Needing Air

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I'm only quoting John Healy the Boston Triumph man about bleed drillings. Apparently, quite a few people fine tune mixture at small throttle openings by enlarging the holes. One thou enlargement at a time was recommended and he said there's a limit at around 50 thou diameter when "the carburettor stops working."

The procedure was something people did to make megaphone exhausts work. Being racers, they didn't tell everyone what they did.

By "air compensation" I mean the air sucked into the central little hole in the intake horn, to go up around the outside of the needle valve into the venturi. That little system prevents the mixture getting richer and richer as air speed over the spray tube increases. It's the difference between 1920s two lever carburettor, on which you had to restrict the air (like using a choke) at low rpm and a modern "single lever" carburettor.

I'm not claiming to know what your modifications will do or that they are bad. I just think you have some dyno time ahead of you and not just at full throttle. If you can analyse mixture with an exhaust sensor, while riding, you may not need so much dynoing.

The idle on my T110 is ok for a rough and ready, slightly overtuned relic.
 
Thanks again Triton Thrasher
I have a dyno organised that will permit me to make my own changes between runs e.g. bowls between carbys sets and changeovers. First, I need something to assess so have devised a number of grouped changes for each throttle opening phase. To reduce the number of runs, these multi-factorial runs on the dyno can then be analysed according to my 'design of experiments' re Taguchi. The variously worn carby bodies allows me to also assess the mods to suit real-world carbys instead of only a pristine pair. For consistent float level on all runs, the float bowls will be changed between carbys - when tuned to my satisfaction I will then experiment with altering float levels.
I assert that, with the annulus seal on place, the larger drilled holes provide equivilant surface area and (hopefully) air volume as the annulus. I think the fuel will be better atomised with the air draw just above the needle jet orifice and the 6 equally spaced bleeds will centralise the needle to the jet (the current system airflow forces the needle forward to chafe the jet).
Ta.
 
Well, scrap that sealed annulus and drilled 106 needle jet idea. Based on annulus = collective hole area, it didn't work first up (too lean) so I'm rethinking that one (maybe increase/drill 1 hole size at a time).
Fitted the 4-ring needles today. They are 2mm longer at the parallel section so the transition from needle jet to main jet will now occur later. The stepped spray nozzle work well to pull fuel better at earlier throttle openings i.e. the transition from pilot and cutaway circuits to needle jets. During the parallel section of the needles the 107 needle jets pass too much fuel with this set of carbys. I dropped in 106's and the AFR improved but still a tad too rich. Tomorrow 105 needle jets should sort that opening range. The needle taper section is next on the list (now 2mm later) so it may be a compomise between 105s and 106s I reckon.. After that, I have a choice of 220 and 260 main jets in the parts box.
Oh, also trialled a closed-off choke hole cover plate: difference yet to be ascertained.
Ta.
 
Triton Thrasher said:
You're really designing a carburettor.

Could keep you out of the pub for some time.
Click to expand...

Naw. Same old mousetrap just snappier.

Hi L.A.B
Thanks for the muffler info. Just what I needed!
Ta.
 
Re: Needing Air with just enough fuel addedd

Jobs to do today.
Buy new 105 needle jets
Taper needle jets nozzle
Fit needle jets
Assess AMAL carbys through to 5000rpm (no load)
Assess noise levels (elbow fitted vs straight exhaust)

All done and I had time to earn an income as well.
The throttle grip has 60+mm of arc length so I marked it roughly according to AMAL's tuning guide i.e. 1/8th of 32mm is 4mm, 1/4 is 8mm, etc. Did you know that 4000rpm is just near the end of cutaway and not quite needle jet parallel phase? and 5000rpm is then on the parallel? That is as far as I went on an unladen motor.
I tapered the new needle jet nozzles even though the new needles are 2mm longer and unlikely to catch on the standard lip (in case I decide to trial the shorter needles at a later date). Decided to go with the 260 main jets.
Tested my muffler noise levels.
Ta.
 
