Amal 289 Throttle Slide Cutaways

[Chris S]

Anyone got any specific measurement details on the cutaways on 289 carb slides?
I understood that the increase in slide cutaway was a linear measurement, but I am now questioning that.
I am setting up a new set of 229/289's on my twin and ordered some 3 cutaway slides. However, the cutaway is not what I expected. I had assumed that a 3 cutaway would be 3/16'' (approx 4.7625mm) but they appear to be far less than that, nearer to 4mm.
The 4 slides that came with the carbs measure 1/4'' (4/16'' or 6.35mm) on the cutaway so I am a bit confused as it would appear that the 3 cutaway is not simply 3 x 1/16''. My understanding was that a 1 cutaway would be 1/16'' a 2 cutaway would be 1/8'' ( 2 x 1/16'') a 3 would be 3/16'' and a 4 would be 1/4'' (4/16'')
Am I missing something or simply misunderstanding the cutaway scale.

 

[davidd]

For those carbs it is 1/16" and it is linear. I am not sure what happened to the number 3's. More important is the height to which you set the cutaways. I think your understanding of that scale is fine.

David

 

[greg brillus]

A 3 will be too rich and a 4 is on the lean side. I start with a number 4 and machine approx. 25 to 30 thou off the bottom to achieve good running without bad spit backs or black smoking with a number 3. The adjustments make a big difference, and the idle low speed circuits often need attention as well, but this involves removing the brass jet block and cleaning/opening the pilot jet passageway. You may need to do this to get any sensible change when adjusting the mixture screws. I have done loads of the new 289 carb's on twins and Singles and they all give similar problems which I have discussed with Burlen's (the manufacturers) about these problems, but nothing much has changed.

 

[Tom Walker]

Chris

I found this a while ago, it contains some useful info.

https://www.voc.uk.com/net/docs/6.1/6.1-764-6.pdf

I would like a diagram of where to measure though, as its not totally obvious. ie what does "linear" actually mean?

Also can you describe, Greg, where you machine the slide to correct slow running?

 

[Tom Walker]

OOps I've just re-read it, and its not much use really!

Ive searched for comprehensive info on the slide cutaway and come up wanting.

It would be great if someone could do an idiots guide on how to make your 3 slide into a 3 1/2, and solve ancilliary problems.

 

[Tom Walker]

I know nothing about Mikuni carbs, but looking on a site for Nortons, this diagram came up showing a Mikuni slide, which shows the cutaway modification,
amazingly in Imperial measurements, uncannily like those on an Amal!

It would be a brave man who attempted to file this by hand, with any kind of accuracy, and even doing it on a milling machine would need a fixture making to hold it without damaging the slide.

 

[greg brillus]

There are two (2) parts to the equation, the first deals with the idle slow running circuit which is within the brass jet block, and the second affects the off idle to normal running which is in the size of the slide cutaway. This applies to the type 29 carb's not so much the 276 found on Rapides as they seem quite good straight out of the box. The 289 carb's once installed can suffer the following running condition......Once started it is found that the mixture screw does very little even when wound fully in (rich) upon opening the throttle the engine spits back constantly out the carb unless the choke (air slide) is dropped near closed and then the engine behaves better. However this takes too long and if you attempt to ride the bike, the engine stumbles because it is too rich, open the chokes and the carbs spit badly again. The real easy fix for the slides is to buy a pair of number 3 1/2 slides from the VOC spares co, as they are not available from Burlen's themselves. Though the slides are quite expensive, this is what prompted me to simply modify either a number 3 or 4 slide to fix the problem. On the number 4 you need to machine off 25 to 30 thou off the flat base of the slide, this will lower the slide within the carb body. On 3 slide you need to machine the cutaway taller at the front by half the amount, use a black texter pen to draw a reference line and use drill bits on a flat bench to check how much you are removing. It sounds all experimental and it basically is.......But it works.......do not get carried away and take too much. The fix for the idle circuit is a bit detailed and I will discuss it if you feel it is causing a problem. It is common for a cold engine to spit back through the carbs as it warms up, but it should not do it all the time, this means it is running lean on that cylinder. If you do decide to machine the base of the number 4 slide, you are best off doing it in a lathe, and we made up an alloy solid slug that slides neatly into the inner bore of the slide so that the sides to not collapse in as the jaws of the chuck grip the slide. They are generally plated brass and machine quite well, just debur the edges carefully afterward and you are good to go. When I modify a slide like in Tom's picture above I use a surface plate and linish the cutaway by hand, it is not that hard to do, but you need to take care that you only remove the metal from the shaded area and not past the bottom arrow point, this is why I mark above the original cutaway with a black texter pen so I can see how I am progressing, then check the heights on a flat surface using number drills as an accurate guide. This has worked many times for me and not just on Vincent's either. Cheers......Greg.

 

[Bill Thomas]

I know nothing about Mikuni carbs, but looking on a site for Nortons, this diagram came up showing a Mikuni slide, which shows the cutaway modification,
amazingly in Imperial measurements, uncannily like those on an Amal!

It would be a brave man who attempted to file this by hand, with any kind of accuracy, and even doing it on a milling machine would need a fixture making to hold it without damaging the slide.

Done that many times, I was talking to a friend with a proper L/ning and said that is what I would do, T.T. carb's, He said what if I take too much off, I said well just take a bit off the bottom !!. He said so it was YOU ??, He had found a few slides that were shorter than standard and I got the blame !!. Cheers Bill.

 

[Chris S]

At least the above confirms that I am not misunderstanding or losing the plot, although my wife says I am always doing that! Thanks for all the input. I have a new pair of 3's arriving today and I will check the cutaway as they are out of the box. I reckon they come out of the box a shade below 4mm cutaway, which is spookily close to 5/32" equating to 2.5 cutaway. Don't know why, but at least I can increase the cutaway by hand until it is where I want it. I have done this on many slides and the old rule applies - measure twice, file once and keep checking. As Greg says, finish on a surface plate and all should be well. Once I get my carbs sorted, I reckon I will have some standard 4's and some modified 3's (3.25 and 3.5) spare which if anyone needs to borrow them to try to sort out their carbs before spending money with Burlens or the spares club, then let me know.

 

[Chris S]

Greg, think I misunderstand a detail in your reply. You say "On the number 4 you need to machine off 25 to 30 thou off the flat base of the slide, this will lower the slide within the carb body. On 3 slide you need to machine the cutaway taller at the front by half the amount"
If I want to increase the cutaway on a 3 to 3.5, would I not need to remove 1/32" as a 3 cutaway is 3/16" (6/32) high and a 4 is 1/4" (8/32) high therefore making a 3.5 equal 7/32"high? This would require me to remove 25 to 30 thou as per your reduction on base of the 4 slide.
Also, if removing material off the base, does this then create the need raise the needle to compensate for it now sitting lower, or is the amount too little to worry about?
Chris