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Amal 2 Stroke, 4 Stroke Differences #50499
12/13/06 3:56 pm
12/13/06 3:56 pm
Joined: Aug 2001
Posts: 332
florida
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cycarmark Offline OP
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florida
I'm working on a '69 Greeves Ranger that is fitted with an AMAL 626 carb. Can someone tell me how to tell if it has the 2 stroke needle jet fitted? I remember there are some differences between the 4 stroke and 2 stroke needle jets. Also who may be able to supply replacement 2 stroke needle jets?

Thanks!

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Re: Amal 2 Stroke, 4 Stroke Differences #50500
12/14/06 1:05 am
12/14/06 1:05 am
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florida
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cycarmark Offline OP
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florida
I just read the posting about tuning AMAL carbs and there was some information about two stroke carbs and four stroke carbs, in particular the spray tube. It said the two stroke carbs had an angled spray tube, does anyone have a picture of one, from what I can tell the spray tube in the 626 carb fitted to the Greeves I am working on is exactly like the spare 928 I have for a BSA A65. Here's another question, would a two stroke fitted with a four stroke carb run way too rich?

Re: Amal 2 Stroke, 4 Stroke Differences #50501
12/14/06 2:49 am
12/14/06 2:49 am
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Winston-Salem N.C.
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D.W.R. Offline
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The rear of the spray tube is cut at about a 45 degree angle on the 2 stroke, the 4 stroke ones are flat on top. A couple of notable exceptions: the 926s on the triples and certain Norton carbs have the angle cut spray tubes. You also have needle differences. I think this was covered recently in a topic of it's own? Not exactly sure, but it has been discussed I know.
In general, yeah they're too rich for most 4 strokes.
Don.R


71 Rocket 3
72 B50 MX
66-71 A 65 Bitsa
96 Trident 900
Re: Amal 2 Stroke, 4 Stroke Differences #50502
12/14/06 8:06 am
12/14/06 8:06 am
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Melbourne Australia
Tiger Offline
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The spray tube is in fact stepped down on the intake side for some 'strokers.

When carb fiddling with 2 strokes it is good to remember that port timing is relatively short compared to valve timing, 2 strokes take a big gulp over a short period and I guess this leads to "inertia" having an effect on the fuel being drawn up past the needle/needle jet ?

The cutaway on the suction side of the spray tube would create a pronounced area of depression at the spray tube and so move more fuel during the short intake cycle of the 2 stroke.

Timely topic, just now I am beginning an experiment with a .0030" semi circular section relief toward the rear of the spray tube on my TR6.
My carb has a Mikuni slide which tends to meter through the needle jet more accurately and the engine seems to need just a little more fuel off idle.
A piece of 3/8" timber dowel will drift the spray tube down and out of the carb body but beware of doing this too often, the spray tube is a knurled fit and eventually this inteference fit may be lost.


1969 TR6R
7.62 x 51 is not a maths puzzle.
Re: Amal 2 Stroke, 4 Stroke Differences #50503
12/14/06 1:04 pm
12/14/06 1:04 pm
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Posts: 332
florida
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cycarmark Offline OP
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florida
Doing a search of the forums on this site, I am now pretty sure I have a 4 stroke carb fitted. The needle has two rings at the top, the needle jet has two small holes drilled in the hex part, and the spray tube is not angled, all seem to indicate 4 stroke.

Thanks to all who replied, and thanks to Britbike in general, what a great resource!

Re: Amal 2 Stroke, 4 Stroke Differences #50504
12/15/06 11:05 pm
12/15/06 11:05 pm
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Posts: 10,205
Boston, Massachusetts
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John Healy Offline
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"The cutaway on the suction side of the spray tube would create a pronounced area of depression at the spray tube and so move more fuel during the short intake cycle of the 2 stroke."

Actually it is the opposite. The slanted spray tube is there on a piston port 2 stroke carb to prevent the engine from taking a big gulp of gasoline at low throttle openings just as the piston rises and clears the intake port.

Piston port 2 stroke engines develop a lot of crankcase vacuum as the piston starts up the bore. As the piston travels up, and before the bottom of the skirt clears the intake port, the rising piston increases vacuum in a non-linear fashion. When the port does open vaccum has increased to a point where it creates a "pop" in the intake as the air rushes in to fill the vacuum. If left alone, this rapid incoming charge at low throttle openings, would cause the mixture to be momentarily abnormally rich. .

I made a set-up to demonstrate this. Remove the float bowl of two similar carburetors. Fit a slant spray tube to one and an straight cut one on the. Attach a clear hose to each main jet. Place the end of each hose in a can of water.

