Caburation in practice

Sat with my first coffee of the day,thinking about my rough tick-over, Yes 45 years on i still find carbs a black art,
Well where to start with is a topic it it's self, I have used colour tunes, and plug chops and balance gauge's..But lets think about whats going on?

Ok, i bet most Commando owners have used Balance gauges...right? But here's my opinion of these; We connect each gauge to the manifolds and frig around with screws.and cable lengths untill the gauge's read about the same...Right? But all "we" are doing is measuring the depression between the slide and valve..am i right so far?

BUT we are not adjusting the air/fuel ratio...if its 14-1 on one side and 12-1 the other there will be a bigger bang on one cylinder..causing a un-balanced engine.
Modern Triumphs Fuel injection systems adjust the air/fuel ratio hundreds of times per second, using a lamba in the zorsts .

So is the BASIC carburation on our Nortons the "added "cause of Vibes? or just the mass of of un-balanced iron flinging about, what i do know is friggin about with settings can cause a better feel..so good carburation air/fuel ratio make's plenty of differance...Any one had better results with exhaust gas meters?

It would be interesting if Jim Comstock gave a bit of a rundown on the FI he is developing for a Commando - CNW says "reeal sooon nooow". ?

But the points you raise are the reason Nortons gave carb specs for the bikes when new.
If you stick to that jetting (on an all stock bike) the jetting is pretty good.

Sure, exhaust gas analysers and dyno runs can get the jetting down fine.
But carbs have to cope with zero degree days and 100 degree days, and zero % humidity and 100% humidity, so all up they do pretty good. ?

P.S. Some years back, a rebuilt 850 would start without choke on a cold day on the old carbs. With new carbs, it needed lotsa choke to start - and the fuel consumption was no comparison.
Keep em within spec ?

HTH.

If the carbs are absolutely identical and set up exactly the same and the depression is the same then the cylinders get the same fuel. Problem is it only takes a small amount of wear/tolerance/small set up differences to throw things out and we all know how original Amals wear but I think the latest spec slides have gone a long way to sorting that.
EFI isn't a universal cure all, there's only one exhaust sensor usually for both cylinders so both injectors try to feed the same fuel regardless of the individual AFR's and injectors can wear or go out of calibration so you could be weak on one rich on the other with the exhaust sensor saying everything is fine.

Dyno's and exhaust gas analysers aren't much use for setting the idle mixture

Setting the idle is a separate process to the initial balancing or synchronising, if that's all you do then the idle will be rough. I usually work from one carb to the other feeling the pressure pules from the exhaust and the temperature of the silencers to work out which cylinder is carrying the load and make small adjustments to even it out, gradually dropping the idle down. Getting a rock solid smooth idle on twin Amals is quite possible.

Dyno's can't measure below like 2000 rpm so useless for idle tuning. Narrow band
O2 sensors are only good in very narrow a/f range at essentially WOT. Wideband
better but not good near idle as O2 can back flush so easy up our short exhausts
and unless battery heated it ain't in the temp zone to function accurately.

Color Tune is made for the idle tuning, but then so are our eyes and ears about as well. Must get timing and spark plug gap and heat range right first of course. Float level must be set as baseline for all rpm tuning procedures too. A proper float level should give best idle at 1.5 pilot turn out on our Amals and them Miki carbs. Then ya can fudge float level a bit for best power or best economy. Lowering spray tubes or notching them can give more accelerator pump like response to low rpm throttle snaps.

Its possible to out run the tiny fuel flow into Amal bowl having to both pass the needle seat then flow pass big needle to reach bowl, a BSA mod is to cut a slot just above the needle seat so it can flow straight into bowl.
http://rides.webshots.com/photo/2904400 ... 1179ffluiN


I just tuned up a spread bore 4 brl carb and studied the online techniques to find the economy guys start from too lean and increase richness to fastest idle then back off towards leaness 1/2 to 1 screw turn, while hot rodders start from too rich slow idle to fastest idle and then back up towards richness 1/2-1 screw turn to dip idle down, then each way diddles the throttle screws for desired idle rpm.

If you don't have electronic ignition, you can just take a plug cap off and set the idle on each cylinder/carb individually. And then just back the settings off a little so the idle speed comes down a little with both cylinders running.