Something different happened today on the way back from the countryside...

Kickstarter ratcheted mid-kick and needed to be gingerly eased through until it catches and can then kick the engine over. Not another worn pawl! and then you recall having bought spares ages ago: but which drawer did you put them in? Found and installed within 2.5 hours. WooHoo. One now left in the small parts drawer.
Ta.
 
some people get obsessed with these things. I like to tease that gettng em to step down easy states of tune preserve wear items but really its poor mechanism ratio on a 200 PSI Combat bomb. I am glad ya reminded me I have one to replace too so should send to cryo temper to see if extends ride time. Just think how bored we would be if Norton actually prefected Commandos for us.
 
Hi hobot.
Yep, pawls are consumables so I grabbed a couple when they came available at a good price.
The run in the countryside trialled my latest carby tweaks. Result: idle through all transitions to needle taper has optimised AFR - flat spot into cutaway phase is gone! Coming onto main jet dropped the AFR to 9-10:1. Changing to the 220 main jets.
Phase 3 - discussions underway - I'm gonna need test pilots.
Ta.
 
I can not follow all of your mixture breather and carb logic nor if I am sensitive enough to notice the slight poor response-stumble phases but do get off on your depth of Commando worship especially as different path than any one else. I have spent so so much time - years down- recovering from various parts breakage-failure to crash crushings other more life priority things finally caught up this last year or so - to follow your tediums is both fun dream machine perfecting progress yet also distasteful as more crap to get done just to enjoy some nice road freedom. My SuVee650 got recovered last week on fuel system issues and used only twice beyond my driveway but had half dozen close call-body/spine/jaw straining crashes going dam careful off pavement I really do not look forward to riding-risking life/limb but on a clunker Commando, when fate allow me to get around to it.
 
I recently read an article about BSA carby mismatch (30mm fitted instead of 28s).
carb-intake-manifold-intake-port-matching-t22127.html
The final sentence mentions "classic" 22 and 7 degree included angles for venturi: does anyone have validated evidence of application?
Has anyone trialled 32mm carbys and parallel manifolds onto a 30mm head with supporting dyno evidence?
Ta.
 
Ventrui angles in and out vary with density of fluid and temps and pressure but BSA learned the best angle for them. We recently had long bickering on BSA accidental power discovery via miss matched big carb and manifold into smaller ports WithOut relieving the flow vortex trip up lip. This accident also occurred with Peel power so good it pissed off about every sportbikeer and most the seasoned experts on public cycle forums as so against the grain to believe w/o being there. You could be the first Nortoneer to get objective measures on this funny flow business like BSA did by track times instead of a road dyno, so we can believe your findings.
Air Flow Optimization via a Venturi Type Air
Restrictor
http://www.iaeng.org/publication/WCE201 ... 7-1851.pdf
 
Re: Needing Air to flow best into the carbys

Thanks hobot, that reference answers my query re venturi intake angles perfectly. Re: BSA emperical track testing and the computer modelling support similar findings.
To test the mismatched carby/port concept, I will need a bike with a 30mm ported head (mine is 32mm) matched with a 32mm manifold (I only have 3 x 32->30mm but am looking to acquire another 32-32mm parallel manifold to make a pair). :D Perhaps a research grant is needed: not sure if my application would be successful. :D
Ta.
 
I did not re-read this article so not up be tested like you now but enough to apply to special exhaust resonator I actually found a hot truck ad for same idea so might be felt or heard better. I did get some rocket science and know what works in dense atmosphere to chase air craft is not efficient in near vacuum orbit.

