Amal needles and poor running (2014)

There is a father and son race team around here who play with matchless singles. I looked down their carb. and it looked really great - the port was huge. I said to them 'why don't you fit a two valve Jawa motor to this bike and you will start where you finished ?' They did that and with a completely unmodified motor immediately beat one of the best who was riding a new Molnar engined manx. They are now trying to modify the Jawa engine by lightening piston etc. Historically the early speedway Jawa (ESO) had a 2mm larger inlet port which was later restricted under the rules to slow the bikes down. So the fellas might get some advantage by going larger. My feeling is that it is often better to keep the gas speeds in the port high.

Bernhard said:

......., I simply cannot deal with someone who appears to think you can fit the biggest bore carb (36mm in this case!) that they can get away with and expect to get uninterrupted power throughout the whole of the engine rev range. It never works.
With these big carbs you are simply going to get a whole lot of sputtering in the lower rev range, try sucking some tap water using a small bore plastic pipe- this should be easy, now try doing it with a 1inch bore pipe and tell me you don’t notice the difference. That is exactly the same conditions you are trying to run your engine.
This engine clearly is a, or close to, a stage 3 tuned engine, with an aim to get the maximum amount of power, regardless of anything else, it can only be made to work cleanly on the upper stages of the engine revs,- some people want to have their cake and eat it.

.

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Hmm..... I hope you didn't mean to be as rude as that seemed to me.

I'm not convinced that the size of the carbs is the real problem given that for the first few years of racing every race I did was a push start. You don't push start a 750 Commando with 10.5 compression unless the carbs work well on the pilots. Something has changed and that's what I'm trying to fault find. Maybe that change is wear in the slides, chokes needles or elsewhere unconnected to the carbs.

Having changed the slides, as I said previously, I can run at 25MPH in top gear, but the plugs are still showing rich.

I went back to basics and went over everything last night. I think I may have found a problem with the ignition. I won't be able to road test it until the weekend, but watch this space.

acotrel said:

There is a father and son race team around here who play with matchless singles. I looked down their carb. and it looked really great - the port was huge. I said to them 'why don't you fit a two valve Jawa motor to this bike and you will start where you finished ?' They did that and with a completely unmodified motor immediately beat one of the best who was riding a new Molnar engined manx. They are now trying to modify the Jawa engine by lightening piston etc. Historically the early speedway Jawa (ESO) had a 2mm larger inlet port which was later restricted under the rules to slow the bikes down. So the fellas might get some advantage by going larger. My feeling is that it is often better to keep the gas speeds in the port high.

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And can this engine tick over reliably at low revs :?:
Re; “My feeling is that it is often better to keep the gas speeds in the port high”
With a big port motor this is only possible at high engine revs, at lower engine revs the gas speed will be slower than a with a smaller inlet port diameter-surely you must know this acotrel as you used to have a TZ350 Yamaha which had as standard fitment of 38mm carbs-that is 1 ¾ inches on the F/G models. This bike had no tickover and no power until you hit the 8,000 revs mark, but I digest this was a Grand Prix winning race tuned two stroke.

I have as an everyday runaround, a GPZ305 twin cylinder bike that runs on a pair of standard fitment 32mm C.V. carbs. These may appear big for such a small bike, but the throttle is operated by a butterfly valve(s) and the slides find their own level according to the amount of vacuum that the engine produces. After tweaking the air/pilot jets with a Colortune kit –which were way out, not just a small amount out as they usually are, it now runs fine but even I must admit there is no substance for a big cube motor.

This large carb issue. Reminds me of...
Mini cooper 998cc would run on a pair of webers DCOE 45's thats a 250cc pulling on a 45mm choke! only minis/fords in the seventies on webers had a large choke tapering down to small inlets, and worked..and sounded fantastic the cams where full race. ..which made low down gas speed even worst :!:

Here are some infro off the internet from two differnt webpages regarding Webbers, this may be a differant carburettor , but the basic principle are still the same; quote:
"Under normal driving conditions, it is my opinion that a well-maintained pair of HS4 or HIF4 SU carburetors is just as good, if not better than, a 45DCOE, considering the complication and expense that tuning these race-application carbs can require.
If you do not have the expertise or equipment (such as a wide-band oxygen sensor and rolling road) to tune these carbs properly, you may be chasing after settings endlessly - the number of variables in tuning these carbs is large and requires an understanding of the theory of operation.
The settings listed here are a starting point only, from which specific modifications will most likely be required.” and......
“When selecting Webers, the most commonly asked question is "Should I have 40s or 45s" coupled with "Surely the 45s will give more power". This shows a basic misunderstanding of the construction and principles of operation of the DCO series. It is not the barrel size (40 or 45) which determines the airflow and therefore potential horsepower; it is the size of the main venturi or choke. Selection of the correct main venturi size is the first step in selecting the carburettor.
It is easy to make the assumption that biggest is best when selecting a main venturi size, but the purpose of the main venturi is to increase the vacuum acting on the main jet in order to draw in and effectively atomise the fuel mixture. The smaller the main venturi, the more effective this action is, but a smaller venturi will inhibit flow. A large venturi may give more power right at the top end of the power band, but will give this at the expense of lower RPM tractability. Only a circuit racer will benefit from this sort of compromise, on a road car, driveability is much more important. 95 percent of the time, a road engine is nowhere near its peak power, but is near its peak torque for 75 percent of the time. It is much more important therefore to select the main venturi for best driveability, once the venturi size has been selected, then the appropriate carburettor size can be arrived at.”

Note these bits; "A large venturi may give more power right at the top end of the power band, but will give this at the expense of lower RPM tractability"

"It is easy to make the assumption that biggest is best when selecting a main venturi size, but the purpose of the main venturi is to increase the vacuum acting on the main jet in order to draw in and effectively atomise the fuel mixture.The smaller the main venturi, the more effective this action is,"

Bernhard, two stroke carburation is very different to four stroke. Your bike probably has reed valves, which create a different situation to piston port motors. It is not so much about gas speeds on a two stroke, but about harmonics. When the exhaust and inlets begin to resonate at 8,000 RPM, you get the big kick in the bum which dictates a point and squirt riding style. With a four stroke, your advantage comes from extra urge and ridability into, around and coming out of corners, so torque is a priority. It is possible to make a piston port two stroke pull, however they lose top end which is their major advantage. The reed valves give a bit of both. If you think you can outblast a TZ350 with a Norton 750 in a straight line by doing 'point and squirt', you are kidding yourself. You have got to come out of the corners a lot faster to get ahead of them.
I would also point out that when Peter Inchley was playing with the Greeves, it did not rev out until the bellmouth fell off the GP carburetor.

acotrel said:

Bernhard, two stroke carburation is very different to four stroke. Your bike probably has reed valves, which create a different situation to piston port motors. It is not so much about gas speeds on a two stroke, but about harmonics. When the exhaust and inlets begin to resonate at 8,000 RPM, you get the big kick in the bum which dictates a point and squirt riding style. With a four stroke, your advantage comes from extra urge and ridability into, around and coming out of corners, so torque is a priority. It is possible to make a piston port two stroke pull, however they lose top end which is their major advantage. The reed valves give a bit of both. If you think you can outblast a TZ350 with a Norton 750 in a straight line by doing 'point and squirt', you are kidding yourself. You have got to come out of the corners a lot faster to get ahead of them.
I would also point out that when Peter Inchley was playing with the Greeves, it did not rev out until the bellmouth fell off the GP carburetor.

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Re; “If you think you can outblast a TZ350 with a Norton 750 in a straight line by doing 'point and squirt', you are kidding yourself. You have got to come out of the corners a lot faster to get ahead of them.”
I don’t think we are talking about the same bike :?:
Probably not, but how come I regularly beat a lot of bikes in 2/3/4 cylinder sizes up to a 1000 cc in 5/8 lap races in places like Brands Hatch & Snetterton ? The season long races like the Champion of Brands, & Snetterton were won by a TZ350 in the 1980s ridden by other riders which means that they were better riders than I.

So, people like the late Jarno Saarinen on a TZ350 also beat the MV500 in the Mallory Park race of the year, and the Dunstall drainpipe 750 E.t.c. in the 1970s :?:
Even John Cooper won this race on his TZ350 powered Yamsel . plus there were numerous other race wins on the British short circuits & elsewhere with this bike in the open 1000cc category. Heaven’s man, one even won the Daytona 200 miler :!:

http://en.wikipedia.org/wiki/Jarno_Saarinen#Career

A late 1990s V twin TZ250 would show a 750 Norton the way home, even today.