Getting the carbs synchronised at speed, so the engine runs smoothly and at its best is where most riders neglect. Tricks like putting a pencil under each slide, so they both lift evenly and equally goes a long way to a smooth running and responsive engine. Some magazine somewhere once quoted that a sample of Commandos showed something like 70% didn't have well synchronised carbs. Don't even mention 4 cylinders, backyard mechanics need not apply... ?

A good mechanical sync of your carbs is as good or better than using a Twin-Max, I've done it both ways and when done properly, mechanically, the Twin-Max simply doesn't move off center. I'll freely admit that with a "ham can" air box syncing is considerably easier with the Twin-Max, but if your cables are "seasoned" syncing can be an annual event.

Given the numerous short-comings of Amal carbs and what I've heard recently it is important to be able to measure your needle jet, with a mic-able gauge, and to check your needle for ware by using a micrometer and taking scores of reading about the diameter of the needle and comparing those readings with readings of the other carbs needle, sameness is virtue. A 106 needle jet can measure anywhere from .101" to .110" depending on when in Amal history you acquired it, get a gauge if you really want to know the size of the orifice. With Amals a 106 needle jet is suppose to be .106" gauges are available, if you can't find them locally PM me I can get you what you want.

If you have old Amals and really want to keep them, then get them sleeved, get new calibrated parts (main jet, needle jet, needle) and mic them; you will still be under the cost of a Mikuni conversion, and if you actually understand the stages of carburetion you'll have a pretty neat experience doing the fine tune, but do make sure that you can chart and clear your pilot air and fuel passage ways in the body of the carb(s) or nothing will go right for you.

RS

A carby thread.. fantastic.
I love to learn new things about carburation. Which is happening right now with this thread.
I like to think that I know a bit about the topic, but that's all based on experience gained from "shoe string racing".
I have tuned, and helped tune 2 strokes and 4 strokes for family and friends for many years, but am being tested by
two pairs of Amals at the moment. One pair on a hot Atlas motor, on Methanol, and a pair of 32s on my 4s Combat,
that have the idle jet modification.
OK forget about the first pair, I think we are getting close there. Thanks ludwig.
Now my carbs... I've played with different cuttaways and needles and was very happy.
Easy start hot or cold. At a 1200 rev hot idle, she will idle unasisted within 30 seconds on a cold start, at about 600 then pick up gradually while I put my helmet on.
Then stay solid all day.I'm happy with that.
Nice colour on the plugs from idle up to 4,500 (according to plug chops), then a tad lighter up to 6,000. After that not concerned at this stage.
However when I ride at a consistant half throttle and need to stop quickly, I am finding that she struggles to idle and even stops, then needing a tickle to start.
These carbies are clean and the filters on the bowl are new. No inline filters. Clean air filter. New needle and seats and stay up floats.
My first thoughts were of a semi blocked tap as i have recently taken to riding with only one tap in use, keeping a reserve, and if it were partially restricted
it may not keep up supply. Had a look, and seems to be a very good flow. Tried both taps open, no different.
I had checked the float levels before installation and was happy with it, but now am reconsidering.
Sounds like float level is too low. ?? Not letting enough in to keep up supply. ??
Any thoughts.
Just realised I may have hijacked a thread here, sorry guys.
AC.

Might try a run with the slides greased to see if worn slide air leaks messing with your pleasure. Won't fix it nor last a full ride but who knows on this learning curve ride. My factory Trixie is behaving this way fine start and slow idle till warmed up then idle raises on 1000 rpm and will not return to idle unless I drag motor down, then stays at low idle fine, unless i get it a tad of throttle it stays fast. I know these slides are worn to point of seeing slides rubbed to fluted state. Open pipes right not and shoot up mufflers not put back on so have not tried the slide grease trick yet.

Tinkering with carbs is one of the best things about owning an old crate. Sticking some ultra reliable guaranteed stay in tune system on a Commando would kinda spoil things, may as well buy a modern.

Rich_j said:

Tinkering with carbs is one of the best things about owning an old crate. Sticking some ultra reliable guaranteed stay in tune system on a Commando would kinda spoil things, may as well buy a modern.

Click to expand...

