Velocity Stacks & Pilot Jet Issues?

Does anyone run velocity stacks on thier concentrics and end up getting poor idling after some numbers of rides? Just thinking dirt may enter the air mixture intake port sitting at carb mouth and either block air path or even make it into pilot mixing chamber/feed to pilot jet ports in carb throat.

I don't run stacks on my commando but do have one on my Velocette and it keeps having idle issues after a number of outings.

Tornado said:

Does anyone run velocity stacks on thier concentrics and end up getting poor idling after some numbers of rides? Just thinking dirt may enter the air mixture intake port sitting at carb mouth and either block air path or even make it into pilot mixing chamber/feed to pilot jet ports in carb throat.

I don't run stacks on my commando but do have one on my Velocette and it keeps having idle issues after a number of outings.

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If I did, I would try to have an airbox built to go with them.

jan nelder said:

If I did, I would try to have an airbox built to go with them.

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Bike did come with an OEM box...a very odd arrangement with lots of weird right angles for air travel around...and a spool of what looks like thick gauge steel wool. As most velo owners back in the day immediately removed these and went with stacks, I did the same. There are modern cone types that can be fitted if you find a small/short enough size (not much space aft of the carb before oil tank position).

Tornado said:

Does anyone run velocity stacks on thier concentrics and end up getting poor idling after some numbers of rides? Just thinking dirt may enter the air mixture intake port sitting at carb mouth and either block air path or even make it into pilot mixing chamber/feed to pilot jet ports in carb throat.

I don't run stacks on my commando but do have one on my Velocette and it keeps having idle issues after a number of outings.

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I was running stacks on my Commando, but with a foam sock type filter to take the worst of the rocks & stuff. The pilot intakes are not covered though, and I've never had a problem. It does depend though on the air quality - if you live down the end of a dirt road you will have more problems that someone up on a mountainside in fresh, clean air.....

Steve Borland, I did the same although I made some coarse-mesh domes for the velocity stacks and used foam microphone covers as filters, they fit perfectly and you can lightly oil them for extra filtering, although I don't bother with this.

I've had the "long" stacks on my Commando for some years now. I do not have any sort of screen or filters on them; never had an issue with idle/pilot jets. Back when I first installed them I was working on modifying the Ham Can filter box to provide filtering for the stacks without disrupting the flow but it became too fiddly so I decided to just run the stacks "wide open."

If this was the old days and the bike was a daily rider as was my original Commando, I wouldn't run it without an air filter but TBH, with the mileage the bike gets nowadays, the additional wear that might occur won't cause a problem in my lifetime. And the bike runs much better with the stacks!

David Vizard put it this way (paraphrased): you may realize a small performance gain by omitting air filtration, but one kicked up cloud of dust inhaled by your engine will reduce your ring sealing to the point where you've lost that advantage.

Tornado said:

Does anyone run velocity stacks on thier concentrics and end up getting poor idling after some numbers of rides? Just thinking dirt may enter the air mixture intake port sitting at carb mouth and either block air path or even make it into pilot mixing chamber/feed to pilot jet ports in carb throat.

I don't run stacks on my commando but do have one on my Velocette and it keeps having idle issues after a number of outings.

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I don't run velocity stacks either, but was having trouble adjusting the idle on a single carb of my pair on the commando. That cylinder was popping randomly but only at idle. I thought it might be the float hanging up so I took the bowl off first and made sure the float was moving freely and the needle was seating correctly. I reassembled it but it was still doing the same thing so I concluded it wasn't the float bowl. Eventually I just gave up and removed the carb from the bike to clean and inspect it. I washed off the slide which felt a little sticky on the down flow side and washed the body too so it would all move freely.

I had previously installed a removable set screw behind the idle jet opposite side from the idle air screw so I can open the port and use a micro drill on a stylus to ream out the jet. The #79 drill (,016") was tight in the jet, so I spun it to get it in then tried to pull it out without spinning to pull the shit sticking to the brass out of the jet. I didn't want to break the tiny drill so it took a lot of spinning and tugging before it would go in and back out without having to spin the drill. I blew everything out good, reassembled the carb on the bike and now the idle air screw works fine. No popping

When the idle air screw won't adjust your idle properly, ..... clean the jet. 99.9% of the time IME, it's a dirty idle jet or idle port hole. Unfortunately the only way to poke the idle jet without disassembling and putting in the clean out port is by glueing the mini drill in a spray tube and pushing it down the idle air adjuster hole since the mini drill stylus is too wide to fit down the air screw hole. The rear port set screw is easy to do. I blue loctite the screw and it's sealed and not going anywhere until I have to service the carb again.... I don't expect many people will drill their old amals and add a clean out port, but it works for me and I got the idea from someone on here somewhere along the line...