I suggest it always depends on the circuit. If it has long straights the Norton won't ever be competitive against guys with savage top end motors. It takes a very good bike to beat an RS125 around a very tight circuit. I once watched one of our top guys trounce all the 600cc sports bikes around Winton Raceway with one. If you want to win on that circuit, superb handling and torque are priorities, otherwise you will be out powered. The straights are long enough for that.

I think it was 1972 at Brands ,jarno was lapping 750's on a 350, John Cooper [Moon eyes] was slow on the BSA , but he was getting over a BIG fall. I remember being total impressed by the flying Finn, he was passing bikes on the hair pin by riding on the grass!

Bernhard said:

acotrel said:

Bernhard, two stroke carburation is very different to four stroke. Your bike probably has reed valves, which create a different situation to piston port motors. It is not so much about gas speeds on a two stroke, but about harmonics. When the exhaust and inlets begin to resonate at 8,000 RPM, you get the big kick in the bum which dictates a point and squirt riding style. With a four stroke, your advantage comes from extra urge and ridability into, around and coming out of corners, so torque is a priority. It is possible to make a piston port two stroke pull, however they lose top end which is their major advantage. The reed valves give a bit of both. If you think you can outblast a TZ350 with a Norton 750 in a straight line by doing 'point and squirt', you are kidding yourself. You have got to come out of the corners a lot faster to get ahead of them.
I would also point out that when Peter Inchley was playing with the Greeves, it did not rev out until the bellmouth fell off the GP carburetor.

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Re; “If you think you can outblast a TZ350 with a Norton 750 in a straight line by doing 'point and squirt', you are kidding yourself. You have got to come out of the corners a lot faster to get ahead of them.”
I don’t think we are talking about the same bike :?:
Probably not, but how come I regularly beat a lot of bikes in 2/3/4 cylinder sizes up to a 1000 cc in 5/8 lap races in places like Brands Hatch & Snetterton ? The season long races like the Champion of Brands, & Snetterton were won by a TZ350 in the 1980s ridden by other riders which means that they were better riders than I.

So, people like the late Jarno Saarinen on a TZ350 also beat the MV500 in the Mallory Park race of the year, and the Dunstall drainpipe 750 E.t.c. in the 1970s :?:
Even John Cooper won this race on his TZ350 powered Yamsel . plus there were numerous other race wins on the British short circuits & elsewhere with this bike in the open 1000cc category. Heaven’s man, one even won the Daytona 200 miler :!:

http://en.wikipedia.org/wiki/Jarno_Saarinen#Career

A late 1990s V twin TZ250 would show a 750 Norton the way home, even today.

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I did a couple of rides yesterday, and made progress then went backwards, then went forwards again

Firstly I found that I had set the ignition timing 10 degrees over advanced!!! Yep , I made a monumental error somehow, or maybe it had slipped since I set it because one of the pillar bolts on the stator was loose.

So, I thought, set the ignition correctly and put everything back to the original setting I used when racing.... ran like shit! So I went back to the settings that worked best with the wrong timing.

When racing I used 25 pilots, 330 mains, #4 slides.

I now have set it to:
Ignition timing at 28 degrees, new needles, new needle jets and 3.5 slides, 20 pilots and it runs quite well. I also blanked off the choke on the RHS in cae it was leaking. Now it's easy to ride in traffic, takes off really well at half to 3/4 throttle ( haven't opened it up more on roads with 60 MPH speed limits) and idles at 1100rpm.

It's not bad at all EXCEPT, it's almost impossible to start hot.


I am currently looking for a colortune and I hope if I can get th epilot system working well it should be a good starter too.

pommie john said:

I did a couple of rides yesterday, and made progress then went backwards, then went forwards again

Firstly I found that I had set the ignition timing 10 degrees over advanced!!! Yep , I made a monumental error somehow, or maybe it had slipped since I set it because one of the pillar bolts on the stator was loose.

So, I thought, set the ignition correctly and put everything back to the original setting I used when racing.... ran like shit! So I went back to the settings that worked best with the wrong timing.

