Carburretor Thoughts

[Vixen]

Hi Roger,

More watchmaking skills in action. 

I recognise the piston but it's a lot smaller than the ones on my MGB

Mike

[Roger B]

Thank you Mike  I would put this one 1/5 to 1/6 full size.

I turned the piston to a measured 12.02mm to match the plug gauge but couldn't check it in the bore due to the length of the plug. The plug was turned to 6.00mm as I can bore to suit. When I removed it from the chuck and tried it I got a good fit. It moves smoothly and if I block the two 0.8 mm holes there is definite resistance 

Next up was to mount the body in the 4 jaw independent to make the hole for the throttle and then turn it round ready to bore the holes for the piston and jet.

[Roger B]

I removed the bulk of the material with a 5mm end mill and then used a 3mm end mill as a boring bar to finish off. Next I faced off the surface where the cover will fit to ensure it was a right angles to the bore. The bore for the jet was more of a problem as I could not see it properly. I started with a2.5 mm drill and then set up a 2mm endmill as a boring bar. I centred this by eye and then fed it in moving the crossslide out in 0.05 mm steps until I got a spiral of swarf to show it was cutting all round. The hole was then finished with a 3mm reamer.

[Roger B]

Next up was to finish the cover fixing flange. This was drilled and milled with the Proxxon mill with plenty of aluminium snow, promptly cleaned up by Henry. To give an idea of the scale I have included an SU number 5 needle as used in a non emission controlled MGB or with a different spring in my much modified 1300 cc Triumph engine.

[Roger B]

After drilling and taping a few holes I assembled the piston end. It moved freely most of the way but kept jamming in the closed position    On close inspection there seemed to be a small ridge at the bottom of the bore  I set the body up again in the 4 jaw and took another skim out. The Plug is 6.00mm diameter and the bore is around 6.08mm. This time there was no jamming so I plugged the jet hole with a piece of 3mm rod, blocked the throttle bore with my finger and gave a suck. The piston rose as I sucked but tended to remain there as there in no spring yet  At least it has a chance of working 

<a href="https://www.youtube.com/watch?v=spE33rp4KrQ" target="_blank">http://www.youtube.com/watch?v=spE33rp4KrQ</a>

The next stage was to make the throttle barrel from 10mm diameter silver steel. The 3mm spigot for the lever was turned and then parted off to 15mm. The cross hole was started in the Proxxon but for the 5.3mm drill and reaming I moved over to the big drill.

[Vixen]

Roger,

Thats looking good, a replica of an SU. carb in miniature. 

What are your thoughts on the fuel supply? A float chamber, or a chicken hopper or a simple clunk tank with the ability to raise and lower the fuel level.

Mike

[Roger B]

Thank you Mike  I am intending to follow the Amsbury route and use a pump and overflow system, but as ever I will see how it works and maybe try other options. If the depression over the jet is more stable maybe the fuel level is less important    That's the fun of experimenting   

Having finished the throttle barrel I tried a few more suction trials but noted that the piston tended to rotate so the 0.8mm ports were on the wrong side. Time to make the anti rotation pin. The Amsbury drawings didn't seem to have any form of rotation control  This was just a stub of 1.5 mm silver steel turned down to 0.9mm at the end. After the initial test I removed it with a magnet (to avoid dismantling it all) and then replaced it with some medium strength thread locker so I could remove it if I had to.

Next up the throttle lever.

[Admiral_dk]

Now you are really going down in size here and like Mike I certainly follow your experiments (though I haven't got many comments so far). It will be interesting to see how far down you can go and still have it working as intended             

The Amsbury drawings didn't seem to have any form of rotation control

I'm sure you remember how the full size works here (no I have never opened an SU - only the Japanese versions on Motorcycles) .... but I do not see a diaphragm in this model    so I understand why you need another device to solve this.

Per

[Art K]

We have some SU carbs at work I can look at them Monday, if I am at work. If I'm lucky they will already be disassembled. Very interesting project Roger I hope it works.
Art

[Roger B]

Per, there were two types of CD carbs in common use for cars. The Stromberg version used a thin rubber diaphragm like the bikes and the SU used a closely machined piston in a cylinder. I don't think I could make a diaphragm in this size but a piston is possible.

[fumopuc]

Hi Roger,
I have seen you there working with little Proxxon lathe.
Are you still happy with it ?

[Roger B]

Hi Achim, yes it does what I want it to do now I have sorted out the collet nut. The supplied 3 jaw chuck was no good however I am thinking of buying some Sherline chucks for it. I also need to sort out a DTI fixing for the crosslide and a toolpost grinder for making needles for this carburettor.

