main jet test

'For normal road use on stock carbs factory settings are pretty close to optimal but when changing to bigger or alternative carbs on a modified motor initial set up can be a crap shoot'

Most old British bikes were set up to be tuned without computers and dynos. If you are fitting a different carburettor, the process is always the same. It is not rocket science and there is no guesswork. Like a lot of things, - once you know how to do it, it is easy.

Some of you guys should try tuning two strokes on methanol. If you are even slightly off the bloody things won't even start. Then when you have got them going, if they starve, they go like buggery and blow-up. All you have to do is forget to drain the carbs just once.

Acotrel

most old british bikes were not tuned with computers all were fitted with Amal so the Jet information was readily available no Rocket science required

When you switch to Mikuni Kehien Delloto things become more complex main jet size can be easily worked a 200 Mikuni jet flows 200cc per min as does the Amal
but each manufacturer has a range of needle tubes / needles for each carburettor get the tube and needle combination wrong and goodbye motor

select the wrong needle tube / needle combination and it will meter less fuel than the main jet size so in effect you can fit a 700 main and the tube will only pass 200cc goodbye motor

i have also raced two strokes and the range of tube / needle combinations is mind boggling the dellorto i used had 20 plus tube options and 26 different needle tapers
Mikuni and kehien also have similar amounts of options N/O/P/Q/R/S and each range have numbers 1234567 etc

Frankie17 said:

Acotrel

most old british bikes were not tuned with computers all were fitted with Amal so the Jet information was readily available no Rocket science required

When you switch to Mikuni Kehien Delloto things become more complex main jet size can be easily worked a 200 Mikuni jet flows 200cc per min as does the Amal
but each manufacturer has a range of needle tubes / needles for each carburettor get the tube and needle combination wrong and goodbye motor

select the wrong needle tube / needle combination and it will meter less fuel than the main jet size so in effect you can fit a 700 main and the tube will only pass 200cc goodbye motor

i have also raced two strokes and the range of tube / needle combinations is mind boggling the dellorto i used had 20 plus tube options and 26 different needle tapers
Mikuni and kehien also have similar amounts of options N/O/P/Q/R/S and each range have numbers 1234567 etc

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Good point Frankie.

In my own experience, Amal needles, needle jets and slide cut aways seldom required vast changes on Brit bikes. But modern carbs are different beasts altogether.

An added problem with FCRs is that the accelerator pump masks all jetting symptoms wen the throttle is opened for acceleration. The best way I could see how my FCR needles were performing was (as posted by someone earlier) by holding the throttle open at fixed points on the dyno with a gas analyser to check AFR.

The way I know when my needle/needle jets are correct is when I get down a certain piece of circuit the fastest. Slightly rich - and the motor is noticeably sluggish, lean - and I can't ride the bike around corners and up and down through the gears without it coughing. When the mixture is correct I cannot whack the throttle open, however when I wind it on with one smooth rapid movement, I get the best acceleration. That is with 34mm carbs with tapered ports. Big carbs affect the way you can use the throttle. I use a quick action Yamaha TZ throttle, only so I never need to have two grabs at the handle. I also usually have three fingers and a thumb operating the twist grip with one or two fingers on the brake. It is all super-light and rapid.
I wonder about testing the AFR at various throttle openings with the bike on a dyno - might work if the AFR really means anything. How do you verify that the specified AFR is giving the best torque and horsepower ? Perhaps you could set the main jets to give the max. outputs at certain revs and then measure the AFR and use that AFR value right down through the full range of throttle openings ? But then you'd have any advance curve to consider. Might be the dog chasing it's tail ?
I'll have a talk to my A grade rider friend who is a well-qualified and competent motor mechanic, he has done a lot of this stuff. Obviously you can do a lot more when you have a computer-controlled engine management system with injection and a selection of advance curves. You have got me interested. However I still think there has to be a correlation between what happens on the dyno and the effect when the bike is raced.

Alan, I think the point here is that the difference between road/track/race testing is really seat of the pants stuff that is very time consuming for some.
Also where do you do this testing , track days or the quiet back roads?
The dyno is a set point to be used to get close to optimum settings very quickly.
I have tried both , and for the extra expense , the dyno is the quickest way under controlled conditions to get the closest to the best AFR at any settings with both fuel and ignition timing and exhaust changes.
Just an opinion.
Also I know now what carb settings I need for differnet tracks with different altitudes and also gearing to suit each track by using the dyno as baseline data
Regards Mike

Old school v new technology


Acotrel posted
I wonder about testing the AFR at various throttle openings with the bike on a dyno - might work if the AFR really means anything. How do you verify that the specified AFR is giving the best torque and horsepower ?