With the idle stop screw set the slide equally on each carb so that the slide is about 1/8 to 3/16" off the bottom (I use a small rod to get them even). Place the end of a vacuum hose over each carb manifold and mark how high the water rises in the tubes. You will notice that the water rises higher in the carb with the straight cut spray tube.

If you carefully raise each slide equally you find that the effect of the spray tube evens out as you lift the slide.

Tiger, Try screwing in the pilot air screw. If your of idle improves, I would drill out the pilot jet a couple of thousandth. In the end you really want to end up with the pilot air screw in the 1 to 1 1/2 position (another story) with the pilot jet adjusted to the proper size for your engine.


Re: Amal 2 Stroke, 4 Stroke Differences [Re: cycarmark] #449386
08/15/12 11:14 pm
08/15/12 11:14 pm
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ed king Offline
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2 stroke or 4 stroke stop the madness google (AMAL 930 Concentric carb) the top heading that comes up should take you to AMAL foctory site-- AMAL uk-- if you click that and in the white area you see notes on rebiulding scroll down it will answer all this with picture ps if you still get lost try (AMAL uk)tecknical then notes on rebuilding your there!! safe ride /also a great site for set up twin carbs fast info (bushmans carb tuning seacrets)

Last edited by ed king; 08/16/12 1:05 am.
Re: Amal 2 Stroke, 4 Stroke Differences [Re: cycarmark] #449392
08/16/12 12:32 am
08/16/12 12:32 am
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Christchurch, NZ
Kevin (NZ). Offline
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Amal Rebuild Website



The following is an excerpt from the website.

Quote:
As a general rule the most noticeable difference between a 2 Stroke and 4 Stroke carburetter is the spray tube, the brass tube visible as you look through the carburetter bore. 4 Stroke spray tubes are cut off straight, 2 Stroke spray tubes are cut off at an angle sloping towards the mounting flange. Some later 850cc Norton Commandos use a spray tube with a square cutaway step while Triumph Triples use an angled spray tube cutaway from the mid point of the spray tube rather than right across as in the 2 stroke version. 2 Stroke and 4 Stroke bodies are not interchangeable and will not run correctly if used on the wrong type of engine.




Last edited by Kevin (NZ).; 08/17/12 8:19 am. Reason: Quoting from website
Re: Amal 2 Stroke, 4 Stroke Differences [Re: cycarmark] #449393
08/16/12 12:33 am
08/16/12 12:33 am
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ed king Offline
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google AMAL uk when you get there at the top see --technical then in light blue -view concentic carb clic that then notes on rebiulding your there the ads at the top of this page AMAL and stedfast you can get the hard to find stuff i bought part from both any one need a 626/8 carb body with #4 slide realy clean i got it on eBay 15 bucks

Last edited by ed king; 08/16/12 1:42 am.
Re: Amal 2 Stroke, 4 Stroke Differences [Re: Kevin (NZ).] #449583
08/17/12 8:14 am
08/17/12 8:14 am
Joined: Nov 2009
Posts: 2,209
Netherlands
Peter R Offline

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Concerning Kevin's remark that 2stroke and 4stroke carb body's are not interchangeable ; I believe that other than the location of the idle jet, there are no differences, and 2 stoke body's can be safely converted to 4 srtoke use, and vice versa


Peter.
1974 Commando 850
1972 Trident T150T
1961 Goldie DBD34
1969 Benelli 250 sport special
Re: Amal 2 Stroke, 4 Stroke Differences [Re: Peter R] #449585
08/17/12 8:17 am
08/17/12 8:17 am
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Christchurch, NZ
Kevin (NZ). Offline
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Hi Peter.
I will amend my post. It was not my remark but from a paragraph on the website that caught my eye.
I had not heard that mentioned before anywhere.

I am not sure how the spraytube is pressed in or indeed if it is removable/replaceable.



Last edited by Kevin (NZ).; 08/17/12 8:21 am.
Re: Amal 2 Stroke, 4 Stroke Differences [Re: cycarmark] #449628
08/17/12 2:00 pm
08/17/12 2:00 pm
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John Healy Offline
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Originally Posted By: Peter R
Concerning Kevin's remark that 2stroke and 4stroke carb body's are not interchangeable ; I believe that other than the location of the idle jet, there are no differences, and 2 stoke body's can be safely converted to 4 srtoke use, and vice versa


Sort of, but as Kevin illustrated the two stroke AMAL uses a different spray tube. I have changed these for years, but as Tiger reminds us, you will not always be successful doing so.

Ed - you should google Gordon Jennings - "Two Stroke Tuner's Handbook" and refer the the carburetor section. I think you will find it interesting!