When racing I used 25 pilots, 330 mains, #4 slides.

I now have set it to:
Ignition timing at 28 degrees, new needles, new needle jets and 3.5 slides, 20 pilots and it runs quite well. I also blanked off the choke on the RHS in cae it was leaking. Now it's easy to ride in traffic, takes off really well at half to 3/4 throttle ( haven't opened it up more on roads with 60 MPH speed limits) and idles at 1100rpm.

It's not bad at all EXCEPT, it's almost impossible to start hot.


I am currently looking for a colortune and I hope if I can get th epilot system working well it should be a good starter too.

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A 20 pilot jet is surely too small for big mkIIs. That was my first thought when reading our post, and then you went on to say it won't start when hot... that seems like proof to me, I ran 25 in 34mm mkIIs, I'd say you should have at least that?

Fast Eddie said:

A 20 pilot jet is surely too small for big mkIIs. That was my first thought when reading our post, and then you went on to say it won't start when hot... that seems like proof to me, I ran 25 in 34mm mkIIs, I'd say you should have at least that?

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I had 25s in there but went leaner when the plugs were running black. I guess it's my next change... again

pommie john said:

Fast Eddie said:

A 20 pilot jet is surely too small for big mkIIs. That was my first thought when reading our post, and then you went on to say it won't start when hot... that seems like proof to me, I ran 25 in 34mm mkIIs, I'd say you should have at least that?

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I had 25s in there but went leaner when the plugs were running black. I guess it's my next change... again

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You don’t mention how many turns you had on your airscrew/ pilot jet :?:

The needle jet should be small enough so that the motor will cough as you lower the needle to the part where the parallel part begins. Then you raise it one notch. As you open the throttle, the metering is done by the parallel part of the needle and the low running jets until you reach about one-third throttle. After that, the taper takes over until you reach about two thirds throttle. Then the metering is done by the main jets.

What amazes me is the difference in the available information on needle tapers for Amal carbs when compared with Mikunis. I use 34mm Amal carbs, but I use the needles from a H2 Kawasaki teo stroke. I use methanol fuel, but the rate of metering as you open the throttle should not be different if was using petrol. Only the needle jet is bigger.
A MK2 Amal carburetor is very similar to a 34mm VM Mikuni.
I bought my MK2 Amals from Steve Maney many years ago and bought them methanol-kitted, so the low running jets and float needles would feed enough. But the needles supplied were Amal X and Amal Y. The tapers were ridiculous.
When I sized my needle jets, I made two at 0,117 thou of an inch but when I lowered the needles, I could not induce the motor to cough, so I made another two at 0,116 of an inch and managed to get the cough, then I raised the needles one notch. There was a very different change in performance. The difference in jet size is half a thou of an inch per side. With petrol as fuel, that half a thou becomes a quarter of a thou. Petrol is twice as critical as methanol when you are jetting for it.
What I am suggesting, is if the needles which are supplied as standard have too much taper when you are using petrol, the motor can be sluggish, but you might not detect it. You only find out the motor can be quicker, when you improve it.
The effect of too much taper might be poor throttle response. The taper is there to compensate for loss of vacuum as you open the throttle. But Commandos do not have huge inlet ports.
When I race my bike, I feed the throttle on, as you would do with a two stroke. The motor spins-up extremely quickly, despite the heavy crank, and regardless of the gearing.
6D Mikuni needles are the correct length for 32mm to 34 mm Amal carbs. And all the needles are the same diameter at the parallel part. 6D are the leanest and 6F are the richest.

Carbs are too big, Use 1 34mm or 2 32mm max for street riding.

Charles Waggett said:

Carbs are too big, Use 1 34mm or 2 32mm max for street riding.

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Hi Charles, do you realize that the last post on this thread was in 2014 ?

cliffa said:

Hi Charles, do you realize that the last post on this thread was in 2014 ?

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Except for the two posts by acotrel in Feb. and March this year, however, I suggest @Charles Waggett checks the date before replying to what are often threads that are several years old.

L.A.B. said:

Except for the two posts by acotrel in Feb. and March this year, however, I suggest @Charles Waggett checks the date before replying to what are often threads that are several years old.

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I don’t see those posts

It is sometimes useful to see an old thread brought back to life despite the fact that we were younger then....:-(