The AFR is the scientific formula for perfect combustion to occur 14.7.1 best power and torque is produced by perfect combustion to check this under real world conditions on track is extremely difficult and time consuming ,by using a rolling road you can determine the AFR and power/ torque figures at set RPM and allow base line adjustments to be done ( before going to the track ) without loosing any precious track time

by using the rolling road set up you can often find a few horses you may have lost or never even new existed

Frankie17 said:

Old school v new technology

The AFR is the scientific formula for perfect combustion to occur 14.7.1 best power and torque is produced by perfect combustion to check this under real world conditions on track is extremely difficult and time consuming ,by using a rolling road you can determine the AFR and power/ torque figures at set RPM and allow base line adjustments to be done ( before going to the track ) without loosing any precious track time

by using the rolling road set up you can often find a few horses you may have lost or never even new existed

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14.7 (stoich) is a theorectical measure whereby all things being equal all the fuel and all the oxygen burn completely. More or less any engine will be toast within minutes at WOT at this ratio. Commoz I think has made some vaild ratio suggestions earlier in the thread, i.e. that you need richer to keep heat down and anyway there is more power available with a richer ratio.

On the other extreme, for part throttle economy, much leaner than stoich can be used effectively, especially in the 'lean burn' engines that were popular before catalysts messed all that up. So Stoich is strictly theorectial and not meaningful in actual use.

My Dyno man pretty much agreed with Jim, you wanna be at around 13 he said, but owt between 12.5 and 13.5 is fine.

Unless you've got Jims clever on board gizmos, its still an awfull lot easier to see how close / far away you are from this by using a Dyno.

You can easily be a long way from it too. Which is kinda eye opening...!

Agree with using slightly richer settings especially on the WOT department.
My race bike with the Premier Amals tends to lean out off the throttle , ( I think this is a Norton thing)so a slightly richer jetting above optimum works for me to cover that issue.
Regards Mike

If you open the throttle too quickly, you lose vacuum. That is probably the reason that the needles are tapered. One thing I have not done with my bike is to try different tapered needles in various situations. So there is probably an opportunity for improvement there. I currently use 6D Mikuni needles in my MK2 Amals with 0.116 inch needle jets for methanol fuel. I always wind the throttle on in a smooth controlled manner. If the needle jets are 0.117 inch, the motor is slightly sluggish as it accelerates. With the Mikuni needles, in #6 s there are two common tapers - a D and an F. - the F is the richer. in both Mikuni and MK2 Amals, the needles are the same diameters at the parallel part and the clip grooves are at the same levels.
Amal methanol needles are just bloody ridiculous. Fortunately, if you run methanol rich, you still get good power (but not best) - NOT SO with petrol !

Perhaps the reason for the pumps on some carbs is to make it possible to whack the throttle open ? If your bike tightens it's line in corners, you are on the gas much further back as you come out of corners, so you would not want the jump in power while you are cranked over anyway.

Doesn't the optimum AFR depend on what fuel you are using ? I distrust blind acceptance of the 14 to 1 figure, or even Jim Comstock's figures. I noticed years ago when I used Avgas, it always ran leaner than standard petrol and so performed better in my motors which were usually jetted slightly rich.

acotrel said:

Doesn't the optimum AFR depend on what fuel you are using ? I distrust blind acceptance of the 14 to 1 figure, or even Jim Comstock's figures. I noticed years ago when I used Avgas, it always ran leaner than standard petrol and so performed better in my motors which were usually jetted slightly rich.

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Of course. They are guideline figures based on assumptions about many factors including the % oxygen content in the air and the make up of the fuel in terms of octane heptane yadda yadda. That doesn't invalidate anything though, just use a margin of safety in your calculations and jetting I would suggest. Like I said before stoich is to be forgotten in the real world.

An engine runs best right before it blows. Best my 750 ever ran was when it blew a head gasket and sucked air until the exhaust valves burned. Cherry red pipes! Didn't take long.

Perhaps if you have your road bike jetted correctly, it should run much better on very cold mornings after you have got some heat into the motor ? The point about my methanol comments was that one thou. of an inch in needle jet diameter is the difference between sluggish and excellent performance. With petrol, one-half a thou. wear should make it sluggish.
About bikes running the best just before they blow up - I forgot to drain the methanol out of my T250 Suzuki racer after I had used it and got the white slime in the carbs. It absolutely flew, then locked up and the bike slid for about 10 metres before I could free the clutch. I don't think I ever got it going that fast again, by down-jetting it intentionally. There is actually a long way between having the black ring in the spark plug and boom. One thing which many people did not know about the fast guys who raced TZs - many of them raced in the jetting danger zone (no black ring in the plugs). In the old days, with those bikes everything cost $1000, so sponsorship was essential.
I recently sold my TZ350G and bought a six speed box for my Seeley with the proceeds - it was a conscious decision.

I think the comment which was made about road bike running for long distances at mid-throttle might have some validity. With a racer it is all about getting the needles and needle jets right for max performance. We are not on the main jets as often as some might believe. I've never heard of anyone burning a valve or piston without being on full throttle. However a friend once replaced the silencer with a megaphone on his Thruxton Velocette exhaust without re-jetting and burnt the piston almost immediately.

Here is the fuel/air ratio target file for my bike. Engine load is vertical and engine rpm is horizontal.

How is the AFR measured - oxygen detector ?

acotrel said:

How is the AFR measured - oxygen detector ?

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Yes, it uses a wideband O2 sensor and compares what comes out the exhaust with what it should be from the chart. The it adjusts the injector pulsewidth to correct any deviation.

It applies the correction to short term memory and then if the correction stays right for a long enough period, it will be burned into long term memory.