Single carb mod for skorpion

[basser23]

I think the spec is 1250-1350 or so...mine is rock steady at 1320 when warm...according to my digital tach,and suprisingly the stock electric tach!

[Linegeist]

There are some of us - old gits like me for example - who think that a large single ticking over at 1300rpm sounds a little too much like a (Japanese) lawnmower, or an angry bee trapped in a bean can. We hanker for the days when one-lung motorcycles had a firing stroke at every other lamp post and you walked behind one ticking over on its stand with care, as the exhaust pulse hit you in the face like a slap from a gay hairdresser on a bad day.

I bought a Skorpion because I heard one being ridden hard several years ago and, to my 1950's-model ears, it sounded exactly like my old Manx Norton, especially on the overrun. It's called 'Nostalgia', and it's why I have resprayed my own Skorpion in 1950's black, polished all the alloy I could find, fitted a cable-driven rev counter and regularly try and position it over oil stains I find in parking bays (as the Skorp' doesn't have any nice nostalgic oil leaks of its own). I also wear an open-face (black) helmet with WWII goggles and a black leather jacket with Levis, plus put I newspaper inside my jacket to keep warm in the winter and smear pork fat on my lips to prevent them chapping in the wind

I can't see why a low tickover should cause engine damage (as long as the machine doesn't spend hours at it, natch') as the motor's not under any appreciable load. The only bearing that's copping any stress at idle is the big end bearing, and oil pressure there is more the product of centrifugal force than the meagre dribblings of the oil pump. The cams can look after themselves with modern lubricants and I suspect the cop cars referred to above clap out because they're driven by gay hairdressers on bad days.

[Bill Jurgenson]

not quite.
the plain bearing cam needs oil pressure to survive. The cranks does not, not do any other of the many ball bearings.
Worst offender is the unbalance of the balancer at such low speed that slaps that stupid cushdrive and the woodruff keys and the gear teeth quite badly.
Take out the balancer at really will tick over like an oldtimer, despite being undersquare. and not just a little bit undersquare, either.

[Linegeist]

I've read your stuff about that cushdrive/balancer Bill and I'm curious - do I need to buy a new, solid, unit or can I simply modify the existing one? If I need a new one, how much would you charge?

As for the camshaft, I'd have thought that, using synthetic oil as I do, the plain bearings aren't going to pick-up after 5 minutes or so of idling at low speed. Whenever I've gone into the head, the cam areas is always awash with healthy amounts of fresh oil, even after standing several weeks. My British bikes all had plain cam bearings and they ticked over at well below 900rpm - AND had dodgy oil pressure and dreadful oil (by today's standards)

I stand to be corrected, especially by someone with your intimate knowledge of these motors, but my engineer's mind's telling me that where there's lots of nice oil sloshing about, even plain bearings would think twice before lunching themselves..................

In my (bike's) defence, I have to say that, ticking over at circa 900rpm, there's very little vibration, and it chuffs away quite happily. It almost feels over-retarded, although that's pretty difficult with a solid-state ignition system. However, I'm paranoid about that dratted woodruff key .................

[Srinath]

Linegeist: - This part - AND had dodgy oil pressure and dreadful oil (by today's standards) - I dont think that is quite true. Yes modern synthetics are better than old conventional. However modern non syntehtic oil - anything newer than SF rating, isn't as good as the older - Pre SE rating oils.

Otherwise - carry on, I am just watching this thread.

Cool.
Srinath.

[Linegeist]

Linegeist: - This part - AND had dodgy oil pressure and dreadful oil (by today's standards) - I dont think that is quite true. Yes modern synthetics are better than old conventional. However modern non syntehtic oil - anything newer than SF rating, isn't as good as the older - Pre SE rating oils.

Can I ask on what you base this statement?

Otherwise - carry on, I am just watching this thread.

Thank you. That's very kind.

Cool.
Srinath.

[Bill Jurgenson]

one thing at a time.
I've no idea what the gear costs in the states and in any event I can't be cheaper than your trustworthy Yamaha dealer.
You definitely should exchange the gear.
Best with that of the Raptor which cannot pose any problems even in the US, unlike parts for bike that are not imported.
balancer gear Raptor 1S31153600
It cost 65€ plus 19% sales tax over here in Germany.
Remove all parts including the thick soft steel washer behind the thin retainer washer of the OEM balancer gear; the Raptor gear is that much thicker.
Disregard the index which is incorrect as the Raptor has two balancers: both woodruff keys must face each other.

