velocity stack - carb settings

hi,

I intend to use this velocity stack

http://www.rgmnorton.co.uk/buy/mk1-conc ... s_1644.htm

on a 1971 750 Norton (standard cam an compression - i guess) with a new Wassel 9/30 carburetor( similar to amal 930 concentric - pilot jet removable). The Carb has already a 106 needle jet, slide 3, 220 main jet.



Can You give me a hint which carb settings would be a good start?


I am currently rebuilding the motor, which is not jet running.

many thanks in advance

Welcome .... one of the experts will have some better info for you .... I'm no expert so I would build the engine then start with carb at factory setting and work from there .... pretty much what I did several years ago and so far no problems , but it did take a few changes to get it right ...
Craig

If this is a street bike, put a proper air cleaner on it and save your new rebuilt engine from disaster.

Velocity stacks are for racing (if they are even good for that). The screens let way too much thru to the engine causing damage.

I use them on my race engine , yes they do make a difference as I proved on a flow bench, especially if the screens are removed.
They 'trick' your engine into thinking it has longer intake manifolds between the carb and the head!!!
Not for everyone, and as others point out , probably not the best choice for a road going bike with no air cleaners.
You may want to increase your main jet jet size.
Regards Mike

dennisgb said:

If this is a street bike, put a proper air cleaner on it and save your new rebuilt engine from disaster.

Velocity stacks are for racing (if they are even good for that). The screens let way too much thru to the engine causing damage.

Click to expand...

If it's for a street bike don't do it, I ran velocity stacks on my Norton just after I converted it to a Featherbed in the early 80s it ran great but the risk of back fire when starting it may also risk a major fire, I nearly lost my bike (major fire) because of them and the worst thinking I was on it at the time, also running without filters will increase wear to your motor sucking all that fine dust particals into the head and motor, a friend of mine ran no filters at all in his Trident for a few years, even after I told him to put filters on, after the motor was pulled down you could see the damage done to it from running without filters.
Run a decent set of filters and it will run just as good and your motor will last a very long time if you keep up the maintenance and running with clean air.
A lot of racers run pod foam filters over the end of velocity stacks but there isn't much room on a Commando.

Ashley

I use them on my race bike. At one meeting I rode through the pits after a race and the rear wheel picked up a stone and chucked it through the 10mm gap between the top of the oil tank and the bottom of the seat. When I tried to start the motor for the next race, it would not start however I knew from experience where to look straight away. The stone was lodged under the slide of one of the carbs. After the meeting I closed the gap under the seat. It is not a good option to fit a wire screen to the open end of the velocity stack because fuel can hang there and cause the carburation to become rich.

There probably isn't much difference between using an air cleaner and jetting for it or using open velocity stacks and jetting for them. Theoretically the standing wave created in the inlet port with open velocity stacks might be better than the situation when an air cleaner is used. However in practice .... ?

Sorry to be another party pooper here... But....

I have run these on several bike road and race, I think they look great, especially on a cafe racer. I had 3 on a Trident that looked proper 'hot rod'!

Personally, I never detected excess engine wear from using them, but I definitely saw increased carb wear of the slide, body, needle and needle jets.

In my personal experience, on a race tuned motor they do make a small BHP difference. Long ones (like theses RGM ones) give a small BHP increase at max RPM. Short fat ones (not commonly available for Amals) give slightly less top end, but a broader spread of power, and were generally what I used on racers.

On a road bike, I never felt any performance gain.

However, these long stacks tended to cause a stutter just off of idle that was very difficult, sometimes impossible, to tune out. On a racer this simply doesn't matter. On a road bike, its a real PITA.

I currently run FCRs on my Commando. I experimented with long and short stacks on the dyno and again favoured the short stacks. However, to prevent excess carb wear I put the stacks inside Ramair foam filters. It might look less 'hot rod' but these FCRs were expensive!

I used these on a road bike a number of years ago with good results. I attached a fairly course domed mesh over the ends and then used a foam cover from a Sure SM58 vocal microphone, it is posible to lightly oil the foam for additional protection, this may well negte any performance gains but it looked good and still offered some filtering.

I rebuilt the top end of my 1973 Trident at 5000 miles running velocity stacks. Replaced them with K&N and never had to do a top end again. I ported the heads, ran a 3-1 open Dunstall and velocity stacks. And I still couldn't catch my buddy's Z-1.

I ran a pair of these when I was searching for a bit more go and they do work to a certain extent but, They fit by screwing three grub screws into the side of the threads on you carb inlet. This trashes the threads and once the vibrations get to the extended length, the grub screws loose their grip on the monkey metal carb mouth. They drop off and dangle on the ty-wrap safety loop. (because you knew they were going to drop off) The jingling sound is a good indicator that they have come off. Look great, not worth the hassle.

thank you all for this good advice. I will consider to use some kind of air filtration.

What mechanism prevents the original air filter assy from taking fire?

some time ago i asked about carb setting with the amal racing velocity stack.

i try to report how things working yet, for anybody who is interested.

i glued the stacks to the carb with with a polyurethan sealent called "sika flex pro II" . this gave a equal surface with no steps in the inlet. i made a test with a piece of aluminium and sealent in petrol with 10% ethanol for half a year. glue looks like when put in.

i also fitted a chinese tea eggs over the stack to prevent eg. stones to be sucked in or fire comming out.

also a wideband lambda sonde was fittet on the pipes near the footrests.

stock default carb settings, only float hight is 1mm under chamber surface.

compression ratio is about 9.3

i did some 120 miles.

