CV carb brainstorming

[Propellor]

Chaps.

I'm wanting to modify the airbox on my gpz500 special. The subframe will be totally reworked so I can work the airbox shape to suit. But just what does the airbox exactly do? The std one is an enormous volume. Why? What if I, say, halved the volume? How critical is the location of the inlet to airbox with respect to location of inlet to carbs? I already found out that the airbox intake aperture area is important. The option of slide carbs is there, but there's the cost....

Sorry that this is an enquiry about a twin, but I'm seeing it as a general technical question, so hopefully you'll forgive me.

[SteveC#222]

http://www.superstreetbike.com/motorcycle-airbox-component-air-force

The more modern the bike the less you can mess around with it. On modern fuel injected, electronically controlled, multi sensored bikes you would probably need a long session on the dyno to get the fueling right.  On an old GPZ you might get away with it a bit more but cv's can be tricky.

[Propellor]

http://www.superstreetbike.com/motorcycle-airbox-component-air-force

The more modern the bike the less you can mess around with it. On modern fuel injected, electronically controlled, multi sensored bikes you would probably need a long session on the dyno to get the fueling right.  On an old GPZ you might get away with it a bit more but cv's can be tricky.

Thanks for the link Steve. The ram air side of things doesn't apply to the gpz. But the airbox resonance thing probably does. But is this a volume thing or a positional thing? In other words, could I have a long slim airbox, which placed the airbox intake snout well away from the carb bell mouth, but had a lower overall volume, and get similar results?

I know (from tinkering) that the intake snout area is important.

[Propellor]

http://advrider.com/index.php?threads/the-bst-40-bible.347184/

I attached the above link in the mikuni bst40 problems thread. I don't want to hijack that thread with the following, but would welcome the thoughts of others.

In the attached link, the guy shows how to improve throttle response in two ways. By drilling the vent holes out slightly larger and by shortening the spring. I think he says he's not convinced by the second and I feel the same.

I've never undertaken either of the above - yet - so if anyone has hard experience, please correct errors!

The drilling of the vent holes I can see how that works. I'd be worried, though, that once drilled there's no turning back if you don't get the desired result. It might be worse! I'd want a spare to work on/test.

The shortening of the spring is something I've mused over myself, but never tried - yet. I can see how the throttle response would be "easier" but, unlike the hole drilling, this mod would surely upset the fuel slope across the range? We're not altering spring rate, but we are altering preload and therefore the height of the slide and needle for a given depression? Have I got that wrong?

I've considered trying this idea myself, not to improve response, but to attempt to bring the fuel slope back into line after changing or even removing the airbox.

Any thoughts?

[SteveC#222]

What about a quick response throttle kit? 1/4 turn from closed to fully open. Popular on dirt bikes but I think you can get them for road/racing bikes too.

[Propellor]

What about a quick response throttle kit? 1/4 turn from closed to fully open. Popular on dirt bikes but I think you can get them for road/racing bikes too.

I don't think it would change the response on a cv carb very much. Isn't one of the "features" of a cv carb that it adjusts itself according to the depression caused by airflow? So even if you whack open the throttle what you are doing is whacking open the butterfies to ask for more airflow, but the diaphragm has to respond to the actual change in depression in its own sweet time. One way to lessen the time is to allow the chamber acting on the diaphragm to empty (or fill, depending on which chamber) quicker, by providing bigger holes. Trimming the spring is supposed to allow quicker action too. I guess it would, but I'm concerned of other side effects. Unless the side effects are what you are looking for!

A side effect to watch out for when opening the holes out, I believe, is slide flutter. I believe that smaller holes help a little in damping out flutter.

Theoretically, I suppose changing chamber volume would affect response time. But probably damping too?

A fast action device would be worth fitting though, to lessen hand movement.

[Oldtimer]

How simple were the old Amal carbs, especially GP carbs

[SteveC#222]

How simple were the old Amal carbs, especially GP carbs

Ahh...how simple were the old K63's on my old Ural, the slide wasn't even solid just a bent piece of metal!

[Propellor]

To be fair though, I wouldn't have thought the ural carb pictured would have any fewer components than a keihin cvk34 fitted to my gpz500. Possibly more.

The lectron is probably the one with fewer components? Or the wal Phillips?

Actually, the carbs on my Yamaha xz550 which I had in the early eighties, had very few components. More like a car style carburettor. Downdraught. The fuelling on it was iffy all the same. It was a pretty crap bike all round in fact. 

[SteveC#222]

To be fair though, I wouldn't have thought the ural carb pictured would have any fewer components than a keihin cvk34 fitted to my gpz500. Possibly more.

Possibly so, but can you balance the carbs on the GPZ by sitting onboard the bike, taking off the air intake hoses, taking your shoes and socks off and sticking a big toe into each carb mouth with the slide resting on each big toe nail and feeling which lifted first when you open the throttle!!... ....seen it done!

[Moto63]

Ha ha ha, one way of clipping ya toe nails I suppose Steve. Personally I just use a good ol fashioned clipper

[Propellor]

To be fair though, I wouldn't have thought the ural carb pictured would have any fewer components than a keihin cvk34 fitted to my gpz500. Possibly more.

Possibly so, but can you balance the carbs on the GPZ by sitting onboard the bike, taking off the air intake hoses, taking your shoes and socks off and sticking a big toe into each carb mouth with the slide resting on each big toe nail and feeling which lifted first when you open the throttle!!... ....seen it done!

I think you may have hit upon the cv carb's biggest drawback!

 

Cheers Steve.

[Oldtimer]

Ha ha ha, one way of clipping ya toe nails I suppose Steve. Personally I just use a good ol fashioned clipper

Personally I use wire side cutters. Much bigger to get hold of and can really cut  them big hard toe nails.

[timbo]

When are you opening the first Thumper Club nail bar? 

[guest564]

The airbox is that size to create a large volume of 'still' air to improve the throttle response. I would have thought that if you were to increase the size of the entry to the airbox that it would help cancel out the effect of a smaller box but at the expense of more noise and possibly a detrimental effect to the jetting. Dynojet used to make jet kits for running without an airbox, the main feature was a smaller air corrector jet but you had to drill and tap to fit it because the stock carbs usually had a fixed orifice.