Re: Amal 2 Stroke, 4 Stroke Differences [Re: Kevin (NZ).] #449644
08/17/12 3:30 pm
08/17/12 3:30 pm
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Netherlands
Peter R Offline

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Kevin,The spray tube can be replaced, I have done this sucessfully . I made a stepped drift from brass on my lathe, and gentle tapping from the topside will move the spray tube out.


Peter.
1974 Commando 850
1972 Trident T150T
1961 Goldie DBD34
1969 Benelli 250 sport special
Re: Amal 2 Stroke, 4 Stroke Differences [Re: cycarmark] #449650
08/17/12 4:06 pm
08/17/12 4:06 pm
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John Healy Offline
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Peter:

The part that can be problematic is the knurled portion of the spray tube. Not all of the spray tubes are knurled exactly the same. This can cause the new spray tube to be loose even when changed by someone who is clever and has the right equipment. As the knurled area is all that retains it, if things go wrong it can fall into the area around the needle jet and upset the flow of air from the main air bleed. Don't be surprised if the experts say it can be done and your experience is different.

Remember these carburetors, and related parts, have been manufactured by at least three different engineering shops. While they are all working to the same drawings they have not uniformly produced exactly the same parts. On occasions there has been a, "I know a better way" attitude. Or we don't have that tool, but this one will do the job.

We routinely change spray tubes when we are short of the Norton carburetors (stepped spray tube). Also British Only dumped several hundred 930 Concentric two stroke carbs into the US market without telling anyone. When owners got frustrated we ended changing a bunch of them back into 4 stroke carburetors.

Yes, it can be done. Yes, you need a special drift to do it successfully. But no, it will not work each and every time.


Re: Amal 2 Stroke, 4 Stroke Differences [Re: cycarmark] #449664
08/17/12 5:23 pm
08/17/12 5:23 pm
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Netherlands
Peter R Offline

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Thanks for this info John, I now know that changing the spray tube sometimes can lead to problems.
Maybe I was just lucky...


Peter.
1974 Commando 850
1972 Trident T150T
1961 Goldie DBD34
1969 Benelli 250 sport special
Re: Amal 2 Stroke, 4 Stroke Differences [Re: cycarmark] #449745
08/18/12 5:43 am
08/18/12 5:43 am
Joined: Mar 2011
Posts: 110
Alaska
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linker48x Offline
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Alaska
Hmm, I have been successfully using a 36mm 2 stroke Concentric, with all the internal parts changed, on my Gold Star Clubman for 20+ years. Took the GP off, for riding on the street, and the Concentric works perfectly, once I changed all the brass, so I suspect the body is fine, as John says. Don't remember now what I did to it to make it work, though. I think this came on a Bultaco, and I got this one new in the box.

Last edited by linker48x; 08/18/12 5:44 am.
Re: Amal 2 Stroke, 4 Stroke Differences [Re: John Healy] #450347
08/22/12 6:55 am
08/22/12 6:55 am
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Hampshire, England
DoubleDiamond Offline
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Originally Posted By: John Healy
"The cutaway on the suction side of the spray tube would create a pronounced area of depression at the spray tube and so move more fuel during the short intake cycle of the 2 stroke."

Actually it is the opposite. The slanted spray tube is there on a piston port 2 stroke carb to prevent the engine from taking a big gulp of gasoline at low throttle openings just as the piston rises and clears the intake port.

Piston port 2 stroke engines develop a lot of crankcase vacuum as the piston starts up the bore. As the piston travels up, and before the bottom of the skirt clears the intake port, the rising piston increases vacuum in a non-linear fashion. When the port does open vaccum has increased to a point where it creates a "pop" in the intake as the air rushes in to fill the vacuum. If left alone, this rapid incoming charge at low throttle openings, would cause the mixture to be momentarily abnormally rich. .

I made a set-up to demonstrate this. Remove the float bowl of two similar carburetors. Fit a slant spray tube to one and an straight cut one on the. Attach a clear hose to each main jet. Place the end of each hose in a can of water.

With the idle stop screw set the slide equally on each carb so that the slide is about 1/8 to 3/16" off the bottom (I use a small rod to get them even). Place the end of a vacuum hose over each carb manifold and mark how high the water rises in the tubes. You will notice that the water rises higher in the carb with the straight cut spray tube.

If you carefully raise each slide equally you find that the effect of the spray tube evens out as you lift the slide.

Tiger, Try screwing in the pilot air screw. If your of idle improves, I would drill out the pilot jet a couple of thousandth. In the end you really want to end up with the pilot air screw in the 1 to 1 1/2 position (another story) with the pilot jet adjusted to the proper size for your engine.


John, that is a very interesting experiment. I was just wondering if you think that there might be a difference between the steady state situation, such as when you have a carb attached to a vacuum pump and when used practically whereby there is a 'pop' created by crankcase vacuum. Presumably it would also be difference on a reed valve 2-stroke where there will be less of a 'pop'? Could this effect be because of the way the air flows in the space underneath the throttle slide?

Thinking about the slanted spray tube, the evidence of the Concentric on the larger engined Norton Commando suggests that a shielded spray tube creates more vacuum. My understanding is that this is how the main jet could be reduced in size on these engines while still maintaining the correct mixture at higher throttle openings.

There is a similar arrangement (slanted spray tube) on the 627 used on the Trident/R3. I had assumed that the 627 was used because a 928 was too big to fit and that the modifications enabled maximum performance/air flow on these slightly smaller than ideal carbs. If you compare the jetting to the BSA B25 (250cc single) they use a 180/200 (dependent on model year) main jet compared to a 150 for the triple. Any thoughts?


BSA: '71 B175; '68 B25; '71 A65; '71 A75
Triumph: '87 T140; '72 T150v
Re: Amal 2 Stroke, 4 Stroke Differences [Re: John Healy] #450351
08/22/12 7:29 am
08/22/12 7:29 am
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Posts: 7,550
scotland
triton thrasher Online content
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Originally Posted By: John Healy

. The slanted spray tube is there on a piston port 2 stroke carb to prevent the engine from taking a big gulp of gasoline at low throttle openings just as the piston rises and clears the intake port.
.


Do the stepped Commando and slanted Trident spray tubes have a similar effect, on four strokes?


Amateur Loctite enthusiast.
Re: Amal 2 Stroke, 4 Stroke Differences [Re: cycarmark] #450352
08/22/12 7:31 am
08/22/12 7:31 am
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scotland
triton thrasher Online content
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I just had a daft thought. How does the carburettor perform with no spray tube?

Or a shorter, straight cut one?

Last edited by triton thrasher; 08/22/12 7:34 am.

Amateur Loctite enthusiast.
Re: Amal 2 Stroke, 4 Stroke Differences [Re: triton thrasher] #450354
08/22/12 7:56 am
08/22/12 7:56 am
Joined: Aug 2012
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Hampshire, England
DoubleDiamond Offline
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Originally Posted By: triton thrasher
I just had a daft thought. How does the carburettor perform with no spray tube?

Or a shorter, straight cut one?


It wouldn't work without a spray tube. It is the increased speed of airflow over the top of the spray tube which lowers the air pressure and draws the mixture into the engine.

A slightly shorter spray tube is likley to make the main jet systems come 'on line' a little sooner and affect the transfer charicteristics from pilot jet to main jet. My experience is that small changes in spray tube height don't have an overall marked impact but they can none-the-less be useful whey trying to fine tune a carb to a particular engine. I have measured a number of 900 series spray tube heights and the difference varies from about 3.5mm to just over 4mm. I have also expiremented with one of just under 3mm and can confirm that the difference in operation is small.


BSA: '71 B175; '68 B25; '71 A65; '71 A75
Triumph: '87 T140; '72 T150v
Re: Amal 2 Stroke, 4 Stroke Differences [Re: cycarmark] #450358
08/22/12 8:12 am
08/22/12 8:12 am
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Thanks, I love to hear from people who've tried things.


Amateur Loctite enthusiast.
Re: Amal 2 Stroke, 4 Stroke Differences [Re: cycarmark] #450391
08/22/12 1:44 pm
08/22/12 1:44 pm
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John Healy Offline
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The increase in speed, and resultant drop on vacuum which draws the fuel from the bowl in our variable venturi carburetors, would be there whether or not the spray tube is present. It is the draw of air, created by the falling piston, through the venturi created by the slide that creates the vacuum signal.

Basically, the shroud around the outlet of the needle jet created by the spray tube, and the air that is drawn through it from the air bleed on the front of the carb inlet, softens the vacuum signal on the needle/main jet at different throttle openings.


Re: Amal 2 Stroke, 4 Stroke Differences [Re: John Healy] #450476
08/22/12 9:52 pm
08/22/12 9:52 pm
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Hampshire, England
DoubleDiamond Offline
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Originally Posted By: John Healy
The increase in speed, and resultant drop on vacuum which draws the fuel from the bowl in our variable venturi carburetors, would be there whether or not the spray tube is present. It is the draw of air, created by the falling piston, through the venturi created by the slide that creates the vacuum signal.

Basically, the shroud around the outlet of the needle jet created by the spray tube, and the air that is drawn through it from the air bleed on the front of the carb inlet, softens the vacuum signal on the needle/main jet at different throttle openings.





OK. Fair point. Perhaps I should have said that without the spray tube the carb won't work properly as it was designed to.


BSA: '71 B175; '68 B25; '71 A65; '71 A75
Triumph: '87 T140; '72 T150v

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