Air Fuel Ratio (AF) in idle is 10.5 . ways to fat but it is working.
accelerating up to 5000rpm AF is moving between 12 an 13.5 and the bike runs really fine (it is my first norton so i can not compare with others). I didnt drive under 3000rpm a lot, because of vibration issues, but i think that is normal.

Plugs are deer-brown on the ground strap. the thread ring is black.

if the weather is going better i will try needle one step down and air screw out. but i am quiet pleased jet.

If your carbs are jetted rich, at least you are safe. The difference in richness between using an air cleaner or velocity stacks might be quite significant. The air cleaner should make it richer. If I was taking the air cleaner off and fitting velocity stacks, I'd be doing a plug chop to make sure the carburation wasn't too lean on the mains.

no750 said:

What mechanism prevents the original air filter assy from taking fire?

Click to expand...

Essentially, you have either a substantial labyrinth (as with the stock ham can), or a good heat sink (the screen meshes that make up the K&N) that is very effective at quenching the fire before it can get out and light any surrounding material. Look on top of an inboard boat motor carburetor; you'll see a flame arrestor. It looks like a large diameter stack of SuperTrapp muffler discs. You can see light right through them, but there's enough surface area presented to the flame that it effectively drops the temp below the point where the flame dies out.

Nathan

Nater_Potater said:

no750 said:

What mechanism prevents the original air filter assy from taking fire?

Click to expand...

Essentially, you have either a substantial labyrinth (as with the stock ham can), or a good heat sink (the screen meshes that make up the K&N) that is very effective at quenching the fire before it can get out and light any surrounding material. Look on top of an inboard boat motor carburetor; you'll see a flame arrestor. It looks like a large diameter stack of SuperTrapp muffler discs. You can see light right through them, but there's enough surface area presented to the flame that it effectively drops the temp below the point where the flame dies out.

Nathan

Click to expand...

Good explanation Nathan. I never understood this either and was a fan of using open bell mouths. After all, they used to be stock fitment and folk used them all the time in the past, so they must be OK right?

Then this happened one Sunday morning. I found it a tad disappointing, spoiled the day somewhat:

Yes, the grub screws ruin the threads on the carburetors and they loosen and fall out; they might even loosen fall out and somehow get into the engine.

That said the design of the those velocity stacks is very good because they present a constant inside diameter to and at the point of the carburetor bellmouth and then that inside diameter continues out to the new bellmouth of the velocity stack. I have a set of worn out 932s where I epoxied those velocity stacks on to the carburetor bodies and filled in the joint at the junction where the carb bell mouth meets the velocity stack. The bike never ran better.

I ran that set of carbs with epoxied bell mouths into the stock him can air cleaner assembly. The velocity stacks, with their two rubbers were run through the two holes in the front plate of the ham can and then they were epoxied to the two carbs. The rubbers for the velocity stacks were on the inside of the ham can's front plate. The stacks' inlets did come close, at their bell mouth entrances to the rear panel of the ham can but that did not seem to affect their performance at least as far as I could tell, with nothing to test it by. If I ever abandon my PWK's and buy a new set of Premiers I will go back to that set up

Nater_Potater said:

no750 said:

What mechanism prevents the original air filter assy from taking fire?

Click to expand...

Essentially, you have either a substantial labyrinth (as with the stock ham can), or a good heat sink (the screen meshes that make up the K&N) that is very effective at quenching the fire before it can get out and light any surrounding material. Look on top of an inboard boat motor carburetor; you'll see a flame arrestor. It looks like a large diameter stack of SuperTrapp muffler discs. You can see light right through them, but there's enough surface area presented to the flame that it effectively drops the temp below the point where the flame dies out.

Nathan

Click to expand...

This. Carburetor fire on a car is bad, on a bike, tank directly above, you have only seconds to extinguish it before you step back & watch it go. Fashion is an expensive hobby. :idea:

Eddie, that hurts just to look at it! I hope the injury didn't go too deep.

Let's see; you lose a little HP without velocity stacks, or



Granted, when you're racing and need every last ounce, some concessions must be made.

For street use where you want a bit more power while retaining filter and spark arrestor function, try one of these (if there's room...)



Maybe not exact dimensions, but you get the idea.

Nathan

Nater_Potater said:

Eddie, that hurts just to look at it! I hope the injury didn't go too deep.

Let's see; you lose a little HP without velocity stacks, or

Granted, when you're racing and need every last ounce, some concessions must be made.

For street use where you want a bit more power while retaining filter and spark arrestor function, try one of these (if there's room...)

Maybe not exact dimensions, but you get the idea.

Nathan

Click to expand...

Nathan, that is precisely the set up I have on the Blue Bomber, alloy velocity stacks inside a Ram Air filter.

Versus straight inlet stubs inside large K&N type filters, my set up gave a whopping increase of just under 2bhp!

So, in reality, it would be practically impossible to detect the difference on the street.

But, 2bhp is 2bhp, onwards and upwards !!

Oh, and there were no injuries in my little mishap Nathan, other than mental trauma and pride !!