[gbritnell]

HI Roger,
When I got interested in motorcycles in my teens my first bike was a Triumph. The Triumphs had Amal carbs, first Monoblocks then Concentrics. These were slide type carbs operated by a cable. I then got into Japanese machines with Mikunis, cable operated. The thing all these carbs had in common was that the slide had a cutaway on the inlet side of the carb. When modifying a Triumph engine for more power you could actually get slides with different amounts of cutaway to tune the throttle response. On the Japanese carbs all you could do was change the jetting.
Now onto the CV type carbs. They have similar features to their cable operated brethren, cutaway slide, tapered needle with jet, low speed circuit etc. but the distinguishing feature is the vacuum operated slide.
My questions to you are, how are you going to regulate the vacuum on the slide, how are you going to calibrate the needle and needle jet size, how are you going to determine the amount of cutaway on the slide.
I only ask these questions because I like you have always wanted to design a more efficient miniature carb that would function properly. The machining to me is the easy part, the stumbling block was trying to answer the questions I posed without tons of experimentation.
gbritnell

[Roger B]

Thank you for your interest George.  I know we have discussed carbs in the past on more than one occasion.

I have attached a diagram of the type of SU I am basing my model on. The pistons (slides) do not have cutaways. The tuning options are the spring for the piston and the needle. There were 2 or 3 jet sizes and needle sets to cope with the range of carbs between 1 1/8” and 2”.

The top of the piston is connected to the manifold depression via the L shaped port(s) the underside is connected to atmosphere by a port or ports in the air cleaner flange (to compensate for pressure drop caused by the filter). The jet can be moved up and down with an adjusting nut to fine tune the mixture. It can also be lowered with a lever to enrich the mixture for starting.

If the manifold depression is sufficient to overcome the weight of the piston and the force from the spring the piston will rise. This opens the venturi and causes the depression to reduce until a balance point is found. Changing the air flow (open or close the throttle) will change the balance point. The needle moves together with the piston and opens and closes the jet orifice to maintain a suitable mixture. The full size version also has a hydraulic damper to aid sudden acceleration. If the throttle is open rapidly the depression will fall rapidly and some of the fuel condenses on the manifold leaning out the mixture and causing a ‘flat spot’. The damper delays the rise of the piston temporarily increasing the depression over the jet and hence the fuel flow.

The set up sequence was to select a suitable spring to suit the airflow range so that most of the piston movement was used. This could be done on a Dyno or be based on experience of similar engines. Next a needle was guessed based on copying a similar engine. The needle was then optimised on a dyno if you were posh or by road testing if not.

That’s the real world so how I am planning to set this up.

First I need to find an suitable weight + spring for the piston. I plan to do this by fitting the carb to the engine with a blank in place of the jet and turning the engine at a range of speeds with the starter/generator and putting small weights on top of the piston rod. A combination of different engine speeds and throttle positions should allow me to find an initial setting.

Second I need to find an initial needle. I can determine from the current carb the jet and needle diameters that it runs with. I will make a needle with that diameter in the middle and a constant taper (10°???). Next I will try and get the engine to start, moving the jet up and down to hopefully find a running position. If it won’t run I can hopefully determine too lean/rich from the plugs. If it will run I can try and determine the optimum jet position for various piston positions by measuring the piston rod height above the cover and the jet height. Knowing the diameter and taper of the needle I should be able to calculate a better profile. Repeat until satisfied.

There are a few more unknowns I may have to deal with. The full size version has a raised bridge across the jet area. I don’t know if I will need one. The weight of the piston will be constant, the force of the spring will rise as the piston rises. I will have to determine if the balance between the two is important.

This is all theory  It may work    If it doesn’t I will go back to fuel injection as I know I can make that work with petrol (gasoline).

[gbritnell]

Hi Roger,
Thanks for the response. Many years ago when the NAMES show started there was always a booth setup from Strictly I.C. Engine magazine. I had won an award through there program and thus became friends with Robert Washburn the editor. He had a friend, Lee Root, who made some remarkable miniature engines. He also contributed an article to the magazine which provided plans for a somewhat replica of a Stromberg 97 carburetor. This carb had a float chamber, air bleeds etc. I built one of them but it was out of scale for my engine so I never used it other than to try it. The biggest issue was getting enough float leverage to open and close the needle properly.
I spoke to Lee on several occasions and found that he was, to me, rather guarded in providing information about his builds.
Lee had built a 4 cylinder engine that had a throttle body fuel injection system on it. <a href="https://www.youtube.com/watch?v=wxsSx7OJ29k" target="_blank">http://www.youtube.com/watch?v=wxsSx7OJ29k</a> The engine ran well and it was intriguing to look into the throat of the carb and watch the minute spray of gas coming out of the nozzle. I questioned Lee several times about how it worked but like I said he was kind of closed mouth about it.
It's a shame that he expertise wasn't passed along but that's the way some fellows are.
I adopted my signature 'Talent unshared is talent wasted' partly because of my experiences with some builders over the years. When you go to meet your maker it doesn't matter how good you were but how you interacted with others.
It would have certainly been a great help for model engine builders to have had some of Lee's knowledge.
gbritnell