True, but a lot of owners like smooth tick overs and opt for a single jap carb, i like stock looks,and will get the amals "good enough"

I have had 3 Nortons, 1 P!! and two Combats work absolute delight from Always 1st kick start, steady slow idle and tire smoke or front lift response just off idle throttle snaps, able to take hi throttle openings at barely over idle speeds to climb slick steeps and not stall or spin out. May not last for 1000's of miles but I know whats possible and take is as personal challenge to keep it that way as much as I can, at least on one Commando. I will invest in the hardened surface slides soon. I do have tested and sorted 34 Mike carb on hand if I have too much down time during Amals recovery. In the end, each one has their own path to Commando-doom and glad of it. If only one Cdo as daily rider a Miki has a lot going for it as does electro ign and belt drive and on and on for shear joy of it.

I thought I was pretty savy regarding Amals ~ but I have had a few issues with my Trident and so decided to pass on a full set and gantry to a retired engineer who is part our small Australian Tripler group !

Bob is a very talented man and only recently shut down his race team but has decided to take on the recovery of as many Triplers as he can save.
He challenges the very accuracy of the Amal factory ~ and has even approached them directly as he agrees many of their new parts are excellent ! But then openly condemns other parts ~


Here is his email as of last week ~ I hang my head !! ( Most of these issues I over looked and probably would have not even considered them !!)


Stuart,

I’ve finished the T150 complete carb set.

Some points I wish to explain.

The manifold plate the carbs bolt onto was bent by .100” and had to be straightened. I guess this was caused by bolting the manifold using the 2 centre bolts to the air filter plate too tightly by someone?

When you fit them don’t do the bolts up too tight please as it will bend again.

If bent again the carb bodies will distort also.

There should be lock nuts to secure the 2 x 5/16” bolts in place at filter plate.

# 3 float bowl needle seat was lose plus the gasket surface was badly pitted and would have only sucked air and not fuel for the idle circuit so I replaced it with a new bowl and needle seat.

All 3 slides were # 3 cutaway and not 3 ½ cutaway so I have fitted Amal’s new Hard Anodized 3 ½ slides. These will never wear out, trust me, they are brilliant.

# 1 carb had a T160, 5 groove identification needle on the middle clip while # 2 & 3 had worn T150 needles but on the top clip position, and # 2 slide was .080” higher up at rest than the others, not good.

All of the 3 piano wires were bent beyond; well I don’t now what.

All of the float levels were set at 3/16” down way too low plus someone has heated and bent the plastic float needle tangs to try and obtain a correct float level. This should never be done.

I have replaced all 3 floats & float pins with new and re-set the floats to the correct level and fitted Viton Alloy float needles.

All of your main jets flowed much higher than they should have and were closer to 160 mains not 150 mains, so I fitted 3 x new flowed mains and 3 x 106 new needle jets including 3 x T150 new 2 groove identification needles. (T150)

All 3 carby top plates were bent so I straightened those and resurfaced them. All 3 x carby body gasket faces were re-surfaced including the float bowl gasket surfaces including the new one as they stupidly come not flat also.

It is very important the carbs can’t suck air here at the gasket face or else it won’t draw fuel from the bowl feed hole for the idle circuit.

All 3 welch plugs were removed and the mixing chamber under was bead blasted and cleaned; they were chockers with crap. Every carb and associated parts were also bead blasted and cleaned.

# 2 carb had an incorrect short Air Mixture Adjusting screw which would not have worked at all to adjust idle mixture and the others were badly pitted and rusty so I replaced all 3 screws with new O rings.

Your carbs were mounted with 1 3/8” factory O rings to the manifold plate but I’ve since learned the O ring receiver groove in the carby body is never deep enough and causes the carby flange to bow, so I have fitted gaskets to the carbs and not O rings, should seal better and not warp the flanges.

Your carby slide springs; well; while they were the correct light type of spring for a Triple they all varied in length greatly which meant one springs was not anywhere near long enough to be effective so I re-set these to 3 1/2 “ the correct free length.

I replaced your stainless steel spring washers on the 10-32 socket head screws with the correct steel type and removed your stainless 5/16” spring washers on the carby mounting bolts with flat AN washers and fitted 5/16” unf AN self locking nuts. Your 5/16” washers need too much pressure to work correctly plus they will bruise and scuff the bolt face plus the carbs should be fitted with self locking nuts not plain nuts and spring washers.