I flipped Peel between the too big manifold and factory match manifolds to head and liked the trip lip. I detest resistive mechanical things, so shot the snot out of couple mower engines and was surprised a ricochet split carb base & exposed funny looking gasket that turned out to almost a mimic of what did on Peel, so depressed with everything wrong about Commando did not bother to clean up a DIY rubber/fiber gasket. Mower has a ring of 1/8 saw teeth bent for slight turbo blade effect. I would next experiment only saw toothing the lower half or 1/3rd of the bore for the short radius port bend energizing turbulence. Better fuel suspension likely a big factor too.

Might consider a G-force type data logger to practice standard runs to monitor real time. Not lab level but realistic enough to know if ya are consistent in tests and if real life detectable.

Peel must have at least one of these helpers
[video]https://www.youtube.com/watch?v=qqpcBpSsj1A[/video]
 
Re: Needing Air to flow best into the carbys

needing said:
Thanks hobot, that reference answers my query re venturi intake angles perfectly. Re: BSA emperical track testing and the computer modelling support similar findings.
To test the mismatched carby/port concept, I will need a bike with a 30mm ported head (mine is 32mm) matched with a 32mm manifold (I only have 3 x 32->30mm but am looking to acquire another 32-32mm parallel manifold to make a pair). :D Perhaps a research grant is needed: not sure if my application would be successful. :D
Ta.
Click to expand...
re; " I will need a bike with a 30mm ported head (mine is 32mm) "


A 30mm bore split steel spacer up to the valve guide will do this job without removing the cylinder head :?:
 
Far out. Just saw an ebay listing by a bike shop for 11 dowty washers and o-rings for a Commando priced at AUD $60. They had want to be both stainless steel and viton at that price but I'm guessing mild steel and nitrile to maximise profit.
Ta.
 
Re: Needing Air to match the pilot circuit fuel

needing said:
Triton Thrasher said:
What do you adjust idle speed with?
Click to expand...
Not with the throttle stop screws! :D
Click picture to start video...
[
Needing Air
Click to expand...

The nice idle you've got with the well sealed slide is impressive - much like a modern fuel injected engine. Since you obviously like experimentation and taking data you may already have answers to the following questions, but if not you seem well equipped to get answers since the engine is well instrumented for such determinations.

My simple questions relate to the A/F ratios in the idling regime. We've likely all set myriad idle circuits of small engines over our life times, but perhaps never known what the specific A/F ratio was when a nice stable idle was achieved, nor what A/F ratio gave the heavy slower rich idle or the higher speed lean idle. So, with that background and because your system has relatively constant air volume and varies primarily in A/F ratio, was wondering about the following.

1. With your present set-up (fixed pilot jet) what range of A/F ratios are obtained via adjustment of only the pilot air screw?

2. Over the range of A/F ratios available via only the pilot air screw adjustment, how much does rpm change with changing A/F ratio (a plot of A/F ratio vs engine rpm would be exemplary), e.g., at a higher A/F ratio of X, idle speed increases to Y, and at lower A/F ratio of A the idle speed drops to B, etc.
 
Re: Needing Air to match the pilot circuit fuel

WZ507 said:
[ So, with that background and because your system has constant air volume and varies only in A/F ratio was wondering about the following.
Click to expand...

Mr Needing's system doesn't have constant air flow, because the idle mixture screw restricts intake of air.

So, screwing it outwards gives a weaker mixture and more air flow. That's one of the reasons most of us find the throttle stop screws useful during adjustment of the idle.
 
Re: Needing Air quantified

Hi WZ507.
I only have a standard Smiths tacho so the incemental RPM changes can only be a guess.
The AFR gauge also fluctuates as seen in the video but I may be able to trial another one.
Which of the recording sheets below best suits your needs?
Ta.
Hi Triton Thrasher.
The manifold vacuum fluctuations are pretty constant at idle as is the fuel charge pulled through the pinhole. The airscrew adjusts the AFR of the fuel charge in the mixing chamber beneath the pinholes. The linked pilot circuits with ball valves in the bowl means fuel draw is one way only and more stable with both carbys drawing. Hope this helps.
Ta.
 
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