto the sealing surface and you want to minimise this as much as possible. You can redrill with an end mill or a flat bottom drill and an accurately drilled (and positioned) guide plate. I've only ever used the Husky head on the earlier style barrels where the head spigots onto the top of the sleeve, and haven't had any sealing issues at all. The spigot also locates the head nicely on the barrel, but I don't see any reason why it wouldn't also work with the later gasketed flat top barrels. You also need to cut way some fin material and spot face the nut seats, especially for the two outer studs. I also cut a seat for the decompressor button and reshaped the bottom fin slightly to match the outline of the barrel fins, just for looks. The original Husky hemi chamber shape was retained and I used a squish band of around 50% area, being careful to match it closely to the piston dome radius. Theoretically you could run a higher CR with the Husky head but be conservative - raising the CR with a two stroke rapidly becomes a game of diminishing returns as improvements in thermal efficiency become outweighed by the reduction in EGT and the resulting reduction in pipe efficiency. I've done some back to back dyno testing of different chamber shapes on a 370 engine, with surprising results. The trench chambers as used on the Pursang engines makes just a tiny bit more power than the others, despite being regarded as outdated by some. I'd like to try a trench chamber with a better-cooled head like the Husky.
Anyhow, once you do all this you'll find it makes no more power than the original on the same tune. Why do it then? There are a few good reasons. One is that cooling is much, much better and you'll be able to maintain full power and WOT indefinitely, unlike the original. With the 350/360/370s especially, once you increase the power output substantially they'll tend to detonate after quite a short period of WOT. Of course you can tune around the problem but it costs power. For me the det disappeared completely with the Husky head and better still it meant the tune didn't have to be compromised so the net result was an improvement in performance. For a methanol engine the head swap probably isn't necessary but with gas it certainly helps.
This is a scan from an old photo from 1995. It's an Astro 360 from I believe 1976. Bultaco got the frame geometry on these bikes perfect. It was stunning just how easy it was to go left on an Astro. In the photo, my Astro was a play bike on the ice, as it had old style, worn studs on it from when I bought it. With the stock bazooka pipe, it was loud. When I say loud, I mean it was really loud.
My first bike. $275.00, no rear brake, no air cleaner..... I remember adding 1 baby food jar of #40 oil to a full tank of gas.
1966 Lobito, 100cc
my uncle loaned me his Alpina for a year when I was 17 years old, should of bought the bike from him.....one of my all time favorites....
Got this in December of 1971 new in Dallas Texas. It was just redone by a very talented friend and has a deserved new home as art. Even the fasteners are replated Bultaco OEM.
I'm the original owner of his M181. 80% of the time it runs 100% of the time...... I'll never part with it though.