Cutaway Size??

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RoadScholar

RoadScholar

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My casual understanding is that the cut away influences air flow at all throttle positions; the Nortons seem to have either 3 1/2s or 3s, I seem to see the 3 1/2s in the 850s. I'm tring to dial in the carbs, which are new and they have chrome slides with 3 1/2 cutaways.

I have a '75 Mk III with Web 12a cam and (roughly) 9.7:1 CR, 932 Amals with 240 mains, .106 needles, in mid position, and 3 1/2 slide cutaways, air screws are out about 2/3 turn each (which seems too tight??). The engine is fitted with a tri spark set at 28 degreesBTD at full advance.

I started tuning with 260 mains the rest as above, I went to 250 mains and got slighly better response, last night I put in 240 mains and got yet another improvement on throttle response; the engine just turned 900 miles from completely fresh.

I'm thinking about trying 230 mains, but have started wondering about the 3 1/2 cut away. Should I be trying a 3 cutaway, would that permit me to open the air screws, and would that give me a virtual 235 main, if I fit the 230 mains???

Your thoughts, experience are greatly appreciated.

RS
 
Why only 28 degrees, should be 31 shouldn't it. These electronic ignitions should advance slower than points.
As for carb settings, I messed with mine after I tuned my motor, never got it quite right. So I set the carbs to the original settings & it was perfect. Make sure you have the correct spray tube. Mine is running 10-1 & a PW3.

The smaller cutaway gives a richer mixture on acceleration & may give a slightly quicker initial acceleration, but could produce a puff of smoke from the exhaust.

Air screws, get engine hot, srew out until engine stops speeding up, then in 1/4 of a turn. Easier said than done though, set many like that, easy when you are used to it. Or, set it with a colourtune, works a treat.
 
With your main jets, when you say you get better throttle response how are you testing that?
You can only check the main jet with wide open throttle, any less than that and you're on the needle.
When you say better response it implies you're doing something active with the throttle, it should be pinned wide open.

You need to get this sorted before playing with anything else.
The order is
1. main jet
2. pilot
3. slide
4. needle and needle jet.

You can't short cut this.
 
Flo,

10-1 and PW3, are these amal terms?

28 degrees is what TriSpark recommends timing be set.

RS
 
RoadScholar said:
My casual understanding is that the cut away influences air flow at all throttle positions;
Click to expand...

The slide cutaway only has a significant influence from approximately 1/8 to 1/4 throttle, where the needle and needle jet take over up to 3/4 throttle, then main jet to full throttle.

http://www.oldbritts.com/amal_tun.html

http://www.hitchcocksmotorcycles.com/am ... nc_ht.html


Flo said:
Why only 28 degrees, should be 31 shouldn't it. These electronic ignitions should advance slower than points.
Click to expand...

No, not for Tri-Sparks.
 
RoadScholar said:
10-1 and PW3, are these amal terms?
Click to expand...


Presumably he means 10:1 compression ratio?

And PW3 = PW3 (Peter Williams) camshaft?
 
L.A.B. said:
The slide cutaway only has a significant influence from approximately 1/8 to 1/4 throttle, where the needle and needle jet take over up to 3/4 throttle, then main jet to full throttle.
Click to expand...

Guessing you mean only a slight influence? Everything i've read states it's mostly an off idle transition.
 
GavinJuice said:
Guessing you mean only a slight influence?
Click to expand...


No.

http://www.hitchcocksmotorcycles.com/am ... _ht_6.html

"THROTTLE VALVE CUTAWAY. The atmospheric side of the throttle is cut away to influence the depression on the main fuel supply and thus gives a means of tuning between the pilot and needle jet range of throttle opening. The amount of cutaway is recorded by a number marked on the throttle, eg: 622/3 means throttle type 600 with number 3 cutaway; Larger cutaways, say 4 and 5, give weaker mixtures, and 2 would give a richer mixture."

Cutaway Size??
 
In case of wires crossed, I meant yes to the compression & cam.
Not to slight influency thing.
 
Flo said:
In case of wires crossed, I meant yes to the compression & cam.
Not to slight influency thing.
Click to expand...


My reply was intended for GavinJuice (see quote) so I don't think anything was misunderstood, just a case of cross-typing I think?
 
Rich_i

Better response with wide open throttle, yes. With the 260s it would accelerate. With the 250s it would actually pull a bit. With the 240s I can begin to feel a bit of the cam.

I was thinking about the #3 cutaway because I have a slight bog coming off the line, but I do have a 21 tooth counter shaft sprocket. My air screws are set at about 2/3 of a turn out which seems too slight. If I had #3 cutaways I might get a better signal to the pilot system which would give me more lattiude with the air screws, no???

Tomorrow I'm going to fit the 230 mains, and turn the air screws in a bit to richen the transition to the needle and see what happens. despite the fact that I live in Ne England, it is suppose to hit near 70 degrees (F), perfect laboratory for jetting; I just won't have the #3 slides.

Thanks to all!

RS
 
Hi RoadScholar,
Seeing that you are in the Boston area I'd like to recommend that you get in touch with http://nneno.org/ ( Northern New England Norton Owners Club). One of their members "DynoDave" has a lot of carb experience, and all the rest of them are a good bunch of fellows.

GB
 
Geo46er,
I belong to NENO, I know Dave Comeau, he knows about all there is to know about Nortons... Thanks for the dose of common sense.

RS
 
RoadScholar said:
Rich_i

My air screws are set at about 2/3 of a turn out which seems too slight.
RS
Click to expand...

Are they newish carbs?, because I've fitted 2 new Amals and my pilot screws are also set fairly close in, about 1/4 turn out on 1 and between 1/2 & 3/4 on the other, I also thought they were too far in and the carbs will spit back if I open the screws any further, I contacted the Amal Co. and spoke to the technical dept , and they said that it was not an issue as long as it was running fine,, I've checked it with my Davida gauges and both are pulling the same, and also with a colour tune which also looks the correct colour; however I wonder if I may have a small airleak between the carb and the head. I don't know what it is but I'm not completely at ease with it and am thinking of going for a single Mikuni Vm 34 conversion.
 
rbt11548,
The carbs are new, now with 1000 miles. What you say about your settings mirrors my own, I used a TwinMax to check the sync, but found that a good mechanical sync works just fine. I am going to order the #3 slides, figuring that the lower cut will increase the vacuum signal to the pilot system and smooth the transition to the needle; if I'm right I'll be able to open the air screws, which will give me a larger hedge against slide/bore ware.
In about 2 hours I am going to step down to 230 mains and see if my cam profile expresses itself. I am close to gettimng this machine broken in and dialed in; so far everything I have done has had positive effect.

RS
 
RoadScholar said:
rbt11548,
The carbs are new, now with 1000 miles. What you say about your settings mirrors my own, I used a TwinMax to check the sync, but found that a good mechanical sync works just fine. I am going to order the #3 slides, figuring that the lower cut will increase the vacuum signal to the pilot system and smooth the transition to the needle; if I'm right I'll be able to open the air screws, which will give me a larger hedge against slide/bore ware.
In about 2 hours I am going to step down to 230 mains and see if my cam profile expresses itself. I am close to gettimng this machine broken in and dialed in; so far everything I have done has had positive effect.

RS
Click to expand...

That main jet sounds very small for a tuned 850. Standard is 260, going smaller is very odd. Have you done plug chop?
 
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