SU carb - too small?

baz said:

Pardon our interruption...

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I was too late....

trident sam said:

That is Baz with a TM Mikuni

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I suggest that it does not matter what carburetor you have, the same bike with fuel injection will always be faster when using petrol as fuel. Carburetors do not offer fine enough mixture control. They only work well with methanol, because it hides-up the tuning errors. Because of it's high latent heat of vaporisation, the jets flow almost twice as much, so the errors are half as significant. If you get a bike going fast when using carburetors with petrol, it is probably due to luck and extreme patience, and the weather would affect it.

acotrel said:

I suggest that it does not matter what carburetor you have, the same bike with fuel injection will always be faster when using petrol as fuel. Carburetors do not offer fine enough mixture control. They only work well with methanol, because it hides-up the tuning errors. Because of it's high latent heat of vaporisation, the jets flow almost twice as much, so the errors are half as significant. If you get a bike going fast when using carburetors with petrol, it is probably due to luck and extreme patience, and the weather would affect it.

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Wrong Al.
Again.

When you say something is wrong, you should probably be able to justify your comment. During WW2, German BF109 and FW190 had fuel injected BMW engines - their speeds were higher. It is dumb to believe that rich mixtures give more power. When we tune a motorcycle for performance, it is normal to start with a rich mixture then lean-off. I do not trust Amal needles. With Mikuni needles, there are lists which give all of the dimensions for the needles, and they are more reliable. With Amal needles, I have never seen lists which were so specific about tapers and diameters. With carburetors, the anulus between the needle jet and the needle determines the metering for mid-throttle opening, It is more important than the main jet size which always supplies more fuel than the needle and needle jet. With a really well-tuned raced bike, winding the throttle on in a controlled fashion is usually necessary to get maximum acceleration. Particularly on tight circuits which have lots of corners. When we use part throttle with a carburetor, the main jet size is not relevant unless it is too small. If you turn out of a corner and onto a straight and whack the throttle open, you will usually be slower then the guy who feeds the throttle on from a long way back in the corner. A lot of guys do not seem to know where their power-band is, in the rev-range. On my bike, it is between 5,500 RPM and 7,300 RPM. When I ride on Winton Raceway - the slowest corner is taken in 2nd gear at about 50 MPH with the rear wheel losing traction, the motor revving at about 6000 RPM. The MK2 Amal carburetors are similar to Mikuni VM. I make my own needle jets, but I use methanol fuel. Petrol is more difficult and ultimately more expensive, I don't think petrol could be tuned with home-made needle jets unless they were calibrated with a flow meter. The only other way might be to adjust the ignition timing on a dyno after using an oxygen probe to set the jetting, The ignition timing can be continuously altered - not in steps.

acotrel said:

During WW2, German BF109 and FW190 had fuel injected BMW engines

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The Bf109 had a Daimler-Benz engine not BMW.

Some of what Al says is correct, whacking open a conventional slide carb will usually result in a bog down, then if really unlucky the power will hit and high side you.
The SU though is a CV carb, hitting the throttle hard will only lift the slide enough to keep the equilibrium in the inlet tract, as the RPM and vacuum increase, the slide lifts further, allowing more fuel and air into the engine. All diaphragm type CV carbs work on this principle too.
If the Bore of the carb is too small, the slide will be fully up at part throttle and / or low RPM and vacuum, effectively strangling the engine.

CV carbs tend to be quite a bit bigger than a slide carb for a similar engine

trappnman said:

When you do get around to it, check the o-ring (or rings on later carbs) on the rotary enricher device. My bike ran fine one day, and then the next really, really rich. The o-ring had become hard, so passed neat fuel all the time. I replaced it with a Viton one, and all good again.

I took me 1 whole riding season to figure this out. I had just rebuilt the engine on my 73 ironhead, and while I suspected carb, I suspected more something in the top end. I cleaned carb multiple times, changed needles, changed spring. Couldn't go 20 miles without really fouling plugs. Didn't even consider that O ring, I didn't realize that that o ring was air flow not gas flow... baffled a few of us. And Im not new to SUs.

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I just found the packet of Viton O rings which I bought, so for reference they are BS013 ( 1.78mm Section 10.82mm Bore).

A small amount of good mixture is often better than a large amount of bad mixture. I suggest the main thing which makes a race bike using petrol slower than a race bike which uses methanol is the fine-ness of tuning. When we use methanol, we can get closer to the optimum tuning. With methanol, we use twice as much, so the tuning errors are half as significant. With an SU carburetor or other constant velocity carb, the jetting is adjusted to suit the vacuum. With a non-CV carb, if the jetting gives the best performance, the trade-off is the rapidity with which the throttle can be opened. So what is quicker - a big gutful of rich mixture, or a fed-on amount of appropriately lean mixture? Most of the Japanese big road bikes of the 1970s had CV carburetors. They make life easier for inexperienced riders.
Smaller ports do not strangle motors unless the rider continually uses full throttle when coming up through the gears, a throttle which is fed-on slower with correct mixture usually gives better results.

It has only been a few years ago that I realised the significance of getting the jetting really right when using methanol. I always thought I had much more lee-way. Petrol is twice as difficult, but has a higher calorific value. Petrol with a CV carb should be as good as a conventional carb using methanol.