Joined: 20 Sep 2005 Posts: 54 Location: Hempstead, New York
Posted: Tue Oct 07, 2008 8:33 am Post subject:
The needle jet size is the exact ID in inches, not the flow rate (the main jet size is the flow rate), but a jet made with a ream will not flow the exact figure as a factory jet.
Joined: 10 Nov 2006 Posts: 448 Location: west cumbria
Posted: Tue Oct 07, 2008 10:22 am Post subject:
Panic,
No argument there it rings a bell.
Years ago and long lost now, I had a complete set of jet drills for Amal from 15 to 300. The main jet number funnily roughly equals the bore radius, I think I'm sure that's how they mic'd up, hell! it was 50 years ago.
Cash
Joined: 20 Sep 2005 Posts: 54 Location: Hempstead, New York
Posted: Tue Oct 07, 2008 10:46 am Post subject:
It's entirely right to be cautious, many motors have been blown by using a drill as a gauge, and assuming that if a drill "feels tight" the jet is the same ID.
The length of the hole, the entry and exit taper, the finish (as seen in a microscope) etc. all affect the flow rate, and 2 "identical", new .106 jets may easily be 5% apart in actual flow. Even flow-rated jets have an "acceptable range"; a 300 may be (example only!!) between 295 and 305. This makes for headaches when you have a twin, especially when you only have 1 correct jet and have to fab the other one.
Normally, for twins I suggest a fabricated jet for testing only (saves money and time to get the general direction of the work), and replace them with new factory parts (once the best size is known) if there are any differences in color, knock sensitivity, carbon, etc. between the cylinders. Use a pin vise (not a drill motor), use a lubricant for brass, work slowly, never cut past any hard resistance, and examine the interior for scrapes, spiral marks etc. - if the surface isn't really clean, use it as a donor for the next larger size because you don't know what it will do.
One of these days, I'm going to make up a test fixture with a big burette on top, a jet-sized female-threaded hole at the bottom, and a paddle switch float in a tall pan underneath. Turn the petcock, start your stop-watch, and a known quantity of liquid (gas is dangerous, but try to use something with similar viscosity, not alcohol) passes through the jet. When the pan level reaches the paddle, it sounds an alarm. The burette level tells you how much passed, the watch tell how long it took (CC of fuel per minute is the Amal main jet size). Now try the other jet.
Trust your sense of when the motor is happy rather than the number on the jet!
Joined: 09 Oct 2004 Posts: 159 Location: Seattle, WA USA
Posted: Tue Oct 07, 2008 12:31 pm Post subject:
Thanks for expanding on the topic. I'd love to try the flow testing, and since I've now got a few pairs of jets kicking around, maybe I will.
I have a pair of .105's in hand and will put those and some 270 mains in this afternoon and see how it "feels".
That brings me to a question. Panic's book says that a lean slide cutaway will hesitate on throttle opening and also says that a megaphone generally requires a slightly richer slide. Am I reading that wrong? Will the .105 lean the off idle mixture without changing the throttle tip-in?
The .1055 reamer and gauge pin are on the way. However, you've got me suitably worried about my ability to do the job accurately. Think I'll hold off for a bit.
Your words "The idle and low-end of the low speed circuit run rich. (Just off idle to 1/4 or 1/3 throttle) "
Right in the area of the cut away and the NJ . Having both things to play with but as Panic says NJ are cheaper and more likely to be off size from the get go. The .1055 pin gage would come in handy for checking the 106's you were running. I know you can feel the oval on a worn pair with this gage.
Has Bruce already started the machining is it too late to try this first?
Joined: 09 Aug 2008 Posts: 51 Location: Upper Black Eddy, PA USA
Posted: Thu Oct 09, 2008 9:20 am Post subject:
The difference between a 105 NJ and a 106 NJ is like night and day. I think you will find the 105s way too lean. On my '72 Combat the 106s when new are just right. But I can start to feel a difference about 2000 miles later and need to replace at 4-5000 miles. The needles do wear also so I replace both at the same time. Probably a 1055 needle jet could extend this replacement interval on my bike. As it stands I'm only putting about 1500 mi a year on now.
The way I gauge the wear is to ride in 2nd gear at about 25mph on a flat surface with no load on the engine. With new needle jets it is smooth. With worn jets it will go uhhhh uh uhhhh uh uhhhh uh. Very scientific description I know.
Joined: 09 Oct 2004 Posts: 159 Location: Seattle, WA USA
Posted: Thu Oct 09, 2008 11:11 am Post subject:
Well, I held off on sending the slides to Bruce until I had a chance to check out the .105 nj's and 270 main.
FWIW, the .106's had maybe 100 miles on 'em as did the needles - so that's not the issue.
Anyway, got the gauge pin and .1055 reamer yesterday. Bet you're surprised (not) that the gauge went cleanly through one of the new needle jets - but not the other. Reamed them both out to .1055 last night and fiddled a bit this morning before work.
The bike seems harder to start. This could be explained by the fact that it was 40 degrees (F) in the garage today - a good 15 deg colder than a few days ago - and I don't have chokes on the carbs. Very unscientific, but it revved easily and pulled away from stops without fuss or hesitation.
Seems the float height auto-adjusts to the highest possible setting. I set them 1mm below the rim of the bowl when I built the carbs. The float needle seats move too easily and sank to the bottom on their travel since then. Likely the source of my over-rich condition.
I've also come to the conclusion that my lean full throttle condition may have been exacerbated by the Andover Norton fuel taps - as they allowed only a dribble of fuel to pass through them. The 270 mains plus the cheapo BAP petcocks flow much better and feel much sturdier. Go figure.
Anyway, a week from tomorrow is dyno day. Fingers crossed.
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At the end of 1967 the Norton Commando was announced.
The Norton Commando was greeted with a certain amount of scepticism because on first sight the commando appeared to comprise of the old Norton Dominator twin cylinder engine mounted at an inclined angle in a set of new cylinder parts.
It was not realized that the new Norton Commando Isolastic method of engine suspension damped out all engine vibration and produced a machine which had uncanny smoothness for a vertical twin. In due course the critics were silenced and the Norton Commando had the distinction of being regarded as the first of todays so called superbikes. There can be little doubt that the original design concept of the Norton Commando has proved correct, since comparatively few modifications of any real consequence have been made since production commenced during 1968.
Now nearly 40 years later Norton Commando riders like us are a breed of our own, and as far as we are concerned its still more fun to go for a blat on the old Norton Commando, and fast. As a Norton Commando owner and enthusiast, my goal here is to promote and give credit to those who keep the Norton name going.
It is more deserving to give credit to the Commando itself, for after all these years it continues to be respected. The original Commando designers like John Favill are those who deserve the credit for developing this incredible motorcycle.
The Norton Commando Roadster and Interstate of the late seventies, never died. Although the Norton Villiers factory dispersed the tradition lived on. Today Kenny Dreer in the USA is developing the new 952 CC Norton. What a great looking bike this is, and its engineering is still based on the original layout. It will be interesting to see how the new 952CC Norton does in todays tough motorcycle market. One thing is for sure, I would own one if I could afford it.
I'm sure that's how they mic'd up, hell! it was 50 years ago.
Very scientific description I know.