o0norton0o said:

I don't run velocity stacks either, but was having trouble adjusting the idle on a single carb of my pair on the commando. That cylinder was popping randomly but only at idle. I thought it might be the float hanging up so I took the bowl off first and made sure the float was moving freely and the needle was seating correctly. I reassembled it but it was still doing the same thing so I concluded it wasn't the float bowl. Eventually I just gave up and removed the carb from the bike to clean and inspect it. I washed off the slide which felt a little sticky on the down flow side and washed the body too so it would all move freely.

I had previously installed a removable set screw behind the idle jet opposite side from the idle air screw so I can open the port and use a micro drill on a stylus to ream out the jet. The #79 drill (,016") was tight in the jet, so I spun it to get it in then tried to pull it out without spinning to pull the shit sticking to the brass out of the jet. I didn't want to break the tiny drill so it took a lot of spinning and tugging before it would go in and back out without having to spin the drill. I blew everything out good, reassembled the carb on the bike and now the idle air screw works fine. No popping

When the idle air screw won't adjust your idle properly, ..... clean the jet. 99.9% of the time IME, it's a dirty idle jet or idle port hole. Unfortunately the only way to poke the idle jet without disassembling and putting in the clean out port is by glueing the mini drill in a spray tube and pushing it down the idle air adjuster hole since the mini drill stylus is too wide to fit down the air screw hole. The rear port set screw is easy to do. I blue loctite the screw and it's sealed and not going anywhere until I have to service the carb again.... I don't expect many people will drill their old amals and add a clean out port, but it works for me and I got the idea from someone on here somewhere along the line...View attachment 119462

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That's basically what The bushmans Guide to Tuning Concentrics details how to do....and uses a 2BA tap to thread the drilled out blanked off port, a cut off idle air mix screw for port block off and some locktite as a sealer.

Check out the Pete Henshaw videos on you tube, he found interesting info when testing velocity stacks just by rolling down hill and watching the tickover change.

When you change the configuration of the carburetor, unless you have it jetted too rich, you cannot expect to make the change without retuning. And the same applies to the exhaust system, a change in restriction changes the jetting needs. With a road bike, slightly too rich is normal. Race bikes are often jetted lean enough to almost cause damage. An air cleaner is a dramatic change.

Madnorton said:

Check out the Pete Henshaw videos on you tube, he found interesting info when testing velocity stacks just by rolling down hill and watching the tickover change.

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Is this Paul Henshaw? If so, he has a heap of videos (2300 according to Youtube !!!!), could you give a bit more info (or preferably a link)?

Its his recent vids on a Commando....here is the testing mentioned:

I have watched his videos and he labours his motor a lot when riding, and no air filter fitted WTF.

When my motor idles, I can almost count the firings. Also this guy's bike is misfiring which means it is jetted a bit lean. The noise recording on the video might be a bit strange - there is a lot of high-pitched rattling. It might have an oil problem.

When the motor is started with an enrichener or choke, it is normal to let it stand idling until it starts to slow because it is too rich for normal riding when it is warm. When the enrichener is taken off, the motor should stand quietly ticking over with no throttle used at any stage, until the bike is ridden. however the motor is set up, you should be able to jet to suit it. The velocity stack might interfere with the holes in the carburetor whet it fits on. Otherwise it should not make much difference, particularly at low revs.

The Commando in the Henshaw videos is clean. Too bad it sounds like an underpowered rototiller running in a field of rocks. Too harsh?

Velocity stacks, no air filters, just need a good backfire from the carbs and if you have a new fuel tap that had a slight leak and I do say new fuel tap and bang, how do it know, the great fire of 82 and I was on it at the time, that was the last time I ran with velocity stacks.
Was at a classic bike race meet at Lakeside raceway back in the days and seen a classic Triumph go up in smoke from a backfire with rider on it, velocity stacks no air filter lucky quick-thinking marshal was handy with fire ext., still it was his race day over, what a mess.

ashman said:

I have watched his videos and he labours his motor a lot when riding, and no air filter fitted WTF.

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I really don't think Paul henshaw needs any advice on how he rides or whether his "customers" bike needs an air filter or bell mouths
You need to look at some of Paul's race bikes and his asbo royal Enfields to see he knows what he is doing