Remember the carbs are not Alloy but merely Pot Metal or almost Zinc in construction and need to be handled carefully or else they buckle.

All 3 slides are now set equal to each other and shouldn’t need further adjustment; the air screws are set at 1 ¼ turns out each so the only adjustment by you when hot is the final air screw adjustment and idle speed, but, I would try it at 1 ¼ turns out first and only adjust the idle speed screw.

However, if when you first start the bike from dead cold and it spits in the carbs if you have adjusted the air screws out to say 1 ½ to 1 ¾ turns out this means they should be screwed back in to richen the idle, that’s why I have them at 1 ¼ turns out. If spitting occurs at idle it is Only the air adjuster screws need adjusting in and nothing else.

The throttle gantry bushes were ok so I left them alone to save you some money and only cleaned and re-greased them and fitted new circlips and washers including the correct gantry return spring retaining end washer with the correct item.

The carbies on the complete set are a mixture and are L15, 1969 Trident, R68, 1973-74 Trident and R66, 1973-74 Trident but that doesn’t matter as they were all 27mm bore carbs except someone had fitted rain hat ticklers to the L15 carb.

I haven’t done anything with the 3 lose carbs, do you want me to repair them?

I can supply 3 x re-sleeved Teflon coated 3 ½ slides for this set but it would have to be done first.

Fit new Rain Hat Ticklers.

Re-surface the carby bowl gasket face and manifold faces.

Bead blast all and remove and fit 3 x new 11/32” welch plugs to the mixing chamber.

Chase all threads and true up carby bodies.

Could be another $200.00 + or so, as I don’t have a firm price on the Teflon Coating, as I’ve only played with this surface treatment and wasn’t charged back then.

Let me know please.

The last post seems to indicate very clearly why attempting to "tune" Amal carbs is not really a job for beginners to carb work!

The last post seems to indicate very clearly why attempting to "tune" Amal carbs is not really a job for beginners to carb work!

Click to expand...

Carbonfibre ~
While I have always enjoyed tinkering and "eventually" getting such a positive result I am now cured..

"Bob" is a wealth of knowledge and experience ~ His shared knowledge on fuels, delivery systems and classic motors coupled with months of toying with Amals, sold me ~ ( Twelve months ago when Bob closed down his dragster race team he never owned a motorcycle ! Now he has an imacculate ES2 and a trio of Tridents and an A75 !)

And after spending five years building my latest project , the T150V I realise that I could have been riding instead of screwing around in the shed a whole ot sooner ~

Of course we all have had some nasty experiences with so called experts.

I was actually the one that introduced Bob to our Ozzie triples group after he bought a new OEM Trident exhaust header system from me via EBay !

So as I have advocated so many times to friends it really pays to "touch base " with vendors or buyers on EBay if they are receptive. it has worked for me quite a few items now and I have made some great friends via EBay !!

And as I have in this evolving scenario ~ I watched and listened to what this man was saying long before I threw him my first set of carbs with gantry attached ~ Personally I don't care what the cost is ~ up to a point of course ~

I am certainly not a millionaire and I work a mundane job but I have been frugal and saved / invested well!

But I feel the $500 I spent on this set of 3 Amals is far better value than ANY new manufacturer's goods straight out of the box. In fact this makes TWO great classic spanners I have encountered ~

No more ~ trying to cheat on costs for this Dog!! :lol:

Thanks Stuart for posting Bob's details as educational to me to learn the old skills-carb details on obsolete motorcycling.
Sounds like carbon would have us remove everything original but the logo. But he is right real Brit Iron takes time and skill beyond modern road appliances. When I have a C'do to ride and its raining or freezing I just put on what's needed to stay comfy but if just my modern working then mostly I just take the cage.

It's Friday night, having a absolute barstard of a tooth ache, I have been surfing the forum and related web sites reading about carbs,
Its hard for me to vision the piot hole fuel oil in the base "sucking "fuel through a air space of 4mm, unless i have got this wrong :?:

The pilot fuel inlet hole sits above the fuel level, and sucks the surface fuel up,crossing the 4mm air space..incredable :shock: surly air is sucked in causing a vacuum,air being easly drawn in before liquid :?: I know it works,but its still a wonder :shock: