1.5 cut away VS. 2.0 cut away

[chuck hahn]

Ive noticed the references to the Cut Away on a slide..Ive also noticed that all the VM 26s I have are all stamped 1.5. So heres the questions..first what is the difference between the 2 ..and what exactkly does the cut away control? Ive read all of section 8 in the GS 1000 service manual on Cliffs website and theres no mention of it or what it does.

[tkent02]

It controls the transition, but I'm not sure exactly how it does.

[chuck hahn]

The only thing tha manual stated was to be sure the slide ( throttle valve ) is all the way down when starting or using the enrichment cicuit or else the negative pressure will drop and no fuel will be drawn in thru the enrichment ports.. What is ( does ) "cut away" mean. Is it an actual feature in the slide ike that hole or something? Ive read a lot of info in the last 5 yers or so but have never gotten a clear picture of what "it" is.

[tkent02]

Look at the bottom edge of the slide, it's cut at a curve, it's lower in front than in back. This effects airflow past the little transition holes in the pilot circuit.. Changes when the carb transitions between being controlled by the pilot circuit and being controlled by the needle jet and jet needle. I have never messed with it, can't remember which way to change it to go richer sooner, or leaner.

[chuck hahn]

Thats what i was suspeting it was but wasnt 100% sure of myself..so when in doubt you come here. Thanks for the insight Tom. Anyone else have additional info??

[chuck hahn]

So the 1.5 transitions a tad SOONER than the 2.0..is that what i gather Tom??

[tkent02]

Again, I don't know. Read about it a while ago, understood it at the time but I have never messed with it on a real motorcycle. I don't know which way it works.....

Have fun finding out...

[chuck hahn]

Ok..so I found this...


The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).

[hampshirehog]

Chuck is on the numbers. Slides are usually the last thing that anyone changes when trying to sort jetting as they, slides, are usually the most expensive things inside a carb. And to a degree that's ok.

Why? Because most 'cruising' riding is on the pilot circuit and most other riding (fast or flat out) is on the needle or main jet. Where the cutaway is wrong then it will show with brisk acceleration - stuttering and bogging are both possible - but it is very easily overcome, usually subconsciously, by twisting the throttle on to the needle circuit.

Our GS bikes in my experience are not overly fussy with slide cutaway size when changing set up, with perhaps the exception of the very first 750s which had big cutaways; when people started putting on bellmouths they ran with a noticeable bog. Easy to sort - put in slides from a later bike.

I have found cutaway much more sensitive on Amal carbs on British twins but fortunately Amal slides made by Burlen are cheap (both as in not expensive and poor quality) and you ony need 2 (of course). On Amal mk11/2s both BSAs and Triumphs are barely rideable if fitted with the wrong slide. I have no idea why because as far as I know the only difference between the mk1 and the mk11/2 is the choke mechanism.