Your old plain bearing brit bikes had bronze or similar bushings, with oil feed and oil grooves.
Ours is running directly in the aluminum of the head there are no bushings and the oil feed is from within the cam. Pressure is at a premium because the feed line is smaller than the sum of the bearings, add to that the pressure losses caused by the auto-deco mechanism. At startup and at very low RPM, the amount of surplus oil flowing around the head and its oil pockets has absolutely impact on the cam bears, the oil pockets are for the rockerarms and cam lobes. Oil pressure must be high enuf to float the cam initially to prevent metal to metal contact. At higher RPM and when warm, the conditions are not so severe. The cam surface runs up onto the oil present, like a boat hydroplaning. This definitely doe not happen at low RPM any more than with cold oil.
Old fashioned single range mineral oils were cold so thick that what amounted to a grease film was always present when cold and classic techniques relied on this film.
For this reason, they also ran with larger tolerance. OTOH, such oils broke down under very high temperature and surface pressures and then metal to metal contact resulted. Modern multi range oils are far less prone to breaking down like that and also need higher temperature to work. Below 80C. (±180F.) is too low for any modern synthetic oil, measured in the return line just behind the engine, not in the tank.

Over square engines, especially those with large bores, burn uncontrollably where the gas column speed and turbulences don't set up as they should. In general, anything larger than 550cc at at ratio of 5:4 (ours is 100:84 or approx 5:4) is considered a problem that cannot be predesigned to work. Gas flow must then be empirically quantified at the bench.
Old long stroke, immensely undersquare designs like practically all brits or e.g. Guzzis, Indians and of course Harleys even today are far easier to control and due to the much lower compression ratio, high combustion pressures are not achieved one way or the other.
Like I tried to explain, true thumpers of old tick over like that and without damage because of all the reasons I have tried to explain, not forgetting the spark advance.
They run at 5-10º BTC some even at 0º where they have manual advance. When you consider this in the context of an engine undersquare to a ratio of 3:4, you can see that there is little pressure and certianly nothing exploding (i.e. burning uncontrolled). Add then the manhole-like flywheel anywhere from 10 to 25lbs and it i easy to understand.
the stock DENSO box advances to abruptly from 3º to 12.9º at 700rpm(that's 10º in a millesec.) and then remains relatively flat over what can be considered an idle range to 2500 and then climbs steeply from there to ridiculous heights. The flat section, that in fact dips, is the main reason for chain grabbing and funky idling, tho the balancer contributes a lot to the poor idle as I have said.

[Linegeist]

An embarrassment of information riches Bill! Thanks for explaining that lot! I'd wondered what was wrong with the ignition timing on the Skorpion - it's obviously a bit radical ........... I'll save for a SilentHektik setup. Meanwhile, armed with that part number, I'm off the the local Yam dealer this morning for a new part.

I'm much obliged.

[Srinath]

Linegeist: - This part - AND had dodgy oil pressure and dreadful oil (by today's standards) - I dont think that is quite true. Yes modern synthetics are better than old conventional. However modern non syntehtic oil - anything newer than SF rating, isn't as good as the older - Pre SE rating oils.

Can I ask on what you base this statement?

Otherwise - carry on, I am just watching this thread.

Thank you. That's very kind.

Cool.
Srinath.

Look at the additive package and the amount of protection additives. The older oils when they burn produce a lot of ashy smoke, that not only riles up environmentalists it also clogs up sensors in the exhaust, so progressively they have been reducing them as the ratings get renewed every few years. New oils are also for fuel conservation, which really isn't a job for the oil, oil is there to lubricate, lubrication sometimes takes up energy, thick grease will make stuff harder to turn than if it had wd40, that doesn't mean you out wd40 where you need grease, unless you need to be replacing it every week.

BTW look @ steering neck bearings on new sport bikes. Not only are they the ball kind instead of a tapered needle bearing they also have no grease at all on them, and they have a light spray of wd40. Yea its easier to steer, but you need to replace it if you had any real use on it. Its a common problem on sv1000's that I know of first hand. I've run original bearings on 20 year old bikes. But the 5 year old sv had a bad lower bearing and it had barely 4500 miles on it. Yea it had a light crash ... but I've crashed older bikes a lot harder ... lighter lubes have taken the place of real oil and real grease these days. But yea the steering bearings dont have anything to do with oil, I was just using it as an example for how it can live with less and light lube, except need to be replaced often.

Cool.
Srinath.

