Hitchcocks Forum

So I got a bit rich on just cracking the throttle. I have never messed with side cut away ever ever in my life but this seemed to be the time.

On the internet-- I read that the slide will richen or weaken the mix at cracking the throttle --well we all know that, but which is which.

I went on to read that a larger cut away measured in mm from base , meaning a 2.5 is a 2.5 mm cut away from the flat bottom of the slide.

Ok so it seemed that according to the internet!!!!!! I need a less cut away to weaken the throttle at crack.

So I scraped off the chrome just on the bottom of the slide on the cutaway, warmed with blow lamp, and tinned the edge with the soldering iron and built up a wall of solder just on the cut away.

Great what a good job after cleaning with a knife as solder don't stick to chrome it was as if it was not there. I reduced the 3 to a 1.

My god it was as rich as hell I took the carb off three times checking choke and for flooding it was that rich when I was expecting lean.

Checked the ---Internet again and another bloke -- not from a forum this time said a big cutaway is lean and the smaller cut out is rich. Oh @#$%^&*(

So-- cut it all off and its back to normal. A whole days messing about. Good job I don't like golf.

QED interesting that the slide cut away could make so much difference.

As you found, it makes a big difference.
If you'd just gone to our host's technical notes you would have found the correct info.
Sticking bits of solder on the cutaway is hardly very precise imho.

Surely a moments thought would suggest that more cut-away means more air. More air means weaker mixture.

Ah, got it now.....you built up the cutaway? You should have cut more metal from the chamfer, to allow more air in, not built it up for less.

I bought a richer slide from Mr. H. I measured it and saw that it was probably a standard one that had been put in a lathe and had the bottom face skimmed to lower it.

Wheaters wrote: ↑

Wed Sep 09, 2020 5:08 pm

Ah, got it now.....you built up the cutaway? You should have cut more metal from the chamfer, to allow more air in, not built it up for less.

I bought a richer slide from Mr. H. I measured it and saw that it was probably a standard one that had been put in a lathe and had the bottom face skimmed to lower it.

I think it's the angled bit at the bottom that makes the difference.

Yes, correct; if its base is skimmed the slide sits lower In the carb. That effectively reduces the height of the chamfered part and the total area of the aperture. That gives a richer mixture. To weaken the mixture the chamfered opening would need to be made taller.

You have also lowered the needle altering mix through the range.
The cut away is the leading edge exposing air flow over the needle jet.

The cut away is ground on one side only.
Solder on brass does not come off.

Precisely. To change the mixture strength controlled by the cutaway, without changing that controlled by the needle, the angle of the cutaway needs to be changed without changing the base of the slide. That’s why I was surprised to find that the “richer” one supplied by our host appeared to have been skimmed on the base, rather than manufactured with a shallower cutaway. The needle would need to be raised to put it back where it originally sat in its jet.

PeteF wrote: ↑

Wed Sep 09, 2020 4:15 pm

As you found, it makes a big difference.
If you'd just gone to our host's technical notes you would have found the correct info.
Sticking bits of solder on the cutaway is hardly very precise imho.

Can you help me with a link. I can not find anything on slide cutaways.

It's all in there on carb tuning.