[Jurrien]

There are some of us - old gits like me for example - who think that a large single ticking over at 1300rpm sounds a little too much like a (Japanese) lawnmower, or an angry bee trapped in a bean can. We hanker for the days when one-lung motorcycles had a firing stroke at every other lamp post and you walked behind one ticking over on its stand with care, as the exhaust pulse hit you in the face like a slap from a gay hairdresser on a bad day.

I bought a Skorpion because I heard one being ridden hard several years ago and, to my 1950's-model ears, it sounded exactly like my old Manx Norton, especially on the overrun. It's called 'Nostalgia', and it's why I have resprayed my own Skorpion in 1950's black, polished all the alloy I could find, fitted a cable-driven rev counter and regularly try and position it over oil stains I find in parking bays (as the Skorp' doesn't have any nice nostalgic oil leaks of its own). I also wear an open-face (black) helmet with WWII goggles and a black leather jacket with Levis, plus put I newspaper inside my jacket to keep warm in the winter and smear pork fat on my lips to prevent them chapping in the wind

I can't see why a low tickover should cause engine damage (as long as the machine doesn't spend hours at it, natch') as the motor's not under any appreciable load. The only bearing that's copping any stress at idle is the big end bearing, and oil pressure there is more the product of centrifugal force than the meagre dribblings of the oil pump. The cams can look after themselves with modern lubricants and I suspect the cop cars referred to above clap out because they're driven by gay hairdressers on bad days.

you should write a book

About the need of oil pressure: I have an old Honda moped with an 90cc 4-stroke engine, in which a few years ago the oil pump crashed. I drove it another 20 km to my home. The engine has, just as the engine discussed here, a steel camshaft rotating directly in the aluminum cilinder head. The head hadn't any sign of damage. I literally drove it half an hour without oil pressure and the camshaft was just lubricated with the oil that was allready up there. Also in the rest of the engine there was no special wear visible.
So personally I don't think there can be any harm in letting the engine rev too low for a short time. But I also think that Bill has a lot more experience with and knowlege about the yamaha engine, so I'll keep the idle just as it is.

But, another question, this time ontopic: I want a single carb as well. There's a lot of info here, which is great. I can't weld aluminium, but I know how to draw in SolidWorks so I want to draw the manifold and get it made. Therefore I have some questions for Bill:

- You told that the length of the manifold is the main reason for the better performance, wich length do you exactly recommend?
- Would you reuse the original rubber joints to connect the manifold to the cilinderhead?
- Is the bikeworx manifold or carb supported by any bracket? Because otherwise it is a lot of weight that must be held by the rubber joints?

Jurrien

[basser23]

JURRIEN,
THE CARB IS ON A RUBBER SPIGOT,THE MANIFOLD IS MOUNTED DIRECTLY TO THE HEAD WITH NO RUBBER IN BETWEEN THE MANIFOLD AND HEAD.

[Jurrien]

Well, this manifold is in the original two rubber joints, right?

[basser23]

Aha! I stand corrected...when morbidelli17 made his,he used a custom made adapter on the head,then a Grizzly manifold bolted to that.FCR carb mounted on a spigot..
So I was assuming that was what we were talking about

[Bill Jurgenson]

you can see that the Bikeworx manifold in stuck into the rubber mounts. and no, there was no additional support.

length depends on purpose, Not owning such manifold anymore,. I can only guess at its length, about 3"
my racer had 2" headers and these were also stuck into the rubber mounts. OTOH, the Grizzly manifold is bolted directly to the head.

[Srinath]

Aha! I stand corrected...when morbidelli17 made his,he used a custom made adapter on the head,then a Grizzly manifold bolted to that.FCR carb mounted on a spigot..
So I was assuming that was what we were talking about

This is what I was thinking. The thing is using cannibalised parts will work for the 1 bike we are fitting these things to. You cannot "manufacture" based on the fact that you will need a grizzly manifold. See a few years ago on the GS500 web site a few guys put their heads together and worked out a proper front end swap for a GS, katana 600 FE, better forks, 2 disk FE, comfortable clip on's etc etc. That summer every katana FE on ebay under 2-300 vanished. It took a few years for the supply of kat FE's to come back, and this was just from some people puttig up pics of what they did.
To make and sell, you cant rely on parts from junkyards.

Anyway I just have a grizzly manifold sitting here looking pretty, nothing more has been done, the MZ runs fine as it sits. I may just leave it as it is.

Cool.
Srinath.